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The core of neutron stars consists of extremely dense matter at relatively low temperatures. In such an environment the appearance of exotic strongly interacting particles beyond nucleons appears quite natural. In this context we consider hybrid stars that, in addition to nucleons and hyperons, also contain quarks as further degrees of freedom. We investigate the impact of quarks on the properties of these compact stars. In addition, we discuss new constraints on such objects arising from the recently measured gravitational wave signal of two merging neutron stars.
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 QCD equation of state at finite baryon density is studied in the framework of a Cluster Expansion Model (CEM), which is based on the fugacity expansion of the net baryon density. The CEM uses the two leading Fourier coefficients, obtained from lattice simulations at imaginary μB, as the only model input and permits a closed analytic form. Excellent description of the available lattice data at both μB = 0 and at imaginary μB is obtained. We also demonstrate how the Fourier coefficients can be reconstructed from baryon number susceptibilities.
The state-of-the-art pattern recognition method in machine learning (deep convolution neural network) is used to identify the equation of state (EoS) employed in the relativistic hydrodynamic simulations of heavy ion collisions. High-level correlations of particle spectra in transverse momentum and azimuthal angle learned by the network act as an effective EoS-meter in deciphering the nature of the phase transition in QCD. The EoS-meter is model independent and insensitive to other simulation inputs including the initial conditions and shear viscosity for hydrodynamic simulations. Through this study we demonstrate that there is a traceable encoder of the dynamical information from the phase structure that survives the evolution and exists in the final snapshot of heavy ion collisions and one can exclusively and effectively decode these information from the highly complex final output with machine learning when traditional methods fail. Besides the deep neural network, the performance of traditional machine learning classifiers are also provided.
A unified chiral mean field approach is presented for QCD thermodynamics in a wide range of temperatures and densities. The model simultaneously gives a satisfactory description of lattice QCD thermodynamics and fulfills nuclear matter and astrophysical constraints. The resulting equation of state can be incorporated in relativistic fluid-dynamical simulations of heavy-ion collisions and neutron stars mergers. Access to different regions of the QCD phase diagram can be obtained in simulations of heavy-ion data and observations of neutron star mergers.
A newly proposed framework of perfect-fluid relativistic hydrodynamics for particles with spin 1/2 is briefly reviewed. The hydrodynamic equations follow entirely from the conservation laws for energy, momentum, and angular momentum. The incorporation of the angular-momentum conservation requires that the spin polarization tensor ωμν is introduced. It plays a role of a Lagrange multiplier conjugated to the spin tensor Sλ,μν. The space-time evolution of the spin polarization tensor depends on the specific form chosen for the spin tensor.
Loosely-bound objects such as light nuclei are copiously produced in proton-proton and nuclear collisions at the Large Hadron Collider (LHC), despite the fact that typical energy scales in such collisions exceed the binding energy of the objects by orders of magnitude. In this review we summarise the experimental observations, put them into context of previous studies at lower energies, and discuss the underlying physics. Most of the data discussed here were taken by the ALICE Collaboration during LHC Run1, which started in 2009 and ended in 2013. Specifically we focus on the production of (anti-)nuclei and (anti-)hypernuclei. Also included are searches for exotic objects like the H-dibaryon, a possible uuddss hexaquark state, or also a possible bound state of a Λ hyperon and a neutron. Furthermore, the study of hyperon-nucleon and hyperon-hyperon interactions through measurements of correlations are briefly discussed, especially in connection with the possible existence of loosely-bound states composed of these baryons. In addition, some results in the strange and charmed hadron sector are presented, to show the capabilities for future measurements on loosely-bound objects in this direction. Finally, perspectives are given for measurements in the currently ongoing Run2 period of the LHC and in the future LHC Run3.
The HADES experiment provides a large acceptance combined with a high mass resolution and therefore makes it possible to study dielectron and hadron production in heavy-ion collisions with unprecedented precision. With the high statistics of seven billion Au+Au collisions at 1.23 AGeV recorded in 2012 the investigation of collective effects and particle correlations is possible with unprecedented accuracy. We present multi-differential data on directed (v1) and elliptic (v2) flow, and the first measurement of triangular flow (v3), of protons and deuterons.
