First measurement of coherent sigma;(0) photoproduction in ultra-peripheral Xe-Xe collisions at root sNN=5.44 TeV

The first measurement of the coherent photoproduction of rho(0) vector mesons in ultra-peripheral Xe- Xe collisions at root s(NN) = 5.44 TeV is presented. This result, together with previous HERA gamma p data and gamma-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 -> rho(0) + Xe + Xe process, measured at midrapidity through the decay channel rho(0)-> pi(+)pi(-), is found to be d sigma/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 rho 0 photoproduction at a centre-of-mass energy per nucleon of the gamma A system of W gamma A,n = 65 GeV is found to be consistent with a power-law behaviour sigma (gamma A -> rho(0) A) proportional to A(alpha) with a slope alpha = 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. (C) 2021 European Organization for Nuclear Research. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.