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R. Baier

Publications and source records attributed to R. Baier.

At least 37 records · Page 2Linked to original sources
arXiv2000

Resonant decay of parity odd bubbles in hot hadronic matter

We investigate the decay of metastable states with broken CP-symmetry which have recently been proposed by Kharzeev, Pisarski and Tytgat to form in hot hadronic matter. We consider the efficiency of the amplification of the $η'$-field via parametric resonance, taking the backreaction into account. For times of the order $t\approx 10 fm$, we find a particle density of about $0.7/fm^3$ and a correlation length of $ξ_{max}\approx 2.5 fm$. The corresponding momentum spectra show a non-thermal behaviour.

hep-ph↗
arXiv2000

Energy loss in perturbative QCD

We review the properties of energetic parton propagation in hot or cold QCD matter, as obtained in recent works. Advances in understanding the energy loss - collisional and radiative - are summarized, with emphasis on the latter: it features very interesting properties which may help to detect the quark-gluon plasma produced in heavy ion collisions. We describe two different theoretical approaches, which lead to the same radiated gluon energy spectrum. The case of a longitudinally expanding QCD plasma is investigated. The energy lost by a jet with given opening angle is calculated in view of making predictions for the suppression (quenching) of hard jet production. Phenomenological implications for the difference between hot and cold matter are discussed. Numerical estimates of the loss suggest that it may be significantly enhanced in hot compared to cold matter.

hep-ph↗
arXiv1999

Hard-thermal-loop resummed pressure of a degenerate quark-gluon plasma

We compute the pressure of a finite density quark-gluon plasma at zero temperature to leading order in hard-thermal-loop perturbation theory, which includes the fermionic excitations and Landau damping. The result is compared with the weak-coupling expansion for finite positive chemical potential $μ$ through order $α_s^2$ and with a quasiparticle model with a mass depending on $μ$.

hep-ph↗
arXiv1999

Dynamical screening in hot systems away from (chemical) equilibrium

Within the Closed Time Path Formalism of Thermal Field Theory we calculate the hard photon emission rate as well as the collisional energy-loss rate for a quark-gluon plasma away from chemical equilibrium. Mass singularities are shown to be dynamically screened within HTL-resummed perturbation theory also away from equilibrium. Additional (pinch) singularities are absent and well defined results are obtained.

hep-ph↗
arXiv1999

Angular Dependence of the Radiative Gluon Spectrum and the Energy Loss of Hard Jets in QCD Media

The induced momentum spectrum of soft gluons radiated from a high energy quark propagating through a QCD medium is derived in the BDMPS formalism. A calorimetric measurement for the medium dependent energy lost by a jet with opening angle $θ_{\rm cone}$ is proposed.The fraction of this energy loss with respect to the integrated one appears to be the relevant observable.It exhibits a universal behaviour in terms 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. Phenomenological implications for the differences between cold and hot QCD matter are discussed.

hep-ph↗
arXiv1998

Radiative Energy Loss of High Energy Partons Traversing an Expanding QCD Plasma

We study analytically the medium-induced energy loss of a high energy parton passing through a finite size QCD plasma, which is expanding longitudinally according to Bjorken's model. We extend the BDMPS formalism already applied to static media to the case of a quark which hits successive layers of matter of decreasing temperature, and we show that the resulting radiative energy loss can be as large as 6 times the corresponding one in a static plasma at the reference temperature $T = T (L)$, which is reached after the quark propagates a distance $L$.

hep-ph↗
arXiv1997

Photon and Dilepton Production from Hot Out-Off-Equilibrium Media

The electromagnetic emissivity from QCD media away from equilibrium is studied in the framework of closed time path thermal field theory. For the dilepton rate a nonequilibrium mesonic medium is considered applying finite temperature perturbation theory for $π-ρ$ interactions. The dilepton rate is derived up to the order $g_ρ^2$. For hard photon production a quark gluon plasma is assumed and calculations are performed in leading order in the strong coupling constant. These examples are chosen in order to investigate the role of possible pinch terms in boson and in fermion propagators, respectively. The implications of the results for phenomenology are also discussed.

hep-ph↗
arXiv1997

Thermal photon production rate from non-equilibrium quantum field theory

In the framework of closed time path thermal field theory we investigate the production rate of hard thermal photons from a QCD plasma away from equilibrium. Dynamical screening provides a finite rate for chemically non-equilibrated distributions of quarks and gluons just as it does in the equilibrium situation. Pinch singularities are shown to be absent in the real photon rate even away from equilibrium.

hep-ph↗
arXiv1996

Thermal Dileptons as a Possible Source of the Soft Dilepton Enhancement Measured in A-A Collisions at SPS Energies