Quarkonia, i.e. bound states of bb‾ and cc‾ quarks, are powerful observables to study the properties of nuclear matter under extreme conditions. The formation of a Quark-Gluon Plasma (QGP), which is predicted by lattice QCD calculations at high temperatures as reached at the LHC energies, has a strong influence on the production and behavior of quarkonia. The latest ALICE results on bottomonium and charmonium production in nucleus−nucleus collisions are presented. This includes measurements of the ϒ(1S) and ϒ(2S) nuclear modification factor (RAA) at forward rapidity and the J/ψ RAA and ν2 as a function of centrality, pT and rapidity in Pb–Pb collisions at sNN=5.02TeV. Also, first results from J/ψ measurements in Xe–Xe collisions at sNN=5.44TeV are presented. Further on, the experimental results are compared to various calculations from theoretical models.
The SLC26 family of transporters maintains anion equilibria in all kingdoms of life. The family shares a 7 + 7 transmembrane segments inverted repeat architecture with the SLC4 and SLC23 families, but holds a regulatory STAS domain in addition. While the only experimental SLC26 structure is monomeric, SLC26 proteins form structural and functional dimers in the lipid membrane. Here we resolve the structure of an SLC26 dimer embedded in a lipid membrane and characterize its functional relevance by combining PELDOR/DEER distance measurements and biochemical studies with MD simulations and spin-label ensemble refinement. Our structural model reveals a unique interface different from the SLC4 and SLC23 families. The functionally relevant STAS domain is no prerequisite for dimerization. Characterization of heterodimers indicates that protomers in the dimer functionally interact. The combined structural and functional data define the framework for a mechanistic understanding of functional cooperativity in SLC26 dimers.
The SLC26 family of transporters maintains anion equilibria in all kingdoms of life. The family shares a 7 + 7 transmembrane segments inverted repeat architecture with the SLC4 and SLC23 families, but holds a regulatory STAS domain in addition. While the only experimental SLC26 structure is monomeric, SLC26 proteins form structural and functional dimers in the lipid membrane. Here we resolve the structure of an SLC26 dimer embedded in a lipid membrane and characterize its functional relevance by combining PELDOR distance measurements and biochemical studies with MD simulations and spin-label ensemble refinement. Our structural model reveals a unique interface different from the SLC4 and SLC23 families. The functionally relevant STAS domain exerts a stabilizing effect on regions central in this dimer. Characterization of heterodimers indicates that protomers in the dimer functionally interact. The combined structural and functional data define the framework for a mechanistic understanding of functional cooperativity in SLC26 dimers.
In LHC Run 3, ALICE will increase the data taking rate significantly to 50 kHz continuous read-out of minimum bias Pb—Pb collisions. The reconstruction strategy of the online-offline computing upgrade foresees a first synchronous online reconstruction stage during data taking enabling detector calibration and data compression, and a posterior calibrated asynchronous reconstruction stage. Many new challenges arise, among them continuous TPC read-out, more overlapping collisions, no a priori knowledge of the primary vertex and of location-dependent calibration in the synchronous phase, identification of low-momentum looping tracks, and sophisticated raw data compression. The tracking algorithm for the Time Projection Chamber (TPC) will be based on a Cellular Automaton and the Kalman filter. The reconstruction shall run online, processing 50 times more collisions per second than today, while yielding results comparable to current offline reconstruction. Our TPC track finding leverages the potential of hardware accelerators via the OpenCL and CUDA APIs in a shared source code for CPUs and GPUs for both reconstruction stages. We give an overview of the status of Run 3 tracking including performance on processors and GPUs and achieved compression ratios.
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.
ALICE (A Large Ion Collider Experiment) is preparing for a major upgrade of the detector, readout and computing systemsfor LHC Run 3. A new facility called O2 (Online-Offline) will play a major role in data compression and event processing. To efficiently operate the experiment, we are designing a monitoring subsystem, which will provide a complete overview of the O2 overall health, detect performance degradation and component failures. The monitoring subsystem will receive and collect up to 600 kHz of performance metrics. It consists of a custom monitoring library and a server-side, distributed software covering five main functional tasks: parameter collection and processing, storage, visualisation and alarms. To select the most appropriate tools for these tasks, we evaluated three options: “Modular Stack”, Zabbix and the currently used ALICE Grid monitoring tool called MonALISA. The former one consists of a toolkit including collectd, Apache Flume, Apache Spark, InfluxDB, Grafana and Riemann. This paper describes the monitoring subsystem functional architecture. It goes through a complete evaluation of the three considered options, the selection process, risk assessment and justification for the final decision. The in-depth comparison includes functional features and throughput measurement to ensure the required processing and storage performance.
