Elsevier

Nuclear Physics A

Volume 926, June 2014, Pages 136-141
Nuclear Physics A

J/ψ photoproduction in Pb–Pb and p–Pb ultra-peripheral collisions with ALICE at LHC

https://doi.org/10.1016/j.nuclphysa.2014.04.024Get rights and content

Abstract

The photoproduction of vector mesons in Ultra-Peripheral Collisions (UPC) is a powerful tool to probe the nuclear gluon distribution, for which there is considerable uncertainty in the low-x region. We present the first measurements in Pb–Pb collisions at sNN=2.76 TeV, performed with the ALICE detector. The J/ψ is identified via its dimuon decay in the forward rapidity region and via dimuon and dielectron decay at midrapidity. The results are compared to theoretical models for coherent J/ψ production and found to be in good agreement with models which include nuclear gluon shadowing. The cross section measurement for incoherent J/ψ and γγe+e at mid-rapidity is reported. Finally we present the first results on p–Pb UPC at sNN=5.02 TeV.

Section snippets

Ultra-peripheral collisions

Ultra-peripheral collisions may occur when two accelerated nuclei are separated by impact parameters larger than the sum of their radii: hadronic interactions are strongly suppressed so that both pure electromagnetic and photonuclear processes can be studied. These interactions can be studied in Pb–Pb collisions at the LHC at unprecedentedly high energies, where the available centre of mass energy per nucleon pair is 2.76 TeV, allowing γγ interactions at 80 GeV and γp at 480 GeV, to be

Physics motivations

Exclusive photoproduction of vector mesons, where a vector meson but no other particles are produced in the event, is of particular interest. Exclusive production of J/ψ in photon–proton interactions, γ+pJ/ψ+p, has been successfully modeled in perturbative QCD in terms of the exchange of two gluons with no net-color transfer [6]. Experimental data on this process from HERA have been used to constrain the proton gluon-distribution at low Bjorken-x [7]. Exclusive vector meson production in

Results and comparison with models

The ALICE detector [10] allows for a study of UPCs in different rapidity ranges. The ALICE sub-detectors used in this analysis can be found in [10], while [12] describes the trigger and data selection.

First results in p–Pb collisions

At the beginning of 2013 ALICE collected a sample of p–Pb collisions at sNN=5.02 TeV provided by LHC. In this case the J/ψ photoproduction is dominated by the γp process and this gives the possibility to study the gluon PDF in the proton down to x105. Considering the different rapidity ranges available for the measurement, ALICE can provide the cross section as a function of the γp centre of mass energy, ranging from 21 GeV up to 1160 GeV. The preliminary result at forward rapidity in p–Pb

Summary

The first measurement of coherent and incoherent J/ψ photoproduction and two-photon production of di-lepton pairs in Pb–Pb collisions at the LHC has been presented and compared with model calculations. The J/ψ photoproduction cross sections provide a powerful tool to constrain the nuclear gluon shadowing in the region x103. The coherent J/ψ cross section is found to be in a good agreement with the model which incorporates the nuclear gluon shadowing according to the EPS09 parameterization

References (14)

  • A.D. Martin et al.

    Phys. Lett. B

    (2008)
  • V. Rebyakova et al.

    Phys. Lett. B

    (2012)
  • E. Fermi

    Nuovo Cimento

    (1925)
  • C.F. Weizsäcker

    Z. Phys.

    (1934)
  • E.J. Williams

    Phys. Rev.

    (1934)
  • A.J. Baltz

    Phys. Rep.

    (2008)
  • C.A. Bertulani et al.

    Annu. Rev. Nucl. Part. Sci.

    (2005)
There are more references available in the full text version of this article.
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