Charmonium Production in Deep Inelastic Scattering at HERA

TL;DR

This study probes charmonium production in deep inelastic e+p scattering at HERA, measuring elastic, quasi-elastic, and inclusive J/psi and psi(2S) production across 2 < Q^2 < 80 GeV^2 and 25 < W < 180 GeV. It tests perturbative QCD predictions through elastic cross sections and their W and t dependences, analyzes decay angular distributions to extract polarization information, and explores NRQCD factorization with color-octet contributions alongside Soft Colour Interactions for inclusive production. Key results show a strong W-dependence and a Q^2 dependence consistent with pQCD expectations, a t-slope similar to photoproduction, and evidence that the psi(2S) to J/psi ratio grows with Q^2; NRQCD predictions face normalization and shape discrepancies, while SCI captures some shapes but not the overall normalization. The findings constrain the gluon density in the proton, test non-perturbative effects in quarkonium formation, and highlight areas where theoretical models need refinement to fully describe DIS charmonium production.

Abstract

The electroproduction of J/psi and psi(2S) mesons is studied in elastic, quasi-elastic and inclusive reactions for four momentum transfers 2 < Q^2 < 80 GeV^2 and photon-proton centre of mass energies 25 < W < 180 GeV. The data were taken with the H1 detector at the electron proton collider HERA in the years 1995 to 1997. The total virtual photon-proton cross section for elastic J/psi production is measured as a function of Q^2 and W. The dependence of the production rates on the square of the momentum transfer from the proton (t) is extracted. Decay angular distributions are analysed and the ratio of the longitudinal and transverse cross sections is derived. The ratio of the cross sections for quasi-elastic psi(2S) and J/psi meson production is measured as a function of Q^2. The results are discussed in terms of theoretical models based upon perturbative QCD. Differential cross sections for inclusive and inelastic production of J/psi mesons are determined and predictions within two theoretical frameworks are compared with the data, the non-relativistic QCD factorization approach including colour octet and colour singlet contributions, and the model of Soft Colour Interactions.

Figures (14)

• Figure 1: Charmonium production mechanisms: a) elastic and b) proton dissociative production via pomeron exchange; c) elastic production via two-gluon exchange; d) leading order diagram in the Colour Singlet Model (the $c\bar{c}$ pair is produced in a colour singlet state); e) leading order Colour Octet Model (the $c\bar{c}$ pair is produced in a colour octet state).
• Figure 2: Mass spectra for events of the quasi-elastic $J/\psi$ selection: a) $\mu^+\mu^-$ pairs, b) $e^+e^-$ pairs. Both particles are detected in the central region. The full lines are the results of a fit using a Gaussian distribution for the signal region (convoluted with an exponential tail to account for energy loss in the case of di-electron decays) and an exponential distribution for the non-resonant background. The mass spectra from two photon processes (LPAIR simulation) are shown as hatched histograms. $N_{\psi}$ is the number of $J/\psi$ events obtained from the fit.
• Figure 3: Mass spectra for events of the $J/\psi$ selection in the low $W$ analysis: a) $\mu^+\mu^-$ pairs for the FMD-FMD sample and b) for the FMD-CTD sample. The open histogram represents the prediction of a Monte Carlo simulation including diffractively produced $J/\psi$ mesons (DIFFVM) and muon pairs from two photon processes (LPAIR) which are also shown separately as hatched histograms.
• Figure 5: Distribution of quasi-elastic $J/\psi$ candidates in the kinematic ($x, Q^2$) plane (data sets I. and II. in Table \ref{['cuts']}), before applying $Q^2$ and $W$ cuts. Lines of constant $W$, energy $E_e$ and polar angle $\theta_e$ of the scattered positron are shown and the different analysis regions are indicated.
• Figure 6: Cross sections for elastic $J/\psi$ production as a function of $W$ at different values of $Q^2$, measured at HERA in this and other analyses H1964Zeus981H1962Zeus971. Data for $Q^2 > 0$ have been scaled by factors $5$, $50$ and $100$ as indicated. The inner error bars on the points from this analysis indicate the statistical errors, while the outer bars show the statistical and systematic uncertainties added in quadrature. The dash-dotted lines are the results of fits of the form $W^{\delta}$ to the data for each $Q^2$. For $Q^2 > 0$ the fits are for H1 data only. The full and dashed lines are results of calculations from Frankfurt et al. F_K_S97 using different parameterizations of the proton gluon densities. The small arrows at $W=200\hbox{GeV}$ indicate the sensitivity of this prediction to a change of the charm quark mass from $1.4\hbox{GeV}$ to $1.5\hbox{GeV}$.