New theoretical results in ultrarelativistic ultraperipheral lead-lead collisions

Abstract

We study dilepton, proton-antiproton and diphoton production in ultraperipheral lead-lead collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 and 5.5 TeV. The nuclear calculations are based on equivalent photon approximation in the impact parameter space. For correct description of the $γγ\to p\bar{p} $ Belle data we include the proton-exchange, the $f_2(1270)$ and $f_2(1950)$ s-channel exchanges, as well as the handbag mechanism. For four muon production, we take into account electromagnetic (two-photon) double-scattering production and direct $γγ$ production of four muons in one scattering. The cross sections for elementary $γγ\to γγ$ subprocess are calculated including three different mechanisms: box diagrams with leptons and quarks in the loops, a VDM-Regge contribution with virtual intermediate hadronic excitations of the photons and the two-gluon exchange contribution. We find that the cross section for elastic $γγ$ scattering could be measured in the lead-lead collisions for the diphoton invariant mass up to $W_{γγ} \approx$ 15 - 20 GeV. Our Standard Model predictions are compared with a recent ATLAS experimental result. We discuss results for PbPb$\to$PbPb$μ^+μ^-μ^+μ^-$, PbPb$\to$PbPb$e^+e^-e^+e^-$, PbPb$\to$PbPb$p\bar{p}$ and PbPb$\to$PbPb$γγ$ reactions at LHC energy.