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The present thesis is primarily concerned with the application of the functional renormalization group (FRG) to spin systems. In the first part, we study the critical regime close to the Berezinskii-Kosterlitz-Thouless (BKT) transition in several systems. Our starting point is the dual-vortex representation of the two-dimensional XY model, which is obtained by applying a dual transformation to the Villain model. In order to deal with the integer-valued field corresponding to the dual vortices, we apply the lattice FRG formalism developed by Machado and Dupuis [Phys. Rev. E 82, 041128 (2010)]. Using a Litim regulator in momentum space with the initial condition of isolated lattice sites, we then recover the Kosterlitz-Thouless renormalization group equations for the rescaled vortex fugacity and the dimensionless temperature. In addition to our previously published approach based on the vertex expansion [Phys. Rev. E 96, 042107 (2017)], we also present an alternative derivation within the derivative expansion. We then generalize our approach to the O(2) model and to the strongly anisotropic XXZ model, which enables us to show that weak amplitude fluctuations as well as weak out-of-plane fluctuations do not change the universal properties of the BKT transition.
In the second part of this thesis, we develop a new FRG approach to quantum spin systems. In contrast to previous works, our spin functional renormalization group (SFRG) does not rely on a mapping to bosonic or fermionic fields, but instead deals directly with the spin operators. Most importantly, we show that the generating functional of the irreducible vertices obeys an exact renormalization group equation, which resembles the Wetterich equation of a bosonic system. As a consequence, the non-trivial structure of the su(2) algebra is fully taken into account by the initial condition of the renormalization group flow. Our method is motivated by the spin-diagrammatic approach to quantum spin system that was developed more than half a century ago in a seminal work by Vaks, Larkin, and Pikin (VLP) [Sov. Phys. JETP 26, 188 (1968)]. By embedding their ideas in the language of the modern renormalization group, we avoid the complicated diagrammatic rules while at the same time allowing for novel approximation schemes. As a demonstration, we explicitly show how VLP's results for the leading corrections to the free energy and to the longitudinal polarization function of a ferromagnetic Heisenberg model can be recovered within the SFRG. Furthermore, we apply our method to the spin-S Ising model as well as to the spin-S quantum Heisenberg model, which allows us to calculate the critical temperature for both a ferromagnetic and an antiferromagnetic exchange interaction. Finally, we present a new hybrid formulation of the SFRG, which combines features of both the pure and the Hubbard-Stratonovich SFRG that were published recently [Phys. Rev. B 99, 060403(R) (2019)].
From the colour glass condensate to filamentation: systematics of classical Yang–Mills theory
(2019)
The non-equilibrium early time evolution of an ultra-relativistic heavy ion collision is often described by classical lattice Yang–Mills theory, starting from the colour glass condensate (CGC) effective theory with an anisotropic energy momentum tensor as initial condition. In this work we investigate the systematics associated with such studies and their dependence on various model parameters (IR, UV cutoffs and the amplitude of quantum fluctuations) which are not yet fixed by experiment. We perform calculations for SU() and SU(), both in a static box and in an expanding geometry. Generally, the dependence on model parameters is found to be much larger than that on technical parameters like the number of colours, boundary conditions or the lattice spacing. In a static box, all setups lead to isotropisation through chromo-Weibel instabilities, which is illustrated by the accompanying filamentation of the energy density. However, the associated time scale depends strongly on the model parameters and in all cases is longer than the phenomenologically expected one. In the expanding system, no isotropisation is observed for any parameter choice. We show how investigations at fixed initial energy density can be used to better constrain some of the model parameters.
The brain is a large complex system which is remarkably good at maintaining stability under a wide range of input patterns and intensities. In addition, such a stable dynamical state is able to sustain essential functions, including the encoding of information about the external environment and storing memories. In order to succeed in these challenging tasks, neural circuits rely on a variety of plasticity mechanisms that act as self-organizational rules and regulate their dynamics. Based on toy models of self-organized criticality, this stable state has been proposed to be a phase transition point, poised between distinct types of unhealthy dynamics, in what has become known as the critical brain hypothesis. It is not yet known, however, if and how self-organization could drive biological neural networks towards a critical state while maintaining or improving their learning and memory functions.
Here, we investigate the emergence of criticality signatures in the form of neuronal avalanches due to self-organizational plasticity rules in a recurrent neural network. We show that power-law distributions of events, widely observed in experiments, arise from a combination of biologically inspired synaptic and homeostatic plasticity but are highly dependent on the external drive. Additionally, we describe how learning abilities and fading memory emerge and are improved by the same self-organizational processes. We finally propose an application of these enhanced functions, focusing on sequence and simple language learning tasks.
Taken together, our results suggest that the same self-organizational processes can be responsible for improving the brain’s spatio-temporal learning abilities and memory capacity while also giving rise to criticality signatures under particular input conditions, thus proposing a novel link between such abilities and neuronal avalanches. Although criticality was not verified, the detailed study of self-organization towards critical dynamics further elucidates its potential emergence and functions in the brain.
We calculate ratios of higher-order susceptibilities quantifying fluctuations in the number of net-protons and in the net-electric charge using the Hadron Resonance Gas (HRG) model. We take into account the effect of resonance decays, the kinematic acceptance cuts in rapidity, pseudo-rapidity and transverse momentum used in the experimental analysis, as well as a randomization of the isospin of nucleons in the hadronic phase. By comparing these results to the latest experimental data from the STAR Collaboration, we determine the freeze-out conditions from net-electric charge and net-proton distributions and discuss their consistency.
Scanning probe microscopy (SPM) has become an essential surface characterization technique in research and development. By concept, SPM performance crucially depends on the quality of the nano-probe element, in particular, the apex radius. Now, with the development of advanced SPM modes beyond morphology mapping, new challenges have emerged regarding the design, morphology, function, and reliability of nano-probes. To tackle these challenges, versatile fabrication methods for precise nano-fabrication are needed. Aside from well-established technologies for SPM nano-probe fabrication, focused electron beam-induced deposition (FEBID) has become increasingly relevant in recent years, with the demonstration of controlled 3D nanoscale deposition and tailored deposit chemistry. Moreover, FEBID is compatible with practically any given surface morphology. In this review article, we introduce the technology, with a focus on the most relevant demands (shapes, feature size, materials and functionalities, substrate demands, and scalability), discuss the opportunities and challenges, and rationalize how those can be useful for advanced SPM applications. As will be shown, FEBID is an ideal tool for fabrication/modification and rapid prototyping of SPM-tipswith the potential to scale up industrially relevant manufacturing.
Five decades of US, UK, German and Dutch music charts show that cultural processes are accelerating
(2019)
Analysing the timeline of US, UK, German and Dutch music charts, we find that the evolution of album lifetimes and of the size of weekly rank changes provide evidence for an acceleration of cultural processes. For most of the past five decades, number one albums needed more than a month to climb to the top, nowadays an album is in contrast top ranked either from the start, or not at all. Over the last three decades, the number of top-listed albums increased as a consequence from roughly a dozen per year, to about 40. The distribution of album lifetimes evolved during the last decades from a log-normal distribution to a power law, a profound change. Presenting an information–theoretical approach to human activities, we suggest that the fading relevance of personal time horizons may be causing this phenomenon. Furthermore, we find that sales and airplay- based charts differ statistically and that the inclusion of streaming affects chart diversity adversely. We point out in addition that opinion dynamics may accelerate not only in cultural domains, as found here, but also in other settings, in particular in politics, where it could have far reaching consequences.
