arXiv subjects

Rainer J. Fries

Publications and source records attributed to Rainer J. Fries.

At least 55 records · Page 3Linked to original sources

arXiv2011

Quark Recombination in Heavy Ion Collisions

Data on high energy nuclear collisions collected at the Relativistic Heavy Ion Collider over the past decade have provided convincing evidence that hadronization is quite different in hot nuclear environments compared to p+p collisions. In particular, the data suggest that we see traces of quark degrees of freedom in elliptic flow, with the implication that collective flow is generated on the parton level and is transfered to hadrons through a simple recombination step. In this contribution we review the experimental evidence for quark recombination and discuss some recombination models which are used to describe these effects.

nucl-th↗

arXiv2010

High Energy Nuclear Collisions: Theory Overview

We review some basic concepts of Relativistic Heavy Ion Physics and discuss our understanding of some key results from the experimental program at the Relativistic Heavy Ion Collider (RHIC). We focus in particular on the early time dynamics of nuclear collisions, some result from lattice QCD, hard probes and photons.

nucl-th↗

arXiv2010

Event-by-Event Jet Quenching and Higher Fourier Moments of Hard Probes

We investigate the effect of event-by-event fluctuations of the fireball created in high energy nuclear collisions on hard probe observables. We show that spatial inhomogeneities lead to changes in the nuclear suppression factor of high momentum hadrons which can be absorbed in the quenching strength q. This can increase the theoretical uncertainty on extracted values of q by up to 50%. We also investigate effects on azimuthal asymmetries v2 and dihadron correlation functions. The latter show a promising residual signal of event-by-event quenching that might allow us to estimate the size of spatial inhomogeneities in the fireball from experimental data.

nucl-th↗

arXiv2010

Event-by-Event Jet Quenching

High momentum jets and hadrons can be used as probes for the quark gluon plasma (QGP) formed in nuclear collisions at high energies. We investigate the influence of fluctuations in the fireball on jet quenching observables by comparing propagation of light quarks and gluons through averaged, smooth QGP fireballs with event-by-event jet quenching using realistic inhomogeneous fireballs. We find that the transverse momentum and impact parameter dependence of the nuclear modification factor R_AA can be fit well in an event-by-event quenching scenario within experimental errors. However the transport coefficient qhat extracted from fits to the measured nuclear modification factor R_AA in averaged fireballs underestimates the value from event-by-event calculations by up to 50%. On the other hand, after adjusting qhat to fit R_AA in the event-by-event analysis we find residual deviations in the azimuthal asymmetry v_2 and in two-particle correlations, that provide a possible faint signature for a spatial tomography of the fireball.

nucl-th↗

arXiv2010

Evaluating Results from the Relativistic Heavy Ion Collider with Perturbative QCD and Hydrodynamics

We review the basic concepts of perturbative quantum chromodynamics (QCD) and relativistic hydrodynamics, and their applications to hadron production in high energy nuclear collisions. We discuss results from the Relativistic Heavy Ion Collider (RHIC) in light of these theoretical approaches. Perturbative QCD and hydrodynamics together explain a large amount of experimental data gathered during the first decade of RHIC running, although some questions remain open. We focus primarily on practical aspects of the calculations, covering basic topics like perturbation theory, initial state nuclear effects, jet quenching models, ideal hydrodynamics, dissipative corrections, freeze-out and initial conditions. We conclude by comparing key results from RHIC to calculations.

nucl-th↗

arXiv2010

Scaling of Elliptic Flow, Recombination and Sequential Freeze-Out of Hadrons in Heavy-Ion Collisions

The scaling properties of elliptic flow of hadrons produced in ultrarelativistic heavy-ion collisions are investigated at low transverse momenta, $p_T\lsim 2$\,GeV. Utilizing empirical parameterizations of a thermalized fireball with collective-flow fields, Resonance Recombination Model (RRM) is employed to describe hadronization via quark coalescence at the hadronization transition. We reconfirm that RRM converts equilibrium quark distribution functions into equilibrated hadron spectra including the effects of space-momentum correlations on elliptic flow. This provides the basis for a controlled extraction of quark distributions of the bulk matter at hadronization from spectra of multi-strange hadrons which are believed to decouple close to the critical temperature. The resulting elliptic flow from empirical fits at RHIC exhibits transverse kinetic-energy and valence-quark scaling. Utilizing the well-established concept of sequential freeze-out we find that the scaling at low momenta can be extended to bulk hadrons ($π$, $K$, $p$) at thermal freeze-out, and thus result in an overall description compatible with both equilibrium hydrodynamics and quark recombination.

nucl-th↗

arXiv2009

High Momentum Probes of Nuclear Matter

We discuss how the chemical composition of QCD jets is altered by final state interactions in surrounding nuclear matter. We describe this process through conversions of leading jet particles. We find that conversions lead to an enhancement of kaons at high transverse momentum in Au+Au collisions at RHIC, while their azimuthal asymmetry v_2 is suppressed.

nucl-th↗

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.

