Diffractive open charm production at HERA

Abstract

We use perturbative QCD to calculate the cross sections ${\sigma }^{L,T}$ for the diffractive production of open charm $(c\overline{c})$ from longitudinally and transversely polarised photons (of virtuality $Q^2$) incident at high energy $(\sqrt{s})$ on a proton target. We study both the $Q^2$ and $M^2$ dependence of the cross sections, where $M$ is the invariant mass of the $c\overline{c}$ pair. Surprisingly, the result for ${\sigma }^T$, as well as for ${\sigma }^L$, is perturbatively stable. We estimate higher-order corrections and find a sizeable enhancement of the cross sections. The cross sections depend on the {\it square} of the gluon density $g(x,K^2)$, and we show that the observation of open charm at the HERA electron-proton collider can act as a sensitive probe of the gluon distribution for $x=(Q^2+M^2)/s$ and scale $K^2=(m_c^2+⟨k_T^2⟩)(1+Q^2/M^2)$ where the average quark transverse momentum squared $⟨k_T^2⟩\sim m_c^2$. As compared to diffractive $J/\psi$ production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the $J/\psi$ wave function. We estimate the fraction of diffractive events that arise from $c\overline{c}$ production.

Comments: 34 pages, LaTeX, 15 Postscript figures included using epsf. The complete paper, including figures, is also available via anonymous ftp at ftp://cpt1.dur.ac.uk/pub/preprints/dtp96/dtp9650/