Fission program at n_TOF
(2019)
Since its start in 2001 the n_TOF collaboration developed a measurement program on fission, in view of advanced fuels in new generation reactors. A special effort was made on measurement of cross sections of actinides, exploiting the peculiarity of the n_TOF neutron beam which spans a huge energy domain, from the thermal region up to GeV. Moreover fission fragment angular distributions have also been measured. An overview of the cross section results achieved with different detectors is presented, including a discussion of the 237Np case where discrepancies showed up between different detector systems. The results on the anisotropy of the fission fragments and its implication on the mechanism of neutron absorption, and in applications, are also shown.
In this thesis, we presented the theoretical description of the magnetic properties of various frustrated spin systems. Especially in search of exotic states, such as quantum spin liquids, magnetically frustrated systems have been subject of intense research within the last four decades. Relating experimental observations in real materials with theoretical models that capture those exotic magnetic phenomena has been one of the great challenges within the field of magnetism in condensed matter.
In order to build such a bridge between experimental observations and theoretical models, we followed two complementary strategies in this thesis. One strategy was based on first principles methods that enable the theoretical prediction of electronic properties of real materials without further experimental input than the crystal structure. Based on these predictions, low-energy models that describe magnetic interactions can be extracted and, through further theoretical modelling, can be compared to experimental observations. The second strategy was to establish low-energy models through comparison of data from experiments, such as inelastic neutron scattering intensities, with calculated predictions based on a variety of plausible magnetic models guided by microscopic insights. Both approaches allow to relate theoretical magnetic models with real materials and may provide guidance for the design of new frustrated materials or the investigation of promising models related to exotic magnetic states.
This work presents the first experimental observation of the attractive strong interaction between a proton and a multi-strange baryon (hyperon) Ξ−. The result is extracted from two-particle correlations of combined p−Ξ−⊕p¯¯¯−Ξ¯¯¯¯+ pairs measured in p-Pb collisions at sNN−−−√=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range [3.6,5.3] is found. Since the measured p−Ξ−⊕p¯¯¯−Ξ¯¯¯¯+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range [1.8,3.7]. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter at saturation densities and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.
This work presents the first experimental observation of the attractive strong interaction between a proton and a multi-strange baryon (hyperon) Ξ−. The result is extracted from two-particle correlations of combined p−Ξ−⊕p¯−Ξ¯+ pairs measured in p-Pb collisions at sNN−−−√=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range [3.6,5.3] is found. Since the measured p−Ξ−⊕p¯−Ξ¯+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range [1.8,3.7]. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter at saturation densities and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.
This Letter presents the first experimental observation of the attractive strong interaction between a proton and a multistrange baryon (hyperon) Ξ−. The result is extracted from two-particle correlations of combined p−Ξ−⊕¯p−¯Ξ+ pairs measured in p−Pb collisions at √sNN=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range [3.6, 5.3] is found. Since the measured p−Ξ−⊕¯p−¯Ξ+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range [1.8, 3.7]. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.
We report on new measurements of Cabibbo-suppressed semileptonic D+s decays using 3.19 fb−1 of e+e− annihilation data sample collected at a center-of-mass energy of 4.178~GeV with the BESIII detector at the BEPCII collider. Our results include branching fractions B(D+s→K0e+νe)=(3.25±0.38(stat.)±0.16(syst.))×10−3 and B(D+s→K∗0e+νe)=(2.37±0.26(stat.)±0.20(syst.))×10−3 which are much improved relative to previous measurements, and the first measurements of the hadronic form-factor parameters for these decays. For D+s→K0e+νe, we obtain f+(0)=0.720±0.084(stat.)±0.013(syst.), and for D+s→K∗0e+νe, we find form-factor ratios rV=V(0)/A1(0)=1.67±0.34(stat.)±0.16(syst.) and r2=A2(0)/A1(0)=0.77±0.28(stat.)±0.07(syst.).
We extend the parton‐hadron‐string dynamics (PHSD) transport approach in the partonic sector by explicitly calculating the total and differential partonic scattering cross sections as a function of temperature T and baryon chemical potential μB on the basis of the effective propagators and couplings from the dynamical quasiparticle model (DQPM) that is matched to reproduce the equation of state of the partonic system above the deconfinement temperature Tc from lattice quantum chromodynamics (QCD). We calculate the collisional widths for the partonic degrees of freedom at finite T and μB in the time‐like sector and conclude that the quasiparticle limit holds sufficiently well. Furthermore, the ratio of shear viscosity η over entropy density s, that is, η/s, is evaluated using the collisional widths and compared to lattice QCD(lQCD) calculations for μB = 0 as well. We find that the ratio η/s does not differ very much from that calculated within the original DQPM on the basis of the Kubo formalism. Furthermore, there is only a very modest change of η/s with the baryon chemical μB as a function of the scaled temperature T/Tc(μB). This also holds for a variety of hadronic observables from central A + A collisions in the energy range 5 GeV urn:x-wiley:00046337:media:asna201913708:asna201913708-math-0001 200 GeV when implementing the differential cross sections into the PHSD approach. Accordingly, it will be difficult to extract finite μB signals from the partonic dynamics based on “bulk” observables.
The ALICE collaboration at the CERN LHC reports novel measurements of jet substructure in pp collisions at s√= 7 TeV and central Pb-Pb collisions at sNN−−−√ = 2.76 TeV. Jet substructure of track-based jets is explored via iterative declustering and grooming techniques. We present the measurement of the momentum sharing of two-prong substructure exposed via grooming, the zg, and its dependence on the opening angle, in both pp and Pb-Pb collisions. We also present the first measurement of the distribution of the number of branches obtained in the iterative declustering of the jet, which is interpreted as the number of its hard splittings. In Pb-Pb collisions, we observe a suppression of symmetric splittings at large opening angles and an enhancement of splittings at small opening angles relative to pp collisions, with no significant modification of the number of splittings. The results are compared to predictions from various Monte Carlo event generators to test the role of important concepts in the evolution of the jet in the medium such as color coherence.
LICE is one of the four major LHC experiments at CERN. When the accelerator enters the Run 3 data-taking period, starting in 2021, ALICE expects almost 100 times more Pb-Pb central collisions than now, resulting in a large increase of data throughput. In order to cope with this new challenge, the collaboration had to extensively rethink the whole data processing chain, with a tighter integration between Online and Offline computing worlds. Such a system, code-named ALICE O2, is being developed in collaboration with the FAIR experiments at GSI. It is based on the ALFA framework which provides a generalized implementation of the ALICE High Level Trigger approach, designed around distributed software entities coordinating and communicating via message passing.
We will highlight our efforts to integrate ALFA within the ALICE O2 environment. We analyze the challenges arising from the different running environments for production and development, and conclude on requirements for a flexible and modular software framework. In particular we will present the ALICE O2 Data Processing Layer which deals with ALICE specific requirements in terms of Data Model. The main goal is to reduce the complexity of development of algorithms and managing a distributed system, and by that leading to a significant simplification for the large majority of the ALICE users.
