Abstract
The first measurement of the coherent photoproduction of ρ0 vector mesons in ultra-peripheral Xe–Xe collisions at sNN=5.44 TeV is presented. This result, together with previous HERA γp data and γ–Pb measurements from ALICE, describes the atomic number (A) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the Xe+Xe→ρ0+Xe+Xe process, measured at midrapidity through the decay channel ρ0→π+π−, is found to be dσ/dy=131.5±5.6(stat.)−16.9+17.5(syst.) mb. The ratio of the continuum to resonant contributions for the production of pion pairs is also measured. In addition, the fraction of events accompanied by electromagnetic dissociation of either one or both colliding nuclei is reported. The dependence on A of cross section for the coherent ρ0 photoproduction at a centre-of-mass energy per nucleon of the γA system of WγA,n=65 GeV is found to be consistent with a power-law behaviour σ(γA→ρ0A)∝Aα with a slope α=0.96±0.02(syst.). This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit.
Highlights
The Large Hadron Collider (LHC) is a source of photon-induced processes
ALICE has previously published [10] the cross section for the coherent production of ρ0 vector mesons in Pb–Pb ultra-peripheral collisions (UPCs) for events with no beam-rapidity neutrons (0n0n, where the first 0n refers to the A-side and the second to the C-side), with one or more neutrons on one side only (0nXn+Xn0n), or on both sides (XnXn)
The trigger requires at least two hits in the inner and in the outer layer of the Silicon Pixel Detector (SPD), at least two pads fired in Time-of-Flight detector (TOF) and no signal in the V0
Summary
The Large Hadron Collider (LHC) is a source of photon-induced processes. The electromagnetic fields of the relativistic nuclei are strongly contracted allowing for their interpretation as a flux of quasi-real photons, which interact with the nuclei travelling in the opposite direction. The coherent production of a ρ0 vector meson off a nucleus allows for the study of shadowing, the experimental fact that the nuclear structure functions are suppressed compared to the superposition of those of their constituent nucleons [11] ALICE has previously published [10] the cross section for the coherent production of ρ0 vector mesons in Pb–Pb UPC for events with no beam-rapidity neutrons (0n0n, where the first 0n refers to the A-side and the second to the C-side), with one or more neutrons on one side only (0nXn+Xn0n), or on both sides (XnXn).
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