The exclusive J/ $$\psi $$ ψ process at the LHC tamed to probe the low x gluon

Abstract

The perturbative QCD expansion for J/ψ photoproduction appears to be unstable: the NLO correction is large (and of opposite sign) to the LO contribution. Moreover, the predictions are very sensitive to the choice of factorisation and renormalisation scales. Here we show that perturbative stability is greatly improved by imposing a 'Q 0 cut' on the NLO coefficient functions; a cut which is required to avoid double counting. Q 0 is the input scale used in the parton DGLAP evolution. This result opens the possibility of high precision exclusive J/ψ data in the forward direction at the LHC being able to determine the low x gluon distribution at low scales.

Strong Interactions - Experimental Overview

“…These measurements extend the W reach by an order of magnitude compared to the measurements performed at HERA. For the exclusive J/ψ production, the results agree with the predictions of the NLO calculations based on the γIP fusion in which the Pomeron consists of two gluons [6], while a power-law behavior constrained by the H1 data [8] predicts too high a cross section. The ratio of ψ(2S) to J/ψ production cross sections, inferred from the LHCb measurements, seems to be W independent contrary to the trend observed in the HERA data [9] of this ratio increasing with W .…”

“…The same processes can be also studied in pp interactions as the protons are not only a source of a IP flux but also of photons. In particular, the exclusive production of heavy vector mesons in pp interactions at the LHC has been advocated as a potential way to constrain the low-x gluon content of the proton [6]. The LHCb experiment presented measurements of exclusive J/ψ and exclusive ψ(2S) production at 7 and 13 TeV as a function of the γ * IP invariant mass W [7].…”

Precision Determination of the Small- x Gluon from Charm Production at LHCb

“…It is important to explore these implications further to inform other applications, such as the modeling of semihard QCD processes at the LHC in Monte Carlo event generators and for calculations of cosmic ray production. Moreover, it would be interesting to compare our determination of the small-x gluon with those that could be achieved from other processes with similar kinematical coverage, such as exclusive production [49] or forward photon production [50,51].…”

Evidence of quasi-partonic higher-twist effects in deep inelastic scattering at HERA at moderate $$Q^2$$ Q 2

“…The higher twist interpretation advocated in this paper should be also confronted with recent interesting results obtained for the vector meson production [43], where power corrections to the coefficient functions in high energy scattering were derived, that do not come from the leading quasipartonic 4-gluon twist 4 exchange. 5 Although we agree that such corrections should also contribute to the DIS cross sections, we stress that the x-dependence of the twist 4 corrections (i.e.…”