First measurement of coherent ${\rho }^0$ photoproduction in ultra-peripheral Xe-Xe collisions at $\sqrt{{\mathbf{s}}_{\mathbf{NN}}}=5.44$ TeV

Abstract

The first measurement of the coherent photoproduction of ${\rho }^0$ vector mesons in ultra-peripheral Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44$ 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 $\mathrm{Xe}+\mathrm{Xe}\to {\rho }^0+\mathrm{Xe}+\mathrm{Xe}$ process, measured at midrapidity through the decay channel ${\rho }^0\to {\pi }^{+}{\pi }^{-}$, is found to be $\mathrm{d}\sigma /\mathrm{d}y=131.5\pm 5.6{(stat.)}_{-16.9}^{+17.5}(syst.)$ 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 $\sigma (\gamma A\to {\rho }^{0}A)\propto A^{\alpha }$ with a slope $\alpha =0.96\pm 0.02(syst.)$. This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit.

21 pages, 4 captioned figures, 2 tables, authors from page 16, published version, figures at http://alice-publications.web.cern.ch/node/6757

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Cited by in corpus (12)

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• Challenges to the Good-Walker paradigm in coherent and incoherent photoproduction
• Exclusive $J/\psi$, $\psi (2s)$, and $e^{+}{e}^{-}$ pair production in Au$+$Au ultra-peripheral collisions at RHIC
• Novel Cross Section Ratios as Possible Signals of Saturation in UPCs
• Photoproduction of K${}^{+}$K${}^{-}$ pairs in ultra-peripheral collisions
• Measurement of the impact-parameter dependent azimuthal anisotropy in coherent ${\rho }^0$ photoproduction in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV
• Light vector meson photoproduction in ultraperipheral heavy ion collisions at the LHC within the Reggeometric Pomeron approach
• Exclusive four pion photoproduction in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV
• Observation of coherent $\varphi$(1020) meson photoproduction in ultraperipheral PbPb collisions at $\sqrt{s_{\text{NN}}}$ = 5.36 TeV
• Diffractive vector meson photo-production in oxygen-oxygen and neon-neon ultraperipheral collisions at energies available at the CERN Large Hadron Collider