Elastic Photoproduction of J/psi and Upsilon Mesons at HERA

TL;DR

This paper reports extended measurements of elastic photoproduction of J/ψ and Υ mesons at HERA, revealing a steep W_γp rise for J/ψ that supports perturbative QCD descriptions with gluon densities such as CTEQ4M and MRSR2 and a two-pomeron mixing framework. The t-dependent analysis yields a finite slope and an effective Regge trajectory α(t) that sits between soft and hard pomeron expectations, with a slope compatible with zero and an intercept consistent with NLO BFKL predictions. Υ production is observed and found to be in reasonable agreement with pQCD predictions once real-part and off-diagonal parton effects are included. Collectively, the results bolster the perturbative description of heavy vector-meson photoproduction and provide a direct, single-experiment extraction of the Regge trajectory for γp → J/ψ p.

Abstract

Cross sections for elastic photoproduction of J/Psi and Upsilon mesons are presented. For J/Psi mesons the dependence on the photon-proton centre-of-mass energy W_gammap is analysed in an extended range with respect to previous measurements of 26<=W_gammap<= 285 GeV. The measured energy dependence is parameterized as sigma_gammap proportional W_gammap^delta with delta=0.83+-0.07. The differential cross section dsigma/dt for J/Psi mesons is derived, its dependence on W_gammap and on t is analysed and the effective trajectory (in terms of Regge theory) is determined to be alpha(t)=(1.27+-0.05)+(0.08+-0.17)*t/GeV^2. Models based on perturbative QCD and on pomeron exchange are compared to the data.

Figures (6)

• Figure 1: Mass spectra for the four $W_{\gamma p}$ regions of the elastic $J/\psi$ selection: a) $\mu^+\mu^-$ pairs in the central region, b) and c) $e^+e^-$ pairs in the backward region, d) $\mu^+\mu^-$ pairs in the forward region. A fit of the signal region as described in the text (full line) and the simulated mass spectra from QED background processes (shaded histograms) are shown.
• Figure 2: a) The cross section $\sigma (\gamma p \rightarrow J/\psi p)$ versus $W_{\gamma p}$ from this analysis and previous HERA results. The inner and outer error bars show the statistical and total errors, respectively. The full line represents a fit of the form $\sigma_{\gamma p}$$\propto W_{\gamma p}^\delta$ to the present H1 data, yielding $\delta=0.83\pm0.07$ with a $\chi^2/\mathit{dof}=4.6/13$. QCD predictions by Frankfurt et al. F_K_S97 using various parameterizations of the gluon density in the proton and the charm quark mass of $1.48\:\hbox{GeV}$ as chosen in F_K_S97 are also shown. b) The same HERA data as in a) and results from fixed-target experiments with a fit of the two-pomeron model by Donnachie and Landshoff (full line; $\chi^2/\mathit{dof}=16.1/22$). Only the contributions of the soft and hard pomeron amplitudes were adjusted allowing for mixing 2pom. The separate contributions are indicated.
• Figure 3: The differential cross section $d\sigma/dt$ for elastic $J/\psi$ photoproduction, as a function of $|t|$, averaged over the energy region $40\leq W_{\gamma p}\leq 150\:\hbox{GeV}$. Only statistical errors are given. Also shown is the result of a fit (dashed line; $\chi^2/\mathit{dof}=4.7/6$) to the data which includes the three different contributions described in the text. The solid curve shows the fit result for elastic $J/\psi$ production assuming an exponential $t$ dependence.
• Figure 4: The differential cross section $d\sigma/dt$ as a function of $W_{\gamma p}$ in five bins of $t$ together with a fit of the form $d\sigma/dt=N\cdot(W_{\gamma p}/W_0)^{4(\alpha(t)-1)}$ (solid line). The inner error bars on the data points show the dominating statistical errors and the outer bars the total error. The predictions of the soft and hard Donnachie-Landshoff pomeron trajectories 2pom are shown as dash-dotted and dashed lines. The prediction based on a NLO BFKL calculation bfklpom is given as a dotted line. All theoretical curves are normalized to the fit at $W_0=90\,$GeV.
• Figure 5: The measured Regge trajectory $\alpha(t)=\alpha_0+\alpha^\prime t$ for the process $\gamma p \rightarrow J/\psi\,p$. The solid line shows the result of the fit. The one standard deviation contour is indicated by a shaded band. Also shown are the soft and the hard Donnachie-Landshoff pomeron trajectories 2pom and a result based on a NLO BFKL calculation bfklpom.