arXiv subjects

Berndt Müller

Publications and source records attributed to Berndt Müller.

At least 91 records · Page 5Linked to original sources

arXiv2011

Entropy Creation in Relativistic Heavy Ion Collisions

We review current ideas on entropy production during the different stages of a relativistic nuclear collision. This includes recent results on decoherence entropy and the entropy produced during the hydrodynamic phase by viscous effects. We start by a discussion of decoherence caused by gluon bremsstrahlung in the very first interactions of gluons from the colliding nuclei. We then present a general framework, based on the Husimi distribution function, for the calculation of entropy growth in quantum field theories, which is applicable to the early ("glasma") phase of the collision during which most of the entropy is generated. The entropy calculated from the Husimi distribution exhibits linear growth when the quantum field contains unstable modes and the growth rateis asymptotically equal to the Kolmogorov-Sinaï (KS) entropy. We outline how the approach can be used to investigate the problem of entropy production in a relativistic heavy-ion reaction from first principles. Finally we discuss some recent results on entropy production in the strong coupling limit, as obtained from AdS/CFT duality.

hep-ph↗

arXiv2011

Entropy production and equilibration in Yang-Mills quantum mechanics

The Husimi distribution provides for a coarse grained representation of the phase space distribution of a quantum system, which may be used to track the growth of entropy of the system. We present a general and systematic method of solving the Husimi equation of motion for an isolated quantum system, and we construct a coarse grained Hamiltonian whose expectation value is exactly conserved. As an application, we numerically solve the Husimi equation of motion for two-dimensional Yang-Mills quantum mechanics (the x-y model) and calculate the time evolution of the coarse grained entropy of a highly excited state. We show that the coarse grained entropy saturates to a value that coincides with the microcanonical entropy corresponding to the energy of the system.

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arXiv2011

Holographic Thermalization

Using the AdS/CFT correspondence, we probe the scale-dependence of thermalization in strongly coupled field theories following a quench, via calculations of two-point functions, Wilson loops and entanglement entropy in d=2,3,4. In the saddlepoint approximation these probes are computed in AdS space in terms of invariant geometric objects - geodesics, minimal surfaces and minimal volumes. Our calculations for two-dimensional field theories are analytical. In our strongly coupled setting, all probes in all dimensions share certain universal features in their thermalization: (1) a slight delay in the onset of thermalization, (2) an apparent non-analyticity at the endpoint of thermalization, (3) top-down thermalization where the UV thermalizes first. For homogeneous initial conditions the entanglement entropy thermalizes slowest, and sets a timescale for equilibration that saturates a causality bound over the range of scales studied. The growth rate of entanglement entropy density is nearly volume-independent for small volumes, but slows for larger volumes.

hep-th↗

arXiv2010

Parton Energy Loss in Strongly Coupled AdS/CFT

This is a brief review of the theory and phenomenology of parton energy loss in strongly coupled field theories with a gravity dual and its comparison with parton energy loss in QCD at weak coupling.

hep-ph↗

arXiv2010

Triangular flow in event-by-event ideal hydrodynamics in Au+Au collisions at $\sqrt{s_{\rm NN}}=200A$ GeV

The first calculation of triangular flow $v_3$ in Au+Au collisions at $\sqrt{s_{\rm NN}}=200A$ GeV from an event-by-event (3+1)-d transport+hydrodynamics hybrid approach is presented. As a response to the initial triangularity $ε_3$ of the collision zone, $v_3$ is computed in a similar way to the standard event-plane analysis for elliptic flow $v_2$. It is found that the triangular flow exhibits weak centrality dependence and is roughly equal to elliptic flow in most central collisions. We also explore the transverse momentum and rapidity dependence of $v_2$ and $v_3$ for charged particles as well as identified particles. We conclude that an event-by-event treatment of the ideal hydrodynamic evolution starting with realistic initial conditions generates the main features expected for triangular flow.

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arXiv2010

Charge Fluctuations from the Chiral Magnetic Effect in Nuclear Collisions

We derive a nonlocal effective Lagrangian for the chiral magnetic effect. An electric field is generated by winding number fluctuations of the nonabelian gauge field in the presence of a strong magnetic field. We estimate the magnitude of charge asymmetry fluctuations with respect to the reaction plane induced by the chiral magnetic effect in relativistic heavy ion collisions to be less than $10^{-6}$, several orders of magnitude smaller than the signal observed in the STAR experiment.

hep-ph↗

arXiv2010

Translation of collision geometry fluctuations into momentum anisotropies in relativistic heavy-ion collisions

We develop a systematic framework for the study of the initial collision geometry fluctuations in relativistic heavy-ion collisions and investigate how they evolve through different stages of the fireball history and translate into final particle momentum anisotropies. We find in our event-by-event analysis that only the few lowest momentum anisotropy parameters survive after the hydrodynamical evolution of the system. The geometry of the produced medium is found to be affected by the pre-equilibrium evolution of the medium and the thermal smearing of the discretized event-by-event initial conditions, both of which tend to smear out the spatial anisotropies. We find such effects to be more prominent for higher moments than for lower moments. The correlations between odd and even spatial anisotropy parameters during the pre-equilibrium expansion are quantitatively studied and found to be small. Our study provides a theoretical foundation for the understanding of initial state fluctuations and the collective expansion dynamics in relativistic heavy-ion collisions.

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arXiv2010

Anomalous Transport Processes in Turbulent non-Abelian Plasmas

Turbulent color fields, which can arise in the early and late stages of relativistic heavy ion collisions, may contribute significantly to the transport processes in the matter created in these collisions. We review the theory of these anomalous transport processes and discuss their possible phenomenology in the glasma and quasistationary expanding quark-gluon plasma.