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.
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 ALICE experimentatthe LHC (CERN)iscurrently developinganew software framework designed for Run3: detector and software will havetocope with Pb–Pb collision rates 100 times higher than today, leadingtothe combinationofcore Online-Offline operations intoasingle framework called O2. The analysis code is expectedtorunonafew large Analysis Facilities counting 20k cores and sustaining a 100 GB/s throughput: this requiresaconjoint effort between the definitionofthe data format, the configurationofthe Analysis Facilities and the developmentofthe Analysis Framework. Wepresent the prototypeofanew Analysis Object Data format basedontimeframes and optimized for continuous readout. Such formatisdesigned tobeextensible and transported efficiently over the network. Wealso present the first iterationofthe Analysis Framework, basedonthe O2 Data Processing Layer and leveraging message passing acrossatopologyofprocesses. Wewill also illustrate the implementation and benchmarkingofacompatibility layer designedtomitigate the transition from the current event-oriented analysis modeltothe new time-oriented one. Finally,wewill giveastatus reportonthe integrationofthe Analysis Framework and Analysis Facilities for Run3into the current organized analysis model in ALICE.
We discuss in some detail the physics content of the new model, QGSJET-III-01, focusing on major problems related to the treatment of semihard processes in the very high energy limit. A special attention has been payed to the main improvement, compared to the QGSJET-II model, which is related to a phenomenological treatment of leading power corrections corresponding to final parton rescattering off soft gluons. In particular, this allowed us to use a twice smaller separation scale between the soft and hard parton physics, compared to the previous model version, QGSJET-II-04. Preliminary results obtained with the new model are also presented.
We report the first measurement of low-energy proton-capture cross sections of 124Xe in a heavy-ion storage ring. 124Xe54+ ions of five different beam energies between 5.5 and 8 AMeV were stored to collide with a windowless hydrogen target. The 125Cs reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and γ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.
The idea of slow-neutron capture nucleosynthesis formulated in 1957 triggered a tremendous experimental effort in different laboratories worldwide to measure the relevant nuclear physics input quantities, namely (n,γ) cross sections over the stellar temperature range (from few eV up to several hundred keV) for most of the isotopes involved from Fe up to Bi. A brief historical review focused on total energy detectors will be presented to illustrate how, advances in instrumentation have led, over the years, to the assessment and discovery of many new aspects of s-process nucleosynthesis and to the progressive refinement of theoretical models of stellar evolution. A summary will be presented on current efforts to develop new detection concepts, such as the Total-Energy Detector with γ-ray imaging capability (i-TED). The latter is based on the simultaneous combination of Compton imaging with neutron time-of-flight (TOF) techniques, in order to achieve a superior level of sensitivity and selectivity in the measurement of stellar neutron capture rates.
An experiment addressing electron capture (EC) decay of hydrogen-like 142Pm60+ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7)s−1 for automatic analysis and 0.0141(7)s−1 for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4).
The radiative electron capture (REC) into the K shell of bare Xe ions colliding with a hydrogen gas target has been investigated. In this study, the degree of linear polarization of the K-REC radiation was measured and compared with rigorous relativistic calculations as well as with the previous results recorded for U92+. Owing to the improved detector technology, a significant gain in precision of the present polarization measurement is achieved compared to the previously published results. The obtained data confirms that for medium-Z ions such as Xe, the REC process is a source of highly polarized x rays which can easily be tuned with respect to the degree of linear polarization and the photon energy. We argue, in particular, that for relatively low energies the photons emitted under large angles are almost fully linear polarized.
The proton-removal mechanism of the 12CB reaction induced on a carbon target via elementary nucleon-nucleon scattering is investigated in exclusive triple-coincidence measurements. The observed two-nucleon angular correlations are found to be consistent with quasi-free scattering of a projectile-like proton off a target-like nucleon. Exclusive cross sections for one-step pp and pn interactions are determined as [formula] and [formula], respectively. The extracted quasi-free component amounts up to 58(4)% of the total proton-removal cross section. The results are compared to total proton-removal cross sections obtained from the experiment and eikonal reaction theory.
