We calculate the production of high energy photons from Compton scattering and annihilation of a quark jet passing through a quark gluon plasma produced in a relativistic heavy ion collision. The contributions are large and reflect the momentum distribution of the jets and the initial conditions of the plasma.Relativistic heavy ion collisions are studied with the aim of producing a plasma of quarks and gluons (QGP). Photons are considered to be an important probe for the investigation of the formation and evolution of such a plasma due to their weak final-state interactions [1]. Once produced they carry the information about the conditions of the environment in which they were created, encoded in their momentum distribution, thus providing a glimpse deep into the bulk of strongly interacting matter. Though most of the measured photons have their origin in the decay of hadrons after the QGP phase, it has become possible to isolate the direct photons produced in such collisions [2].The sources of direct photons considered so far include quark annihilation, Compton scattering, and bremsstrahlung following the initial hard scattering of partons of the nuclei [3], as well as thermal photons from the QGP [4,5,6,7,8] and from hadronic interactions in the hot hadronic gas after the hadronization of the plasma [4,9]. The pre-equilibrium production of photons has also been investigated by several authors [10]. Results are available for production from the entire history of the system [11].In this letter we study a new source of direct photons originating from the passage of the produced high energy quark jets through the QGP (jet-photon conversion). A fast quark passing through the plasma will produce photons by Compton scattering with the thermal gluons and annihilation with the thermal antiquarks. This process is higher order in α s compared with photons from initial hard scatterings, but it is not a subleading contribution, since it corresponds to double scattering, which is enhanced by the size of the system. For cold nuclear matter this effect is encoded in multi-parton matrix elements which are enhanced by powers of A 1/3 [12]. Below, we find that this source is at least comparable in strength to the other direct photon sources and even dominates in the range p ⊥ ≤ 6 GeV for Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC).We also demonstrate that the p ⊥ -distribution of these photons is directly proportional to the momentum distribution of jets at an early stage after their production, before they have lost energy on their travel through the plasma. Since the measured high-p ⊥ hadron spectrum is proportional to the spectrum of partons after they have left the plasma, a comparision of both spectra could provide a quantitative determination of the energy loss and help confirm the mechanism of jet quenching [13,14].Furthermore, the photon yield depends on the integrated density of the matter traversed by the jets and thus can provide a measurement of this quantity. We emphasize that our mechanism is dis...

We predict the transverse momentum (p T ) dependence of elliptic flow of thermal photons for Au Au collisions at the BNL Relativistic Heavy Ion Collider. We model the system hydrodynamically, with a thermalized quark-gluon plasma at early times followed by hadronization and decoupling. Photons are emitted throughout the expansion history. Contrary to hadron elliptic flow, which increases monotonically with p T , the elliptic flow v 2 p T of thermal photons is predicted to first rise and then fall again. Photon elliptic flow at high p T reflects the quark momentum anisotropy at early times when it is small, while at low p T it mirrors the large pion momentum anisotropy during the late hadronic emission stage. An interesting structure is predicted at intermediate p T 0:4 GeV=c, where photon elliptic flow reflects the momenta and the (compared to pions) reduced v 2 of heavy vector mesons in the late hadronic phase.

This writeup is a compilation of the predictions for the forthcoming Heavy Ion Program at the Large Hadron Collider, as presented at the CERN Theory Institute ‘Heavy Ion Collisions at the LHC—Last Call for Predictions’, held from 14th May to 10th June 2007.