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The cosmological implications of the Covariant Canonical Gauge Theory of Gravity (CCGG) are investigated. CCGG is a Palatini theory derived from first principles using the canonical transformation formalism in the covariant Hamiltonian formulation. The Einstein-Hilbert theory is thereby extended by a quadratic Riemann-Cartan term in the Lagrangian. Moreover, the requirement of covariant conservation of the stress-energy tensor leads to necessary presence of torsion. In the Friedman universe that promotes the cosmological constant to a time-dependent function, and gives rise to a geometrical correction with the EOS of dark radiation. The resulting cosmology, compatible with the ΛCDM parameter set, encompasses bounce and bang scenarios with graceful exits into the late dark energy era. Testing those scenarios against low-z observations shows that CCGG is a viable theory.
In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being geared toward the implementation of THz imaging systems in real operational conditions. The article will touch upon the advanced solid-state-based THz imaging systems, including room temperature THz sensors and arrays, as well as their on-chip integration with diffractive THz optical components. We will cover the current-state of compact room temperature THz emission sources, both optolectronic and electrically driven; particular emphasis is attributed to the beam-forming role in THz imaging, THz holography and spatial filtering, THz nano-imaging, and computational imaging. A number of advanced THz techniques, such as light-field THz imaging, homodyne spectroscopy, and phase sensitive spectrometry, THz modulated continuous wave imaging, room temperature THz frequency combs, and passive THz imaging, as well as the use of artificial intelligence in THz data processing and optics development, will be reviewed. This roadmap presents a structured snapshot of current advances in THz imaging as of 2021 and provides an opinion on contemporary scientific and technological challenges in this field, as well as extrapolations of possible further evolution in THz imaging.
We examine the thermodynamic behavior of a static neutral regular (non-singular) black hole enclosed in a finite isothermal cavity. The cavity enclosure helps us investigate black hole systems in a canonical or a grand canonical ensemble. Here we demonstrate the derivation of the reduced action for the general metric of a regular black hole in a cavity by considering a canonical ensemble. The new expression of the action contains quantum corrections at short distances and concludes to the action of a singular black hole in a cavity at large distances. We apply this formalism to the noncommutative Schwarzschild black hole, in order to study the phase structure of the system. We conclude to a possible small/large stable regular black hole transition inside the cavity that exists neither at the system of a classical Schwarzschild black hole in a cavity, nor at the asymptotically flat regular black hole without the cavity. This phase transition seems to be similar with the liquid/gas transition of a Van der Waals gas.
When a very strong light field is applied to a molecule an electron can be ejected by tunneling. In order to quantify the time-resolved dynamics of this ionization process, the concept of the Wigner time delay can be used. The properties of this process can depend on the tunneling direction relative to the molecular axis. Here, we show experimental and theoretical data on the Wigner time delay for tunnel ionization of H2 molecules and demonstrate its dependence on the emission direction of the electron with respect to the molecular axis. We find, that the observed changes in the Wigner time delay can be quantitatively explained by elongated/shortened travel paths of the emitted electrons, which occur due to spatial shifts of the electrons’ birth positions after tunneling. Our work provides therefore an intuitive perspective towards the Wigner time delay in strong-field ionization.
Chiral symmetry represents a fundamental concept lying at the core of particle and nuclear physics. Its spontaneous breaking in vacuum can be exploited to distinguish chiral hadronic partners, whose masses differ. In fact, the features of this breaking serve as guiding principles for the construction of effective approaches of QCD at low energies, e.g., the chiral perturbation theory, the linear sigma model, the (Polyakov)–Nambu–Jona-Lasinio model, etc. At high temperatures/densities chiral symmetry can be restored bringing the chiral partners to be nearly degenerated in mass. At vanishing baryochemical potential, such restoration follows a smooth transition, and the chiral companions reach this degeneration above the transition temperature. In this work I review how different realizations of chiral partner degeneracy arise in different effective theories/models of QCD. I distinguish the cases where the chiral states are either fundamental degrees of freedom or (dynamically-generated) composed states. In particular, I discuss the intriguing case in which chiral symmetry restoration involves more than two chiral partners, recently addressed in the literature.