Ziel der Simulationsstudien in dieser Arbeit war es, die Leistungsfähigkeit des Transition Radiation Detectors zur Identifikation von leichten Kernen und Hyperkernen im CBM-Experiment zu untersuchen. Die Trennung von Helium und Deuterium
mithilfe ihres spezifischen Energieverlustes im TRD ist zentral, um eine Rekonstruktion des seltenen Hyperkerns 6 ΛΛHe mit einem hohen Signal-zu-Untergrund-Verhältnisse zu leisten. Zur Erfüllung der Anforderungen, die sich aus dem CBM-Forschungsprogramm ergeben, wird eine Auflösung des Energieverlustes dEdx von Helium von höchstens 30 % verlangt...
Measurements of the π±, K±, and proton double differential yields emitted from the surface of the 90-cm-long carbon target (T2K replica) were performed for the incoming 31 GeV/c protons with the NA61/SHINE spectrometer at the CERN SPS using data collected during 2010 run. The double differential π± yields were measured with increased precision compared to the previously published NA61/SHINE results, while the K± and proton yields were obtained for the first time. A strategy for dealing with the dependence of the results on the incoming proton beam profile is proposed. The purpose of these measurements is to reduce significantly the (anti)neutrino flux uncertainty in the T2K long-baseline neutrino experiment by constraining the production of (anti)neutrino ancestors coming from the T2K target.
We investigate the QCD phase diagram for nonzero background magnetic fields using first-principles lattice simulations. At the physical point (in terms of quark masses), the thermodynamics of this system is controlled by two opposing effects: magnetic catalysis (enhancement of the quark condensate) at low temperature and inverse magnetic catalysis (reduction of the condensate) in the transition region. While the former is known to be robust and independent of the details of the interactions, inverse catalysis arises as a result of a delicate competition, effective only for light quarks. By performing simulations at different quark masses, we determine the pion mass above which inverse catalysis does not take place in the transition region anymore. Even for pions heavier than this limiting value — where the quark condensate undergoes magnetic catalysis — our results are consistent with the notion that the transition temperature is reduced by the magnetic field. These findings will be useful to guide low-energy models and effective theories of QCD.
The coordinate and momentum space configurations of the net baryon number in heavy ion collisions that undergo spinodal decomposition, due to a first-order phase transition, are investigated using state-of-the-art machine-learning methods. Coordinate space clumping, which appears in the spinodal decomposition, leaves strong characteristic imprints on the spatial net density distribution in nearly every event which can be detected by modern machine learning techniques. On the other hand, the corresponding features in the momentum distributions cannot clearly be detected, by the same machine learning methods, in individual events. Only a small subset of events can be systematically differ- entiated if only the momentum space information is available. This is due to the strong similarity of the two event classes, with and without spinodal decomposition. In such sce- narios, conventional event-averaged observables like the baryon number cumulants signal a spinodal non-equilibrium phase transition. Indeed the third-order cumulant, the skewness, does exhibit a peak at the beam energy (Elab = 3–4 A GeV), where the transient hot and dense system created in the heavy ion collision reaches the first-order phase transition.
Measurement of the production of charm jets tagged with D0 mesons in pp collisions at √s = 7 TeV
(2019)
The production of charm jets in proton-proton collisions at a center-of-mass energy of s√=7 TeV was measured with the ALICE detector at the CERN Large Hadron Collider. The measurement is based on a data sample corresponding to a total integrated luminosity of 6.23 nb−1, collected using a minimum-bias trigger. Charm jets are identified by the presence of a D0 meson among their constituents. The D0 mesons are reconstructed from their hadronic decay D0 →K−π+. The D0-meson tagged jets are reconstructed using tracks of charged particles (track-based jets) with the anti-kT algorithm in the jet transverse momentum range 5<pchT,jet< 30 GeV/c and pseudorapidity |ηjet| < 0.5. The fraction of charged jets containing a D0-meson increases with pchT,jet from 0.042 ± 0.004 (stat) ± 0.006 (syst) to 0.080 ± 0.009 (stat) ± 0.008 (syst). The distribution of D0-meson tagged jets as a function of the jet momentum fraction carried by the D0 meson in the direction of the jet axis (zch∥) is reported for two ranges of jet transverse momenta, 5<pchT,jet< 15 GeV/c and 15<pchT,jet< 30 GeV/c in the intervals 0.2<zch∥∥<1.0 and 0.4<zch∥∥<1.0, respectively. The data are compared with results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and with a Next-to-Leading-Order perturbative Quantum Chromodynamics calculation, obtained with the POWHEG method and interfaced with PYTHIA 6 for the generation of the parton shower, fragmentation, hadronisation and underlying event.
