Coherent photoproduction of vector mesons in heavy ion ultraperipheral collisions: Update for run 2 at the CERN Large Hadron Collider

TL;DR

The paper provides Run 2 predictions for coherent vector-meson photoproduction in Pb-Pb ultraperipheral collisions at √s_NN = 5.02 TeV, extending prior 2.76 TeV insights. It blends a modified vector-meson dominance with Gribov–Glauber shadowing for light mesons and LO pQCD with leading-twist gluon shadowing for J/ψ and ψ(2S), with NLO gluons needed for Υ to match data, and incorporates neutron-tagged channels to access higher photon energies. The study finds strong inelastic nuclear shadowing for ρ/φ and substantial gluon shadowing for heavy quarkonia, with the ψ(2S)/J/ψ ratio largely determined by the proton-level cross sections; it also shows that forward-neutron tagging (0nXn/XnXn) enables probing smaller x values by accessing larger W_γp. Electromagnetic excitations thus broaden the kinematic reach and provide a crucial testbed for nuclear gluon distributions in nuclei at small x.

Abstract

We make predictions for the cross sections of coherent photoproduction of $ρ$, $φ$, $J/ψ$, $ψ(2S)$, and $Υ(1S)$ mesons in Pb-Pb ultraperipheral collisions (UPCs) at $\sqrt{s_{NN}}=5.02$ TeV in the kinematics of run 2 at the Large Hadron Collider extending the approaches successfully describing the available Pb-Pb UPC data at $\sqrt{s_{NN}}=2.76$ TeV. Our results illustrate the important roles of hadronic fluctuations of the photon and inelastic nuclear shadowing in photoproduction of light vector mesons on nuclei and the large leading twist nuclear gluon shadowing in photoproduction of quarkonia on nuclei. We show that the ratio of $ψ(2S)$ and $J/ψ$ photoproduction cross sections in Pb-Pb UPCs is largely determined by the ratio of these cross sections on the proton. We also argue that UPCs with electromagnetic excitations of the colliding ions followed by the forward neutron emission allows one to significantly increase the range of photon energies accessed in vector meson photoproduction on nuclei.

Figures (8)

• Figure 1: The $\gamma p \to \phi p$ cross section as a function of $W_{\gamma p}$. The fixed-target Egloff:1979mgBusenitz:1989gq and HERA Derrick:1996afBerger_thesis data vs. the Donnachie--Landshoff (DL) model Donnachie:1994zb.
• Figure 2: The $d \sigma_{Pb Pb \to \rho Pb Pb}(y)/dy$ cross section as a function of the $\rho$ meson rapidity $y$ at $\sqrt{s_{NN}}=5.02$ TeV. Predictions of the combination of the modified VMD and GG models (mVMD-GGM) are shown for the four considered reaction channels. The solid curves correspond to the calculation using the nominal value of $\omega^{\rho}_{\sigma}$; the shaded areas show the theoretical uncertainty in modeling of this quantity. The dashed curves labeled "one-side" show the contribution of the first term in Eq. (\ref{['eq:cs_upc']}).
• Figure 3: The $d \sigma_{Pb Pb \to \phi Pb Pb}(y)/dy$ cross section as a function of the $\phi$ meson rapidity $y$ at $\sqrt{s_{NN}}=5.02$ TeV. For notation, see Fig. \ref{['fig:rho']}.
• Figure 4: The $d \sigma_{Pb Pb \to J/\psi Pb Pb}(y)/dy$ cross section as a function of the $J/\psi$ rapidity $y$ at $\sqrt{s_{NN}}=5.02$ TeV: predictions of LO pQCD with the gluon shadowing ratio $R_g(x,\mu^2)$ of the leading twist nuclear shadowing model. The shaded areas span the range of predictions corresponding to the upper and lower limits on $R_g(x,\mu^2)$.
• Figure 5: The $d \sigma_{Pb Pb \to J/\psi Pb Pb}(y)/dy$ cross section as a function of the $J/\psi$ rapidity $y$ at $\sqrt{s_{NN}}=5.02$ TeV: predictions of LO pQCD with the EPS09 gluon shadowing ratio $R_g(x,\mu^2)$. The shaded areas show the uncertainties of $R_g(x,\mu^2)$. The dashed curves labeled "one-side" show the contribution of the first term in Eq. (\ref{['eq:cs_upc']}).