Exclusive production of [formula omitted] pairs in γγ collisions at RHIC

M Kłusek,W Schäfer,A Szczurek

doi:10.1016/j.physletb.2009.03.006

Abstract

We discuss exclusive electromagnetic production of two neutral ρ mesons in coherent photon–photon processes in ultrarelativistic heavy-ion collisions. The cross section is calculated in the equivalent photon approximation (EPA). Both uncertainties of the flux factors and photon–photon cross sections are discussed in detail. We show that inclusion of precise charge densities in nuclei is essential for realistic evaluations of the nuclear photon–photon cross sections. We find that the cross section, especially with realistic flux factors, is sensitive to low energy in the subsystem γγ→ρ0ρ0. The experimental data for the γγ→ρ0ρ0 cross section extracted from e+e− collisions are parametrized and used to estimate the nucleus–nucleus cross section. In addition, we include vector-dominance-model (VDM)–Regge contribution which becomes important at large photon–photon energy. Large nuclear cross sections are obtained. We discuss a possibility of focusing on the large-energy component. We find that both ρ0 mesons are produced predominantly at midrapidities and could be measured by the STAR Collaboration at RHIC.

Highlights

Exclusive production of elementary particles or mesons in ultrarelativistic heavy ion collisions is an interesting and quickly growing field [1, 2, 3] of theoretical investigation
Only integrated cross section for AA → AAρ0ρ0 was roughly estimated in the literature [5], using a pQCD–inspired modelling of the Pomeron exchange between mesons for AA → AAJ/ΨJ/Ψ and AA → AAρ0J/Ψ and assuming Regge factorization
If the ρ0ρ0 invariant mass is very large, the two π+π−–pairs will be separated by a large rapidity gap

Summary

INTRODUCTION

Exclusive production of elementary particles (lepton pairs, Higgs, etc.) or mesons (vector mesons, pair of pseudoscalar mesons, etc.) in ultrarelativistic heavy ion collisions is an interesting and quickly growing field [1, 2, 3] of theoretical investigation. Only single-ρ0 exclusive cross section AA → AAρ0 was measured [4]. Only integrated cross section for AA → AAρ0ρ0 was roughly estimated in the literature [5], using a pQCD–inspired modelling of the Pomeron exchange between mesons for AA → AAJ/ΨJ/Ψ and AA → AAρ0J/Ψ and assuming Regge factorization. One observes a huge enhancement close to the threshold. The origin of this enhancement was never understood. The huge cross section at W ≈ 1.5-2 GeV was interpreted as a tensor resonance decaying into ρ0ρ0 channel [9]. In the following we shall see the consequences of the anomalous behaviour seen in the e+e− data for the two-photon production of the ρ0ρ0 pair in the nucleus-nucleus collisions

Equivalent photon approximation for nuclei

RESULTS

CONCLUSIONS