arXiv subjects

Christoph Herold

Publications and source records attributed to Christoph Herold.

At least 37 records · Page 2Linked to original sources

arXiv2020

Unsupervised Outlier Detection in Heavy-Ion Collisions

We present different methods of unsupervised learning which can be used for outlier detection in high energy nuclear collisions. The UrQMD model is used to generate the bulk background of events as well as different variants of outlier events which may result from misidentified centrality or detector malfunctions. The methods presented here can be generalized to different and novel physics effects. To detect the outliers, dimensional reduction algorithms are implemented, specifically the Principle Component Analysis (PCA) and Autoencoders (AEN). We find that mainly the reconstruction error is a good measure to distinguish outliers from background. The performance of the algorithms is compared using a ROC curve. It is shown that the number of reduced (encoded) dimensions to describe a single event contributes significantly to the performance of the outlier detection task. We find that the model which is best suited to separate outlier events requires a good performance in reconstructing events and at the same time a small number of parameters.

hep-ex↗

arXiv2020

Deuteron and Antideuteron Coalescence in Heavy-Ion Collisions: Energy Dependence of the Formation Geometry

We investigate the collision energy dependence of deuteron and antideuteron emission in the RHIC-BES low- to mid-energy range $\sqrt{s_{NN}} = 4.6-200$ GeV where the formation rate of antinuclei compared to nuclei is strongly suppressed. In the coalescence picture, this can be understood as bulk emission for nuclei in contrast to surface emission for antinuclei. By comparison with experimental data on the coalescence parameter $B_2$, we are able to extract the respective source geometries. This interpretation is further supported by results from the UrQMD transport model, and establishes the following picture: At low energies, nucleons freeze out over the total fireball volume, while antinucleons are annihilated inside the nucleon-rich fireball and can only freeze out on its surface. Towards higher energies, this annihilation effect becomes irrelevant (due to the decreasing baryochemical potential) and the system's freeze-out is driven by the mesons. Thus, the nucleon and antinucleon freeze-out distributions become similar with increasing energy.

hep-ph↗

arXiv2020

Influence of centrality definition and detector efficiency on the net-proton kurtosis

We study the influence of the centrality definition and detector efficiency on the net-proton kurtosis for minimum bias Au+Au collisions at a beam energy of $\sqrt{s_{\mathrm{NN}}}= 7.7$ GeV by using the UrQMD model. We find that different ways of defining the centrality lead to different cumulant ratios. Moreover, we demonstrate that the kurtosis is suppressed for central collisions when a wider transverse momentum acceptance is used. Finally, the influence of a detector efficiency on the measured cumulant ratios is estimated.

nucl-th↗

arXiv2019

Delta mass shift as a thermometer of kinetic decoupling in Au+Au reactions at 1.23 AGeV

The HADES experiment at GSI will soon provide data on the production and properties of $Δ$(1232) baryons from Au+Au reactions at 1.23 AGeV. Using the UrQMD model, we predict the yield and spectra of $Δ$(1232) resonances. In addition we show that one expects to observe a mass shift of the $Δ$(1232) resonance on the order of 50 MeV in the reconstructable $Δ$(1232) mass distribution. This mass shift can be understood in terms of late stage $Δ$(1232) formation with limited kinetic energy. We show how the mass shift can be used to constrain the kinetic decoupling temperature of the system.

nucl-th↗

arXiv2019

First, second, third and fourth flow harmonics of deuterons and protons in Au+Au reactions at 1.23 A GeV

We explore the directed, elliptic, triangular and quadrangular flow of deuterons in Au+Au reactions at a beam energy of 1.23 AGeV within the UrQMD approach. These investigations are of direct relevance for the HADES experiment at GSI that has recently presented first data on the flow of light clusters in Au+Au collisions at 1.23 AGeV. To address the deuteron flow, UrQMD has been extended to include deuteron formation by coalescence. We find that this ansatz provides a very good description of the measured deuteron flow data, if a hard equation of state is used for the simulation. In addition we show that light cluster formation has a sizable impact on the proton flow and has to be taken into account to obtain reliable results in the forward/backward region. Based on the observed scaling of the flow, which is a natural result of coalescence, we conclude that deuteron production at GSI energies is a final state recombination effect. Finally, we also discuss the scaling relations of the higher order flow components up to $v_4$. We show that $v_3 \sim v_1v_2$ and $v_4 \sim v_2^2$ as function of transverse momentum and that the integrated $v_2^2 \sim v_4$ over the investigated energy range from $E_{lab}$=0.1 AGeV to 40 AGeV.

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

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

Dynamical formation of center domains in quark-gluon plasma

We study the formation of domain structures due to spontaneous breakdown of center symmetry at high temperatures in quenched QCD. We develop a phenomenological model for the explicit propagation of the Polyakov loop as the relevant order parameter of the deconfinement phase transition. The surface tension in the equation of motion is fit in comparison with lattice QCD data. Results give insight into the dynamical formation of center domains as well as the formation of energy bands along domain walls and let us estimate the required time to form such structures above the critical temperature.

hep-ph↗

arXiv2017

Mapping of Deformation to Apparent Young's Modulus in Real-Time Deformability Cytometry

As described in the work of Mietke et al. (1) the deformation (defined as 1 - circularity [see (2)]) of a purely elastic, spherical object deformed in a real-time deformability cytometry (RT-DC) experiment can be mapped to its apparent Young's Modulus. This note is supposed to help a fast and correct mapping of RT-DC results - namely, deformation and size - to values of the apparent Young's Modulus E.

cond-mat.soft↗

arXiv2016

Phenomena at the QCD phase transition in nonequilibrium chiral fluid dynamics (N$χ$FD)

