A fresh look at diffractive $J/ψ$ photoproduction at HERA, with predictions for THERA

TL;DR

This work analyzes exclusive diffractive $J/\psi$ photoproduction within a QCD-improved dipole framework to quantify perturbative and non-perturbative effects via the universal cross section ${\hat{\sigma}}(b^2,x)$. It extends the model with a $b$-dependent scale, running charm mass, skewedness, energy-dependent slope, and a treatment of the real part of the amplitude, while enforcing unitarity taming at high energy. The study finds a significant hard contribution persists into THERA energies, implying gradual taming of the energy growth; skewedness contributes about a 10% cross-section enhancement, and the dominant uncertainties arise from the small-$x$ gluon distribution and PDF choices (CTEQ4L vs MRST). It also demonstrates that the balance between perturbative and non-perturbative physics is sensitive to the scaling parameter $\lambda$, with THERA predictions requiring precise knowledge of the gluon density and diffractive dynamics.

Abstract

We quantify perturbative and non-perturbative QCD effects in the exclusive $J/ψ$-photoproduction cross section, and in the shrinkage of the differential cross section with respect to momentum transfer, $t$. We predict that in the high energy THERA region there will always be a significant contribution to this process that rises quickly with energy. This implies that the taming of the rise of the cross section with energy, due to both the expansion of spatially-small fluctuations in the photon and to higher twist effects, is rather gradual.

Figures (16)

• Figure 1: A comparison of the $J/\psi$ photoproduction cross section, using conventional CTEQ4L parton distribution function (PDF) for the gluon and $\beta = 0$, with data fixedtargeth100bruniosaka. The solid curve has fixed mass and slope $B = 4.0$ GeV$^{-2}$, the long and short dashed curves include running charm quark mass and $W$-dependent slope, respectively. The dotted curve includes both effects.
• Figure 2: The ratio of running charm quark mass squared to fixed mass squared as a function of transverse size using eq.(\ref{['mqrun']}) and incorporating four light flavours, $n_{f} = 4$.
• Figure 3: The integrand multiplying ${\hat{\sigma}}$ for fixed and running charm quark mass. The running mass effect is implemented for $b < 0.3$ fm and causes a suppression of the small $b$ region.
• Figure 4: The effective momentum fraction at which the gluon is sampled, divided by $\delta$, for several values of the scaling parameter $\lambda$.
• Figure 5: The effective four-momentum scale at which the gluon is sampled for several values of the scaling parameter $\lambda$.