Quasi-universal behavior of the threshold mass in unequal-mass, spinning binary neutron star mergers
(2021)
The lifetime of the remnant produced by the merger of two neutron stars can provide a wealth of information on the equation of state of nuclear matter and on the processes leading to the electromagnetic counterpart. Hence, it is essential to determine when this lifetime is the shortest, corresponding to when the remnant has a mass equal to the threshold mass, Mth, to prompt collapse to a black hole. We report on the results of more than 360 simulations of merging neutron-star binaries covering 40 different configurations differing in mass ratio and spin of the primary. Using this data, we have derived a quasi-universal relation for Mth and expressed its dependence on the mass ratio and spin of the binary. The new expression recovers the results of Koeppel et al. for equal-mass, irrotational binaries and reveals that Mth can increase (decrease) by 5% (10%) for binaries that have spins aligned (antialigned) with the orbital angular momentum and provides evidence for a nonmonotonic dependence of Mth on the mass asymmetry in the system. Finally, we extend to unequal masses and spinning binaries the lower limits that can be set on the stellar radii once a neutron star binary is detected, illustrating how the merger of an unequal-mass, rapidly spinning binary can significantly constrain the allowed values of the stellar radii.
Based on recent perturbative and non-perturbative lattice calculations with almost quark flavors and the thermal contributions from photons, neutrinos, leptons, electroweak particles, and scalar Higgs bosons, various thermodynamic quantities, at vanishing net-baryon densities, such as pressure, energy density, bulk viscosity, relaxation time, and temperature have been calculated up to the TeV-scale, i.e., covering hadron, QGP, and electroweak (EW) phases in the early Universe. This remarkable progress motivated the present study to determine the possible influence of the bulk viscosity in the early Universe and to understand how this would vary from epoch to epoch. We have taken into consideration first- (Eckart) and second-order (Israel–Stewart) theories for the relativistic cosmic fluid and integrated viscous equations of state in Friedmann equations. Nonlinear nonhomogeneous differential equations are obtained as analytical solutions. For Israel–Stewart, the differential equations are very sophisticated to be solved. They are outlined here as road-maps for future studies. For Eckart theory, the only possible solution is the functionality, H(a(t)), where H(t) is the Hubble parameter and a(t) is the scale factor, but none of them so far could to be directly expressed in terms of either proper or cosmic time t. For Eckart-type viscous background, especially at finite cosmological constant, non-singular H(t) and a(t) are obtained, where H(t) diverges for QCD/EW and asymptotic EoS. For non-viscous background, the dependence of H(a(t)) is monotonic. The same conclusion can be drawn for an ideal EoS. We also conclude that the rate of decreasing H(a(t)) with increasing a(t) varies from epoch to epoch, at vanishing and finite cosmological constant. These results obviously help in improving our understanding of the nucleosynthesis and the cosmological large-scale structure.
Consequences of minimal length discretization on line element, metric tensor, and geodesic equation
(2021)
When minimal length uncertainty emerging from a generalized uncertainty principle (GUP) is thoughtfully implemented, it is of great interest to consider its impacts on gravitational Einstein field equations (gEFEs) and to try to assess consequential modifications in metric manifesting properties of quantum geometry due to quantum gravity. GUP takes into account the gravitational impacts on the noncommutation relations of length (distance) and momentum operators or time and energy operators and so on. On the other hand, gEFE relates classical geometry or general relativity gravity to the energy–momentum tensors, that is, proposing quantum equations of state. Despite the technical difficulties, we intend to insert GUP into the metric tensor so that the line element and the geodesic equation in flat and curved space are accordingly modified. The latter apparently encompasses acceleration, jerk, and snap (jounce) of a particle in the quasi-quantized gravitational field. Finite higher orders of acceleration apparently manifest phenomena such as accelerating expansion and transitions between different radii of curvature and so on.
