Abstract
Based on accurate calculations of the flux of equivalent photons of the proton and heavy nuclei and the pQCD framework for the gluon distribution in the proton and nuclei, we analyze the rapidity and momentum transfer distributions of coherent J/ψ photoproduction in ultraperipheral proton-Pb collisions at the LHC. We demonstrate that unlike the case of proton-proton UPCs marred by certain theoretical uncertainties and experimental limitations, after a cut excluding the region of small momentum transfers, ultraperipheral proton-Pb collisions offer a clean way to study the gluon distribution in the proton down to x ≈ 10−5. Our analysis of the momentum transfer distributions shows that an interplay of J/ψ production by low-energy photons on the nucleus and by high-energy photons on the proton in proton-Pb UPCs can result in some excess of events at small p t in a definite region of the rapidity y.
Highlights
Our analysis of the momentum transfer distributions shows that an interplay of J/ψ production by low-energy photons on the nucleus and by high-energy photons on the proton in proton-Pb UPCs can result in some excess of events at small pt in a definite region of the rapidity y
In obtaining the results presented below, we used the exact results for the photon flux of the proton, Nγ/p(ω) [eq (2.1) and the red solid curve in figure 2] and for the photon flux of Pb, Nγ/P b(ω) [eqs. (2.3)–(2.5) and the red solid curve in figure 3] and the results of our leading order (LO) pQCD analysis of exclusive J/ψ photoproduction on the proton and nuclei [8]
One expects that the photon-proton contribution should by far dominate the photon-nucleus one because (i) the nuclear enhancement of the photon flux emitted by the nucleus is much stronger than that of the t-integrated coherent photon-nucleus cross section and (ii) because nuclear shadowing suppresses the photon-nucleus cross section
Summary
Equation (1.1) allows one to calculate the rapidity distribution of J/ψ photoproduction in proton-nucleus UPCs. In obtaining the results presented below, we used the exact results for the photon flux of the proton, Nγ/p(ω) [eq (2.1) and the red solid curve in figure 2] and for the photon flux of Pb, Nγ/P b(ω) [eqs. One should note that with the considered beam directions, the LHCb detector, which covers the 2 < y < 4.5 range, can measure photoproduction on the proton by high-energy photons emitted by lead and, can access the gluon distribution in the proton down to xp ≈ 10−5.
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