Measurements of K∗(892)0 and ϕ(1020) resonance production in Pb-Pb and pp collisions at sNN−−−√ = 5.02 TeV with the ALICE detector at the Large Hadron Collider are reported. The resonances are measured at midrapidity (|y| < 0.5) via their hadronic decay channels and the transverse momentum (pT) distributions are obtained for various collision centrality classes up to pT = 20 GeV/c. The pT-integrated yield ratio K∗(892)0/K in Pb-Pb collisions shows significant suppression relative to pp collisions and decreases towards more central collisions. In contrast, the ϕ(1020)/K ratio does not show any suppression. Furthermore, the measured K∗(892)0/K ratio in central Pb-Pb collisions is significantly suppressed with respect to the expectations based on a thermal model calculation, while the ϕ(1020)/K ratio agrees with the model prediction. These measurements are an experimental demonstration of rescattering of K∗(892)0 decay products in the hadronic phase of the collisions. The K∗(892)0/K yield ratios in Pb-Pb and pp collisions are used to estimate the time duration between chemical and kinetic freeze-out, which is found to be ∼ 4-7 fm/c for central collisions. The pT-differential ratios of K∗(892)0/K, ϕ(1020)/K, K∗(892)0/π, ϕ(1020)/π, p/K∗(892)0 and p/ϕ(1020) are also presented for Pb-Pb and pp collisions at sNN−−−√ = 5.02 TeV. These ratios show that the rescattering effect is predominantly a low-pT phenomenon.
We study the hadronic decays of Λ+c to the final states Σ+η and Σ+η′, using an e+e− annihilation data sample of 567 pb−1 taken at a center-of-mass energy of 4.6 GeV with the BESIII detector at the BEPCII collider. We find evidence for the decays Λ+c→Σ+η and Σ+η′ with statistical significance of 2.5σ and 3.2σ, respectively. Normalizing to the reference decays Λ+c→Σ+π0 and Σ+ω, we obtain the ratios of the branching fractions B(Λ+c→Σ+η)B(Λ+c→Σ+π0) and B(Λ+c→Σ+η′)B(Λ+c→Σ+ω) to be 0.35±0.16±0.03 and 0.86±0.34±0.07, respectively. The upper limits at the 90\% confidence level are set to be B(Λ+c→Σ+η)B(Λ+c→Σ+π0)<0.58 and B(Λ+c→Σ+η′)B(Λ+c→Σ+ω)<1.2. Using BESIII measurements of the branching fractions of the reference decays, we determine B(Λ+c→Σ+η)=(0.41±0.19±0.05)% (<0.68%) and B(Λ+c→Σ+η′)=(1.34±0.53±0.21)% (<1.9%). Here, the first uncertainties are statistical and the second systematic. The obtained branching fraction of Λ+c→Σ+η is consistent with the previous measurement, and the branching fraction of Λ+c→Σ+η′ is measured for the first time.
Event-shape engineering for the D-meson elliptic flow in mid-central Pb–Pb collisions at √sNN = 5.02
(2019)
The production yield of prompt D mesons and their elliptic flow coefficient v2 were measured with the Event-Shape Engineering (ESE) technique applied to mid-central (10-30% and 30-50% centrality classes) Pb-Pb collisions at the centre-of-mass energy per nucleon pair sNN−−−√=5.02 TeV, with the ALICE detector at the LHC. The ESE technique allows the classification of events, belonging to the same centrality, according to the azimuthal anisotropy of soft particle production in the collision. The reported measurements give the opportunity to investigate the dynamics of charm quarks in the Quark-Gluon Plasma and provide information on their participation in the collective expansion of the medium. D mesons were reconstructed via their hadronic decays at mid-rapidity, |η|<0.8, in the transverse momentum interval 1<pT<24 GeV/c. The v2 coefficient is found to be sensitive to the event-shape selection confirming a correlation between the D-meson azimuthal anisotropy and the collective expansion of the bulk matter, while the per-event D-meson yields do not show any significant modification within the current uncertainties.
The production yield of prompt D mesons and their elliptic flow coefficient v2 were measured with the Event-Shape Engineering (ESE) technique applied to mid-central (10–30% and 30–50% centrality classes) Pb-Pb collisions at the centre-of-mass energy per nucleon pair sNN−−−√=5.02 TeV, with the ALICE detector at the LHC. The ESE technique allows the classification of events, belonging to the same centrality, according to the azimuthal anisotropy of soft particle production in the collision. The reported measurements give the opportunity to investigate the dynamics of charm quarks in the Quark-Gluon Plasma and provide information on their participation in the collective expansion of the medium. D mesons were reconstructed via their hadronic decays at mid-rapidity, |η| < 0.8, in the transverse momentum interval 1 < pT < 24 GeV/c. The v2 coefficient is found to be sensitive to the event-shape selection confirming a correlation between the D-meson azimuthal anisotropy and the collective expansion of the bulk matter, while the per-event D-meson yields do not show any significant modification within the current uncertainties.
Two-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative momentum q ∝ 1/R. It was previously observed that in pp collisions at s√ = 7TeV the average pair transverse momentum kT range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity ST into spherical (ST > 0:7) and jet-like (ST < 0:3) events a method was developed that allows for the determination of source radii for much larger values of kT for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the kT dependence of the radii. The emission source size in spherical events shows a substantially diminished kT dependence, while jet-like events show indications of a negative trend with respect to kT in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one.
Two-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative momentum q∝1/R. It was previously observed that in pp collisions at s√=7 TeV the average pair transverse momentum kT range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity ST into spherical (ST>0.7) and jet-like (ST<0.3) events a method was developed that allows for the determination of source radii for much larger values of kT for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the kT dependence of the radii. The emission source size in spherical events shows a substantially diminished kT dependence, while jet-like events show indications of a negative trend with respect to kT in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one.
Two-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative momentum q∝1/R. It was previously observed that in pp collisions at s√=7 TeV the average pair transverse momentum kT range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity ST into spherical (ST>0.7) and jet-like (ST<0.3) events a method was developed that allows for the determination of source radii for much larger values of kT for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the kT dependence of the radii. The emission source size in spherical events shows a substantially diminished kT dependence, while jet-like events show indications of a negative trend with respect to kT in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one.
The Compressed Baryonic Matter (CBM) experiment is a dedicated heavy ion collision experiment at the FAIR facility. It will be one of the first HEP experiments which works in a triggerless mode: data received in the DAQ from the detectors will not be associated with events by a hardware trigger anymore. All raw data within a giventime period will be collected continuously in containers, so-called time-slices. The task of the reconstruction algorithms is to create events out of this raw data stream. In this contribution, the optimization of the reconstruction software in the RICH detector to the free-streaming data flow is presented. The implementation of ring reconstruction algorithms which use time measurements of the hits as an additional parameter is discussed.
We have identified a mistake in how Fig. 1 is referenced in the text of the article Eur. Phys. J. C 77 (2017) no. 8, 569 which affected three paragraphs of the results section. The corrected three paragraphs as well as the unmodified accompanying figure are reproduced in this document with the correct labeling.
In addition, an editing issue led to a missing acknowledgements section. The missing section is reproduced at the end of this document in the manner in which it should have appeared in the published article.
We study the production of entropy in the context of a nonequilibrium chiral phase transition. The dynamical symmetry breaking is modeled by a Langevin equation for the order parameter coupled to the Bjorken dynamics of a quark plasma. We investigate the impact of dissipation and noise on the entropy and explore the possibility of reheating for crossover and first-order phase transitions, depending on the expansion rate of the fluid. The relative increase in is estimated to range from 10% for a crossover to 100% for a first-order phase transition at low beam energies, which could be detected in the pion-to-proton ratio as a function of beam energy.
