The ALICE Collaboration at the LHC has mea- sured the J/ψ and ψ′ photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at √sNN = 2.76 TeV. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be mini- mal. The analysis is based on an event sample correspond- ing 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 dσcoh/dy = J/ψ 2.38+0.34(sta+sys) mb and dσinc /dy = 0.98+0.19(sta + −0.24 J/ψ −0.17 sys) mb, respectively. The results are compared to theoret- ical models for J/ψ production and the coherent cross sec- tion is found to be in good agreement with those models in- corporating moderate nuclear gluon shadowing at Bjorken- x around 10−3, such as EPS09 parametrization. In addition the cross section for the process γ γ → e+e− has been mea- sured and found to be in agreement with models implement- ing QED at leading order.

### Charmonium and e + e − pair photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76\ \mbox{TeV}$

#### Abstract

The ALICE Collaboration at the LHC has mea- sured the J/ψ and ψ′ photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at √sNN = 2.76 TeV. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be mini- mal. The analysis is based on an event sample correspond- ing 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 dσcoh/dy = J/ψ 2.38+0.34(sta+sys) mb and dσinc /dy = 0.98+0.19(sta + −0.24 J/ψ −0.17 sys) mb, respectively. The results are compared to theoret- ical models for J/ψ production and the coherent cross sec- tion is found to be in good agreement with those models in- corporating moderate nuclear gluon shadowing at Bjorken- x around 10−3, such as EPS09 parametrization. In addition the cross section for the process γ γ → e+e− has been mea- sured and found to be in agreement with models implement- ing QED at leading order.
##### Scheda breve Scheda completa Scheda completa (DC)
73
2617
2617
http://arxiv.org/abs/1305.1467
charmonium; PAIR PRODUCTION; mid-rapidity Pb-Pb collisions
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/2318/143058