arXiv subjects

Marcus Bleicher

Publications and source records attributed to Marcus Bleicher.

At least 91 records · Page 5Linked to original sources

arXiv2019

Benchmark values for the net proton number fluctuations

We formulate a baseline model for the net-proton number fluctuations in ultrarelativistic heavy-ion collisions. Our model includes total baryon number conservation, fluctuations of the participating nucleon number, limited acceptance, and a possibility that isospin is remembered by the participating wounded nucleons. Most importantly, we formulated the model as consisting of two components: wounded nucleons and the produced nucleon-antinucleon pairs. Those two components have different rapidity distributions. Owing to this feature we obtain also predictions for the rapidity dependence of the cumulants of the net-proton number distribution and their ratios.

nucl-th↗

arXiv2019

Elliptic flow and $R_{AA}$ of D mesons at FAIR comparing the UrQMD hybrid model and the coarse-graining approach

We present a study of the elliptic flow and $R_{AA}$ of D and anti-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 mesons elliptic flow and that the subsequent hadronic interactions seem to play only a minor role. This indicates that 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.

hep-ph↗

arXiv2018

Evolution of the moments of multiplicity distributions

Measured moments of the multiplicity distribution for a given sort of particles are used in the literature for the determination of the phase transition parameters of hot QCD matter in ultrarelativistic heavy-ion collisions. We argue that the subsequent cooling in the hadronic phase, however, may drive the multiplicity distribution out of equilibrium. We use a master equation for the description of the evolution of the multiplicity distribution to demonstrate how the different moments depart away from their equilibrium values. If such moments were measured and interpreted as if they were equilibrated, one would obtain different apparent temperatures from different moments.

nucl-th↗

arXiv2018

Susceptibilities of strongly interacting matter in a finite volume

We investigate possible finite-volume effects on baryon number susceptibilities of strongly interacting matter. Assuming that a hadronic and a deconfined phase both contribute to the thermodynamic state of a finite system due to fluctuations, it is found that the resulting shapes of the net-baryon number distributions deviate significantly from the infinite volume limit for a given temperature $T$ and baryochemical potential $μ_B$. In particular, the constraint on color-singletness for the finite quark-gluon phase contribution leads to a change of the temperature dependence of the susceptibilities in finite volumes. According to the model, the finite-volume effect depends qualitatively on the value of $μ_B$.

nucl-th↗

arXiv2018

The QCD Phase Diagram from Statistical Model Analysis

In high multiplicity nucleus-nucleus collisions baryon-antibaryon annihilation and regeneration occur during the final hadronic expansion phase, thus distorting the initial equilibrium multiplicity ratios. We quantify the modifications employing the hybrid UrQMD transport model and apply them to the grand canonical partition functions of the Statistical Hadronization Model(SHM). We analyze minimum bias and central Pb+Pb collision data at SPS and LHC energy. We explain the Pion to Proton ratio puzzle. We also reproduce the deuteron to proton ratio at LHC energy by the SHM, and by UrQMD after attaching a phase space coalescence process. We discuss the resulting (T,$μ_{B}$) diagram.

nucl-th↗

arXiv2018

Entropy production and reheating at the chiral phase transition

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 S/N is estimated to range from 10% for a crossover to 200% 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.

hep-ph↗

arXiv2018

Event-by-event fluctuations in p+p and central A+A collisions within relativistic transport models

Event-by-event multiplicity fluctuations in nucleus-nucleus collisions are studied within the relativistic transport models: EPOS, PHSD, and UrQMD. As measures of particle number fluctuations we consider the scaled variances $ω[X]$ for positive, negative, and all charged hadrons, and the strongly intensive quantities $Δ[K^+,π^+],$ $Σ[K^+,π^+]$ for $K^+$ and $π^+$ yields. At the SPS energy range the fluctuation measures are calculated for proton-proton, Ar+Sc, and Pb+Pb collisions. Comparison with recent NA61/SHINE and older NA49 measurements of the multiplicity fluctuations is done. A validity of the model of independent sources, a role of the experimental acceptance, and the centrality selection procedure are studied.Event-by-event multiplicity fluctuations in nucleus-nucleus collisions are studied within the relativistic transport models: EPOS, PHSD, and UrQMD. As measures of particle number fluctuations we consider the scaled variances $ω[X]$ for positive, negative, and all charged hadrons, and the strongly intensive quantities $Δ[K^+,π^+],$ $Σ[K^+,π^+]$ for $K^+$ and $π^+$ yields. At the SPS energy range the fluctuation measures are calculated for proton-proton, Ar+Sc, and Pb+Pb collisions. Comparison with recent NA61/SHINE and older NA49 measurements of the multiplicity fluctuations is done. A validity of the model of independent sources, a role of the experimental acceptance, and the centrality selection procedure are studied.