The QCD phase-diagram is studied, at finite magnetic field. Our calculations are based on the QCD effective model, the SU(3) Polyakov linear-sigma model (PLSM), in which the chiral symmetry is integrated in the hadron phase and in the parton phase, the up-, down- and strange-quark degrees of freedom are incorporated besides the inclusion of Polyakov loop potentials in the pure gauge limit, which are motivated by various underlying QCD symmetries. The Landau quantization and the magnetic catalysis are implemented. The response of the QCD matter to an external magnetic field such as magnetization, magnetic susceptibility and permeability has been estimated. We conclude that the parton phase has higher values of magnetization, magnetic susceptibility, and permeability relative to the hadron phase. Depending on the contributions to the Landau levels, we conclude that the chiral magnetic field enhances the chiral quark condensates and hence the chiral QCD phase-diagram, i.e. the hadron-parton phase-transition likely takes place, at lower critical temperatures and chemical potentials.
Die vorliegende Dissertation stellt die Strahldynamikdesigns zweier Hochfrequenzquadrupol-Linearbeschleuniger bzw. Radio Frequency Quadrupoles (RFQs) vor: das fur den RFQ des Protonen-Linearbeschleunigers (p-Linac) des FAIR2-Projekts an der GSI3 Darmstadt sowie einen ersten Designentwurf für einen kompakten RFQ, der u.a. zur Erzeugung von Radioisotopen für medizinische Zwecke genutzt werden könnte. Der Schwerpunkt liegt auf dem ersten Design.
The topic of this thesis is the theoretical description of the hadron gas stages in heavy-ion collisions. The overall addressed question hereby is: How does the hadronic medium evolve i.e. what are the relevant microscopic reaction mechanisms and the properties of the involved degrees of freedom? The main goal is to address this question specifically for hadronic multi-particle interactions. For this goal, the hadronic transport approach SMASH is extended with stochastic rates, which allow to include detailed balance fulfilling multi-particle reactions in the approach. Three types of reactions are newly-accounted for: 3-to-1, 3-to-2 and 5-to-2 reactions. After extensive verifications of the stochastic rates approach, they are used to study the effect of multi-particle interactions, particularly in afterburner calculations.
These studies follow complementary results for the dilepton and strangeness production with only binary reactions, which show that hadronic transport approaches are capable of describing observables when employed for the entire evolution of low-energy heavy-ion collisions. This is illustrated by the agreement of dilepton and strangeness production for smaller systems with SMASH calculations. It is, in particular, possible to match the measured strangeness production of phi and Xi hadrons via additional heavy nucleon resonance decay channels. For larger systems or higher energies, hadronic transport cascade calculations with vacuum resonance properties can point to medium effects. This is demonstrated extensively for the dilepton emission in comparisons to the full set of HADES dielectron data. The dilepton invariant mass spectra are sensitive to a medium modification of the vector meson spectral function for large collision systems already at low beam energies. The sensitivity to medium modifications is mapped out in detail by comparisons to a coarse-graining approach, which employs medium-modified spectral functions and is based on the same evolution.
The theoretical foundation of stochastic rates are collision probabilities derived from the Boltzmann equation's collision term with the assumption of a constant matrix element. This derivation is presented in a comprehensive and pedagogical fashion. The derived collision probabilities are employed for a stochastic collision criterion and various detailed-balance fulfilling multi-particle reactions: the mesonic Dalitz decay back-reaction (3-to-1), the deuteron catalysis (3-to-2) and the proton-antiproton annihilation back-reaction (5-to-2). The introduced stochastic rates approach is extensively verified by studies of the numerical stability and comparisons to previous results and analytic expectations. The stochastic rates results agree perfectly with the respective analytic results.
Physically, multi-particle reactions are demonstrated to be significant for different observables, most notably the yield of the partaking particles, even in the late dilute stage of heavy-ion reactions. They lead to a faster equilibration of the system than equivalent binary multi-step treatments. The difference in equilibration consequently influences the yield in afterburner calculations. Interestingly, the interpretation of results is not dependent on employing multi-particle or multi-step treatments, which a posteriori validates the latter.
As the first test case of multi-particle reactions in heavy-ion reactions, the mesonic 3-to-1 Dalitz decay is found to be dominated by the omega Dalitz decay back-reaction. While the effect on the medium is found to be negligible overall, the regeneration is found to be sizable: up to a quarter of Dalitz decays are regenerated.
