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A. Peshier

Publications and source records attributed to A. Peshier.

At least 19 recordsLinked to original sources
arXiv2017

Tracing the pressure of the gluon plasma

Being interested in how a strongly coupled system approaches asymptotic freedom, we re-examine existing precision lattice QCD results for thermodynamic properties of the gluon plasma in a large temperature range. We discuss and thoroughly test the applicability of perturbative results, on which grounds we then infer that the pressure and other bulk properties approach the free limit somewhat slower than previously thought. We also revise the value of the first non-perturbative coefficient in the weak-coupling expansion.

hep-ph↗
arXiv2007

Collisional Energy Loss of a Fast Muon in a Hot QED Plasma

We calculate the collisional energy loss of a muon of high energy $E$ in a hot QED plasma beyond logarithmic accuracy, i.e., we determine the constant terms of order O(1) in $-dE/dx \propto \ln{E}+ O(1)$. Considering first the $t$-channel contribution to $-dE/dx$, we show that the terms $\sim O(1)$ are sensitive to the full kinematic region for the momentum exchange $q$ in elastic scattering, including large values $q \sim O(E)$. We thus redress a previous calculation by Braaten and Thoma, which assumed $q << E$ and could not find the correct constant (in the large $E$ limit). The relevance of 'very hard' momentum transfers then requires, for consistency, that $s$ and $u$-channel contributions from Compton scattering must be included, bringing a second modification to the Braaten-Thoma result. Most importantly, Compton scattering yields an additional large logarithm in $-dE/dx$. Our results might have implications in the QCD case of parton collisional energy loss in a quark gluon plasma.

hep-ph↗
arXiv2006

Collisional jet quenching becomes probable

It was argued recently that loop corrections to tree-level amplitudes are essential in the discussion of the collisional energy loss of energetic partons in the hot quark gluon plasma: Instead of $dE_{\rm coll}^B/dx \sim α^2 T^2 \ln(ET/m_D^2)$, as first derived by Bjorken (assuming a constant $α$), the mean energy loss actually behaves as $dE_{\rm coll}/dx \sim α(m_D^2)T^2$. Here we calculate, within this resummation-improved framework, the probability distribution functions (`quenching weights') of the collisional energy loss. First results from a Monte Carlo implementation of this probabilistic collisional quenching shed new light on the interpretation of jet suppression in heavy ion collisions.

hep-ph↗
arXiv2006

Rethinking the QCD collisional energy loss

It is shown that to leading order the collisional energy loss of an energetic parton in the hot quark gluon plasma reads $dE/dx \sim α(m_D^2)T^2$, where the scale of the coupling is determined by the (parametrically soft) Debye screening mass. Compared to previous expressions derived by Bjorken and other authors, $dE^B/dx \sim α^2 T^2 \ln(ET/m_D^2)$, the rectified result takes into account the running of the coupling, as dictated by quantum corrections beyond tree level. As one significant consequence, due to asymptotic freedom, the QCD collisional energy loss becomes independent of the jet energy in the limit $E \gg T$. It is advocated that this resummation improved perturbative result might be useful to (re-)estimate the collisional energy loss for temperatures relevant in heavy ion phenomenology.

hep-ph↗
arXiv2006

The QCD collisional energy loss revised

It is shown that to leading order the QCD collisional energy loss reads $dE/dx \sim α(m_D^2)T^2$. Compared to prevalent expressions, $dE^B/dx \sim α^2 T^2 \ln(ET/m_D^2)$, which could be considered adaptions of the (QED) Bethe-Bloch formula, the rectified result takes into account the running coupling, as dictated by renormalization. As one significant consequence, due to asymptotic freedom, the collisional energy loss becomes independent of the jet energy $E$. Some implications with regard to heavy ion collisions are pointed out.

hep-ph↗
arXiv2006

Running coupling and screening in the (s)QGP

Emphasizing the importance of renormalization in the context of thermal field theory in general, it is pointed out that the Debye mass in the hot quark gluon plasma is determined by the coupling at the scale $m_D$, not $T$ as commonly presumed. The mended result agrees quantitatively with lattice QCD calculations in the strong coupling regime almost down to $T_c$.