Heavy-ion collisions performed in the beam energy range accessible by the NICA collider facility are expected to produce systems of extreme net-baryon densities and can thus reach yet unexplored regions of the QCD phase diagram. Here, one expects the phase transition between the plasma of deconfined quarks and gluons and the hadronic matter to be of first order. A discovery of the first-order phase transition would as well prove the existence of the QCD critical point, a landmark in the phase diagram. In order to understand possible signals of the first-order phase transition in heavy-ion collision experiments it is very important to develop dynamical models of the phase transition. Here, we discuss the opportunities of studying dynamical effects at the QCD first-order phase transition within our model of nonequilibrium chiral fluid dynamics.

nucl-th↗

arXiv2016

Dynamical net-proton fluctuations near a QCD critical point

We investigate the evolution of the net-proton kurtosis and the kurtosis of the chiral order parameter near the critical point in the model of nonequilibrium chiral fluid dynamics. The order parameter is propagated explicitly and coupled to an expanding fluid of quarks and gluons in order to describe the dynamical situation in a heavy-ion collision. We study the critical region near the critical point on the crossover side. There are two sources of fluctuations: non-critical initial event-by-event fluctuations and critical fluctuations. These fluctuations can be distinguished by comparing a mean-field evolution of averaged thermodynamic quantities with the inclusion of fluctuations at the phase transition. We find that while the initial state fluctuations give rise to flat deviations from statistical fluctuations, critical fluctuations reveal a clear structure of the phase transition. The signals of the critical point in the net-proton and sigma field kurtosis are affected by the nonequilibrium dynamics and the inhomogeneity of the space-time evolution but develop clearly.

hep-ph↗

arXiv2014

Multiplicity fluctuations at the quark-hadron phase transition from a fluid dynamical model

The region of large net-baryon densities in the QCD phase diagram is expected to exhibit a first-order phase transition. Experimentally, its study will be one of the primaryobjectives for the upcoming FAIR accelerator. We model the transition between quarks and hadrons in a heavy-ion collision using a fluid which is coupled to the explicit dynamics of the chiral order parameter and a dilaton field. This allows us to investigate signals stemming from the nonequilibrium evolution during the expansion of the hot plasma. Special emphasis is put on an event-by-event analysis of baryon number fluctuations which have long since been claimed to be sensitive to a critical point.

hep-ph↗

arXiv2014

Net-baryon number variance and kurtosis within nonequilibrium chiral fluid dynamics

We study the variance and kurtosis of the net-baryon number in a fluid dynamical model for heavy-ion collisions. It is based on an effective chiral model with dilatons for the strong coupling regime of QCD. Taking into account spinodal instabilities, we demonstrate that this model exhibits a diverging quark number susceptibility and kurtosis all along the spinodal lines of the first-order phase transition, with a change of universality class at the critical end point. During the (3+1) dimensional expansion of a hot and dense fireball, instabilities are created by fluctuations in the explicitly propagated chiral and dilaton field. We find a clear enhancement of event-by-event fluctuations of the baryon number at the critical point and first-order phase transition in comparison with an evolution through the crossover region.

hep-ph↗

arXiv2014

Formation of droplets with high baryon density at the QCD phase transition in expanding matter

We consider the (3+1) dimensional expansion and cooling of the chirally-restored and deconfined matter at finite net-baryon densities as expected in heavy-ion collisions at moderate energies. In our approach, we consider chiral fields and the Polyakov loop as dynamical variables coupled to a medium represented by a quark-antiquark fluid. The interaction between the fields and the fluid leads to dissipation and noise, which in turn affect the field fluctuations. We demonstrate how inhomogeneities in the net-baryon density may form during an evolution through the spinodal region of the first-order phase transition. For comparison, the dynamics of transition through the crossover and critical end point is also considered.

nucl-th↗

arXiv2013

Fluid dynamics near the QCD critical point

We present a fully dynamical model to study the chiral and deconfinement transition of QCD simultaneously. The quark degrees of freedom constitute a heat bath in local equilibrium for both order parameters, the sigma field and a dynamical Polyakov loop. The nonequilibrium evolution of these fields is described by Langevin equations including dissipation and noise. In several quench scenarios we are able to observe a delay in the relaxation times near the transition temperature for a critical point as well as a first-order phase transition scenario. During the hydrodynamical expansion of a hot quark fluid we find a strong enhancement of thermal fluctuations at the first-order transition compared to a scenario with a critical point.

nucl-th↗

arXiv2013

Influence of an inhomogeneous and expanding medium on signals of the QCD phase transition

According to a fluid dynamic expansion of the fireball we investigate how the inhomogeneity of the system influences the chiral phase transition of QCD. We compare the averaged values of the order parameter in equilibrium with that of a homogeneous system. If the temperature is averaged over a certain region of the fireball the corresponding correlation length does not diverge in an expansion with a critical point.

nucl-th↗

arXiv2013

Chiral fluid dynamics with explicit propagation of the Polyakov loop

We present a fully dynamical model to study nonequilibrium effects in both the chiral and the deconfinement phase transition. The sigma field and the Polyakov loop as the corresponding order parameters are propagated by Langevin equations of motion. The locally thermalized background is provided by a fluid of quarks and antiquarks. Allowing for an exchange of energy and momentum through dissipative and stochastic processes we ensure that the total energy of the coupled system remains conserved. We study its relaxational dynamics in different quench scenarios and are able to observe critical slowing down as well as the enhancement of long wavelength modes at the critical point. During the fluid dynamical expansion of a hot plasma fireball typical nonequilibrium effects like supercooling and domain formation occur when the system evolves through the first order phase transition.

nucl-th↗