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

International audienceThe ALICE Collaboration has measured the energy dependence of exclusive photoproduction of $\mathrm {J}/\psi $ vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_\mathrm{NN}} = 5.02$ TeV. The e$^+$ e$^-$ and $\mu ^+\mu ^-$ decay channels are used to measure the cross section as a function of the rapidity of the $\mathrm {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}}