The first measurement of the cross section for coherent J/$\psi$ photoproduction as a function of $|t|$, the square of the momentum transferred between the incoming and outgoing target nucleus, is presented. The data were measured with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV with the J/$\psi$ produced in the central rapidity region $|y|<~0.8$, which corresponds to the small Bjorken-$x$ range $(0.3-1.4) \times 10^{-3}$. The measured $|t|$-dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky-Kovchegov equation. These new results are therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken-$x$.
PLB 817 (2021) 136280
HEP Data
e-Print: arXiv:2101.04623 | PDF | inSPIRE
CERN-EP-2021-003
Figure group
Figure 1
Invariant-mass distributions for different \pttwo intervals with the global fit described in the text shown with the blue line. The exponential part of the fit model, representing the $\gamma\gamma\to\mu^+\mu^-$ background, is shown in red. |       |
Figure 2
Dependence on \mant of the photonuclear cross section for the coherent photoproduction of $\Jpsi$ off Pb compared with model predictions (top panel), where for LTA the {\em low shadowing} case is shown (see text). Model to data ratio for each prediction in each measured point (bottom panel). The uncertainties are split to those originating from experiment and to those originating from the correction to go from the UPC to the photonuclear cross section. |  |