nucl-th↗

arXiv2018

Directed, elliptic and triangular flow of protons in Au+Au reactions at 1.23 AGeV: A theoretical analysis of the recent HADES data

Recently, the HADES experiment at GSI has provided preliminary data on the directed flow, $v_1$ elliptic flow, $v_2$ and triangular flow, $v_3$ of protons in Au+Au reactions at a beam energy of 1.23 A GeV. Here we present a theoretical discussion of these flow harmonics within the UrQMD transport approach. We show that all flow harmonics, including the triangular flow, provide a consistent picture of the expansion of the system, if potential interactions are taken into account. Investigating the dependence of the flow harmonics on the nuclear interaction potentials it is shown that especially $v_3$ can serve as a sensitive probe for the nuclear equation of state at such low energies. The triangular flow and its excitation function with respect to the reaction-plane were calculated for the first time and indicate a complex interplay of the time-evolution of the system and the initial conditions at low beam-energies. Our study also indicates a significant softening of the equation of state at beam energies above $E_{\mathrm{lab}}> 7$A GeV which can be explored by at the future FAIR facility.

nucl-th↗

arXiv2018

Testing Charm Quark Equilibration in Ultra-High Energy Heavy Ion Collisions with Fluctuations

Recent lattice QCD data on higher order susceptibilities of Charm quarks provide the opportunity to explore Charm quark equilibration in the early quark gluon plasma (QGP) phase. Here, we propose to use the lattice data on second and fourth order net Charm susceptibilities to infer the Charm quark equilibration temperature and the corresponding volume, in the early QGP stage, via a combined analysis of experimentally measured multiplicity fluctuations. Furthermore, the first perturbative results for the second and fourth order Charm quark susceptibilities and their ratio are presented.

hep-ph↗

arXiv2018

Broadening of the chiral critical region in a hydrodynamically expanding medium

We investigate higher cumulants of the sigma field as the chiral order parameter at the QCD phase transition. We derive a thermodynamic expression for the skewness and kurtosis from susceptibilities and use these to determine $Sσ$ and $κσ^2$ for the sigma field in equilibrium. In a next step, we study the behavior of these cumulants in both an equilibrated static medium and an expanding medium resembling the hydrodynamic stage of a heavy-ion collision. For the latter one, we find a significant broadening of the critical region.

hep-ph↗

arXiv2017

Rapidity Gap Survival in Enhanced Pomeron Scheme

We apply the phenomenological Reggeon field theory framework to investigate rapidity gap survival (RGS) probability for diffractive dijet production in proton-proton collisions. In particular, we study in some detail rapidity gap suppression due to elastic rescatterings of intermediate partons in the underlying parton cascades, described by enhanced (Pomeron-Pomeron interaction) diagrams. We demonstrate that such contributions play a subdominant role, compared to the usual, so-called "eikonal", rapidity gap suppression due to elastic rescatterings of constituent partons of the colliding protons. On the other hand, the overall RGS factor proves to be sensitive to color fluctuations in the proton. Hence, experimental data on diffractive dijet production can be used to constrain the respective model approaches.

hep-ph↗

arXiv2017

Unruh thermal hadronization and the cosmological constant

We use black holes with a negative cosmological constant to investigate aspects of the freeze-out temperature for hadron production in high energy heavy-ion collisions. The two black hole solutions present in the anti-de Sitter geometry have different mass and are compared to the data showing that the small black hole solution is in good agreement. This is a new feature in the literature since the small black hole in general relativity has different thermodynamic behavior from that of the large black hole solution. We find that the inclusion of the cosmological constant (which can be interpreted as the plasma pressure) leads to a lowering of the temperature of the freeze-out curve as a function of the baryochemical potential, improving the description previously suggested by Castorina, Kharzeev, and Satz.

