First measurement of the ${\mathbf{|\textit t|}}$-dependence of coherent $\mathbf{\rm{J/ψ}}$ photonuclear production

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The first measurement of the dependence on $|t|$, the square of the momentum transferred between the incoming and outgoing target nucleus, of coherent J/$\psi$ photoproduction 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 $(0.3-1.4)\times10^{-3}$. 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. This new observable is therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken-$x$.

 

Accepted by: PLB
HEP Data
e-Print: arXiv:2101.04623 | PDF | inSPIRE
CERN-EP-2021-003

Figure 2

< p>Dependence on \mant of the photonuclear cross section for the coherent photoproduction of $\Jpsi$ off Pb compared with model predictions~ (top panel). 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.< /p>