photoproduction in Pb–Pb and p–Pb ultra-peripheral collisions with ALICE at LHC
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 , allowing γγ interactions at and at , 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 in photon–proton interactions, , 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 provided by LHC. In this case the photoproduction is dominated by the process and this gives the possibility to study the gluon PDF in the proton down to . Considering the different rapidity ranges available for the measurement, ALICE can provide the cross section as a function of the 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 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 photoproduction cross sections provide a powerful tool to constrain the nuclear gluon shadowing in the region . The coherent cross section is found to be in a good agreement with the model which incorporates the nuclear gluon shadowing according to the EPS09 parameterization
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