Non-equilibrium rescattering effects are shown to be relevant for late collision stages for two particle species: deuteron and protons. In both cases, the relevant rescatterings involve multiple particles.
The deuteron pion and nucleon catalysis reactions equilibrate quickly in the afterburner stage at intermediate energies. The constant formation and destruction keeps the yield constant and microscopically explains the "snowballs in hell"-paradox. The yield is also generated with no d present at early times, which explains why coalescence models can also match the multiplicity.
New is the study of the 5-body back-reaction of proton-antiproton annihilations. This work marks the first realization of microscopic 5-body reactions in a transport approach to fulfill detailed balance for such reactions. A sizable regeneration due to the back-reaction of up to half of the proton-antiproton pairs lost due to annihilations is found. Consequently, both annihilation and regeneration in the late non-equilibrium stage are shown to have a significant effect on the p yield.
Ziel dieser Dissertation ist es, die Gleichgewichts- und Nichtgleichgewichts-Eigenschaften des stark wechselwirkenden QGP-Mediums nahe dem Phasenübergang unter extremen Bedingungen von hohen T und hohen Baryonendichten mit Hilfe der kinetischen Theorie im Rahmen von effektiven Modellen zu untersuchen. Wir werden zunächst die thermodynamischen und Transporteigenschaften des QGPs in der Nähe des Gleichgewichts auf der Basis des DQPM im Bereich moderater chemischer Baryonenpotentiale μB ≥ 0.5 GeV untersuchen. Insbesondere werden die EoS und die Schallgeschwindigkeit sowie die Transportkoeffizienten des QGP auf der Grundlage des DQPM bei endlichen T und μB berechnet. Transportkoeffizienten sind besonders interessant, da sie Informationen über die Wechselwirkungen im Medium erlauben, das im Gleichgewicht durch eine Temperatur T und ein chemisches Potential μB charakterisiert werden kann. Unter Berücksichtigung der Transportkoeffizienten und der EoS der QGP-Phase vergleichen wir unsere Ergebnisse mit verschiedenen Resultaten aus der Literatur, in denen Transportkoeffizienten des QGPs auf Basis von effektiven Modellen vorwiegend bei Null oder kleinem chemischen Potentialen untersucht wurden.
Darüber hinaus werden in Kapitel 3 die Gleichgewichtseigenschaften des QGPs und insbesondere die Auswirkungen der μB-Abhängigkeit der thermodynamischen und Transporteigenschaften des QGPs im Rahmen des erweiterten PHSD-Transportansatzes untersucht, der die vollständige Entwicklung des Systems einschließlich der partonischen Phase umfasst. Die Entwicklung des PHSD-Transportansatzes wird in der partonischen Phase erweitert, indem explizit die gesamt- und differentiellen partonischen Streuquerschnitte auf der Grundlage des DQPM berechnet und bei der tatsächlichen Temperatur T und dem baryonischen chemischen Potential μB in jeder einzelnen Raum-Zeit-Zelle, in der die partonische Streuung stattfindet, ausgewertet werden.
Um die Spuren der μB-Abhängigkeit des QGPs in den Observablen zu untersuchen, werden die Ergebnisse von PHSD5.0 (mit μB-Abhängigkeiten) mit den Ergebnissen von PHSD5.0 für μB = 0 sowie mit PHSD4.0, in dem die Massen/Breiten der Quarks und Gluonen sowie deren Wechselwirkungsquerschnitte nur von T abhängen, verglichen. Wir diskutieren die PHSD-Ergebnisse für verschiedene Observablen: (i) Rapiditäts- und pT -Verteilungen von identifizierten Hadronen für symmetrische Au+Au- und Pb+Pb- Kollisionen bei Energien von 30 AGeV (zukünftige NICA-Energie) sowie für die RHIC-Spitzenenergie von √sNN = 200 GeV; (ii) gerichteter Fluss v1 von identifizierten Hadronen für Au + Au bei invarianter Energie √sNN = 27 GeV und 200 GeV; (iii) elliptischer Fluss v2 der identifizierten Hadronen für Au+Au bei invarianten Energien √sNN = 27 und 200 GeV. Der Vergleich der "Bulk"-Observablen für Au+Au-Kollisionen innerhalb der drei PHSD-Einstellungen hat gezeigt, dass sie eine recht geringe Empfindlichkeit gegenüber den μB -Abhängigkeiten der Partoneigenschaften (Massen und Breiten) und ihrer Wechselwirkungsquerschnitte aufweisen, sodass die Ergebnisse von PHSD5.0 mit und ohne μB sehr nahe beieinander liegen. Nur im Fall von Kaonen, Antiprotonen ̄p und Antihyperonen ̄Λ + ̄Σ0 konnte ein kleiner Unterschied zwischen PHSD4.0 und PHSD5.0 bei den höchsten SPS- und RHIC-Energien festgestellt werden.
