Measurement of Inelastic J/Psi Photoproduction at HERA

TL;DR

This study measures inelastic J/ψ photoproduction in e^+p collisions at HERA using the ZEUS detector, focusing on direct photon-gluon fusion and separating contributions via z and p_T distributions. J/ψ decays to μ^+μ^- and e^+e^- are analyzed in specific z and W ranges, with backgrounds from diffractive and resolved processes subtracted. The results are compared to NLO QCD predictions within the color-singlet framework and show good agreement for p_T^2>1 GeV^2 and z<0.8, while LO color-octet models fail to describe the data. The measured cross sections rise with W and are compatible with gluon densities from MRSD′/GRV, reinforcing the CS mechanism and constraining alternative production scenarios. Overall, the work provides a precise test of QCD in heavy quarkonia photoproduction at high energies and informs gluon density determinations in the proton.

Abstract

We present a measurement of the inelastic, non diffractive J/$ψ$ photoproduction cross section in the reaction $e^{+} p \to e^{+} {J}/ψX$ with the ZEUS detector at HERA. The J/$ψ$ was identified using both the $μ^{+}μ^{-}$ and $e^{+}e^{-}$ decay channels and events were selected within the range $0.4<z<0.9$ ($0.5<z<0.9$) for the muon (electron) decay mode, where $z$ is the fraction of the photon energy carried by the J/$ψ$ in the proton rest frame. The cross section, the $p^2_T$ and the $z$ distributions, after having subtracted the contributions from resolved photon and diffractive proton dissociative processes, are given for the photon-proton centre of mass energy range $50<W<180$ GeV; $p^2_T$ is the square of the J/$ψ$ transverse momentum with respect to the incoming proton beam direction. In the kinematic range $0.4 < z < 0.9$ and $p^2_T > 1$ GeV$^2$, NLO calculations of the photon-gluon fusion process based on the colour-singlet model are in good agreement with the data. The predictions of a specific leading order colour-octet model, as formulated to describe the CDF data on J/$ψ$ hadroproduction, are not consistent with the data.

Figures (7)

• Figure 1: Dominant inelastic J/$\psi$ production mechanisms at HERA. Photon-gluon fusion is described by diagram (a). Diagrams (b) and (c) correspond to diffractive proton dissociation and resolved photon J/$\psi$ production, respectively.
• Figure 2: Invariant mass spectrum for the muon pair sample (a) for $0.4 < z <0.9$ and (b) for $z < 0.4$, in the $W$ range 50 to 180 GeV. The invariant mass spectrum for the electron pair sample ($0.5 < z <0.9$ and $90 < W < 180$ GeV) is shown in (c). The muon mass spectrum (a) was fitted to the sum of a Gaussian and a flat background; the spectrum (b) was fitted to the sum of a Gaussian and a linear background. The electron mass spectrum (c) was fitted to the sum of the convolution of a Gaussian and a bremsstrahlung function plus a linear background.
• Figure 3: In (a) the uncorrected $W$ distribution of the $\mu^{+}\mu^{-}$ data (full dots) with $0.4 < z < 1$ is compared to the mixture of HERWIG and EPSOFT (continuous histogram) described in section \ref{['s:mcacc']}. In (b), (c) and (d) similar comparisons between data and the Monte Carlo mixture are shown for the distributions of $p_T$ and polar angle $\theta$ of the J/$\psi$ and for the energy in the forward calorimeter $E_{FCAL}$, respectively. The Monte Carlo mixture is normalized to the number of measured events.
• Figure 4: The direct inelastic J/$\psi$ photoproduction cross section as a function of $W$ for $z<0.9$. Data from ZEUS, H1 H1, FTPS FTPS, NA14 NA14 and EMC EMC2 are shown. The ZEUS result at the lowest $W$ value is obtained with the muon channel only. The inner error bar indicates the statistical uncertainty, the outer error bar the quadratic sum of the statistical and systematic uncertainties. This is also true for the results from H1, FTPS and EMC Collaborations. The other three ZEUS measurements come from the combination of the electron and muon results as described in the text. The inner error bars represent the statistical and decay channel specific errors added in quadrature, the outer ones the statistical, decay specific and common systematic errors added in quadrature. The lines correspond to the NLO prediction from kramer assuming the GRV grv (continuous), MRSG mrs2 (dashed) and CTEQ3M cteq (dotted-dashed) gluon distributions with $m_c=1.4$ GeV and $\Lambda_{QCD}=300$ MeV, the dotted curve was obtained with GRV, $m_c=1.55$ GeV and $\Lambda_{QCD}=215$ MeV . The curves are scaled up by a factor of 1.15 to take into account the contribution from $\psi' \rightarrow \hbox{J}/\psi X$.
• Figure 5: The direct inelastic J/$\psi$ photoproduction cross section as a function of $W$ for $z<0.8$ and $p_T^2 > 1$ GeV$^2$. Data from ZEUS and H1 H1 are shown. The ZEUS results in the lowest two $W$ bins are obtained with the muon channel only. The inner error bars indicate the statistical uncertainties, the outer error bars the quadratic sum of the statistical and systematic uncertainties. This is also true for the results from the H1 Collaboration. The ZEUS measurement in the highest $W$ bin comes from the combination of the electron and muon results as described in the text. The inner error bar represents the statistical and decay channel specific errors added in quadrature, the outer one the statistical, decay specific and common systematic errors added in quadrature. The lines correspond to the NLO prediction from kramer assuming the GRV grv (continuous), MRSG mrs2 (dashed) and CTEQ3M cteq (dotted-dashed) gluon distributions with $m_c=1.4$ GeV and $\Lambda_{QCD}=300$ MeV. The curves are scaled up by a factor of 1.15 to take into account the contribution from $\psi' \rightarrow \hbox{J}/\psi X$.