Diffractive Photoproduction of J/psi Mesons with Large Momentum Transfer at HERA

TL;DR

This study measures diffractive photoproduction of $J/\psi$ mesons with large momentum transfer at HERA, focusing on $2<|t|<30$ GeV$^2$ and $50<W_{\gamma p}<150$ GeV. It provides differential cross sections $d\sigma/dt$ and energy-dependent cross sections $\sigma_{\gamma p}$ in three $t$ intervals, and extracts the effective Pomeron trajectory $\alpha(t)$ from the $W_{\gamma p}$ dependence. The data are compared to perturbative QCD models based on BFKL and DGLAP evolution, with BFKL LL (and some NL corrections) generally describing the large-$|t|$ region and DGLAP LL describing the small-$|t|$ domain; spin-density matrix elements are consistent with SCHC. The results constrain the dynamics of diffractive exchange at high momentum transfer and test the applicability of pQCD frameworks to exclusive diffractive processes in ep collisions.

Abstract

The diffractive photoproduction of J/psi mesons is measured with the H1 detector at the ep collider HERA using an integrated luminosity of 78 pb^-1. The differential cross section d sigma(gamma p -> J/psi Y) / d t is studied in the range 2 < |t| < 30 GeV^2, where t is the square of the four-momentum transferred at the proton vertex. The cross section is also presented as a function of the photon-proton centre-of-mass energy W in three t intervals, spanning the range 50 < W < 200 GeV. A fast rise of the cross section with W is observed for each t range and the slope for the effective linear Pomeron trajectory is measured to be alpha^\prime= -0.0135 \pm 0.0074 (stat.) \pm 0.0051 (syst.) GeV^-2. The measurements are compared with perturbative QCD models based on BFKL and DGLAP evolution. The data are found to be compatible with s-channel helicity conservation.

Figures (6)

• Figure 1: The $\mu^+ \mu^-$ invariant mass distribution in the kinematic region $50 < W_{\gamma p} <150 \ {\rm GeV}$, $z>0.95$ and $|t| > 2~{\rm GeV^{2}}$. The histogram shows the sum of the Monte Carlo simulations of $J/\psi$ production using HITVM (open histogram), the contribution from lepton pair production as simulated by the LPAIR program (dark shaded histogram) and the contribution from diffractive $\psi(2S)$ events as simulated with the DIFFVM program (light shaded histogram).
• Figure 2: Kinematic distributions of the dimuon sample in the mass range $2.9 < M_{\mu^{+} \mu^{-}} < 3.3 \ {\rm GeV}$. a) The polar angle $\theta^{\mu}$ and b) the transverse momentum $p_t^{\mu}$ of the muon tracks. c) The elasticity $z$ and d) the photon-proton centre-of-mass energy $W_{\gamma p}$. e) The distribution of the cosine of the polar angle and f) the azimuthal distribution of the positively charged decay muon in the helicity frame. g) The distribution of the squared dimuon transverse momentum.
• Figure 3: The photon-proton differential cross section ${\rm d}\sigma/{\rm d}t$ for $J/\psi$ production in the kinematic range $50 < W_{\gamma p} < 150 \ {\rm GeV}$, $|t|>2.0 \ {\rm GeV}^{2}$ and $z>0.95$. The inner error bars correspond to the statistical error and the outer error bars are the statistical and systematic errors added in quadrature. The solid line shows the prediction from the BFKL calculation in the leading logarithmic approximation for fixed $\alpha_s$Enberg:2002zy. The dashed (dotted) curve corresponds to the BFKL calculation including non-leading corrections and using a fixed (running) $\alpha_s$Enberg:2002zy. The dashed-dotted curve, shown in the range $|t| < M_{J/\psi}^2$, shows a calculation based on the DGLAP equation in the leading logarithmic approximation Gotsman:2001ne.
• Figure 4: The photon-proton cross section as a function of $W_{\gamma p}$ in three bins of $|t|$. The inner error bars correspond to the statistical error and the outer error bars are the statistical and systematic errors added in quadrature. The solid lines show the predictions from the BFKL calculation in the leading logarithmic approximation and the dashed lines correspond to the BFKL calculation including non-leading corrections using a fixed $\alpha_s$Enberg:2002zy. The dashed-dotted curve is the result of a calculation based on the DGLAP equation in the leading logarithmic approximation Gotsman:2001ne.
• Figure 5: Normalised decay angular distributions for $J/\psi$ meson production in three bins of $|t|$: a,b) $2<|t|<5 \ {\rm GeV}^2$; c,d) $5<|t|<10 \ {\rm GeV}^2$ and e,f) $10<|t|<30 \ {\rm GeV}^2$. The left column (a,c,e) shows the azimuthal distributions of the positively charged decay muon in the helicity frame and the right column (b,d,f) shows the polar angle distributions. The inner error bars show the statistical error and the outer error bars show the statistical and systematic errors added in quadrature. The solid lines show the results of a two-dimensional fit to the data (see text). The shaded band represents the statistical uncertainty for the fit. The dashed line shows the expectation from $s$-channel helicity conservation.