Wir finden nur geringe Unterschiede zwischen den Ergebnissen von PHSD4.0 und PHSD5.0 für die hier betrachteten hadronischen Observablen sowohl bei hohen als auch bei mittleren Energien. Dies hängt damit zusammen, dass bei hohen Energien, wo die Materie vom QGP dominiert wird, ein sehr kleines chemisches Baryonenpotential μB in zentralen Kollisionen bei mittlerer Rapidität gemessen wird, während mit abnehmender Energie und größerem μB der Anteil des QGPs rapide abnimmt, sodass die endgültigen Beobachtungswerte insgesamt von den Hadronen dominiert werden, die an der hadronischen Rückstreuung teilgenommen haben, und somit die Information über ihren QGP-Ursprung verwaschen oder verloren geht.
In Kapitel 4 betrachten wir die Transportkoeffizienten von QGP-Materie im erweiterten Polyakov-NJL-Modell entlang der Übergangslinie für moderate Werte des chemischen Baryonenpotenzials 0 ≤ μB ≤ 0.9 GeV sowie in der Nähe des kritischen Endpunkts(CEP) und bei großem chemischen Baryonenpotenzial μB = 1.2 GeV, wo ein Phasenübergang erster Ordnung stattfindet. Wir untersuchen, wie die Natur der Freiheitsgrade die Transporteigenschaften des QGPs beeinflusst. Darüber hinaus demonstrieren wir die Auswirkungen des Phasenübergangs erster Ordnung und des CEP auf die Transportkoeffizienten im dekonfinierten QCD-Medium.
Darüber hinaus wird in Kapitel 5 eine phänomenologische Erweiterung des DQPM auf große baryonchemische Potentiale μB einschließlich der Region mit einem möglichen CEP und späterem Phasenübergang erster Ordnung betrachtet. Eines der wichtigsten Merkmale des Modells ist das Auftreten einer ’kritischen‘ Skalierung in der Nähe des CEP. Das Hauptziel des vorgestellten Modells besteht darin, die mikroskopischen und makroskopischen Eigenschaften der partonischen Freiheitsgrade für den Bereich des Phasendiagramms bereitzustellen, der durch moderates T und moderates oder hohes μB gekennzeichnet ist.
...
Abstract
The primary immunological target of COVID-19 vaccines is the SARS-CoV-2 spike (S) protein. S is exposed on the viral surface and mediates viral entry into the host cell. To identify possible antibody binding sites, we performed multi-microsecond molecular dynamics simulations of a 4.1 million atom system containing a patch of viral membrane with four full-length, fully glycosylated and palmitoylated S proteins. By mapping steric accessibility, structural rigidity, sequence conservation, and generic antibody binding signatures, we recover known epitopes on S and reveal promising epitope candidates for structure-based vaccine design. We find that the extensive and inherently flexible glycan coat shields a surface area larger than expected from static structures, highlighting the importance of structural dynamics. The protective glycan shield and the high flexibility of its hinges give the stalk overall low epitope scores. Our computational epitope-mapping procedure is general and should thus prove useful for other viral envelope proteins whose structures have been characterized.
Author summary
The SARS-CoV-2 virus has caused a global health crisis. The spike protein exposed at its surface is key for infection and the primary antibody target. However, spike is covered by highly mobile glycan molecules that could impair antibody binding. To identify accessible epitopes, we performed molecular dynamics simulations of an atomistic model of glycosylated spike embedded in a membrane. By combining extensive simulations with bioinformatics analyses, we recovered known antibody binding sites and identified several epitope candidates as targets for further vaccine development.
