The ALICE Collaboration reports three measurements in ultra-peripheral proton$-$lead collisions at forward rapidity. The exclusive two-photon process \ggmm and the exclusive photoproduction of J/$\psi$ are studied. J/$\psi$ photoproduction with proton dissociation is measured for the first time at a hadron collider. The cross section for the two-photon process of dimuons in the invariant mass range from 1 to 2.5 GeV/$c^2$ agrees with leading order quantum electrodynamics calculations. The exclusive and dissociative cross sections for J/$\psi$ photoproductions are measured for photon$-$proton centre-of-mass energies from 27 to 57 GeV. They are in good agreement with HERA results.
Phys. Rev. D 108, 112004 (2023)
HEP Data
e-Print: arXiv:2304.12403 | PDF | inSPIRE
CERN-EP-2023-059
Figure group
Figure 1
Transverse momentum distribution of opposite-sign dimuons with $1.5 < M_{\mu\mu} < 2.0$ GeV/$c^2$ and $2.5 < y < 4.0$. The data are represented by full circles with vertical bars for the statistical uncertainties and horizontal bars for the interval width. The solid line represents the fit to the data, and the dashed and dot-dashed lines represent the fit components. |  |
Figure 3
Left: dimuon invariant mass distribution using the selection given in Sec. 3.2, fitted with a two-component model to separate J/$\psi$ events from two-photon interactions in the continuum. Right: $\pt$ distribution of the exclusive $\gamma \gamma \rightarrow \mu\mu$ continuum extracted using the sPlot technique. The distribution is fitted with a Landau distribution. |   |
Figure 5
Differential cross sections for exclusive $\gamma \gamma \rightarrow \mu\mu$ production measured by ALICE in p$-$Pb UPCs at $\sqrt{s_{\rm NN}}$ = 8.16 TeV, as a function of $M_{\mu\mu}$, for $2.5 < y < 3.25$ (left) and $3.25 < y < 4$ (right). The vertical error bars represent the statistical and systematic uncertainties summed in quadrature. The results are compared with the prediction from STARlight and from SuperChic. |   |
Figure 6
Exclusive J/$\psi$ photoproduction cross section off protons measured as a function of the centre-of-mass energy of the photon--proton system $W_{\gamma {\rm p}}$ by ALICE in p$-$Pb UPCs and compared with previous measurements and with next-to-leading-order JMRT and CCT models. The power-law fit to the ALICE data is also shown. The uncertainties of the data points are the quadratic sum of the statistical and systematic uncertainties. |  |
Figure 7
Dissociative J/$\psi$ photoproduction cross section off protons measured by ALICE in p$-$Pb UPCs at $\sqrt{s_{\rm NN}}$ = 8.16 TeV and compared with H1 data. A comparison with the CCT model is shown. The uncertainties of the data points are the quadratic sum of the statistical and systematic uncertainties. |  |
Figure 8
Ratio of dissociative-to-exclusive J/$\psi$ photoproduction cross sections measured by ALICE in p$-$Pb UPCs at $\sqrt{s_{\rm NN}}$ = 8.16 TeV and compared with H1 measurements. The uncertainties of the data points are the quadratic sum of the statistical andsystematic uncertainties. The experimental uncertainties for the H1 data are computed assuming completely independent uncertainties for the exclusive and dissociative cross sections. The measurements are compared with the CCT model and a model by Mäntysaari--Schenke (MS). The uncertainty band of the MS model corresponds to the statistical uncertainty of the calculation. |  |