Coherent J/$ψ$ photoproduction at forward rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$ TeV

The ALICE collaboration performed the first rapidity-differential measurement of coherent J/$\psi$ photoproduction in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV. The J/$\psi$ is detected via its dimuon decay in the forward rapidity region ($-4.0 <~ y <~ -2.5$) for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 750 $\mu$b$^{-1}$. The cross section for coherent J/$\psi$ production is presented in six rapidity bins. The results are compared with theoretical models for coherent J/$\psi$ photoproduction. These comparisons indicate that gluon shadowing effects play a role in the photoproduction process. The ratio of $\psi'$ to J/$\psi$ coherent photoproduction cross sections was measured and found to be consistent with that measured for photoproduction off protons.


Phys.Lett. B798 (2019) 134926
HEP Data
e-Print: arXiv:1904.06272 | PDF | inSPIRE

Figure 1

Left: Invariant mass distribution for muon pairs satisfying the event selection described in the text. The dashed green line corresponds to the background. The solid magenta and red lines correspond to Crystal Ball functions representing \jpsi and \psip signals, respectively. The solid blue line corresponds to the sum of background and signal functions. Right: transverse momentum distribution for muon pairs in the range $2.85 < m_{\mu\mu} < 3.35$ GeV/$c^{2}$(around the \jpsi mass).

Figure 4

Measured coherent differential cross section of \jpsi photoproduction in ultra-peripheral \PbPb collisions at \fivenn. The error bars represent the statistical uncertainties, the boxes around the points the systematic uncertainties. The theoretical calculations  described in the text are also shown. The green band represents the uncertainties of the EPS09 LO calculation.