Publication Date:
2021-08-01
Description:
The coherent photoproduction of $$mathrm{J}/psi $$ J / ψ and $${uppsi '}$$ ψ ′ mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy $$sqrt{s_{mathrm {NN}}}~=~5.02$$ s NN = 5.02 TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The $$mathrm{J}/psi $$ J / ψ is reconstructed using the dilepton ($$l^{+} l^{-}$$ l + l - ) and proton–antiproton decay channels, while for the $${uppsi '}$$ ψ ′ the dilepton and the $$l^{+} l^{-} pi ^{+} pi ^{-}$$ l + l - π + π - decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 $$mu {mathrm{b}}^{-1}$$ μ b - 1 . The results are compared with theoretical models for coherent $$mathrm{J}/psi $$ J / ψ and $${uppsi '}$$ ψ ′ photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around $$6imes 10^{-4}$$ 6 × 10 - 4 , such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent $$mathrm{J}/psi $$ J / ψ cross section including ALICE measurements at forward rapidity. The ratio of $${uppsi '}$$ ψ ′ to $$mathrm{J}/psi $$ J / ψ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.
Print ISSN:
1434-6044
Electronic ISSN:
1434-6052
Topics:
Physics
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