The production of soft dileptons in a thermal mesonic medium is discussed in the context of recent CERN experimental data reported by the CERES Collaboration. We do not intend to give a general and critical review, but instead concentrate mainly on our approach, however, incorporating many of the recent attempts in the literature. We calculate the contributions to the dilepton yield arising from pion annihilation and $π-ρ$ scattering. It is shown that thermal dileptons from $π-ρ$ scattering give a significant contribution to the low-mass yield, however, it can only partly account for the experimentally observed soft dilepton excess seen in S-Au and Pb-Au collisions at SPS energy. The out off-equilibrium effects as well as a dropping vector meson mass are discussed in the context of the thermal dilepton yield. We emphasize, following the results of Li, Ko, and Brown, that, until now, the best way to provide a quantitative explanation of the observed enhancement of low-mass dileptons by the CERN experiments is the assumption of a decreasing vector meson mass in a high density thermal medium.

hep-ph↗
arXiv1996

Thermal Dileptons from pi - rho Interactions in a Hot Pion Gas

A systematic study of low mass dilepton production from $π-ρ$ interactions in a hot medium is presented. Applying finite temperature perturbation theory the dilepton rate, respectively the virtual photon rate, is computed up to order $g_ρ^2$. For dilepton masses below the $ρ$ the two-body reactions $ππ\to ργ^*$, $πρ\to πγ^*$, and the decay process $ρ\to ππγ^*$ give significant contributions. Non-equilibrium contributions to the thermal rate are estimated, including the modification of the particle distribution function with non-zero pion chemical potential. The comparison of the dilepton rate with the recent data measured in nucleus-nucleus collisions at SPS energy by the CERES Collaboration is also performed. It is shown that the additional thermal dileptons from $π-ρ$ interactions can partially account for the access of the soft dilepton yield seen experimentally.

hep-ph↗
arXiv1996

Anomalous electromagnetic processes at high temperatures

Chiral theories of constituent quarks interacting with bosons and photons at high temperatures are studied. In the expected chirally symmetric phase effective electromagnetic anomalous couplings for e.g. $πσ\to γγ, ~ γ\to πππσ, $ etc., are derived by applying functional methods.

hep-ph↗
arXiv1996

Radiative energy loss and $p_{\perp}$-broadening of high energy partons in nuclei

The medium-induced $p_{\perp}$-broadening and induced gluon radiation spectrum of a high energy quark or gluon traversing a large nucleus is studied. Multiple scattering of the high energy parton in the nucleus is treated in the Glauber approximation. We show that -dE/dz, the radiative energy loss of the parton per unit length, grows as L, the length of the nuclear matter, as does the characteristic transverse momentum squared of the parton $p_{\perp W}^2$. We find dE/dz = (1/8)α_s N_c $p_{\perp W}^2$ holds independent of the details of the parton-nucleon scatterings so long as L is large. Numerical estimates suggest that $p_{\perp}$-broadening and energy loss may be significantly enhanced in hot matter as compared to cold matter, thus making the study of such quantities a possible signal for quark-gluon plasma formation.

hep-ph↗
arXiv1996

Radiative energy loss of high energy quarks and gluons in a finite volume quark-gluon plasma

The medium induced energy loss spectrum of a high energy quark or gluon traversing a hot QCD medium of finite volume is studied. We model the interaction by a simple picture of static scattering centres. The total induced energy loss is found to grow as $L^2$, where $L$ is the extent of the medium. The solution of the energy loss problem is reduced to the solution of a Schrödinger-like equation whose ``potential'' is given by the single-scattering cross section of the high energy parton in the medium. These resuls should be directly applicable to a quark-gluon plasma.

hep-ph↗
arXiv1996

The Landau-Pomeranchuk-Migdal effect in QED

The calculation of the radiative energy loss encountered by a fast charged particle which undergoes multiple scattering is being investigated. A detailed derivation of the Landau-Pomeranchuk-Migdal coherent effect in QED is given, focusing on the specific feature of the Coulomb interaction. As a result the radiation intensity per unit length in the coherent regime is shown to be proportional to $\sqrt ω$ for a photon energy $ω$ times a logarithmic enhancement which is determined exactly.

hep-ph↗
arXiv1996

Anomalous Processes in Effective Chiral Theories at High Temperatures

We discuss chiral theories of constituent quarks interacting with bosons at high temperatures. In the chirally symmetric phase we demonstrate by applying functional methods the presence of effective anomalous couplings for e.g. $πσ\to γγ, ~ γ\to πππσ, ~K K \to πππσ$, etc., as they have recently been discussed by Pisarski.

hep-ph↗
arXiv1994

Induced Gluon Radiation in a QCD Medium

Soft gluon radiation induced by multiple scattering of a fast quark or gluon propagating through QCD matter is discussed. After revisiting the Landau-Pomeranchuk-Migdal effect in QED we show that large formation times of bremsstrahlung quanta determine the QCD radiation intensity (in analogy to QED) and derive the gluon energy spectrum. Coherent suppression takes place as compared to the Bethe-Heitler situation of independent emissions. As a result the energy loss of fast partons in a QCD medium depends on the incident energy $E$ similarly to QED, $-dE/dz \propto \sqrt{E}$.

hep-ph↗