Exclusive photoproduction of J/psi mesons at HERA

TL;DR

This ZEUS study measures exclusive photoproduction of J/ψ mesons in ep collisions at HERA over $W$ from $20$ to $290$ GeV and $|t|$ up to $1.8$ GeV$^2$, using decays to $\mu^+\mu^-$ and $e^+e^-$. It finds that $s$-channel helicity conservation holds within $2\sigma$ and that the cross section rises steeply with $W$, consistent with perturbative QCD expectations tied to gluon-density growth. The differential cross section $d\sigma/dt$ is well described by an exponential with a slope $b(W)$ that increases with $W$, allowing a direct extraction of the Pomeron trajectory: $\alpha_{I\!P}(0) = 1.200\pm0.009\;\text{(stat)}^{+0.004}_{-0.010}\,(\syst)$ and $\alpha'_{I\!P} = 0.115\pm0.018\;\text{(stat)}^{+0.008}_{-0.015}\,(\syst)\;\mathrm{GeV}^{-2}$. The results, in agreement with several pQCD-based models, challenge the soft-Pomeron picture and provide important constraints on gluon PDFs and diffractive dynamics in exclusive vector meson production.

Abstract

The exclusive photoproduction of J/psi mesons, gamma p->J/psi p, has been studied in ep collisions with the ZEUS detector at HERA, in the kinematic range 20<W<290 GeV, where W is the photon-proton centre-of-mass energy. The J/psi mesons were reconstructed in the muon and the electron decay channels using integrated luminosities of 38 pb^-1 and 55 pb^-1, respectively. The helicity structure of J/psi production shows that the hypothesis of s-channel helicity conservation is satisfied at the two standard-deviation level. The total cross section and the differential cross-section dsigma/dt, where t is the squared four-momentum transfer at the proton vertex, are presented as a function of W, for |t|<1.8 GeV^2. The t distribution exhibits an exponential shape with a slope parameter increasing logarithmically with W with a value b=4.15 \pm 0.05 (stat.)^{+0.30}_{-0.18} (syst.) GeV^-2 at W=90 GeV. The effective parameters of the Pomeron trajectory are alphapom(0) = 1.200 \pm 0.009(stat.)^{+0.004}_{-0.010}(syst.) and alphappom= 0.115 \pm 0.018(stat.)^{+0.008}_{-0.015}(syst.) GeV^-2.

Figures (8)

• Figure 1: Invariant-mass distributions of the $\mu^+\mu^-$ pairs in the different $\hbox{${W}$}$ regions: (a) $20<W<30 {\,\text{Ge}\text{V}}$, (b) $30<W<70 {\,\text{Ge}\text{V}}$, (c) $70<W<110 {\,\text{Ge}\text{V}}$, (d) $110<W<150 {\,\text{Ge}\text{V}}$ and (e) $150<W<170 {\,\text{Ge}\text{V}}$. The histograms represent the LPAIR distributions of the non-resonant background.
• Figure 3: The acceptance-corrected decay angular distributions for exclusive $J/\psi$ photoproduction in the kinematic range $30<\hbox{${W}$}<170 {\,\text{Ge}\text{V}}$ and $|t|<1 {\,\text{Ge}\text{V}}^2$. The non-resonant background has been subtracted. The results of both the $\mu^+ \mu^-$ and $e^+e^-$ decay channels are presented. The vertical bars indicate the statistical uncertainties only. The curves are the results of the fits to Eqs. (\ref{['hel_theta']}) and (\ref{['hel_phi']}), as described in the text.
• Figure 4: The exclusive $J/\psi$ photoproduction cross section as a function of $\hbox{${W}$}$ for $J/\psi \rightarrow \hbox{$\mu^+\mu^-$}$ and $J/\psi \rightarrow e^+e^-$. The inner bars indicate the statistical uncertainties; the outer bars are the statistical and systematic uncertainties added in quadrature. Results from the H1 H1001, E401 E401 and E516 E516 experiments are also shown. The solid line is the result of a fit to the ZEUS data of the form $\sigma \propto (W/90 {\,\text{Ge}\text{V}})^\delta$ and the dotted line is the extrapolation of the fit.
• Figure 5: The exclusive $J/\psi$ photoproduction cross section as a function of $\hbox{${W}$}$ obtained in the two leptonic decay channels, $J/\psi \rightarrow \hbox{$\mu^+\mu^-$}$ and $J/\psi \rightarrow e^+e^-$. The inner bars indicate the statistical uncertainties, the outer bars are the statistical and systematic uncertainties added in quadrature. The experimental results are compared to the QCD predictions of MRT MRT99, using two different parameterisations of the gluon PDF in the proton, MRST99 mrst99 (dotted curve) and CTEQ5M cteq5m (dashed curve). The solid curve shows the QCD prediction of FMS jhep:103:45 using $\lambda = 4$ and the CTEQ4L cteq4l PDF. The dash-dotted curve displays the prediction soares00 based on the colour-dipole model pr:d59:014017*pr:d60:114023 with a double-Gaussian $J/\psi$ wave-function; this prediction was re-scaled to the $b$-slope measured in this paper.
• Figure 6: The differential cross-section $d\sigma_{\gamma p \rightarrow J/\psi p}/dt$ for exclusive $J/\psi$ photoproduction for representative bins of $\hbox{${W}$}$ and for the decay channels, $J/\psi \rightarrow \hbox{$\mu^+\mu^-$}$ (squares) and $J/\psi \rightarrow e^+e^-$ (points). The vertical bars indicate the statistical uncertainties only. The full lines represent the results of a fit of the form $d\sigma/dt = d\sigma/dt|_{t=0} \cdot e^{bt}$ performed in the range $-t<1.2 {\,\text{Ge}\text{V}}^2$ for the muon channel and in the range $-t<1.25 {\,\text{Ge}\text{V}}^2$ for the electron channel.