Hadrons from ion electromagnetic dissociation break exclusivity conditions in ultraperipheral collisions, resolving tensions in exclusive muon-pair and coherent J/ψ measurements at the LHC.
Charmonium and $e^+e^-$ pair photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}$=2.76 TeV
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abstract
The ALICE Collaboration at the LHC has measured the J/$\psi$ and $\psi^{'}$ photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 $\mu\rm{b}^{-1}$. The cross section for coherent and incoherent J/$\psi$ production in the rapidity interval $-0.9<y<0.9$, are $\mathrm{d}\sigma_{{\rm J}/\psi}^{\rm coh}/\mathrm{d}y = 2.38^{+0.34}_{-0.24}\big(\mathrm{sta+sys}\big)$mb. and $\mathrm{d}\sigma_{{\rm J}/\psi}^{\rm inc}/\mathrm{d}y = 0.98^{+0.19}_{-0.17}\big(\mathrm{sta+sys}\big)$mb, respectively. The results are compared to theoretical models for J/$\psi$ production and the coherent cross section is found to be in good agreement with those models incorporating moderate nuclear gluon shadowing, such as EPS09 parametrization. In addition the cross section for the process $\gamma\gamma\rightarrow e^+ e^-$ has been measured and found to be in agreement with models implementing QED at leading order.
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ATLAS reports differential cross sections for coherent exclusive J/ψ → μ⁺μ⁻ in Pb+Pb ultraperipheral collisions at √s_NN=5.36 TeV, agreeing with models but in tension with prior Run 2 data at central rapidity.
ALICE reports new rapidity-differential cross sections for coherent J/ψ and ψ(2S) photoproduction and exclusive dimuons in forward rapidity UPC Pb-Pb collisions at 5.36 TeV, showing nuclear shadowing ratios of approximately 0.76 and 0.71.
A suppression factor R_G=0.85 is tuned to F2 and rho photoproduction data and used to predict observables for rho(2S), omega, and phi states via holographic wave functions in the dipole picture.
A review summarizing experimental evidence, theoretical models of nuclear shadowing, model-agnostic nuclear PDF extractions, and prospects for LHC and EIC studies.
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Measurement of coherent exclusive $J/\psi\to\mu^+\mu^-$ production in ultraperipheral Pb+Pb collisions at $\sqrt{s_{\textrm{NN}}}=5.36$ TeV with the ATLAS detector
ATLAS reports differential cross sections for coherent exclusive J/ψ → μ⁺μ⁻ in Pb+Pb ultraperipheral collisions at √s_NN=5.36 TeV, agreeing with models but in tension with prior Run 2 data at central rapidity.