The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of J/ψ vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per nucleon pair sNN−−−√=5.02 TeV. The e+e− and μ+μ− decay channels are used to measure the cross section as a function of the rapidity of the J/ψ in the range −2.5<y<2.7, corresponding to an energy in the γp centre-of-mass in the interval 40<Wγp<550 GeV. The measurements, which are consistent with a power law dependence of the exclusive J/ψ photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements.
Measurements of anisotropic flow coefficients with two- and multi-particle cumulants for inclusive charged particles in Pb-Pb collisions at sNN−−−√=5.02 and 2.76 TeV are reported in the pseudorapidity range |η|<0.8 and transverse momentum 0.2<pT<50 GeV/c. The full data sample collected by the ALICE detector in 2015 (2010), corresponding to an integrated luminosity of 12.7 (2.0) μb−1 in the centrality range 0-80%, is analysed. Flow coefficients up to the sixth flow harmonic (v6) are reported and a detailed comparison among results at the two energies is carried out. The pT dependence of anisotropic flow coefficients and its evolution with respect to centrality and harmonic number n are investigated. An approximate power-law scaling of the form vn(pT)∼pn/3T is observed for all flow harmonics at low pT (0.2<pT<3 GeV/c). At the same time, the ratios vn/vn/mm are observed to be essentially independent of pT for most centralities up to about pT=10 GeV/c. Analysing the differences among higher-order cumulants of elliptic flow (v2), which have different sensitivities to flow fluctuations, a measurement of the standardised skewness of the event-by-event v2 distribution P(v2) is reported and constraints on its higher moments are provided. The Elliptic Power distribution is used to parametrise P(v2), extracting its parameters from fits to cumulants. The measurements are compared to different model predictions in order to discriminate among initial-state models and to constrain the temperature dependence of the shear viscosity to entropy-density ratio.
Chirality is omnipresent in living nature. On the single molecule level, the response of a chiral species to a chiral probe depends on their respective handedness. A prominent example is the difference in the interaction of a chiral molecule with left or right circularly polarized light. In the present study, we show by Coulomb explosion imaging that circularly polarized light can also induce a chiral fragmentation of a planar and thus achiral molecule. The observed enantiomer strongly depends on the orientation of the molecule with respect to the light propagation direction and the helicity of the ionizing light. This finding might trigger new approaches to improve laser-driven enantioselective chemical synthesis.
Cortical circuits exhibit highly dynamic and complex neural activity. Intriguingly, cortical activity exhibits consistently two key features across observed species and brain areas. First, individual neurons tend to be co-active in spatially localized domains forming orderly arranged, modular layouts with a typical spatial scale. Second, cortical elements are correlated in their activity over large distances reflecting long-range network interactions distributed over several millimeters. Currently, it is unclear how these two fundamental properties emerge in the early developing cortical activity.
Here, I aim to fill this gap by combining analyses of chronic imaging data and network models of developing cortical activity. Neural recordings of spontaneous and visually evoked activity in primary visual cortex of ferrets during their early cortical development were obtained using in vivo 2-photon and widefield epi-fluorescence calcium imaging. Spontaneous activity was used to probe the early state of cortical networks as its spatiotemporal organization is independent of a stimulus-imposed structure, and it is already present early in cortical development prior to reliably evoked responses. To assess the mature functional organization of distributed networks in cortex, the tuning of neural responses to stimulus features, in particular to the orientation of an edge-like stimulus, was assessed. Cortical responses to moving gratings of varying orientations form an orderly arranged layout of orientation domains extending over several millimeters.
To begin with, I showed that spontaneous activity correlations extend over several millimeters, supporting the assumption of using spontaneous activity to assess distributed networks in cortex.
Next, I asked how distributed networks in the mature visual cortex - assessed by spontaneous activity correlations - are related to its fine-scale functional organization. I found that the spatially extended and modular spontaneous correlation patterns accurately predict the fine spatial structure of visually evoked orientation domains several millimeters away. These results suggest a close relation between spontaneous correlations and visually evoked responses on a fine spatial scale and across large spatial distances.
As the principles governing the functional organization and development of distributed network interactions in the neocortex remain poorly understood, I next asked how long range correlated activity arises early in development. I found that key features of mature spontaneous activity introduced in this work, including long-range spontaneous correlations, were present already early in cortical development prior to the maturation of long-range, horizontal connections, and the predicted mature orientation preference layout. Even after silencing feed-forward input drive by inactivating retina or thalamus, long-range correlated and modular activity robustly emerged in early cortex. These results suggest that local recurrent connections in early cortical circuits can generate structured long-range network correlations that guide the formation of visually-evoked distributed functional networks.
To investigate how these large-scale cortical networks emerge prior to the maturation and elaboration of long-range horizontal connectivity, I examined a statistical network model describing an ensemble of spatially extended spontaneous activity patterns. I found a direct relationship between the dimensionality of this ensemble of activity patterns and the decay of its correlation structure. Specifically, reducing the dimensionality of the ensemble leads to an increase in the spatial range of the correlation structure.
To test whether this mechanism could generate a long-range correlation structure in cortical circuits, I studied a dynamical network model implementing a dimensionality reduction mechanism. Based on previous work demonstrating that network heterogeneity reduces the dimensionality of activity patterns, I showed that by increasing the degree of heterogeneity in the network, the dimensionality of the ensemble of activity patterns decreases and in turn their correlations extend over a greater range. A comparison to experimental data revealed a quantitative match between the network model and the observations in vivo in several of the key features of the early cortex including the spatial scale of correlations. Low dimensionality of spontaneous activity thus might provide an organizational principle explaining the observed long-range correlation structure in the early cortex.
Finally, I asked whether a network with a biologically plausible architecture can generate modular activity. Several classical models showed that modular activity patterns can emerge via an intracortical mechanism involving lateral inhibition. However, this assumption appears to be in conflict with current experimental evidence. Moreover, these network models were not experimentally tested, so far. Here, I showed by using linear stability analysis that spatially localized self-inhibition relaxes the constraints on the connectivity structure in a network model, such that biologically more plausible network motifs with shorter ranging inhibition than excitation can robustly generate modular activity.
Importantly, I also provided several model predictions to make the class of network models experimentally testable in view of recent technological advancements in imaging and manipulation of cortical circuits. A critical prediction of the model is the decrease in spacing of active domains when the total amount of inhibition increases. These results provide a novel mechanism of how cortical circuits with short-range inhibition can form modular activity.
Taken together, this thesis provides evidence that the two described fundamental features of neural activity are already present in the early cortex and shows that activity with those features can be generated in network models with an architecture consistent with the early cortex using basic principles.
We present a study of the elliptic flow and RAA of D and D¯ mesons in Au+Au collisions at FAIR energies. We propagate the charm quarks and the D mesons following a previously applied Langevin dynamics. The evolution of the background medium is modeled in two different ways: (I) we use the UrQMD hydrodynamics + Boltzmann transport hybrid approach including a phase transition to QGP and (II) with the coarse-graining approach employing also an equation of state with QGP. The latter approach has previously been used to describe di-lepton data at various energies very successfully. This comparison allows us to explore the effects of partial thermalization and viscous effects on the charm propagation. We explore the centrality dependencies of the collisions, the variation of the decoupling temperature and various hadronization parameters. We find that the initial partonic phase is responsible for the creation of most of the D/D¯ mesons elliptic flow and that the subsequent hadronic interactions seem to play only a minor role. This indicates that D/D¯ mesons elliptic flow is a smoking gun for a partonic phase at FAIR energies. However, the results suggest that the magnitude and the details of the elliptic flow strongly depend on the dynamics of the medium and on the hadronization procedure, which is related to the medium properties as well. Therefore, even at FAIR energies the charm quark might constitute a very useful tool to probe the quark–gluon plasma and investigate its physics.
