Inelastic Photoproduction of J/Psi Mesons at HERA

TL;DR

This work presents a comprehensive measurement of inelastic J/$\psi$ photoproduction at HERA over $60<W_{\gamma p}<260$ GeV, $0.05<z<0.9$, and $1<p_t^2<60$ GeV$^2$, including the first cross sections at $z<0.3$. The data are confronted with several QCD-based predictions: NLO Colour Singlet Model for direct photons, LO $k_t$-factorisation (CASCADE), and LO NRQCD with color-singlet and color-octet contributions for direct and resolved photons, as well as NRQCD resummations. The results show good agreement with NLO CS predictions in the medium-$z$ range, and the $k_t$-factorisation and NRQCD approaches can describe broader kinematic regions with appropriate LDME choices; polarization measurements are broadly consistent with these models and offer guidance for constraining long-distance matrix elements. Overall, the study advances the understanding of charmonium production mechanisms in ep and gamma-p interactions, contributing to a unified description across different collision systems and informing LDME determinations.

Abstract

An analysis of inelastic photoproduction of J/Psi mesons is presented using data collected at the ep collider HERA corresponding to an integrated luminosity of above 80pb-1. Differential and double differential cross sections are measured in a wide kinematic region: 60<W_gammap<260 GeV, 1<p_t^2< 60 GeV^2 and 0.05<z<0.9, where z is the fraction of the energy of the exchanged photon transferred to the J/Psi meson in the rest frame of the target proton. Cross sections at z<0.3 are presented for the first time. Theoretical calculations within the Colour Singlet Model at NLO for direct photon processes are shown to give a good description of the data in the medium z region (0.3<z<0.9) up to the highest p_t^2 values. A calculation using a k_t factorisation approach in LO in the Colour Singlet Model is also able to describe these data. The data in the full z range are also compared to LO calculations within a non-relativistic QCD framework including color octet and colour singlet contributions for direct and resolved photons. It seems possible to reconcile data and theory with modest contributions from colour octet processes. The polarisation of the J/Psi meson is measured as a function of z and p_t,psi and is reasonably described by the theoretical predictions.

Figures (10)

• Figure 1: Generic Feynman diagrams for inelastic $J/\psi$ production. a,b) direct photon processes; c) resolved photon process. In diagrams a) and c) the $\mit c\overline{c}$ pair leading to the formation of the $J/\psi$ can be in a colour singlet or octet state while in b) it can only be in a colour octet state. Additional soft gluons emitted during the hadronisation process are not shown.
• Figure 2: Distribution of the invariant mass of the muon pairs after all selection cuts a) in the range $0.3<z<0.9$, $60<W_{\gamma p}<240\,{\rm GeV}$ and $p_{t,\psi}>1\,{\rm GeV}$ (dataset I) and b) in the range $0.05<z<0.45$, $120<W_{\gamma p}<260\,{\rm GeV}$ and $p_{t,\psi}>1\,{\rm GeV}$ (dataset II). The data points with statistical error bars show the unlike sign combinations, the histograms the like sign combinations. The lines are fits as described in the text.
• Figure 3: Comparison between data and Monte Carlo simulations after all selection cuts and after subtraction of non-resonant background. a) $W_{\gamma p}$, b) $z$ and c) $p_{t,\psi}^2$ distributions in the range $0.3<z<0.9$, $60<W_{\gamma p}<240\,{\rm GeV}$ and $p_{t,\psi}^2>1\,{\rm GeV}^2$ (dataset I). d) $W_{\gamma p}$, e) $z$ and f) $p_{t,\psi}^2$ distributions in the range $0.05<z<0.45$, $120<W_{\gamma p}<260\,{\rm GeV}$ and $p_{t,\psi}^2>1\,{\rm GeV}^2$ (dataset II). The error bars on the data points are statistical only. The EPJPSI simulation of direct + resolved photon processes (full histograms) and resolved photon processes (shaded histograms) are compared with the data (for the relative normalisation see text). An estimate of the direct photon contribution from $b\rightarrow J/\psi+X$ from EPJPSI (which is not included in the full histogram) is also shown as a dotted histogram in d)--f).
• Figure 4: Inelastic $J/\psi$ production in the region $60<W_{\gamma p}<240\,\,\hbox{\rm GeV}$, $0.3<z<0.9$ and $p_{t,\psi}^2$$>1\,\,\hbox{,\rm GeV$^2$}$. a) Total cross section as a function of $W_{\gamma p}$, b) differential cross sections ${\rm d}\sigma/{\rm d}z$ and c) ${\rm d}\sigma/{\rm d}p_{t,\psi}^{2}$. The H1 data are shown together with NLO calculations in the Colour Singlet Model csm_nlo with MRST mrst parton density functions ($W_{\gamma p}$$=115\,\,\hbox{\rm GeV}$ for b) and c)). The shaded band reflects the uncertainties in $m_c$ and $\alpha_s$ (see Table \ref{['tab:qcd']}); the dashed (dash-dotted) curve is calculated with $m_c = 1.3 (1.4)\,{\rm GeV}$, $\alpha_s(M_Z) = 0.1175 (0.1225)$. In c) a CSM LO calculation with MRST parton distributions, $m_c = 1.3\,{\rm GeV}$, $\alpha_s(M_Z) = 0.1225$ is also shown (dotted).
• Figure 5: Double differential cross section ${\rm d}^2\sigma/ {\rm d}p^{2}_{t,\psi}\,{\rm d}z$ in the same kinematic region and compared with the same CSM NLO calculations as in Fig. \ref{['fig1']}.