Bremsstrahlung of a quark propagating through a nucleus

Abstract

The density of gluons produced in the central rapidity region of a heavy ion collision is poorly known. We investigate the influence of the effects of quantum coherence on the transverse momentum distribution of photons and gluons radiated by a quark propagating through nuclear matter. We describe the case that the radiation time substantially exceeds the nuclear radius (the relevant case for RHIC and LHC energies), which is different from what is known as Landau-PomeranchukMigdal effect corresponding to an infinite medium. We find suppression of the radiation spectrum at small transverse photon/gluon momentum k T , but enhancement for k T > 1 GeV. Any nuclear effects vanish for k T ≥ 10 GeV. Our results allow also to calculate the k T dependent nuclear effects in prompt photon, light and heavy (Drell-Yan) dilepton and hadron production.

Diffraction in QCD

“…Namely, the forward cross section of diffractive radiation qp →llqp is zero [30]. Indeed, according to (10) the forward diffractive cross section is given by the dispersion of the eigen amplitude distribution.…”

“…This is not a surprize, since the two processes are related by QCD factorization. The cross section of heavy photon (γ * →ll) radiation by a quark reads [29][30][31][32],…”

Diffractive production of Drell–Yan pairs and heavy flavors

“…The distribution function W qQQ (α, r T ) for a qQQ fluctuation has the same form as W qll (α, r T ), except replacement α 2 em → 2α 2 s /3. The cross section ofQQ production in q − N interaction turns out to be equal to the color dipole cross section for interaction of a colorless systemqqg * with a nucleon (compare with (4)) σ(q →QQq) = σq qg * (α, r T ) [1,16] which has a form [17], σq qg * (α, r T ) = 8 9 [σq q ((1 − α)r T ) + σq q (r T )] − 1 8 σq q (αr T ) .…”

Charmonium propagation through a dense medium

“…One has to sum up all possible trajectories of propagation of quark and antiquark in order to incorporate the effects of fluctuation of the dipole separation. In the light-cone variables the Green function of the dipole propagating in a medium satisfies the 2-dimensional Schrödinger-type equation [6,15],…”

“…A rigorous quantum-mechanical description of the propagation of a colourlesscc can be performed with the path-integral method [6,15]. One has to sum up all possible trajectories of propagation of quark and antiquark in order to incorporate the effects of fluctuation of the dipole separation.…”