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arXiv2010

Hadron Mass Spectrum from Lattice QCD

Finite temperature lattice simulations of quantum chromodynamics (QCD) are sensitive to the hadronic mass spectrum for temperatures below the "critical" temperature T_c ~ 160 MeV. We show that a recent precision determination of the QCD trace anomaly shows evidence for the existence of a large number of hadron states beyond those known from experiment. The lattice results are well represented by an exponentially growing hadron mass spectrum up to a temperature T = 155 MeV. Using simple parametrizations we show how one may estimate the total spectral weight in these yet undermined states.

hep-ph↗

arXiv2010

Electric Charge Separation in Strong Transient Magnetic Fields

We discuss various mechanisms for the creation of an asymmetric charge fluctuation with respect to the reaction plane among hadrons emitted in relativistic heavy-ion collisions. We show that such mechanisms exist in both, the hadronic gas and the partonic phases of QCD. The mechanisms considered here all require the presence of a strong magnetic field (the ``chiral magnetic effect''), but they do not involve parity or charge-parity violations. We analyze how a transient local electric current fluctuation generated by the chiral magnetic effect can dynamically evolve into an asymmetric charge distribution among the final-state hadrons in momentum space. We estimate the magnitude of the event-by-event fluctuations of the final-state charge asymmetry due to the partonic and hadronic mechanisms.

hep-ph↗

arXiv2010

Physics With Two Time Dimensions

We explore the properties of physical theories in space-times with two time dimensions. We show that the common arguments used to rule such theories out do not apply if the dynamics associated with the additional time dimension is thermal or chaotic and does not permit long-lived time-like excitations. We discuss several possible realizations of such theories, including holographic representations and the possibility that quantum dynamics emerges as a consequence of a second time dimension.

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arXiv2009

Aspects of thermal strange quark production: the deconfinement and chiral phase transitions

We study the gluonic sector of the three-flavor PNJL model by obtaining the adjoint Polyakov loop and the gluon distribution function in the mean-field approximation. Besides, we explore the thermal strange quark pair-production processes, $q\bar {q} \to s\bar {s}$ and $gg \to s \bar {s}$, with the aid of the three-flavor PNJL model. The results help us identify the temperature where the gluonic contribution to the production rate becomes dominant, which is an innovative phenomenon compared with the result obtained in free perturbation theory.

hep-ph↗

arXiv2009

Phenomenology of the three-flavor PNJL model and thermal strange quark production

We study the temperature dependence of the adjoint Polyakov loop and its implication for the momentum spectrum of gluons in the mean-field approximation. This allows us to calculate the contribution of the thermal (transverse) gluons to the thermodynamic pressure. As an application, we evaluate the rates for the strange quark pair-production processes $q\bar{q} \to s\bar{s}$ and $gg \to s\bar{s}$ as functions of temperature including thermal effects on quark deconfinement and chiral symmetry breaking.

hep-ph↗

arXiv2008

The Flavours of the Quark-Gluon Plasma

Quarks of other flavours than up and down, i.e. $s$, $c$, and $b$ quarks, have been long recognized as effective probes of the structure of hot QCD matter. In this talk, I review some of the motivations for their investigation and discuss the salient results obtained so far, with a focus on the results from the Relativistic Heavy Ion Collider (RHIC). Some ideas for future studies are also mentioned.

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arXiv2008

The "Perfect" Fluid Quenches Jets Almost Perfectly

The QCD matter produced in nuclear collisions at the Relativistic Heavy Ion Collider (RHIC) has been found to have a very low shear viscosity, which is close to the lower bound allowed by unitarity. The matter has also been found to strongly suppress the emission of energetic hadrons. This phenomenon, called "jet quenching" is interpreted to be the result of a large energy loss by the precursor parton on its path through the dense matter, primarily due to gluon radiation. I discuss how the two phenomena are related. The RHIC data suggest, in some scenarios of jet quenching, that the quark-gluon plasma created in nuclear collisions is characterized by strong coupling, but still admits a quasi-particle description.

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arXiv2008

Stress Tensor and Bulk Viscosity in Relativistic Nuclear Collisions

We discuss the influence of different initial conditions for the stress tensor and the effect of bulk viscosity on the expansion and cooling of the fireball created in relativistic heavy-ion collisions. In particular, we explore the evolution of longitudinal and transverse components of the pressure and the extent of dissipative entropy production in the one-dimensional, boost-invariant hydrodynamic model. We find that a bulk viscosity consistent with recent estimates from lattice QCD further slows the equilibration of the system, however it does not significantly increase the entropy produced.

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arXiv2007

From Quark-Gluon Plasma to the Perfect Liquid

After reviewing some basic concepts of the theory of strongly interacting matter above nuclear energy density and reviewing some salient results of the experimental program at the Relativistic Heavy Ion Collider (RHIC), these lectures explain why the quark-gluon plasma observed in the RHIC experiments has been called a ``perfect liquid.'' They then give an introduction to some recent ideas concerning the possible origin of the nearly inviscid nature of the quark-gluon plasma and discuss the connection between low viscosity and strong parton energy loss of hot QCD matter.

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arXiv2006

Anomalous Transport Processes in Anisotropically Expanding Quark-Gluon Plasmas

We derive an expression for the anomalous viscosity in an anisotropically expanding quark-gluon-plasma, which arises from interactions of thermal partons with dynamically generated color fields. The anomalous viscosity dominates over the collisional viscosity for large velocity gradients or weak coupling. This effect may provide an explanation for the apparent ``nearly perfect'' liquidity of the matter produced in nuclear collisions at RHIC without the assumption that it is a strongly coupled state.

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