High-Energy Photons from Passage of Jets through Quark-Gluon Plasma

Abstract

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...

A systematic study of direct photon production in heavy ion collisions

“…It does not reflect, however, the possibly large cancellation between the bremsstrahlung associated with hard jet production and the subsequent quark scattering, which was found to control induced gluon emission in finite nonAbelian plasmas [3,10,11]. Inverse Compton scattering in the QGP [12] has also been suggested as the dominant source of intermediate p T photons if the γ acquires most of the momentum of the incoming quark. Such enhancement effects, in conjunction with their associated negative azimuthal asymmetry coefficient v 2 (p T ) [8,13], do not appear to be compatible with existing direct photon data at RHIC [2,14,15].…”

“…For such conversions, in Eq. (24) the p γ ≈ p c approximation [12] that stems from the limit of small tchannel momentum transfers to energetic jets is implicit. For a parent quark propagating through the QGP:…”

Quark and Gluon Degrees of Freedom in High-Energy Heavy Ion Collisions

“…In 2002, we suggested that jets traveling through the plasma can radiate photons (and dileptons) as well and that this might be an important process [20,21]. The leading order channels for real photons are annihilation, q (jet)+q (medium) → γ + g and Compton scattering q (jet)+g (medium) → γ + q.…”

Effect of running coupling on photons from jet–plasma interaction in relativistic heavy-ion collisions

“…However, while evaluating jet-photon the authors in Ref. [2] assumes that the largest contribution to photons corresponds to p γ ∼ p q (pq). This implies that the annihilating quark (anti-quark) directly converts into a photon.…”

“…These apart, there exits another class of photon emission process via the jet conversion mechanism (jet-plasma * Electronic address: lusaka.bhattacharya@saha.ac.in † Electronic address: pradipk.roy@saha.ac.in interaction) [2] which occurs when a high energy jet interacts with the medium constituents via annihilation and Compton processes. It might be noted that this phenomenon (for Compton process) has been illustrated quite some time ago [3] in the context of estimating photons from equilibrating plasma.…”