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Register a new userPenetrating probes: Jets and photons in a non-equilibrium quark-gluon plasma
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We employ new field-theoretical tools to study photons and jets in a non-equilibrium quark-gluon plasma. Jet broadening and photon emission takes place through radiation which is suppressed by repeated and coherent interaction with the medium. We analyze this physics in an anisotropic plasma such as is created in the early stages of heavy-ion collisions. The anisotropy introduces an angular dependence in radiation and reduces its overall rate. This can affect phenomenological predictions of the rapidity dependence and angular flow of jets and photons.
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Jets are a promising way to probe the non-equilibrium physics of quark-gluon plasma (QGP). We study how an out-of-equilibrium medium induces a jet particle to emit gluons. Evaluation of the emission rate is complicated by Weibel instabilities which lead to an exponential growth of chromomagnetic fields. Deriving a quantum field theoretical description of an unstable QGP medium, we show that the chromomagnetic fields deflect jet particles during the gluon emission.
Jets and photons could play an important role in finding the transport coefficients of the quark-gluon plasma. To this end we analyze their interaction with a non-equilibrium quark-gluon plasma. Using new field-theoretical tools we derive two-point correlators for the plasma which show how instabilities evolve in time. This allows us, for the first time, to derive finite rates of interaction with the medium. We furthermore show that coherent, long-wavelength instability fields in the Abelian limit do not modify the rate of photon emission or jet-medium interaction.
Photons are a penetrating probe of the hot medium formed in heavy-ion collisions, but they are emitted from all collision stages. At photon energies below 2-3 GeV, the measured photon spectra are approximately exponential and can be characterized by their inverse logarithmic slope, often called effective temperature $T_mathrm{eff}$. Modelling the evolution of the radiating medium hydrodynamically, we analyze the factors controlling the value of $T_mathrm{eff}$ and how it is related to the evolving true temperature $T$ of the fireball. We find that at RHIC and LHC energies most photons are emitted from fireball regions with $T{,sim,}T_mathrm{c}$ near the quark-hadron phase transition, but that their effective temperature is significantly enhanced by strong radial flow. Although a very hot, high pressure early collision stage is required for generating this radial flow, we demonstrate that the experimentally measured large effective photon temperatures $T_mathrm{eff}{,>,}T_mathrm{c}$, taken alone, do not prove that any electromagnetic radiation was actually emitted from regions with true temperatures well above $T_mathrm{c}$. We explore tools that can help to provide additional evidence for the relative weight of photon emission from the early quark-gluon and late hadronic phases. We find that the recently measured centrality dependence of the total thermal photon yield requires a larger contribution from late emission than presently encoded in our hydrodynamic model.
In this article we investigate how the drag coefficient $A$ and $hat{q}$, the transverse momentum transfer by unit length, of charm quarks are modified if the QGP is not in complete thermal equilibrium using the dynamical quasi-particle model (DQPM) which reproduces both, the equation-of-state of the QGP and the spatial diffusion coefficient of heavy quarks as predicted by lattice QCD calculations. We study three cases: a) the QGP has an anisotropic momentum distribution of the partons which leads to an anisotropic pressure b) the QGP partons have higher or lower kinetic energies as compared to the thermal expectation value, and c) the QGP partons have larger or smaller pole masses of their spectral function as compared to the pole mass from the DQPM at the QGP temperature. In the last two cases we adjust the number density of partons to obtain the same energy density as in an equilibrated QGP. In the first scenario we find that if the transverse pressure exceeds the longitudinal one for small heavy quark momenta $A$ becomes larger and $hat{q}$ smaller as compared to an isotropic pressure. For heavy quarks with large momentum both, $A$ and $hat{q}$ , approach unity. If the partons have less kinetic energy or a smaller pole mass as compared to a system in equilibrium charm quarks lose more energy. In the former case $hat{q}$ decreases whereas in the latter case it increases for charm quark with a low or intermediate transverse momentum. Thus each non-equilibrium scenario affects $A$ and $hat{q}$ of charm quarks in a different way. The modifications in our scenarios are of the order 20-50% at temperatures relevant for heavy ion reactions. These modifications have to be considered if one wants to determine these coefficients by comparing heavy ion data with theoretical predictions from viscous hydrodynamics or Langevin equations.
The photon emission from a non-equilibrium quark-gluon plasma (QGP) is analyzed. We derive an integral equation that describes photon production through quark-antiquark annihilation and quark bremsstrahlung. It includes coherence between different scattering sites, also known as the Landau-Pomeranchuk-Migdal effect. These leading-order processes are studied for the first time together in an out-of-equilibrium field theoretical treatment that enables the inclusion of viscous corrections to the calculation of electromagnetic emission rates. In the special case of an isotropic, viscous, plasma the integral equation only depends on three constants which capture the non-equilibrium nature of the medium.
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