We report on the successful implementation and characterization of a cryogenic solid hydrogen target in experiments on high-power laser-driven proton acceleration. When irradiating a solid hydrogen filament of 10 μm diameter with 10-Terawatt laser pulses of 2.5 J energy, protons with kinetic energies in excess of 20 MeV exhibiting non-thermal features in their spectrum were observed. The protons were emitted into a large solid angle reaching a total conversion efficiency of several percent. Two-dimensional particle-in-cell simulations confirm our results indicating that the spectral modulations are caused by collisionless shocks launched from the surface of the the high-density filament into a low-density corona surrounding the target. The use of solid hydrogen targets may significantly improve the prospects of laser-accelerated proton pulses for future applications.
The effect of a non-zero strangeness chemical potential on the strong interaction phase diagram has been studied within the framework of the SU(3) quark-hadron chiral parity-doublet model. Both, the nuclear liquid-gas and the chiral/deconfinement phase transitions are modified. The first-order line in the chiral phase transition is observed to vanish completely, with the entire phase boundary becoming a crossover. These changes in the nature of the phase transitions are expected to modify various susceptibilities, the effects of which might be detectable in particle-number distributions resulting from moderate-temperature and high-density heavy-ion collision experiments.
Early, non-invasive sensing of sustained hyperglycemia in mice using millimeter-wave spectroscopy
(2019)
Diabetes is a very complex condition affecting millions of people around the world. Its occurrence, always accompanied by sustained hyperglycemia, leads to many medical complications that can be greatly mitigated when the disease is treated in its earliest stage. In this paper, a novel sensing approach for the early non-invasive detection and monitoring of sustained hyperglycemia is presented. The sensing principle is based on millimeter-wave transmission spectroscopy through the skin and subsequent statistical analysis of the amplitude data. A classifier based on functional principal components for sustained hyperglycemia prediction was validated on a sample of twelve mice, correctly classifying the condition in diabetic mice. Using the same classifier, sixteen mice with drug-induced diabetes were studied for two weeks. The proposed sensing approach was capable of assessing the glycemic states at different stages of induced diabetes, providing a clear transition from normoglycemia to hyperglycemia typically associated with diabetes. This is believed to be the first presentation of such evolution studies using non-invasive sensing. The results obtained indicate that gradual glycemic changes associated with diabetes can be accurately detected by non-invasively sensing the metabolism using a millimeter-wave spectral sensor, with an observed temporal resolution of around four days. This unprecedented detection speed and its non-invasive character could open new opportunities for the continuous control and monitoring of diabetics and the evaluation of response to treatments (including new therapies), enabling a much more appropriate control of the condition.
We study how the mass and magnetic moment of the quarks are dynamically generated in nonequilibrium quark matter. We derive the equal-time transport and constraint equations for the quark Wigner function in a magnetized quark model and solve them in the semi-classical expansion. The quark mass and magnetic moment are self-consistently coupled to the Wigner function and controlled by the kinetic equations. While the quark mass is dynamically generated at the classical level, the quark magnetic moment is a pure quantum effect, induced by the quark spin interaction with the external magnetic field.
We consider a simple model of modified gravity interacting with a single scalar field ϕ with weakly coupled exponential potential within the framework of non-Riemannian spacetime volume-form formalism. The specific form of the action is fixed by the requirement of invariance under global Weyl-scale symmetry. Upon passing to the physical Einstein frame we show how the non-Riemannian volume elements create a second canonical scalar field u and dynamically generate a non-trivial two-scalar-field potential Ueff(u,ϕ) with two remarkable features: (i) it possesses a large flat region for large u describing a slow-roll inflation; (ii) it has a stable low-lying minimum w.r.t. (u,ϕ) representing the dark energy density in the “late universe”. We study the corresponding two-field slow-roll inflation and show that the pertinent slow-roll inflationary curve ϕ = ϕ(u) in the two-field space (u,ϕ) has a very small curvature, i.e., ϕ changes very little during the inflationary evolution of u on the flat region of Ueff(u,ϕ). Explicit expressions are found for the slow-roll parameters which differ from those in the single-field inflationary counterpart. Numerical solutions for the scalar spectral index and the tensor-to-scalar ratio are derived agreeing with the observational data.
Die vorliegende Dissertation untersucht die Nichtgleichgewichtsdynamik von relativistischen Schwerionenkollisionen ausgehend von der anfänglichen Produktion von Teilchen durch den Zerfall von Strings, der Bildung eines Quark-Gluon-Plasmas (QGP), dessen kinetische und chemische Äquilibrierung als Funktion der Zeit sowie seine Transporteigenschaften im Gleichgewicht bei endlicher Temperatur und endlichem chemischen Potential. Ein Verständnis der frühen Phase der Schwerionenkollisionen ist insbesondere von großen Interesse, da letztere eine Verbindung zwischen den ersten Nukleon-Nukleon Kollisionen und der Quark-Gluon-Plasma Phase herstellen, die zu einem späteren Zeitpunkt ein gewisses Maß an Thermalisierung zeigt. Allerdings können nur Nichtgleichgewichts-Theorien eine Verbindung zwischen dem anfänglichen QGP und seiner - zumindest partiellen - Thermalisierung herstellen. Um die Dynamik eines stark wechselwirkenden Mediums wie des Quark-Gluon-Plasmas zu beschreiben, reichen übliche Transportgleichungen (basierend auf der Boltzmann-Gleichung) nicht aus und es müssen komplexere Theorien, die auch für stark korrelierte Medien geeignet sind, angewendet werden. Hier kommen hydrodynamische Simulationen oder Transportrechnungen - basierend auf verallgemeinerten Transportgleichungen - zum Einsatz. Solche verallgemeinerte Transportgleichungen, wie die Kadanoff-Baym-Gleichungen, ergeben sich aus der quantenmechanischen Nichtgleichgewichts-Vielteilchentheorie, in der Green’s- Funktionen in Minkowski Raum-Zeit die interessierenden Größen sind, um die Dynamik des betrachteten Mediums zu beschreiben. Mit geeigneten Näherungen kann man so kinetische Transportgleichungen erhalten, die eine einheitliche Behandlung von stabilen und instabilen Teilchen auch außerhalb des Gleichgewichts ermöglichen. Diese Bestandteile bilden die Basis des Transportmodells Parton-Hadron-String Dynamics (PHSD), welches daher ein geeignetes ’Instrument’ ist um die verschiedenen Phasen einer Schwerionenkollision zu analysieren, egal ob die verschiedenen Formen der Materie im Gleichgewicht sind oder nicht.
In dieser Arbeit wird zunächst die Quantenchromodynamik (QCD) vorgestellt und erklärt, wie diese Theorie im Laufe der Jahre entwickelt wurde um ein wichtiger Bestandteil des Standardmodells der Teilchenphysik zu werden. Wir werden weiterhin die verbleibenden Herausforderungen in unserem Verständnis der QCD vorstellen, die sich primär auf das Phasendiagramm der stark wechselwirkenden Materie konzentrieren.
