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R. Baier

Publications and source records attributed to R. Baier.

At least 19 recordsLinked to original sources
arXiv2018

Light-by-Light Scattering in the Presence of Magnetic Fields

The low-energy light-by-light cross section as determined by the nonlinear Euler-Heisenberg QED Lagrangian is evaluated in the presence of constant magnetic fields in the center-of-mass system of the colliding photons. This cross section has a complicated dependence on directions and polarizations. The overall magnitude decreases as the magnetic field is increased from zero, but this trend is reversed for ultrastrong magnetic fields $B\gtrsim B_c$, where the cross section eventually grows quadratically with the magnetic field strength perpendicular to the collision axis. This effect is due to interactions involving the lowest Landau level of virtual Dirac particles; it is absent in scalar QED. An even more dramatic effect is found for virtual charged vector mesons where the one-loop cross section diverges at the critical field strength due to an instability of the lowest Landau level and the possibility of the formation of a superconducting vacuum state. We also discuss (the absence of) implications for the recent observation of light-by-light scattering in heavy-ion collisions.

hep-ph↗
arXiv2015

Scalar field collapse with negative cosmological constant

The formation of black holes or naked singularities is studied in a model in which a homogeneous time-dependent scalar field with an exponential potential couples to four dimensional gravity with negative cosmological constant. An analytic solution is derived and its consequences are discussed. The model depends only on one free parameter, which determines the equation of state and decides the fate of the spacetime. Without fine tuning the value of this parameter the collapse ends in a generic formation of a black hole or a naked singularity. The latter case violates the cosmic censorship conjecture.

gr-qc↗
arXiv2013

Predictions for $p+$Pb Collisions at sqrt s_NN = 5 TeV

Predictions for charged hadron, identified light hadron, quarkonium, photon, jet and gauge bosons in p+Pb collisions at sqrt s_NN = 5 TeV are compiled and compared. When test run data are available, they are compared to the model predictions.

hep-ph↗
arXiv2008

Relativistic viscous hydrodynamics, conformal invariance, and holography

We consider second-order viscous hydrodynamics in conformal field theories at finite temperature. We show that conformal invariance imposes powerful constraints on the form of the second-order corrections. By matching to the AdS/CFT calculations of correlators, and to recent results for Bjorken flow obtained by Heller and Janik, we find three (out of five) second-order transport coefficients in the strongly coupled N=4 supersymmetric Yang-Mills theory. We also discuss how these new coefficents can arise within the kinetic theory of weakly coupled conformal plasmas. We point out that the Mueller-Israel-Stewart theory, often used in numerical simulations, does not contain all allowed second-order terms and, frequently, terms required by conformal invariance.

hep-th↗
arXiv2007

How does transverse (hydrodynamic) flow affect jet-broadening and jet-quenching ?

We give the modification of formulas for $p_{\perp}$-broadening and energy loss which are necessary to calculate parton interactions in a medium with flow. Arguments are presented leading to the conclusion that for large $p_{\perp}$-spectra observed in heavy ion collisions at RHIC, the influence of transverse flow on the determination of the "quenching power" of the produced medium is small. This leaves open the question of the interpretation of data in a consistent perturbative framework.

nucl-th↗
arXiv2005

Kinetic equations for Bose-Einstein condensates from the 2PI effective action

We use the 2PI effective action of a relativistic scalar field theory to derive kinetic equations for a Bose-condensed system near the phase transition.We start from equations of motion derived within a 1/N-expansion at NLO. In taking the non-relativistic limit we obtain a generalized Gross-Pitaevskii equation for the condensate field. Within the Popov approximation we explicitly compute the collision term up to order g^2 using the Kadanoff-Baym formalism. For the sake of self-consistency we derive in the same way a Boltzmann equation for the non-condensate distribution function. The final results are in agreement to those previously obtained by Griffin, Nikuni and Zaremba and by Stoof.

hep-ph↗
arXiv2004

Saturation and shadowing in high-energy proton-nucleus dilepton production

We discuss the inclusive dilepton cross section for proton (quark)-nucleus collisions at high energies in the very forward rapidity region. Starting from the calculation in the quasi-classical approximation, we include low-x evolution effects in the nucleus and predict leading twist shadowing together with anomalous scaling behaviour.

hep-ph↗
arXiv2002

Jet Quenching

A short summary of the physics underlying jet quenching is given

hep-ph↗
arXiv2002

Far-from-equilibrium dynamics with broken symmetries from the 2PI-1/N expansion

We derive the nonequilibrium real-time evolution of an O(N) - invariant scalar quantum field theory in the presence of a nonvanishing expectation value of the quantum field. Using a systematic 1/N expansion of the 2PI effective action to next-to-leading order, we obtain nonperturbative evolution equations which include scattering and memory effects. The equivalence of the direct method, which requires the resummation of an infinite number of skeleton diagrams, with the auxiliary-field formalism, which involves only one diagram at next-to-leading order, is shown.

hep-ph↗
arXiv2002

Does parton saturation at high density explain hadron multiplicities at RHIC ?

We discuss the recent claim that hadron multiplicities measured at RHIC energies are directly described in terms of gluon degrees of freedom fixed from the initial conditions of central heavy ion collisions. The argument is based on the parton saturation scenario expected to be valid at high parton densities and on the assumption of conserved gluon number. Alternatively we conjecture that "bottom-up" equilibration before hadronization modifies this picture, due to nonconservation of the number of gluons.

hep-ph↗
arXiv2001

Quenching of hadron spectra in media

We determine how the yield of large transverse momentum hadrons is modified due to induced gluon radiation off a hard parton traversing a QCD medium. The quenching factor is formally a collinear- and infrared-safe quantity and can be treated perturbatively. In spite of that, in the $p_\perp$ region of practical interest, its value turns out to be extremely sensitive to large distances and can be used to unravel the properties of dense quark-gluon final states produced in heavy ion collisions. We also find that the standard modelling of quenching by shifting $p_\perp$ in the hard parton cross section by the mean energy loss is inadequate.

hep-ph↗
arXiv2001

Parametric amplification versus collisions: An illustrative application

We investigate the time dependence of the quantum statistical model with $λϕ^4/4!$ self-interaction and consider the resulting induced particle number density. For a special example in which the classical approximation exhibits parametric resonance, the effects of the back reaction and especially of collisions, treated in a perturbative way, are analysed.

hep-ph↗
arXiv2001

On the Angular Dependence of the Radiative Gluon Spectrum

The induced momentum spectrum of soft gluons radiated from a high energy quark produced in and propagating through a QCD medium is reexamined in the BDMPS formalism. A mistake in our published work (Physical Review C60 (1999) 064902) is corrected. The correct dependence of the fractional induced loss $R(θ_{\rm cone})$ as a universal function of the variable $θ^2_{\rm cone} L^3 \hat q$ where $L$ is the size of the medium and $\hat q$ the transport coefficient is presented. We add the proof that the radiated gluon momentum spectrum derived in our formalism is equivalent with the one derived in the Zakharov-Wiedemann approach.

hep-ph↗
arXiv2001

Bottom-up thermalization in heavy ion collisions

We describe how thermalization occurs in heavy ion collisions in the framework of perturbative QCD. When the saturation scale $Q_s$ is large compared to $Λ_{QCD}$, thermalization takes place during a time of order $α^{-13/5}Q_s^{-1}$ and the maximal temperature achieved is $α^{2/5}Q_s$.

hep-ph↗
arXiv2000

Comments on two papers by Kapusta and Wong

We critically examine recently published results on the thermal production of massive vector bosons in a quark-gluon plasma. We claim the production rate is a collinear safe observable.

hep-ph↗