hep-th↗

arXiv2017

The QCD Phase Diagram from Statistical Model Analysis

Ideally, the Statistical Hadronization Model (SHM) freeze-out curve should reveal the QCD parton-hadron phase transformation line in the ($T$,$μ_B$) plane. We discuss the effects of various final state interaction phenomena, like baryon-antibaryon annihilation, core-corona effects or QCD critical point formation, which shift or deform the SHM freezeout curve. In particular, we present a method to remove the annihilation effects by quantifying them with the microscopic hadron transport model UrQMD. We further discuss the new aspects of hadronization that could be associated with the relatively broad cross-over phase transformation as predicted by lattice-QCD theory at low $μ_B$. That opens up the possibility that various observables of hadronization, e.g. hadron formation or susceptibilities of higher order (related to grand canonical fluctuations of conserved hadronic charges) may freeze out at different characteristic temperatures. This puts into question the concept of a universal \textit{(pseudo-)critical} temperature, as does the very nature of a cross-over phase transformation.

hep-ph↗

arXiv2017

Clusters and higher moments of proton number fluctuations

Production of deuterons by coalescence has an important influence on the moments of the observed proton number distribution. Therefore, this effect must be taken into account when physics conclusions about baryon number fluctuation are drawn from the measurement of proton number fluctuations. We also show that a measurement of the third and fourth moments of the deuteron number distribution would allow to distinguish whether deuterons are produced statistically or by coalescence.

nucl-th↗

arXiv2017

Black Holes and High Energy Physics: From Astrophysics to Large Extra Dimensions

Up to now, Einstein's general theory of relativity has passed all experimental tests. But yet we know that it is not a fundamental theory and that it is incompatible with quantum theory. While several extended and improved gravitational theories on classical and quantum level are nowadays available, it is a great challenge to find experimental setups to check their signatures. We discuss recent developments in direct observation of black holes comprising gravitational waves from black hole mergers, radio interferometry images of black hole shadows, and Hawking radiation of black holes in particle accelerators. These investigations cover the full black hole mass range from microscopic to stellar and supermassive black holes. We comment on the associated strong-field tests of Einstein's general theory of relativity and implications for quantum gravity. Special emphasis lies upon the physics of large extra dimensions and black hole evaporation, the existence of a minimal black hole mass, and the cross sections of higher-dimensional black holes. We complete this short review with the latest experimental constraints at the Large Hadron Collider.

gr-qc↗

arXiv2017

AdS/CFT far from equilibrium in a Vaidya setup

In this paper we apply the AdS/CFT correspondence to study a strongly coupled plasma far from equilibrium with a strong emphasis on the shear behavior. The plasma serves as a model for an electrically charged quark-gluon plasma. On the gravitational side, we use an ingoing Vaidya black brane spacetime. The highest rate of mass infall is confined to a short time interval.

hep-th↗

arXiv2017

Planck scale black holes - Theory vs. observations

In this paper we present the status of the physics of Planck scale black holes with particular reference to their conjectured production in particle accelerator experiments at the terascale. After reviewing some open issues of fundamental interactions and introducing the physics in the large extra-dimensional scenario, we present the expected signatures left by a microscopic black hole in a particle detector. The final part of the paper is devoted to the latest experimental bounds on the sought black hole signals.

hep-ph↗

arXiv2017

Numerical magneto-hydrodynamics for relativistic nuclear collisions

We present an improved version of the ECHO-QGP numerical code, which self-consistently includes for the first time the effects of electromagnetic fields within the framework of relativistic magnetohydrodynamics (RMHD). We discuss results of its application in relativistic heavy-ion collisions in the limit of infinite electrical conductivity of the plasma. After reviewing the relevant covariant $3\!+\!1$ formalisms, we illustrate the implementation of the evolution equations in the code and show the results of several tests aimed at assessing the accuracy and robustness of the implementation. After providing some estimates of the magnetic fields arising in non-central high-energy nuclear collisions, we perform full RMHD simulations of the evolution of the Quark-Gluon Plasma in the presence of electromagnetic fields and discuss the results. In our ideal RMHD setup we find that the magnetic field developing in non-central collisions does not significantly modify the elliptic-flow of the final hadrons. However, since there are uncertainties in the description of the pre-equilibrium phase and also in the properties of the medium, a more extensive survey of the possible initial conditions as well as the inclusion of dissipative effects are indeed necessary to validate this preliminary result.

hep-ph↗