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.
Inclusive J/ψ production is studied in minimum-bias proton-proton collisions at a centre-of-mass energy of s√ = 5.02 TeV by ALICE at the CERN LHC. The measurement is performed at mid-rapidity (|y| < 0.9) in the dielectron decay channel down to zero transverse momentum pT, using a data sample corresponding to an integrated luminosity of Lint = 19.4 ± 0.4 nb−1. The measured pT-integrated inclusive J/ψ production cross sec- tion is dσ/dy = 5.64 ± 0.22(stat.) ± 0.33(syst.) ± 0.12(lumi.) μb. The pT-differential cross section d2σ/dpTdy is measured in the pT range 0–10 GeV/c and compared with state-of- the-art QCD calculations. The J/ψ 〈pT〉 and ⟨p2T⟩ are extracted and compared with results obtained at other collision energies.
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.
Production cross sections of muons from semi-leptonic decays of charm and beauty hadrons were measured at forward rapidity (2.5 < y < 4) in proton-proton (pp) collisions at a centre-of-mass energy s√ = 5.02 TeV with the ALICE detector at the CERN LHC. The results were obtained in an extended transverse momentum interval, 2 < pT< 20 GeV/c, and with an improved precision compared to previous measurements performed in the same rapidity interval at centre-of-mass energies s√ = 2.76 and 7 TeV. The pT- and y-differential production cross sections as well as the pT-differential production cross section ratios between different centre-of-mass energies and different rapidity intervals are described, within experimental and theoretical uncertainties, by predictions based on perturbative QCD.
The measurement of the production of prompt D0, D+, D*+, and D+S mesons in proton–lead (p–Pb) collisions at the centre-of-mass energy per nucleon pair of sNN−−−√ = 5.02 TeV, with an integrated luminosity of 292 ± 11 μb−1, are reported. Differential production cross sections are measured at mid-rapidity (−0.96 < ycms< 0.04) as a function of transverse momentum (pT) in the intervals 0 < pT< 36 GeV/c for D0, 1 < pT< 36 GeV/c for D+ and D*+, and 2 < pT< 24 GeV/c for D+ mesons. For each species, the nuclear modification factor RpPb is calculated as a function of pT using a proton-proton (pp) ref- erence measured at the same collision energy. The results are compatible with unity in the whole pT range. The average of the non-strange D mesons RpPb is compared with theoretical model predictions that include initial-state effects and parton transport model predictions. The pT dependence of the D0, D+, and D*+ nuclear modification factors is also reported in the interval 1 < pT< 36 GeV/c as a function of the collision centrality, and the central-to-peripheral ratios are computed from the D-meson yields measured in different centrality classes. The results are further compared with charged-particle measurements and a similar trend is observed in all the centrality classes. The ratios of the pT-differential cross sections of D0, D+, D*+, and D+S mesons are also reported. The D+S and D+ yields are compared as a function of the charged-particle multiplicity for several pT intervals. No modification in the relative abundances of the four species is observed with respect to pp collisions within the statistical and systematic uncertainties.
The transverse structure of jets was studied via jet fragmentation transverse momentum (jT) distributions, obtained using two-particle correlations in proton-proton and proton-lead collisions, measured with the ALICE experiment at the LHC. The highest transverse momentum particle in each event is used as the trigger particle and the region 3 < pTt < 15GeV/c is explored in this study. The measured distributions show a clear narrow Gaussian component and a wide non-Gaussian one. Based on Pythia simulations, the narrow component can be related to non-perturbative hadronization and the wide component to quantum chromodynamical splitting. The width of the narrow component shows a weak dependence on the transverse momentum of the trigger particle, in agreement with the expectation of universality of the hadronization process. On the other hand, the width of the wide component shows a rising trend suggesting increased branching for higher transverse momentum. The results obtained in pp collisions at s√=7 TeV and in p–Pb collisions at sNN−−−√=5.02 TeV are compatible within uncertainties and hence no significant cold nuclear matter effects are observed. The results are compared to previous measurements from CCOR and PHENIX as well as to PYTHIA 8 and Herwig 7 simulations.
