Interference Phenomena in Medium Induced Radiation

We consider the interference pattern for the medium-induced gluon radiation produced by a color singlet quark-antiquark antenna embedded in a QCD medium with size $L$ and `jet quenching' parameter $\hat q$. Within the BDMPS-Z regime, we demonstrate that, for a dipole opening angle $\theta_{q\bar q} \gg\theta_c\equiv {2}/{\sqrt{\hat q L^3}}$, the interference between the medium--induced gluon emissions by the quark and the antiquark is suppressed with respect to the direct emissions. This is so since direct emissions are delocalized throughout the medium and thus yield contributions proportional to $L$ while interference occurs only between emissions at early times, when both sources remain coherent. Thus, for \tqq \gg\theta_c, the medium-induced radiation is the sum of the two spectra individually produced by the quark and the antiquark, without coherence effects like angular ordering. For \tqq \ll\theta_c, the medium--induced radiation vanishes.Comment: 4 pages, 2 figures; Proceedings of the "Quark Matter 2011" conferenc

A New Mechanism of Quark Energy Loss

We show that a heavy quark moving sufficiently fast through a quark-gluon plasma may lose energy by Cherenkov-radiating mesons. We demonstrate that this takes place in all strongly coupled, large-Nc plasmas with a gravity dual. The energy loss is exactly calculable in these models despite being an O(1/Nc)-effect. We discuss phenomenological implications for heavy-ion collision experiments.Comment: 4 pages, 4 figures; v2: plot modified, conclusions unchange

Prediction of a Photon Peak in Heavy Ion Collisions

We show that if a flavour-less vector meson remains bound after deconfinement, and if its limiting velocity in the quark-gluon plasma is subluminal, then this meson produces a distinct peak in the spectrum of thermal photons emitted by the plasma. We also demonstrate that this effect is a universal property of all strongly coupled, large-Nc plasmas with a gravity dual. For the J/psi the corresponding peak lies between 3 and 5 GeV and could be observed at LHC.Comment: 4 pages, 5 figures. Minor corrections; references adde

Angular Dependence of Jet Quenching Indicates Its Strong Enhancement Near the QCD Phase Transition

We study dependence of jet quenching on matter density, using "tomography" of the fireball provided by RHIC data on azimuthal anisotropy $v_2$ of high $p_t$ hadron yield at different centralities. Slicing the fireball into shells with constant (entropy) density, we derive a "layer-wise geometrical limit" $v_2^{max}$ which is indeed above the data $v_2<v_2^{max}$. Interestingly, the limit is reached only if quenching is dominated by shells with the entropy density exactly in the near-$T_c$ region. We show two models that simultaneously describe the high $p_t$ $v_2$ and $R_{AA}$ data and conclude that such a description can be achieved only if the jet quenching is few times stronger in the near-$T_c$ region relative to QGP at $T>T_c$. One possible reason for that may be recent indications that the near-$T_c$ region is a magnetic plasma of relatively light color-magnetic monopoles.Comment: 4 pages, 4 figures. Final version published as PRL102,202302(2009

Calculating the jet quenching parameter in the plasma of NCYM theory from gauge/gravity duality

A particular decoupling limit of non-extremal (D1, D3) brane bound state system of type IIB string theory is known to give the gravity dual of space-space non- commutative Yang-Mills (NCYM) theory at finite temperature. We use a string probe in this background to compute the jet quenching parameter in the strongly coupled plasma of hot non-commutative Yang-Mills theory in (3+1)-dimensions from gauge/gravity duality. We give the expressions for the jet quenching parameter for both small and large non-commutativity. For small non-commutativity, we find that the value of the jet quenching parameter gets reduced from its commutative value. The reduction is enhanced with temperature as T^{7} for fixed non-commutativity and the fixed 't Hooft coupling. We further generalize the results for the non-commutative Yang-Mills theories in diverse dimensions.Comment: Latex file,12 pages,No figure; v2: 17 pages, Motivation, various clarifications,references added. An estimation of the correction with the known experimental bound on $\theta$ is given, v3: added more clarifications, one reference and acknowledgment,version to appear in Phys. Rev.