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Energy dependence of exclusive $J/ψ$ photoproduction off protons in ultra-peripheral p-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV

The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of $J/\psi$ vector mesons off proton targets in ultra-peripheral p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV. The e$^+$e$^-$ and $\mu^+\mu^-$ decay channels are used to measure the cross section as a function of the rapidity of the $J/\psi$ in the range $-2.5 <~ y <~ 2.7$, corresponding to an energy in the $\gamma$p centre-of-mass in the interval $40 <~ W_{\gamma\mathrm{p}}<~550$ GeV. The measurements, which are consistent with a power law dependence of the exclusive $J/\psi$ photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements.

 

Submitted to: EPJC
e-Print: arXiv:1809.03235 | PDF | inSPIRE

Figures

Figure 1

Mass distributions of selected dileptons for the dielectron (upper left) and dimuon (upper right) samples for the central analysis and dimuon samples for the semi-forward (lower left) and semi-backward (lower right) analyses. In all cases the data are represented by points with error bars The solid blue line is a fit to a Crystal-Ball function plus an exponential distribution, where this last contribution is shown by a dotted red line.
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Figure 2

Transverse momentum distributions of dileptons around the \jpsi mass for the dielectron (upper left) and dimuon (upper right) samples for the central analysis and dimuon samples for the semi-forward (lower left) and semi-backward (lower right) analyses. In all cases the data are represented by points with error bars. The blue, magenta (dash) and green (dash-dot-dot) lines correspond to Monte Carlo templates for \jpsi\ coming from exclusive photoproduction off protons or off lead and continuous dilepton production respectively. The red (dash-dot) line is a template for dissociative and hadronic background obtained from data. The solid black line is the sum of all contributions.
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Figure 3

(Upper panel) ALICE data (red symbols) on exclusive photoproduction of \jpsi off protons as a function of the centre-of-mass energy of the photon--proton system \Wgp, obtained in collisions of protons and lead nuclei at $\snn=5.02$ TeV, including results from~, compared to a power-law fit, to data from HERA, to the solutions from LHCb\ and to theoretical models (see text). The uncertainties are the quadratic sum of the statistical and systematic uncertainties. (Lower panel) Ratio of the models shown in the upper panel\ to the power law fit through the ALICE data points. The Bjorken x value corresponding to \Wgp is also displayed on the top of the Figure, see text for details.
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