Abstract
We make predictions for the cross sections of diffractive dijet photoproduction in $pp$, $pA$ and $AA$ ultraperipheral collisions (UPCs) at the LHC during Runs 1 and 2 using next-to-leading perturbative QCD. We find that the resulting cross sections are sufficiently large and, compared to lepton-proton scattering at HERA, have an enhanced sensitivity to small observed momentum fractions in the diffractive exchange, commonly denoted $z_{P}^{\rm jets}$, and an unprecedented reach in the invariant mass of the photon-nucleon system $W$. We examine two competing schemes of diffractive QCD factorization breaking, which assume either a global suppression factor or a suppression for resolved photons only and demonstrate that the two scenarios can be distinguished by the nuclear dependence of the distributions in the observed parton momentum fraction in the photon $x_{\gamma}^{\rm jets}$.
Highlights
Pb-Pb collisions at sNN = 2.76 TeV by the ALICE collaboration [10,11,12]
A comparison of our predictions for pA ultraperipheral collisions (UPCs) shown in figures 8 and 9 to those for pp UPCs shown in figures 4 and 5 demonstrates that the general trends for the dependence of the cross section on various variables are similar in the pp and pA cases: very roughly, the pA results can be obtained from the pp ones by multiplying them by the scaling factor of (1/2)Z2(0.7 fm/RA) ≈ 350, where took into account that the photon spectra of the proton and a nucleus extend up to x ∼ 1/bmin = 1/(0.7 fm) and bmin ∼ 1/RA, respectively, and that in the pA UPC cross section, the contribution of the minus-term in eq (3.1) is generally small
Using general kinematic conditions and cuts on the final state, we found that the values of the cross section as a function of various variables are sufficiently large, i.e., this process can be observed
Summary
2.1 General expression for the cross section The mechanism of diffractive dijet photoproduction in ultraperipheral collisions of relativistic ions A and B is illustrated in figure 1. Where a and b are parton flavors; fγ/p(y) is the flux of equivalent photons of the proton, which depends on the photon light-cone momentum fraction y; fa/γ(xγ, μ2) is the parton distribution function (PDF) of the photon, which depends on the parton light-cone momentum fraction xγ and the factorization scale μ; fbD/p(4)(xP, zP, t, μ2) is the diffractive PDF of the proton; dσa(nb→) jets is the elementary cross section for the production of an n-parton final state in the interaction of partons a and b; and the sum over a involves quarks and gluons (the resolved photon contribution) and the direct photon contribution with a = γ, which has support at LO only at xγ = 1. Where the sum jets runs over the hadronic final states labeled “i”, which are included in the jets
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