The second (v2) and third (v3) flow harmonic coefficients of J/ψ mesons are measured at forward rapidity (2.5 < y < 4.0) in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the LHC. Results are obtained with the scalar product method and reported as a function of transverse momentum, pT, for various collision centralities. A positive value of J/ψ v3 is observed with 3.7σ significance. The measurements, compared to those of prompt D0 mesons and charged particles at mid-rapidity, indicate an ordering with vn(J/ψ) < vn(D0) < vn(h±) (n = 2, 3) at low and intermediate pT up to 6 GeV/c and a convergence with v2(J/ψ) ≈ v2(D0) ≈ v2(h±) at high pT above 6–8 GeV/c. In semi-central collisions (5–40% and 10–50% centrality intervals) at intermediate pT between 2 and 6 GeV/c, the ratio v3/v2 of J/ψ mesons is found to be significantly lower (4.6σ) with respect to that of charged particles. In addition, the comparison to the prompt D0-meson ratio in the same pT interval suggests an ordering similar to that of the v2 and v3 coefficients. The J/ψ v2 coefficient is further studied using the Event Shape Engineering technique. The obtained results are found to be compatible with the expected variations of the eccentricity of the initial-state geometry.
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.
Lattice QCD with heavy quarks reduces to a three-dimensional effective theory of Polyakov loops, which is amenable to series expansion methods. We analyse the effective theory in the cold and dense regime for a general number of colours, Nc. In particular, we investigate the transition from a hadron gas to baryon condensation. For any finite lattice spacing, we find the transition to become stronger, i.e. ultimately first-order, as Nc is made large. Moreover, in the baryon condensed regime, we find the pressure to scale as p ∼ Nc through three orders in the hopping expansion. Such a phase differs from a hadron gas with p ∼ N0c, or a quark gluon plasma, p ∼ N2c, and was termed quarkyonic in the literature, since it shows both baryon-like and quark-like aspects. A lattice filling with baryon number shows a rapid and smooth transition from condensing baryons to a crystal of saturated quark matter, due to the Pauli principle, and is consistent with this picture. For continuum physics, the continuum limit needs to be taken before the large Nc limit, which is not yet possible in practice. However, in the controlled range of lattice spacings and Nc-values, our results are stable when the limits are approached in this order. We discuss possible implications for physical QCD.
We present a study of the inclusive charged-particle transverse momentum (pT) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity, dNch/dη, in pp collisions at s√=5.02 and 13 TeV covering the kinematic range |η|<0.8 and 0.15<pT<20 GeV/c. The results are presented for events with at least one charged particle in |η|<1 (INEL>0). The pT spectra are reported for two multiplicity estimators covering different pseudorapidity regions. The pT spectra normalized to that for INEL>0 show little energy dependence. Moreover, the high-pT yields of charged particles increase faster than the charged-particle multiplicity density. The average pT as a function of multiplicity and transverse spherocity is reported for pp collisions at s√=13 TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the average pT is higher (smaller) than that measured in INEL>0 pp collisions. Within uncertainties, the functional form of ⟨pT⟩(Nch) is not affected by the spherocity selection. While EPOS LHC gives a good description of many features of data, PYTHIA overestimates the average pT in jet-like events.
Measurement of the inclusive isolated photon production cross section in pp collisions at √s = 7 TeV
(2019)
The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at a centre-of-momentum energy of s√= 7 TeV. The measurement is performed with the electromagnetic calorimeter EMCal and the central tracking detectors, covering a range of |η|<0.27 in pseudorapidity and a transverse momentum range of 10<pγT<60 GeV/c. The result extends the pT coverage of previously published results of the ATLAS and CMS experiments at the same collision energy to smaller pT. The measurement is compared to next-to-leading order perturbative QCD calculations and to the results from the ATLAS and CMS experiments. All measurements and theory predictions are in agreement with each other.