hep-ph↗
arXiv2005

The hot non-perturbative gluon plasma is an almost ideal colored liquid

We study properties of a gluon plasma above the critical temperature $T_c$ in a generalized quasi-particle approach with a Lorentz spectral function. The model parameters are determined by a fit of the entropy $s$ to lattice QCD data. The effective degrees of freedom are found to be rather heavy and of a sizeable width. With the spectral width being closely related to the interaction rate, we find a large effective cross section, which is comparable to the typical distance squared of the quasiparticles. This suggests that the system should be viewed as a liquid as also indicated by an estimate of the plasma parameter $Γ$. Furthermore, within the quasiparticle approach we find a very low viscosity to entropy ratio, $η/s \sim 0.2$ for $T > 1.05 T_c$, supporting the recent conjecture of an almost ideal quark-gluon liquid seen at RHIC.

hep-ph↗
arXiv2004

Hard parton damping in hot QCD

The gluon and quark collisional widths in hot QCD plasmas are discussed with emphasis on temperatures near Tc, where the coupling is large. Considering the effect on the entropy, which is known from lattice calculations, it is argued that the width of the partons, which in the perturbative limit is given by gamma ~ g^2 ln(1/g) T, should be sizeable at intermediate temperatures but has to be small close to Tc. This behavior implies a substantial reduction of the radiative energy loss of jets near Tc.

hep-ph↗
arXiv2004

Hard gluon damping in hot QCD

The gluon collisional width in hot QCD plasmas is discussed with emphasis on temperatures near $T_c$, where the coupling is large. Considering its effect on the entropy, which is known from lattice calculations, it is argued that the width, which in the perturbative limit is given by $γ\sim g^2 \ln(1/g) T$, should be sizeable at intermediate temperatures but has to be small close to $T_c$. Implications of these results for several phenomenologically relevant quantities, such as the energy loss of hard jets, are pointed out.

hep-ph↗
arXiv2003

The QCD equation of state and quark star properties

We review our quasiparticle model for the thermodynamics of strongly interacting matter at high temperature, and its extrapolation to non-zero chemical potential. Some implications of the resulting soft equation of state of quark matter at low temperatures are pointed out.

hep-ph↗
arXiv2003

From QCD lattice calculations to the equation of state of quark matter

We describe two-flavor QCD lattice data for the pressure at finite temperature and zero chemical potential within a quasiparticle model. Relying only on thermodynamic selfconsistency, the model is extended to nonzero chemical potential. The results agree with lattice calculations in the region of small chemical potential.

hep-ph↗
arXiv2002

Quasi-particle model for deconfined matter

Our quasi-particle model for deconfined matter near T_c is reviewed. The extrapolation of lattice QCD data to a finite baryo-chemical potential is discussed. Determined by the chiral transition temperature T_c, the resulting equation of state of neutral and beta-stable deconfined matter is soft and limits size and mass of pure quark stars.

hep-ph↗
arXiv2002

Probing colored glass via $q\bar{q}$ photoproduction II: diffraction

In this paper, we consider the diffractive photoproduction of quark-antiquark pairs in peripheral heavy ion collisions. The color field of the nuclei is treated within the Colored Glass Condensate model. The cross-section turns out to be very sensitive to the value of the saturation scale.

hep-ph↗
arXiv2001

Resummation of the QCD thermodynamic potential

It is argued why thermodynamic approximations in terms of dressed propagators may, at larger coupling strength, be better behaved than perturbative results, and why in hot QCD the hard thermal loop approximation of the thermodynamic potential cannot be expected to work close to the phase transition.

hep-ph↗
arXiv2001

Probing colored glass via $q\bar{q}$ photoproduction

In this paper, we calculate the cross-section for the photoproduction of quark-antiquark pairs in the peripheral collision of ultra-relativistic nuclei, by treating the color field of the nuclei within the Color Glass Condensate model. We find that this cross-section is sensitive to the saturation scale $Q_s^2$ that characterizes the model. In particular, the transverse momentum spectrum of the produced pairs could be used to measure the properties of the color glass condensate.

hep-ph↗
arXiv2001

Coulomb corrections to $e^+ e^-$ production in ultra-relativistic nuclear collisions

The purpose of this paper is to explain the discrepancies existing in the literature relative to $e^+e^-$ pair production in peripheral heavy ion collisions at ultra-relativistic energies. A controversial issue is the possible cancellation of Coulomb corrections to the Born term in the pair production cross-section. Such a cancellation has been observed in a recent approach based on finding retarded solutions of the Dirac equation, but does not seem to hold in a perturbative approach. We show in this paper that the two approaches are in fact calculating different observables: the perturbative approach gives the exclusive cross-section of single pair production, while the other method gives the inclusive cross-section. We have also performed a thorough study of the electron propagator in the non-static background field of the two nuclei, the conclusion of which is that the retarded propagator is in the ultra-relativistic limit a much simpler object than the Feynman propagator, and can be calculated exactly.

nucl-th↗