Living cells constantly remodel the shape of their lipid membranes. In the endoplasmic reticulum (ER), the reticulon homology domain (RHD) of the reticulophagy regulator 1 (RETR1/FAM134B) forms dense autophagic puncta that are associated with membrane removal by ER-phagy. In molecular dynamics (MD) simulations, we find that FAM134B-RHD spontaneously forms clusters, driven in part by curvature-mediated attractions. At a critical size, as in a nucleation process, the FAM134B-RHD clusters induce the formation of membrane buds. The kinetics of budding depends sensitively on protein concentration and bilayer asymmetry. Our MD simulations shed light on the role of FAM134B-RHD in ER-phagy and show that membrane asymmetry can be used to modulate the kinetic barrier for membrane remodeling.
Living cells constantly remodel the shape of their lipid membranes. In the endo-plasmic reticulum (ER), the reticulon homology domain (RHD) of the reticulophagy regulator 1 (RETR1/FAM134B) forms dense autophagic puncta that are associated with membrane removal by ER-phagy. In molecular dynamics (MD) simulations, we find that FAM134B-RHD spontaneously forms clusters, driven in part by curvature-mediated attraction. At a critical size, the FAM134B-RHD clusters induce the formation of membrane buds. The kinetics of budding depends sensitively on protein concentration and bilayer asymmetry. Our MD simulations shed light on the role of FAM134B-RHD in ER-phagy and show that membrane asymmetry can be used to modulate the kinetics barrier for membrane remodeling.
During infection the SARS-CoV-2 virus fuses its viral envelope with cellular membranes of its human host. The viral spike (S) protein mediates both the initial contact with the host cell and the subsequent membrane fusion. Proteolytic cleavage of S at the S2′ site exposes its fusion peptide (FP) as the new N-terminus. By binding to the host membrane, the FP anchors the virus to the host cell. The reorganization of S2 between virus and host then pulls the two membranes together. Here we use molecular dynamics (MD) simulations to study the two core functions of the SARS-CoV-2 FP: to attach quickly to cellular membranes and to form an anchor strong enough to withstand the mechanical force during membrane fusion. In eight 10 μs long MD simulations of FP in proximity to endosomal and plasma membranes, we find that FP binds spontaneously to the membranes and that binding proceeds predominantly by insertion of two short amphipathic helices into the membrane interface. Connected via a flexible linker, the two helices can bind the membrane independently, yet binding of one promotes the binding of the other by tethering it close to the target membrane. By simulating mechanical pulling forces acting on the C-terminus of the FP, we then show that the bound FP can bear forces up to 250 pN before detaching from the membrane. This detachment force is more than 10-fold higher than an estimate of the force required to pull host and viral membranes together for fusion. We identify a fully conserved disulfide bridge in the FP as a major factor for the high mechanical stability of the FP membrane anchor. We conclude, first, that the sequential binding of two short amphipathic helices allows the SARS-CoV-2 FP to insert quickly into the target membrane, before the virion is swept away after shedding the S1 domain connecting it to the host cell receptor. Second, we conclude that the double attachment and the conserved disulfide bridge establish the strong anchoring required for subsequent membrane fusion. Multiple distinct membrane-anchoring elements ensure high avidity and high mechanical strength of FP–membrane binding.
