Elastic and Inelastic Photoproduction of \jpsi Mesons at HERA

TL;DR

This study presents comprehensive J/ψ photoproduction measurements at HERA in the Q^2 ≈ 0 regime, separating elastic, proton-dissociative, and inelastic channels using H1 data. It tests diffractive mechanisms (pomeron exchange) against perturbative QCD predictions (Ryskin) by analyzing cross sections, p_t^2 distributions, and decay angular distributions across W_{γp} up to 150 GeV, and compares elastic results with ZEUS and fixed-target data. The elastic cross section shows a strong energy rise, σ_{γp} ∝ W_{γp}^{0.64–0.90}, consistent with gluon-density-driven predictions (MRSA') in perturbative QCD; proton-dissociation has a similar magnitude and a steeper energy dependence, while inelastic J/ψ production agrees with NLO color-singlet calculations under strong kinematic cuts, constraining low-x gluon densities. A first ψ′ diffractive measurement yields a ψ′/J/ψ ratio of about 0.20, reinforcing the diffractive picture at HERA energies. These results together advance understanding of diffractive mechanisms and the gluon content of the proton at low x, with implications for perturbative QCD descriptions of heavy quarkonia production.

Abstract

Results on \jpsi production in $e p$ interactionsin the H1 experiment at HERA are presented. The \jpsi mesons are produced by almost real photons ($Q^2\approx 0$) and detected via their leptonic decays. The data have been taken in 1994 and correspond to an integrated luminosity of $2.7 {pb}^{-1}$. The $γp$ cross section for elastic \jpsi production is observed to increase strongly with the \cm energy. The cross section for diffractive $J/ψ$ production with proton dissociation is found to be of similar magnitude as the elastic cross section. Distributions of transverse momentum and decay angle are studied and found to be in accord with a diffractive production mechanism. For inelastic \jpsi production the total $γp$ cross section, the distribution of transverse momenta, and the elasticity of the \jpsi are compared to NLO QCD calculations in a colour singlet model and agreement is found. Diffractive \psiprime production has been observed and a first estimate of the ratio to \jpsi production in the HERA energy regime is given.

Figures (9)

• Figure 1: $J/\psi$ production mechanisms: $(a)$ elastic $J/\psi$ production via pomeron exchange; $(b)$ diffractive proton dissociation; $(c)$ elastic $J/\psi$ production in perturbative QCD: exchange of a gluon ladder Ryskin; $(d)$ photon gluon fusion model for inelastic $J/\psi$ production (colour singlet model Berger_Jones).
• Figure 2: Mass distribution for $\mu^+\mu^-$ (a) and $e^+e^-$ pairs (b) for the two track selection above $2 \,\hbox{GeV}$. The curves are fits of a Gaussian plus a polynomial background to the $J/\psi$ mass region. The shaded histogram shows the contribution of QED lepton pairs. For $\mu^+\mu^-$ the maximum of the fit is at $(3.10 \pm 0.01)\,\hbox{GeV}$ with a width of 76 MeV. For $e^+e^-$ the maximum is at $(3.08 \pm 0.02)\,\hbox{GeV}$ and the width is 77 MeV. For both cases the detector simulation yields a width of 65 MeV.
• Figure 3: a) Total cross section for elastic$J/\psi$ photoproduction. The inner error bars of the HERA points are statistical, the outer ones contain statistical and systematic errors added in quadrature. The data at lower cms energies are from previous experiments de687ee40182fftps; they were corrected with the new $J/\psi$ decay branching ratio where necessary and include systematic errors (added in quadrature). A parametrisation of the energy dependence as $W_{\gamma p}^{\delta}$ with $\delta=0.90$ (full curve) and $\delta=0.32$ (long dashed) normalized to the E 516 point is shown. Also shown is the result of calculations according to the Ryskin model Ryskin_et_al using the MRSA' (dashed) and GRV parametrisations (dash dotted) of the gluon density. b) Total cross section for $J/\psi$-production with proton dissociation. The inner error bars of the H1 points are statistical, the outer ones contain statistical and systematic errors added in quadrature. The full line represents a fit to $W_{\gamma p}^{\delta}$ with $\delta = 1.2 \pm 0.2$.
• Figure 4: a)$d\sigma/dp_{t}^2$ for elastic$J/\psi$ production integrated over $30\,\hbox{GeV}\leq W_{\gamma p} \leq 150\,\hbox{GeV}$. The error bars contain statistical and systematic errors added in quadrature. The straight line is a fit to the data in the range $p_t^2\leq 1 \,\hbox{GeV}^2$ of the form $\exp(-bp_t^2)$ with $b=4.0\pm0.3~\,\hbox{,GeV$^{-2}$}$. The histogram shows the $p_{t}^2$ distribution of the DIFFVM Monte Carlo. b)$d\sigma_{\gamma p}/dp_{t}^2$ for $J/\psi$ meson production with proton dissociation integrated over $30\,\hbox{GeV}\leq W_{\gamma p} \leq 150\,\hbox{GeV}$. The straight line is an exponential fit to the data in the range $p_t^2\leq 1 \,\hbox{GeV}^2$ which yields a value of $b=1.6\pm0.3~\,\hbox{,GeV$^{-2}$}$, the dashed line represents the DIFFVM Monte Carlo calculation.
• Figure 5: $d\sigma/d\cos\theta^*$ for diffractive $J/\psi$ production (elastic and proton dissociation). The inner error bars are statistical, the outer ones contain statistical and systematic errors added in quadrature. The curve is a fit of the form $\propto (1+\hbox{const}\,\cos^2\theta^*)$.