Abstract
There are several predictions for the behaviour of the gluon distribution in nuclei at small Bjorken-x and experimental data are needed to choose among them. This is achieved by measuring the cross section of processes specially sensitive to this parton distribution.The high flux of photons from lead ions at the LHC allows us to study photon-induced reactions in ultra-peripheral collisions of Pb–Pb nuclei, in particular of those producing a J/ψ meson exclusively. The study of these collisions, where projectiles do not overlap, provides information about the initial state of nuclei.The newest ALICE results on vector meson photoproduction are presented. The increased statistics and higher collision energy of of Run 2 allow us to put new constraints on available models.
Highlights
Parton density functions (PDFs) of gluons and quarks inside the proton were measured with high precision at small Bjorken-x at HERA [1]
According to the equivalent photon approximation, a large flux of quasi-real photons accompanies the electrically charged lead ions circulating at the LHC during Pb–Pb data taking opening the possibility to use ultra-peripheral collisions (UPC) of lead ions to study photoninduced processes
In the infinite momentum frame where the photons live a long time, when the quasi-real photon emitted by one of the lead ions approaches the vicinity of the target nucleus, it may fluctuate into a quark-antiquark pair
Summary
Parton density functions (PDFs) of gluons and quarks inside the proton were measured with high precision at small Bjorken-x at HERA [1]. A measurement of the kinematics of the meson and the cross section of this process [2] is sensitive in a leading-order approach in perturbative QCD to the gluon distribution function of the nucleus [3]. Measurements of J/ψ photoproduction in UPC with the ALICE detector [5] allow us to reach Bjorken-x of 10−2 in forward and 10−3 in central rapidity.
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