Im zweiten Kapitel untersuchen wir die Nichtgleichgewichts-Feldtheorie und die damit verbundenen Techniken - wie die Keldysh-Kontur - zur Beschreibung der Green’schen Funktionen als wesentlichen Freiheitsgrade. Wir leiten die Evolutionsgleichung für die Green’schen Funktionen her, d. h. die Kadanoff Baym-Gleichungen am Beispiel einer skalaren Feldtheorie.
Im nächsten Kapitel wird das Transportmodell Parton-Hadron-String Dynamics (PHSD), welches die Anwendung der verallgemeinerten Transportgleichungen zur Beschreibung relativistischer Schwerionenkollisionen darstellt, vorgestellt.
Wir beginnen im Kapitel 4 mit der Untersuchung der Nichtgleichgewichtseigenschaften des Quark-Gluon-Plasmas, welches bei relativistischen Schwerionenkollisionen erzeugt wird. Zu diesem Zweck vergleichen wir die Quark-Gluon-Plasmaentwicklung aus dem PHSD mit einem viskosen hydrodynamischen Modell, bei dem ein lokales kinetisches und chemisches Gleichgewicht angenommen wird.
Im Kapitel 5 konzentrieren wir uns auf das frühe Vorgleichgewichtsstadium ultra-relativistischer Schwerionenkollisionen und insbesondere auf die Freiheitsgrade der QGP-Phase in diesem Stadium. Wir untersuchen die Auswirkungen eines QGP, welches anfänglich entweder aus einem System aus massiven Gluonen (Szenario I) oder alternativ aus Quarks und Antiquarks (Szenario II) besteht. Das nächste Kapitel wird ebenfalls die Produktion von Teilchen im Frühstadium von Schwerionenkollisionen behandeln, jedoch bei niedrigeren Kollisionsenergien. Hier wird eine mikroskopische Beschreibung des K+/pi+-Verhältnisses im Vordergrund stehen, d. h. die Erklärung des Maximums in diesem Verhältnis bei etwa 30 A GeV ("Horn") in zentralen Au+Au (oder Pb+Pb) Kollisionen. Insbesonders werden wir die Modifikation des String-Fragmentierungsprozesses (über den Schwinger-Mechanismus) in einer Umgebung mit hoher hadronischer Dichte aufgrund der teilweisen Wiederherstellung der chiralen Symmetrie untersuchen.
In Kapitel 7 erweitern wir das Parton-Hadron-String Dynamics (PHSD)-Transportmodell im partonischen Sektor, indem wir explizit die totalen und differentiellen partonischen Streuungsquerschnitte als Funktion der Temperatur T und des baryochemischen Potentials μB berechnen auf der Basis der effektiven Propagatoren und Kopplungen des Dynamical QuasiParticle Models (DQPM), welches auch die generelle Zeitentwicklung der partonischen Freiheitsgrade beschreibt. Wir finden nur eine sehr bescheidene Änderung von n/s mit dem baryonchemischen Potential μB in Abhängigkeit von der skalierten Temperatur T/Tc(μB). Dies gilt auch für eine Vielzahl von hadronischen Observablen aus zentralen A+A Kollisionen im Energiebereich von 5 GeV < vsNN < 200 GeV bei der Implementierung der differentiellen Querschnitte in das PHSD-Modell. Da wir in Schwerionen-Observablen nur kleine Spuren einer μB-Abhängigkeit finden - obwohl die effektiven Partonenmassen und Kollisionsbreiten sowie deren Partonenquerschnitte eindeutig von μB abhängen - impliziert dies, dass man eine beträchtliche Partonendichte und ein großes Raum-Zeit-QGP-Volumen zur Untersuchung der Dynamik in der partonischen Phase benötigt. Diese Bedingungen sind nur bei hohen Kollisionsenergien erfüllt, bei denen μB jedoch eher niedrig ist. Wenn andererseits die Kollisionsenergie verringert und somit μB erhöht wird, wird die hadronische Phase dominant und dementsprechend wird es zunehmend schwieriger, Signale aus der Partonendynamik auf der Basis von "Bulk"-Observablen zu extrahieren.
Application of the Luttinger theorem to the Kondo lattice YbRh2Si2 suggests that its large 4f-derived Fermi surface (FS) in the paramagnetic (PM) regime should be similar in shape and volume to that of the divalent local-moment antiferromagnet (AFM) EuRh2Si2 in its PM regime. Here we show by angle-resolved photoemission spectroscopy that paramagnetic EuRh2Si2 has a large FS essentially similar to the one seen in YbRh2Si2 down to 1 K. In EuRh2Si2 the onset of AFM order below 24.5 K induces an extensive fragmentation of the FS due to Brillouin zone folding, intersection and resulting hybridization of the Fermi-surface sheets. Our results on EuRh2Si2 indicate that the formation of the AFM state in YbRh2Si2 is very likely also connected with similar changes in the FS, which have to be taken into account in the controversial analysis and discussion of anomalies observed at the quantum critical point in this system.
Measurements of inclusive and direct photon production at mid-rapidity in pp collisions at s√=2.76 and 8 TeV are presented by the ALICE experiment at the LHC. The results are reported in transverse momentum ranges of 0.4<pT<10 GeV/c and 0.3<pT<16 GeV/c, respectively. Photons are detected with the electromagnetic calorimeter~(EMCal) and via reconstruction of e+e− pairs from conversions in the ALICE detector material using the central tracking system. For the final measurement of the inclusive photon spectra the results are combined in the overlapping pT interval of both methods. Direct photon spectra, or their upper limits at 90% C.L. are extracted using the direct photon excess ratio Rγ, which quantifies the ratio of inclusive photons over decay photons generated with a decay-photon simulation. An additional hybrid method, combining photons reconstructed from conversions with those identified in the EMCal, is used for the combination of the direct photon excess ratio Rγ, as well as the extraction of direct photon spectra or their upper limits. While no significant signal of direct photons is seen over the full pT range, Rγ for pT>7 GeV/c is at least one σ above unity and consistent with expectations from next-to-leading order pQCD calculations.
Measurements of inclusive and direct photon production at mid-rapidity in pp collisions at s√=2.76 and 8 TeV are presented by the ALICE experiment at the LHC. The results are reported in transverse momentum ranges of 0.4<pT<10 GeV/c and 0.3<pT<16 GeV/c, respectively. Photons are detected with the electromagnetic calorimeter~(EMCal) and via reconstruction of e+e− pairs from conversions in the ALICE detector material using the central tracking system. For the final measurement of the inclusive photon spectra the results are combined in the overlapping pT interval of both methods. Direct photon spectra, or their upper limits at 90% C.L. are extracted using the direct photon excess ratio Rγ, which quantifies the ratio of inclusive photons over decay photons generated with a decay-photon simulation. An additional hybrid method, combining photons reconstructed from conversions with those identified in the EMCal, is used for the combination of the direct photon excess ratio Rγ, as well as the extraction of direct photon spectra or their upper limits. While no significant signal of direct photons is seen over the full pT range, Rγ for pT>7 GeV/c is at least one σ above unity and consistent with expectations from next-to-leading order pQCD calculations.
