Photon-induced Processes in Semi-central Nucleus--Nucleus Collisions

Abstract

We calculate total and differential cross sections for $J/\psi$ photoproduction in ultrarelativistic lead-lead collisions at the LHC energy $\sqrt{s_{NN}}=2.76$ TeV. We use a simple model based on vector dominance picture and multiple scattering of the hadronic ($c \bar c$) state in a cold nucleus. In our analysis we use Glauber formulae for calculating $\sigma_{tot,J/\psi Pb}$ which is a building block of our model. For semi-central collisions a modification of the photon flux is necessary. We discuss how to effectively correct photon fluxes for geometry effects. We try to estimate the cross sections for different centrality bins and for $J/\psi$ mesons emitted in forward rapidity range ($2.5<y<4$) corresponding to the ALICE experimental results. We discuss similar analysis for dilepton production in ultrarelativistic heavy-ion collisions at very low pair transverse momenta, $P_T\leq 0.15$\,GeV. We investigate the interplay of thermal radiation with photon annihilation processes, $\gamma \gamma \to l^+ l^-$, due to the coherent electromagnetic fields of the colliding nuclei. For the thermal radiation, we employ the emission from the QGP and hadronic phases with in-medium vector spectral functions. We first verify that the combination of photon fusion, thermal radiation and final-state hadron decays gives a fair description of the low-$P_T$ invariant-mass as well as $P_T$ distributions as measured recently by the STAR collaboration in $\sqrt{s_{NN}}$=200\,GeV Au+Au collisions for different centralities. The coherent contribution dominates in peripheral collisions, while thermal radiation shows a significantly stronger increase with centrality. We also provide predictions for the ALICE experiment at the LHC. The resulting excitation function reveals a nontrivial interplay of photoproduction and thermal radiation.