nucl-th↗

arXiv2008

Coalescence Models For Hadron Formation From Quark Gluon Plasma

We review hadron formation from a deconfined quark gluon plasma (QGP) via coalescence or recombination of quarks and gluons. We discuss the abundant experimental evidence for coalescence from the Relativistic Heavy Ion Collider (RHIC) and compare the various coalescence models advocated in the literature. We comment on the underlying assumptions and remaining challenges as well as the merits of the models. We conclude with a discussion of some recent developments in the field.

nucl-th↗

arXiv2008

Decoherence and Entropy Production in Relativistic Nuclear Collisions

Short thermalization times of less than 1 fm/c for quark and gluon matter have been suggested by recent experiments at the Relativistic Heavy Ion Collider (RHIC). It has been difficult to justify this rapid thermalization in first-principle calculations based on perturbation theory or the color glass condensate picture. Here, we address the related question of the decoherence of the gluon field, which is a necessary component of thermalization. We present a simplified leading-order computation of the decoherence time of a gluon ensemble subject to an incoming flux of Weizsacker-Williams gluons. We also discuss the entropy produced during the decoherence process and its relation to the entropy in the final state which has been measured experimentally.

nucl-th↗

arXiv2007

Quark and Gluon Degrees of Freedom in High-Energy Heavy Ion Collisions

I discuss some recent progress in our understanding of high energy nuclear collisions. I will focus on two topics which I was lucky to co-pioneer in the recent past. One is recombination of quarks and its interpretation as a signal for deconfinement, the second is electromagnetic radiation from jets passing through a quark gluon plasma. This talk was given during the award ceremony for the 2007 IUPAP Young Scientist Award.

nucl-th↗

arXiv2007

Photons and Dileptons at LHC

We discuss real and virtual photon sources in heavy ion collisions and present results for dilepton yields in Pb+Pb collisions at the LHC at intermediate and large transverse momentum p_T.

nucl-th↗

arXiv2007

Hadron Correlations and Parton Recombination

Parton recombination has been found to be an extremely useful model to understand hadron production at the Relativistic Heavy Ion Collider. It is particularly important to explore its connections with hard processes. This article reviews some of the aspects of the quark recombination model and places particular emphasis on hadron correlations.

nucl-th↗

arXiv2007

Early Time Evolution of High Energy Heavy Ion Collisions

We solve the Yang-Mills equations in the framework of the McLerran-Venugopalan model for small times tau after a collision of two nuclei. An analytic expansion around tau=0 leads to explicit results for the field strength and the energy momentum tensor of the gluon field at early times. We then discuss constraints for the energy density, pressure and flow of the plasma phase that emerges after thermalization of the gluon field.

nucl-th↗

arXiv2006

Hadronization of Dense Partonic Matter

The parton recombination model has turned out to be a valuable tool to describe hadronization in high energy heavy ion collisions. I review the model and revisit recent progress in our understanding of hadron correlations. I also discuss higher Fock states in the hadrons, possible violations of the elliptic flow scaling and recombination effects in more dilute systems.

nucl-th↗

arXiv2006

High Momentum Dilepton Production from Jets in a Quark Gluon Plasma

We discuss the emission of high momentum lepton pairs in central Au+Au collisions at RHIC ($\sqrt{s_{NN}}=$200 GeV) and Pb+Pb collisions at the LHC ($\sqrt{s_{NN}}=$5500 GeV). Yields of dileptons produced through interactions of jets with thermal partons have been calculated, with next-to-leading order corrections through hard thermal loops (HTL) resummation. They are compared to thermal dilepton emission and the Drell-Yan process. A complete leading order treatment of jet energy loss has been included. Jet-plasma interactions are found to dominate over thermal dilepton emission for all values of the invariant mass $M$. Drell-Yan is the dominant source of high momentum lepton pairs for $M >$ 3 GeV at RHIC, after the background from heavy quark decays is subtracted. At LHC, the range $M<7$ GeV is dominated by jet-plasma interactions. Effects from jet energy loss on jet-plasma interactions turn out to be weak, but non-negligible, reducing the yield of low-mass dileptons by about 30%.

hep-ph↗

arXiv2006

Azimuthal Asymmetry of Direct Photons in High Energy Nuclear Collisions

We show that a sizeable azimuthal asymmetry, characterized by a coefficient v_2, is to be expected for direct photons produced in non-central high energy nuclear collisions. This signal is generated by photons radiated by jets interacting with the surrounding hot plasma. The anisotropy is out of phase by an angle $π/2$ with respect to that associated with the elliptic anisotropy of hadrons, leading to negative values of v_2. Such an asymmetry, if observed, could be a signature for the presence of a quark gluon plasma and would establish the importance of jet-plasma interactions as a source of electromagnetic radiation.

hep-ph↗

arXiv2005

From Color Fields to Quark Gluon Plasma

We discuss a model for the energy distribution and the early space-time evolution of a heavy ion collision. We estimate the gluon field generated in the wake of hard processes and through primordial fluctuations of the color charges in the nuclei. Without specifying the dynamical mechanism of thermalization we calculate the energy momentum tensor of the following plasma phase. The results of this model can be used as initial conditions for a further hydrodynamic evolution.

hep-ph↗