The elliptic flow of inclusive and direct photons was measured at mid-rapidity in two centrality classes 0-20% and 20-40% in Pb-Pb collisions at sNN−−−√ = 2.76 TeV by ALICE. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the detector material with the e+e− pairs reconstructed in the central tracking system. The results of the two methods were combined and the direct photon elliptic flow was extracted in the transverse momentum range 0.9<pT<6.2 GeV/c. A comparison to RHIC data shows a similar magnitude of the measured direct-photon elliptic flow. Hydrodynamic and transport model calculations are systematically lower than the data, but are found to be compatible.
Surface plasmon polaritons on (silver) nanowires are promising components for future photonic technologies. Here, we study near-field patterns on silver nanowires with a scattering-type scanning near-field optical microscope that enables the direct mapping of surface waves. We analyze the spatial pattern of the plasmon signatures for different excitation geometries and polarization and observe a plasmon wave pattern that is canted relative to the nanowire axis, which we show is due to a superposition of two different plasmon modes, as supported by electromagnetic simulations including the influence of the substrate. These findings yield new insights into the excitation and propagation of plasmon polaritons for applications in nanoplasmonic devices.
We discuss the diffusion currents occurring in a dilute system and show that the charge currents do not only depend on gradients in the corresponding charge density, but also on the other conserved charges in the system—the diffusion currents are therefore coupled. Gradients in one charge thus generate dissipative currents in a different charge. In this approach, we model the Navier-Stokes term of the generated currents to consist of a diffusion coefficient matrix, in which the diagonal entries are the usual diffusion coefficients and the off-diagonal entries correspond to the coupling of different diffusion currents. We evaluate the complete diffusion matrix for a specific hadron gas and for a simplified quark-gluon gas, including baryon, electric and strangeness charge. Our findings are that the off-diagonal entries can range within the same magnitude as the diagonal ones.
The first measurement of e+e− pair production at mid-rapidity (|ηe| < 0.8) in pp collisions at s√=7 TeV with ALICE at the LHC is presented. The dielectron production is studied as a function of the invariant mass (mee < 3.3 GeV/c2), the pair transverse momentum (pT,ee < 8 GeV/c), and the pair transverse impact parameter (DCAee), i.e., the average distance of closest approach of the reconstructed electron and positron tracks to the collision vertex, normalised to its resolution. The results are compared with the expectations from a cocktail of known hadronic sources and are well described when PYTHIA is used to generate the heavy-flavour contributions. In the low-mass region (0.14 < mee < 1.1 GeV/c2), prompt and non-prompt e+e− sources can be separated via the DCAee. In the intermediate-mass region (1.1 < mee < 2.7 GeV/c2), a double-differential fit to the data in mee and pT,ee and a fit of the DCAee distribution allow the total cc¯¯ and bb¯¯¯ cross sections to be extracted. Two different event generators, PYTHIA and POWHEG, can reproduce the shape of the two-dimensional mee and pT,ee spectra, as well as the shape of the DCAee distribution, reasonably well. However, differences in the cc¯¯ and bb¯¯¯ cross sections are observed when using the generators to extrapolate to full phase space. Finally, the ratio of inclusive to decay photons is studied via the measurement of virtual direct photons in the transverse-momentum range 1 < pT < 8 GeV/c. This is found to be unity within the statistical and systematic uncertainties and consistent with expectations from next-to-leading order perturbative quantum chromodynamic calculations.
Der Urknall vor ungefähr 13.8 Milliarden Jahren markiert die Entstehung des Universums. Die gesamte Energie und Materie war in einem Punkt konzentriert und expandiert seitdem kontinuierlich. Wenige Sekundenbruchteile nach dem Urknall war die Temperatur und Dichte dieser Materie extrem hoch und die erschaffenen Elementarteilchen, speziell Quarks und Gluonen, durchliefen einen Zustand den man als Quark-Gluon-Plasma (QGP) bezeichnet und innerhalb dessen die starke Wechselwirkung dominiert. Innerhalb dieses Plasmas können Quarks und Gluonen, welche sonst in Hadronen gebunden sind, sich frei bewegen. Die direkte Beobachtung des frühzeitlichen QGPs ist mit heutigen Mitteln nicht möglich. Allerdings ist es möglich die Dynamik und Kinematik innerhalb eines künstlich erzeugten QGPs zu erforschen und damit Rückschlüsse auf die Vorgänge während des Urknalls zu machen.
Um künstliche QGPs unter kontrollierten Bedingungen zu erzeugen, werden heutzutage ultrarelativistische Schwerionen zur Kollision gebracht. Der stärkste je gebaute Schwerionenbeschleuniger LHC befindet sich am Kernforschungzentrum CERN in der Nähe von Genf. Das ALICE Experiment, als eines der vier großen Experimente am LHC, wurde speziell gebaut um das QGP näher zu untersuchen. Vollständig ionisierte Bleikerne werden mit nahezu Lichtgeschwindigkeit in den Experimenten zur Kollision gebracht. Die deponierte Energie lässt die Temperatur der Quarks und Gluonen innerhalb der kollidierenden Nukleonen ansteigen bis eine kritische Temperatur überschritten wird und ein Phasenübergang in das QGP erfolgt. Im Laufe der Kollision kühlt das Medium ab und gelangt unter die kritische Temperatur. Nun werden aus den ehemals freien Quarks Hadronen gebildet. Diese Hadronen oder Zerfallsprodukte dieser Hadronen können daraufhin in die Detektoren des Experiments fliegen und werden dann dort gemessen.
Es gibt mehrere mögliche Observablen des QGP, die messbar mit dem ALICE Experiment sind. Die Observablen, die in dieser Arbeit detailliert untersucht werden, sind die invariante Masse und der Paartransversalimpuls eines Dielektrons. Ein Dielektron besteht aus einem Elektron und einem Positron, welche miteinander korreliert sind. Dielektronen sind ideale Sonden zur Vermessung des QGPs. Sie werden durch verschiedene Prozesse während allen Kollisionsphasen produziert, wie beispielsweise bei den initialen, harten Stößen der kollidierenden Nukleonen oder durch den elektromagnetischen Zerfall verschiedener Hadronen wie π0 und J/ψ. Zusätzlich strahlt das QGP Dielektronen abhängig von seiner Temperatur ab. Theoretisch erlaubt dies die direkte Temperaturmessung des QGPs. Ein weiterer Vorteil der Dielektronenmessung gegenüber der Messung von Hadronen liegt darin, dass Elektronen und Positronen keine Farbladungen tragen und somit auch nicht mit der dominierenden starken Wechselwirkung innerhalb des QGPs interagieren und somit unbeeinflusst Informationen über seine Dynamik liefern können.
In dieser vorliegenden Arbeit werden Dielektronenspektren als Funktion der invarianten Masse und des Paartransversalimpulses in Blei-Blei-Kollisionen mit einer Schwerpunktsenergie von √sNN = 5.02 TeV gemessen. Das erste Mal in Schwerionenkollisionen konnte an einem der großen LHC Experimente der minimale Transversalimpuls der gemessenen Elektronen und Positronen auf peT > 0.2 GeV/c minimiert werden. Dies gibt im Vergleich zu der publizierten Messung mit peT > 0.4 GeV/c die Möglichkeit auch sogenannte weiche Prozesse zu messen, erhöht aber auch den Komplexit ätsgrad der Messung durch massiv gesteigerten Untergrund. Zusätzlich ist die Messung zentralitäsabhängig durchgeführt. Zentralität ist ein Maß für den Abstand der beiden Bleikerne zum Zeitpunkt der Kollision. Je zentraler eine Kollision, desto größer ist die deponierte Energie und desto größer und heißer ist das erzeugte QGP und die daraus resultierenden Effekte.