The measurements of the production of prompt D0, D+, D∗+, and D+s mesons in proton–proton (pp) collisions at s√=5.02 TeV with the ALICE detector at the Large Hadron Collider (LHC) are reported. D mesons were reconstructed at mid-rapidity (|y|<0.5) via their hadronic decay channels D0→K−π+, D+→K−π+π+, D∗+→D0π+→K−π+π+, D+s→ϕπ+→K+K−π+, and their charge conjugates. The production cross sections were measured in the transverse momentum interval 0<pT<36 GeV/c for D0, 1<pT<36 GeV/c for D+ and D∗+, and in 2<pT<24 GeV/c for D+s mesons. Thanks to the higher integrated luminosity, an analysis in finer pT bins with respect to the previous measurements at s√=7 TeV was performed, allowing for a more detailed description of the cross-section pT shape. The measured pT-differential production cross sections are compared to the results at s√=7 TeV and to four different perturbative QCD calculations. Its rapidity dependence is also tested combining the ALICE and LHCb measurements in pp collisions at s√=5.02 TeV. This measurement will allow for a more accurate determination of the nuclear modification factor in p–Pb and Pb–Pb collisions performed at the same nucleon–nucleon centre-of-mass energy.
Charged-particle pseudorapidity density at mid-rapidity in p-Pb collisions at √sNN = 8.16 TeV
(2019)
The pseudorapidity density of charged particles, dNch/dη, in p–Pb collisions has been measured at a centre-of-mass energy per nucleon–nucleon pair of sNN−−−√ = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, |η|<1.8. The dNch/dη value is 19.1±0.7 at |η|<0.5. This quantity divided by ⟨Npart⟩ / 2 is 4.73±0.20, where ⟨Npart⟩is the average number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at sNN−−−√ = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agree within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the dNch/dη distribution. Saturation-based models reproduce the distributions well for η>−1.3. The dNch/dη is also measured for different centrality estimators, based both on the charged-particle multiplicity and on the energy deposited in the Zero-Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity.
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.
First results on K/π, p/π and K/p fluctuations are obtained with the ALICE detector at the CERN LHC as a function of centrality in Pb--Pb collisions at sNN−−−√=2.76 TeV. The observable νdyn, which is defined in terms of the moments of particle multiplicity distributions, is used to quantify the magnitude of dynamical fluctuations of relative particle yields and also provides insight into the correlation between particle pairs. This study is based on a novel experimental technique, called the Identity Method, which allows one to measure the moments of multiplicity distributions in case of incomplete particle identification. The results for p/π show a change of sign in νdyn from positive to negative towards more peripheral collisions. For central collisions, the results follow the smooth trend of the data at lower energies and νdyn exhibits a change in sign for p/π and K/p.
The quasi-free scattering reactions 11C(p, 2p) and 10,11,12C(p, pn) have been studied in inverse kinematics at beam energies of 300–400 MeV/u at the R3B-LAND setup. The outgoing proton-proton and protonneutron pairs were detected in coincidence with the reaction fragments in kinematically complete measurements. The efficiency to detect these pairs has been obtained from GEANT4 simulations which were tested using the 12C(p, 2p) and 12C(p, pn) reactions. Experimental cross sections and momentum distributions have been obtained and compared to DWIA calculations based on eikonal theory. The new results reported here are combined with previously published cross sections for quasi-free scattering from oxygen and nitrogen isotopes and together they enable a systematic study of the reduction of singleparticle strength compared to predictions of the shell model over a wide neutron-to-proton asymmetry range. The combined reduction factors show a weak or no dependence on isospin asymmetry, in contrast to the strong dependency reported in nucleon-removal reactions induced by nuclear targets at lower energies. However, the reduction factors for (p, 2p) are found to be ’significantly smaller than for (p, pn) reactions for all investigated nuclei.
