Measurement of inelastic $J/\psi$ production in deep inelastic scattering at HERA

Abstract.

The inelastic production of $J/\psi$ mesons in ep collisions has been studied with the ZEUS detector at HERA using an integrated luminosity of $109{ \text{pb}^{-1}}$. The $J/\psi$ mesons were identified using the decay channel $J/\psi\to\mu^{ + }\mu^{-}$. The measurements were performed in the kinematic range $2 < Q^2 < 80 {\rm GeV}^2$, $50 < W < 250 \text{GeV}$, 0.2 < z < 0.9 and $-1.6 < Y_{\text{lab}} < 1.3$, where Q² is the virtuality of the exchanged photon, W is the photon-proton centre-of-mass energy, z is the fraction of the photon energy carried by the $J/\psi$ meson in the proton rest frame and $Y_{\text{lab}}$ is the rapidity of the $J/\psi$ in the laboratory frame. The measured cross sections are compared to theoretical predictions within the non-relativistic QCD framework including colour-singlet and colour-octet contributions, as well as to predictions based on the k _T -factorisation approach. Calculations of the colour-singlet process generally agree with the data, whereas inclusion of colour-octet terms spoils this agreement.