Coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}} $$ = 5.02 TeV

Abstract

Cross sections for the coherent photoproduction of ρ 0 vector mesons in ultraperipheral Pb-Pb collisions at √ s NN = 5.02 TeV are reported. The measurements, which rely on the π + π − decay channel, are presented in three regions of rapidity covering the range |y| < 0.8. For each rapidity interval, cross sections are shown for different nuclearbreakup classes defined according to the presence of neutrons measured in the zero-degree calorimeters. The results are compared with predictions based on different models of nuclear shadowing. Finally, the observation of a coherently produced resonance-like structure with a mass around 1.7 GeV/c 2 and a width of about 140 MeV/c 2 is reported and compared with similar observations from other experiments.

Incoherent ρ meson photoproduction in ultraperipheral nuclear collisions at the CERN Large Hadron Collider

“…Our results show that within the adopted model for the hadronic structure of the incident photon, the effect of the inelastic nuclear shadowing in the incoherent cross sections is significant. This observation complements a similar conclusion in the coherent case [8], which was confirmed by a comparison of our calculations with the ALICE measurements [17,18]. We also find very significant differences of our results for the incoherent photoproduction cross section from predictions of the STARlight Monte Carlo framework, which is frequently used in the UPC experimental data processing and analysis [14].…”

“…It leads to a strong energy-dependent violation of the optical theorem and a suppression of the coherent cross section by approximately a factor of three in the LHC kinematics (a factor of four in the asymptotic black body limit). An interplay of these two effects results in an overall suppression by approximately a and √ s NN = 5.02 TeV [18] are shown by the filled circle and the squares with error bars, respectively.…”

“…6. Also, the scaled results of the STAR measurement of coherent ρ photoproduction in Au-Au UPCs at √ s NN = 200 GeV [43] and ALICE measurements of coherent ρ photoproduction in Pb-Pb UPCs at √ s NN = 2.76 TeV [17] and √ s NN = 5.02 TeV [18] are shown by the filled circle and the squares with error bars, respectively. One can see that the predictions of our approach are in excellent agreement with the ALICE data.…”

The cos 2ϕ azimuthal asymmetry in ρ0 meson production in ultraperipheral heavy ion collisions

“…Among many exciting directions of UPC studies, see for example [1,2], diffractive vector mesons photoproduction on nuclei provide access to the three dimensional gluon tomography of nucleus as well as stringent tests of the color glass condensate (CGC) description of saturation physics. Because of this, such processes have been extensively studied from both theoretical [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and experimental [22][23][24][25][26][27][28] sides during the past few decades.…”

ALICE measurements of coherents rho0 photoproduction in Pb--Pb ultraperipheral collisions

“…RHIC and first LHC results have deviated from some Glauber+VDM calculations, which thus call for new data. The ALICE Collaboration reports [1] the first measurements of coherent rho photoproduction accompanied by electromagnetic dissociation (EMD) with data taken at √ NN = 5.02 TeV. The rapidity-dependent cross section of coherent 0 photoproduction is measured and it is compared with theoretical models.…”