During infection the SARS-CoV-2 virus fuses its viral envelope with cellular membranes of its human host. Initial contact with the host cell and membrane fusion are both mediated by the viral spike (S) protein. Proteolytic cleavage of S at the S2′ site exposes its 40 amino acid long fusion peptide (FP). Binding of the FP to the host membrane anchors the S2 domain of S in both the viral and the host membrane. The reorganization of S2 then pulls the two membranes together. Here we use molecular dynamics (MD) simulations to study the two core functions of the SARS-CoV-2 FP: to attach quickly to cellular membranes and to form an anchor strong enough to withstand the mechanical force during membrane fusion. In eight 10 μs-long MD simulations of FP in proximity to endosomal and plasma membranes, we find that FP binds spontaneously to the membranes and that binding proceeds predominantly by insertion of two short amphipathic helices into the membrane interface. Connected via a flexible linker, the two helices can bind the membrane independently, yet binding of one promotes the binding of the other by tethering it close to the target membrane. By simulating mechanical pulling forces acting on the C-terminus of the FP we then show that the bound FP can bear forces up to 250 pN before detaching from the membrane. This detachment force is more than ten-fold higher than an estimate of the force required to pull host and viral membranes together for fusion. We identify a fully conserved disulfide bridge in the FP as a major factor for the high mechanical stability of the FP membrane anchor. We conclude, first, that the sequential binding of two short amphipathic helices allows the SARS-CoV-2 FP to insert quickly into the target membrane, before the virion is swept away after shedding the S1 domain connecting it to the host cell receptor. Second, we conclude that the double attachment and the conserved disulfide bridge establish the strong anchoring required for subsequent membrane fusion. Multiple distinct membrane-anchoring elements ensure high avidity and high mechanical strength of FP-membrane binding.
The main subject of this thesis is the study of hadron and photon production in relativistic heavy-ion collisions by means of hydrodynamics+transport approaches. Two different kinds of such hybrid approaches are employed in this work, the SMASH-vHLLE-hybrid and a MUSIC+SMASH hybrid. While the former is capable of simulating heavy-ion collisions covering a wide range of collision energies down to √s = 4.3 GeV, reproducing the correct baryon stopping powers, the latter provides a framework to consistently model photon production in the hadronic stage of high-energy heavy-ion collisions.
The SMASH-vHLLE-hybrid is a novel state-of-the-art hybrid approach whose development constitutes a major contribution to this thesis. It couples the hadronic transport SMASH to the 3+1D viscous hydrodynamics approach vHLLE. Therein, SMASH is employed to provide the fluctuating 3D initial conditions and to model the late hadronic rescattering stage, and vHLLE for the fluid dynamical evolution of the hot and dense fireball. The initial conditions are provided on a hypersurface of constant proper time, and the macroscopic evolution of the fireball is carried out down to an energy density of ecrit = 0.5 GeV/fm3, where particlization occurs. Consistency at the interfaces is verified in view of global, on-average quantum number conservation and the SMASH-vHLLE-hybrid is validated by comparison to SMASH+CLVisc as well as UrQMD+vHLLE hybrid approaches. The establishment of the SMASH-vHLLE-hybrid to theoretically describe heavy-ion collisions at intermediate and high collision energies forms a basis for a range of extensions and future research projects. It is further made available to the heavy-ion community by virtue of being published on Github.
The SMASH-vHLLE-hybrid is applied to simulate Au+Au/Pb+Pb collisions between √s = 4.3 GeV and √s = 200.0 GeV. A good agreement with the experimentally measured rapidity and transverse mass spectra is obtained. In particular the baryon stopping dynamics are well reproduced at low, intermediate, and high collision energies. Excitation functions for the mid-rapidity yield and mean transverse momentum of pions, protons and kaons are demonstrated to agree well with their experimentally measured counterpart. These results further validate the approach and provide a solid baseline for potential future studies. The importance of annihilations and regenerations of protons and anti-protons is additionally investigated in Au+Au/Pb+Pb collisions between √s = 17.3 GeV and √s = 5.02 TeV with the SMASH-vHLLE-hybrid. It is found that, regarding the p + p ̄ ↔ 5 π reaction, 20-50% (depending on the rapidity range) of the (anti-)proton yield lost to annihila- tions in the hadronic rescattering stage is restored owing to the back reaction. The back reaction thus constitutes a non-negligible contribution to the final (anti-)proton yield and should not be neglected when modelling the late rescattering stage of heavy-ion collisions.
