Elastic J/Psi Production at HERA

TL;DR

Elastic J/ψ production in ep collisions at HERA is analyzed across photoproduction and electroproduction to test Regge-based and perturbative QCD descriptions. The study finds a hard QCD-like W_{γp} dependence and a Q^2 dependence consistent with σ ∝ (M_ψ^2+Q^2)^{-n}, with n ≈ 2.49, and observes a t-dependent slope b that shrinks with increasing |t| but shows shrinkage behavior weaker than soft Pomeron expectations. The data are well described by pQCD models (notably MRT) with strong sensitivity to the gluon density at low x, and the angular analyses support s-channel helicity conservation, yielding R = σ^L/σ^T in line with theory. Overall, the results constrain gluon distributions and generalized parton densities in a low-x, low-Q^2 regime and reinforce the applicability of perturbative approaches to diffractive vector-meson production.

Abstract

Cross sections for elastic production of J/Psi mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb^{-1}. Results are presented for photon virtualities Q^2 up to 80 GeV^2. The dependence on the photon-proton centre of mass energy W_{gamma p} is analysed in the range 40 < \Wgp < 305 GeV in photoproduction and 40 < \Wgp < 160 GeV in electroproduction. The \Wgp dependences of the cross sections do not change significantly with Q^2 and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q^2. Differential cross sections dσ/dt, where t is the squared four-momentum transfer at the proton vertex, are measured in the range |t|<1.2 GeV^2 as functions of \Wgp and Q^2. Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/Psi production and decay angular distributions are consistent with s-channel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q^2 and is found to be described by perturbative QCD based models.

Figures (14)

• Figure 1: Elastic $J/\psi$ production, a) in an approach based on Pomeron (I P) exchange and b) in a pQCD approach via two gluon exchange. The kinematic variables are indicated in a).
• Figure 2: The dilepton invariant mass distributions (data and fits) in the four kinematic regions defined in table \ref{['tab:sel']}.
• Figure 3: Observed event distributions for the four data sets (points) defined in table \ref{['tab:sel']}, omitting the forward detector cuts against proton dissociative events. The first two rows correspond to the selected $J/\psi \to\mu^+\mu^-$ candidates where the small non-resonant background has been subtracted. The data are shown with the elastic (signal) simulations (DIFFVM el., white area) and proton dissociation MC (DIFFVM pdiss., shaded area). Rows three and four correspond to $J/\psi \to e^+e^-$ candidates, where the non-resonant background is not subtracted. Here, in addition to the elastic and proton dissociative $J/\psi$ simulations the contributions from $\gamma \gamma \rightarrow e^+e^-$ (LPAIR) and Compton scattering (COMPTON) are shown. The normalisations are obtained from a fit of the overall mass peak of each data set. The variables $W_{\gamma p}$ , $Q^2$, $t$ and $p^2_{t,\psi}$ are defined in the text. $\theta_\mu$ refers to the decay muons of data set II. In row three $\theta_{1}$ and $\theta_{2}$ refer to the decay electrons which are selected in different polar angular regions. In row four $E_{1}$ and $E_2$ refer to the energies of the decay electrons.
• Figure 4: a) Total cross section for elastic $J/\psi$ production as a function of $Q^2$ in the range $|t|<1.2\,\mathrm{GeV}^2$ at $W_{\gamma p} =90\,\mathrm{GeV}$. The inner error bars show the statistical errors, while the outer error bars show the statistical and systematic uncertainties added in quadrature. The solid line is a fit to the H1 data of the form $\sigma_{\gamma p }\propto (M_\psi^2+Q^2)^{-n}$. Data from the ZEUS experiment Chekanov:2002xiChekanov:2004mw are also shown. b) The ratio of the MRT calculations Martin:1999wb to the fit from a). The MRT QCD predictions are based on different gluon distributions Pumplin:2002vwMartin:2001esAdloff:2003uhChekanov:2002pv. The curves are individually normalised to the measurements across the complete $Q^2$ range yielding factors between 1.5 and 2.8. The shaded band represents the uncertainty of the fit result.
• Figure 5: a) Total cross sections for elastic $J/\psi$ production as a function of $W_{\gamma p}$ in the range $|t|<1.2\,\mathrm{GeV}^2$ in photoproduction. The inner error bars show the statistical errors, while the outer error bars show the statistical and systematic uncertainties added in quadrature. The solid line shows a fit to the H1 data of the form $\sigma\propto W_{\gamma p} ^{\delta}$. Results from the ZEUS experiment Chekanov:2002xi in a similar kinematic range are also shown. b) The ratio of theoretical predictions to the fit to the H1 data in a). The shaded band represents the uncertainties of the fit result. Predictions from MRT QCD calculations Martin:1999wb and a dipole model (FMS, Frankfurt:2000ez) based on different gluon distributions Pumplin:2002vwMartin:2001esAdloff:2003uhChekanov:2002pvLai:1996mg are shown. For the MRT curves the normalisation factors determined from the $Q^2$ distributions are used. The FMS prediction is normalised to the fit result at $W_{\gamma p} =90\,\mathrm{GeV}$.