Abstract
The ALICE Collaboration at the LHC has measured the J/ψ and ψ′ photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at sqrt{s_{mathrm{NN}}}=2.76~mathrm{TeV}.The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 μb−1. The cross section for coherent and incoherent J/ψ production in the rapidity interval −0.9<y<0.9, are mathrm{d}sigma_{mathrm{J}/psi}^{mathrm{coh}}/mathrm{d}y = 2.38^{+0.34}_{-0.24} (mathrm{sta+sys} )~mathrm{mb} and mathrm{d}sigma_{mathrm{J}/psi}^{mathrm{inc}}/mathrm{d}y = 0.98^{+0.19}_{-0.17} (mathrm{sta}+mathrm{sys})~mathrm{mb}, respectively. The results are compared to theoretical models for J/ψ production and the coherent cross section is found to be in good agreement with those models incorporating moderate nuclear gluon shadowing at Bjorken-x around 10−3, such as EPS09 parametrization. In addition the cross section for the process γγ→e + e − has been measured and found to be in agreement with models implementing QED at leading order.
Highlights
Exclusive production of charmonium in photon–proton interactions at HERA, γ + p → J/ψ(ψ ) + p, has been successfully modelled in perturbative QCD in terms of the exchange of two gluons with no net-colour transfer [3]
We found no hits in the Forward Multiplicity Detector (FMD) detector for the selected barrel ultra-peripheral collision trigger (BUPC) events, confirming that the VZERO inefficiency is very small
The first measurement of coherent and incoherent J/ψ photoproduction and two-photon production of dilepton pairs in Pb–Pb collisions at mid-rapidity at the LHC has been presented and compared with model calculations
Summary
Exclusive production of charmonium in photon–proton interactions at HERA, γ + p → J/ψ(ψ ) + p, has been successfully modelled in perturbative QCD in terms of the exchange of two gluons with no net-colour transfer [3]. Incoherent production, corresponding to quasi-elastic scattering off a single nucleon, is characterized by a somewhat higher transverse momentum ( pT ≈ 500 MeV/c) In this case the nucleus interacting with the photon breaks up, but, apart from single nucleons or nuclear fragments in the very forward region, no other particles are produced. Mx e=nt(MatJ/mψi/d√-raspNiNd)iteyxpa(ll±owy)s the exploration ≈ 10−3, where of at the region present the uncertainty in the nuclear gluon shadowing distribution is rather large [7] This analysis is focused both on coherently and incoherently produced J/ψ mesons. Two-photon production of di-lepton pairs in heavy-ion interactions is of great interest, as it probes Quantum Electrodynamics in the regime of strong fields This process is sensitive to the effect √produced by the strong fields of the nuclei. The results are compared with predictions by models neglecting higherorder effects discussed above
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