J/ψ suppression has long been considered a sensitive signature of the formation of the Quark-Gluon Plasma (QGP) in relativistic heavy-ion collisions. In this letter, we present the first measurement of inclusive J/ψ production at mid-rapidity through the dimuon decay channel in Au+Au collisions at √sNN = 200 GeV with the STAR experiment. These measurements became possible after the installation of the Muon Telescope Detector was completed in 2014. The J/ψ yields are measured in a wide transverse momentum (pT) range of 0.15 GeV/c to 12 GeV/c from central to peripheral collisions. They extend the kinematic reach of previous measurements at RHIC with improved precision. In the 0-10% most central collisions, the J/ψ yield is suppressed by a factor of approximately 3 for pT > 5 GeV/c relative to that in p + p collisions scaled by the number of binary nucleon-nucleon collisions. The J/ψ nuclear modification factor displays little dependence on pT in all centrality bins. Model calculations can qualitatively describe the data, providing further evidence for the color-screening effect experienced by J/ψ mesons in the QGP.
The jet radial structure and particle transverse momentum (pT) composition within jets are presented in centrality-selected Pb–Pb collisions at √sNN = 2.76 TeV. Track-based jets, which are also called charged jets, were reconstructed with a resolution parameter of R = 0.3 at midrapidity |ηch jet| < 0.6 for transverse momenta pT, ch jet = 30–120 GeV/c. Jet–hadron correlations in relative azimuth and pseudorapidity space (Δϕ, Δη) are measured to study the distribution of the associated particles around the jet axis for different pT,assoc-ranges between 1 and 20 GeV/c. The data in Pb–Pb collisions are compared to reference distributions for pp collisions, obtained using embedded PYTHIA simulations. The number of high-pT associate particles (4 < pT,assoc < 20 GeV/c) in Pb–Pb collisions is found to be suppressed compared to the reference by 30 to 10% depending on centrality. The radial particle distribution relative to the jet axis shows a moderate modification in Pb–Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb–Pb collisions compared to the reference, while low-pT associate particles tend to be broadened. The results, which are presented for the first time down to pT, ch jet = 30 GeV/c in Pb–Pb collisions, are compatible with both previous jet–hadron-related measurements from the CMS Collaboration and jet shape measurements from the ALICE Collaboration at higher pT, and add further support for the established picture of in-medium parton energy loss.
Charged-particle spectra at midrapidity are measured in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair √sNN = 5.02 TeV and presented in centrality classes ranging from most central (0–5%) to most peripheral (95–100%) collisions. Possible medium effects are quantified using the nuclear modification factor (RAA) by comparing the measured spectra with those from proton–proton collisions, scaled by the number of independent nucleon–nucleon collisions obtained from a Glauber model. At large transverse momenta (8 < pT < 20 GeV/c), the average RAA is found to increase from about 0.15 in 0–5% central to a maximum value of about 0.8 in 75–85% peripheral collisions, beyond which it falls off strongly to below 0.2 for the most peripheral collisions. Furthermore, RAA initially exhibits a positive slope as a function of pT in the 8–20 GeV/c interval, while for collisions beyond the 80% class the slope is negative. To reduce uncertainties related to event selection and normalization, we also provide the ratio of RAA in adjacent centrality intervals. Our results in peripheral collisions are consistent with a PYTHIA-based model without nuclear modification, demonstrating that biases caused by the event selection and collision geometry can lead to the apparent suppression in peripheral collisions. This explains the unintuitive observation that RAA is below unity in peripheral Pb–Pb, but equal to unity in minimum-bias p–Pb collisions despite similar charged-particle multiplicities.
73Ge(n, γ ) cross sections were measured at the neutron time-of-flight facility n_TOF at CERN up to neutron energies of 300 keV, providing for the first time experimental data above 8 keV. Results indicate that the stellar cross section at kT = 30 keV is 1.5 to 1.7 times higher than most theoretical predictions. The new cross sections result in a substantial decrease of 73Ge produced in stars, which would explain the low isotopic abundance of 73Ge in the solar system.
We present the first measurement of the proton–Ω correlation function in heavy-ion collisions for the central (0–40%) and peripheral (40–80%) Au + Au collisions at √sNN = 200 GeV by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Predictions for the ratio of peripheral collisions to central collisions for the proton–Ω correlation function are sensitive to the presence of a nucleon– bound state. These predictions are based on the proton– interaction extracted from (2 + 1)-flavor lattice QCD calculations at the physical point. The measured ratio of the proton–Ω correlation function between the peripheral (small system) and central (large system) collisions is less than unity for relative momentum smaller than 40 MeV/c. Comparison of our measured correlation ratio with theoretical calculation slightly favors a proton– bound system with a binding energy of ∼ 27 MeV.