Die gemessenen Dielektronenverteilungen werden mit dem erwarteten Beiträgen aus hadronischen Zerfällen verglichen. Die Messung ergibt, dass der Beitrag aus semileptonischen Zerfällen von Charmquarks gemessen im Vakuum, welcher mit der Anzahl der binären Nukleon-Nukleon-Kollisionen in Blei-Blei-Ereignissen hochskaliert ist, nicht das Dielektronenspektrum beschreibt. Eine Modifizierung des Beitrag gemäß des unabhängig gemessenen nuklearen Modifikationsfaktors für einzelne Elektronen aus Charm- und Beautyquarks verbessert die Beschreibung des Dielektronenspektrums. Zusätzlich wurde der Beitrag virtueller direkter Photonen abgeschätzt. Die gemessenen Werte sind vergleichbar mit vorangegangenen Messungen bei einer niedrigeren Schwerpunktsenergie. Ebenso ist es möglich in periphären Kollisionen einen Beitrag durch eine Quelle zu vermessen, die Dielektronen bei niedrigem Transversalimpuls pT,ee < 0.15 GeV/c aussendet.
The production of low-mass dielectrons is one of the most promising tools for the investigation of chiral symmetry restoration and thermal radiation from the QGP created in heavy-ion collisions. To single out the signal characteristics of the QGP, it is crucial to understand the primordial e+e− pair production in vacuum, i.e. in inelastic proton-proton (pp) collisions. Low-mass dielectrons have been measured with ALICE at the LHC in pp collisions at s=7and13TeV, and in Pb–Pb collisions at sNN=2.76TeV. An overview of the results on dielectron production is presented, together with their implications for the direct-photon and heavy-quark production.
The measurement of dielectron production is presented as a function of invariant mass and transverse momentum (pT) at midrapidity (|ye|<0.8) in proton-proton (pp) collisions at a centre-of-mass energy of s√=13 TeV. The contributions from light-hadron decays are calculated from their measured cross sections in pp collisions at s√=7 TeV or 13 TeV. The remaining continuum stems from correlated semileptonic decays of heavy-flavour hadrons. Fitting the data with templates from two different MC event generators, PYTHIA and POWHEG, the charm and beauty cross sections at midrapidity are extracted for the first time at this collision energy: dσcc¯/dy|y=0=974±138(stat.)±140(syst.) μb and dσbb¯/dy|y=0=79±14(stat.)±11(syst.) μb using PYTHIA simulations and dσcc¯/dy|y=0=1417±184(stat.)±204(syst.) μb and dσbb¯/dy|y=0=48±14(stat.)±7(syst.) μb for POWHEG. These values, whose uncertainties are fully correlated between the two generators, are consistent with extrapolations from lower energies. The different results obtained with POWHEG and PYTHIA imply different kinematic correlations of the heavy-quark pairs in these two generators. Furthermore, comparisons of dielectron spectra in inelastic events and in events collected with a trigger on high charged-particle multiplicities are presented in various pT intervals. The differences are consistent with the already measured scaling of light-hadron and open-charm production at high charged-particle multiplicity as a function of pT. Upper limits for the contribution of virtual direct photons are extracted at 90% confidence level and found to be in agreement with pQCD calculations.
Using a 3.19 fb−1 data sample collected at an 𝑒+𝑒− center-of-mass energy of 𝐸cm=4.178 GeV with the BESIII detector, we measure the branching fraction of the leptonic decay 𝐷+𝑠→𝜇+𝜈𝜇 to be ℬ𝐷+𝑠→𝜇+𝜈𝜇=(5.49±0.16stat±0.15syst)×10−3. Combining our branching fraction with the masses of the 𝐷+𝑠 and 𝜇+ and the lifetime of the 𝐷+𝑠, we determine 𝑓𝐷+𝑠|𝑉𝑐𝑠|=246.2±3.6stat±3.5syst MeV. Using the 𝑐→𝑠 quark mixing matrix element |𝑉𝑐𝑠| determined from a global standard model fit, we evaluate the 𝐷+𝑠 decay constant 𝑓𝐷+𝑠=252.9±3.7stat±3.6syst MeV. Alternatively, using the value of 𝑓𝐷+𝑠 calculated by lattice quantum chromodynamics, we find |𝑉𝑐𝑠|=0.985±0.014stat±0.014syst. These values of ℬ𝐷+𝑠→𝜇+𝜈𝜇, 𝑓𝐷+𝑠|𝑉𝑐𝑠|, 𝑓𝐷+𝑠 and |𝑉𝑐𝑠| are each the most precise results to date.
The long-awaited detection of a gravitational wave from the merger of a binary neutron star in August 2017 (GW170817) marks the beginning of the new field of multi-messenger gravitational wave astronomy. By exploiting the extracted tidal deformations of the two neutron stars from the late inspiral phase of GW170817, it is now possible to constrain several global properties of the equation of state of neutron star matter. However, the most interesting part of the high density and temperature regime of the equation of state is solely imprinted in the post-merger gravitational wave emission from the remnant hypermassive/supramassive neutron star. This regime was not observed in GW170817, but will possibly be detected in forthcoming events within the current observing run of the LIGO/VIRGO collaboration. Numerous numerical-relativity simulations of merging neutron star binaries have been performed during the last decades, and the emitted gravitational wave profiles and the interior structure of the generated remnants have been analysed in detail. The consequences of a potential appearance of a hadron-quark phase transition in the interior region of the produced hypermassive neutron star and the evolution of its underlying matter in the phase diagram of quantum cromo dynamics will be in the focus of this article. It will be shown that the different density/temperature regions of the equation of state can be severely constrained by a measurement of the spectral properties of the emitted post-merger gravitational wave signal from a future binary compact star merger event.
The ALICE experiment at the Large Hadron Collider (LHC) at CERN is planned to be operated in a continuous data-taking mode in Run 3. This will allow to inspect data from all Pb-Pb collisions at a rate of 50 kHz, giving access to rare physics signals embedded in a large background.
Based on experience with real-time reconstruction of particle trajectories and event properties in the ALICE High Level Trigger, the ALICE O2 facility is currently designed and developed to support processing of a continuous, triggerless stream of data segmented into entities referred to as timeframes.
Both raw data input into the ALICE O2 system and the actual processing of aggregated timeframes are distributed among multiple processes on a manynode cluster. Process communication is based on the asynchronous message passing paradigm.
This paper presents the basic concept for identification of data in the distributed system together with prototype implementations and performance measurements.
The single crystal growth of 19 different intermetallic compounds within the LnT2X2 family (with Ln = lanthanides, T = Co, Ru, Rh, Ir, and X = Si, P) is presented, by employing a high-temperature metal-flux technique. The habitus of the obtained crystals is platelet-like with the crystallographic c direction perpendicular to the surface and with individual masses between 1 and 100 mg. The magnetic properties of these crystals are characterized by magnetization, heat-capacity, and resistivity measurements. These crystals form the materials basis for a thorough study of exciting surface properties by angle-resolved photoemission spectroscopy.