Measurement of Elastic $J/ψ$ Photoproduction at HERA

TL;DR

This study measures elastic J/ψ photoproduction in e+p collisions at HERA with the ZEUS detector, focusing on the W dependence, momentum-transfer distribution, and decay angular properties. Using a dipole-gluon-based Monte Carlo and careful background subtraction, the analysis yields a steep W rise characterized by δ ≈ 0.92 and an exponential t-slope around b ≈ 4.6 GeV^-2. The results favor perturbative QCD descriptions with gluon ladders over soft pomeron models, and the angular distributions confirm s-channel helicity conservation, implying transverse photon dominance. Together, these measurements constrain the small-x gluon density and the effective scales governing exclusive vector-meson photoproduction at high energies.

Abstract

The reaction gamma p -> J/Psi p has been studied in ep interactions using the ZEUS detector at HERA. The cross section for elastic J/Psi photoproduction has been measured as a function of the photon-proton centre of mass energy W in the range 40 < W < 140 GeV at a median photon virtuality Q^2 of 5*10^{-5} GeV^2. The photoproduction cross section, sigma_{gamma p -> J/Psi p}, is observed to rise steeply with W. A fit to the data presented in this paper to determine the parameter $δ$ in the form sigma_{gamma p -> J/Psi p} \propto W^δ yields the value δ= 0.92 \pm 0.14 \pm 0.10. The differential cross section dsigma/d|t| is presented over the range |t| < 1.0 GeV^2 where t is the square of the four-momentum exchanged at the proton vertex. dσ/d|t| falls exponentially with a slope parameter of 4.6 \pm 0.4 (+0.4-0.6) GeV^{-2}. The measured decay angular distributions are consistent with s-channel helicity conservation.

Figures (5)

• Figure 1: Schematic diagrams for diffractive $J/\psi$ electroproduction. (a) The mechanism for elastic vector meson production. (b) Proton dissociative $J/\psi$ photoproduction where the proton dissociates into a hadronic system of invariant mass $M_N$.
• Figure 2: (a) The mass distribution of the events in the electron pair sample. A clear peak at the $J/\psi$ mass is observed. The solid line shows the result of a fit in which a Gaussian resolution function has been convoluted with a radiative $J/\psi$ mass spectrum and added to a polynomial background. (b) The mass distribution for events in the muon pair sample. The solid line shows the result of a fit in which a Gaussian resolution function has been added to a flat background function. For both the electron and muon channels the contribution of events from the Bethe-Heitler process is shown as the hatched area.
• Figure 3: The elastic $J/\psi$ photoproduction cross section as a function of $W$. (a) Shows the results of this analysis. The error bars represent the statistical and systematic errors added in quadrature. The solid line shows the result of the fit to the data using the expression $\sigma_{\gamma p \rightarrow J/\psi p} \propto W^{\delta}$. As described in the text the value $\delta = 0.92 \pm 0.14 \pm 0.10$ was obtained. The dashed line shows the prediction of a soft pomeron model Ref:DL in which $\delta \approx 0.22$. (b) The results of this analysis (solid circles) are compared to data from H1, ZEUS and the results of lower energy measurements Ref:FixedTrgtJPsiRef:E401. The result of a pQCD calculation Ref:RyskinRoberts in which the MRS-A$^{\prime}$Ref:MRSG parton distributions have been used is shown as the solid line. The result of the calculation presented in Ref:Kaidalov is shown by the dotted line. The result of the calculation presented in Ref:Paccanoni is shown by the long dash dotted line.
• Figure 4: The distribution of transverse momentum squared for $J/\psi$ produced in the reaction $\gamma p \rightarrow J/\psi p$ in the kinematic range $40 < W < 140$ GeV. (a) The differential cross section ${d\sigma}/{dp_{T J/\psi}^2}$. The data are shown as the points and the result of the exponential fit in the range $p_{T}^2<1$ GeV$^2$ is shown as the solid line. (b) The correction factor, $F$, required to obtain the $|t|$ distribution from the $p^{2}_{T J/\psi}$ distribution by accounting for the $Q^2$ of the photon. (c) The differential cross section ${d\sigma}/{d|t|}$. The result of the exponential fit in the range $|t|<1$ GeV$^2$ is shown as the solid line. In (a) and (c) the inner error bars represent the statistical and decay-channel-specific errors added in quadrature, the outer ones statistical, decay-channel-specific errors and common systematic errors added in quadrature.
• Figure 5: Acceptance corrected decay angular distributions for the $J/\psi$ in the reaction $e p \rightarrow e J/\psi p$ in the kinematic range $40 < W < 140$ GeV. No subtraction of the proton dissociative contribution to the sample has been made for the data presented in this figure since the angular dependence of the proton dissociative and elastic $J/\psi$ production is assumed to be the same. The curves are the results of the fits described in the text. The error bars represent the statistical, decay-channel-specific errors and common systematic errors added in quadrature.