Event-by-event jet quenching

Abstract

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 coefficientq 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 adjustingq to fit R AA in the eventby-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. We discuss a correlation function that is a measure for spatial inhomogeneities in a collision and can be constrained from data.

Effects of initial-state fluctuations on jet-energy loss

“…The smaller R AA in the case of fluctuating initial conditions obtained in the present study is opposite to the results reported in previous studies based on a static medium [24] or 2+1 dimensional ideal hydrodynamics [25], where a larger R AA was obtained when initial fluctuations were included. To understand this difference, we define the jet energy loss difference δ ∆E ≡ ∆E fluc − ∆E smth between the average energy loss calculated with fluctuating initial conditions and that with smooth initial conditions.…”

“…Our results are opposite to those found in Ref. [24] for a static medium and also those in Ref. [25] using a 2+1 ideal hydrodynamics and with the jet energy loss depending on the local energy density, which show a reduced jet energy loss in the QGP for the fluctuating initial conditions.…”

“…[24] based on a static medium, the relative difference between the nuclear modification factors for the fluctuating and smooth initial conditions is then positive as shown by open diamonds in Fig. 4 for all values of α, indicating that the jet energy loss is reduced after including initial fluctuations.…”

“…Our results show that including transverse expansion of the medium and assuming that the jet energy loss depends on the local parton density slightly enhance the energy loss of jets, contrary to the reduced jet energy loss found in Refs. [24,25]. We further investigate the effect of initial fluctuations for different path-length dependence of jet energy loss in the medium.…”

QCD Back-Scattering Photons in Relativistic Heavy ion Collisions

“…First we have calculated the prompt photon (direct + fragmentation) spectra and the fast quark spectra at LO of strong coupling (α s ) with the code JETPHOX (Aurenche et al, 2006). Next we have used the fireball model PPM (Rodriguez et al, 2010) to evaluate the back-scattering photon yield while accounting for (i) the energy loss of partons and (ii) the energy loss trigger of jets.…”