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-$x$.

 

Submitted to: PLB
e-Print: arXiv:2101.04623 | PDF | inSPIRE
CERN-EP-2021-003

Figure 1

< p>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.< /p>

Figure 1

< p>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.< /p>