The MUSIC+SMASH hybrid is a hybrid approach ideally suited to model the production of photons in relativistic heavy-ion collisions. Therein, the macroscopic production of photons in the hadronic stage in MUSIC relies on the identical effective field theories as the photon cross sections implemented in SMASH for the microscopic production. The MUSIC+SMASH hybrid thus provides the first consistent framework to the end of hadronic photon production. It accounts for 2 → 2 scattering processes of the kind π + ρ → π + γ and pion bremsstrahlung processes π + π → π + π + γ. The MUSIC+SMASH hybrid is employed in an ideal 2D setup to systematically assess the importance of non-equliibrium dynamics in the hadronic rescattering stage on mid-rapidity transverse momentum spectra and elliptic flow of photons at RHIC/LHC energies. This is achieved by comparing the outcome of the MUSIC+SMASH hybrid, involving an out-of-equilibrium late rescattering stage, to macroscopically approximating late stage photon production by means of MUSIC, employed down to temperatures well below the switching temperature. It is found that non-equilibrium dynamics have only minor implications for photon transverse momentum spectra, but significantly enhance the photon elliptic flow. At RHIC energies, an enhancement of up to 70%, and at LHC of up to 65% is observed in the non-equilibrium afterburner as compared to its hydrodynamical counterpart. In combination with the large amount of photons produced above the particlization temperature, these differences are modest regarding the transverse momentum spectra, but a significant enhancement of the elliptic flow is observed at low transverse momenta. Below pT ≈ 1.4 GeV, the combined v2 is enhanced by up to 30% at RHIC, and up to 20% at the LHC within the non-equilibrium setup as compared to its approximation via hydrodynamics. Non-equilibrium dynamics in the hadronic rescattering stage are hence important, especially in view of momentum anisotropies at low transverse momenta. These findings thus contribute to the understanding of low-pT photons produced in heavy-ion collisions at RHIC/LHC energies and the MUSIC+SMASH hybrid employed for this study provides a baseline for additional studies regarding photon production in the future.
To summarize, the approaches and frameworks presented in this thesis provide a good baseline for further extensions and studies in order to improve the understanding of hadron and photon production in relativistic heavy-ion collisions across a wide range of collision energies. More broadly, such future studies of hadrons and photons may contribute to enhance the understandig of the properties of the fundamental building blocks of matter, of which everything that surrounds us is made of.
Recurrent cortical networks provide reservoirs of states that are thought to play a crucial role for sequential information processing in the brain. However, classical reservoir computing requires manual adjustments of global network parameters, particularly of the spectral radius of the recurrent synaptic weight matrix. It is hence not clear if the spectral radius is accessible to biological neural networks. Using random matrix theory, we show that the spectral radius is related to local properties of the neuronal dynamics whenever the overall dynamical state is only weakly correlated. This result allows us to introduce two local homeostatic synaptic scaling mechanisms, termed flow control and variance control, that implicitly drive the spectral radius toward the desired value. For both mechanisms the spectral radius is autonomously adapted while the network receives and processes inputs under working conditions. We demonstrate the effectiveness of the two adaptation mechanisms under different external input protocols. Moreover, we evaluated the network performance after adaptation by training the network to perform a time-delayed XOR operation on binary sequences. As our main result, we found that flow control reliably regulates the spectral radius for different types of input statistics. Precise tuning is however negatively affected when interneural correlations are substantial. Furthermore, we found a consistent task performance over a wide range of input strengths/variances. Variance control did however not yield the desired spectral radii with the same precision, being less consistent across different input strengths. Given the effectiveness and remarkably simple mathematical form of flow control, we conclude that self-consistent local control of the spectral radius via an implicit adaptation scheme is an interesting and biological plausible alternative to conventional methods using set point homeostatic feedback controls of neural firing.
Charts are used to measure relative success for a large variety of cultural items. Traditional music charts have been shown to follow self-organizing principles with regard to the distribution of item lifetimes, the on-chart residence times. Here we examine if this observation holds also for (a) music streaming charts (b) book best-seller lists and (c) for social network activity charts, such as Twitter hashtags and the number of comments Reddit postings receive. We find that charts based on the active production of items, like commenting, are more likely to be influenced by external factors, in particular by the 24 h day–night cycle. External factors are less important for consumption-based charts (sales, downloads), which can be explained by a generic theory of decision-making. In this view, humans aim to optimize the information content of the internal representation of the outside world, which is logarithmically compressed. Further support for information maximization is argued to arise from the comparison of hourly, daily and weekly charts, which allow to gauge the importance of decision times with respect to the chart compilation period.