Inclusive production of D $^{\ast\pm}$ mesons in photon-photon collisions at $\sqrt{s}_{\rm ee}=183$ and 189 GeV and a first measurement of $F^\gamma _{2,{\rm c}}$

Abstract

The inclusive production of ${{\rm D}^{\ast\pm}}$ mesons in photon-photon collisions has been measured using the OPAL detector at LEP at $\mboxe^+\mboxe^-$ centre-of-mass energies $\sqrt{s_{\rm ee}}$ of 183 and 189 GeV. The ${{\rm D}^{\ast+}}$ mesons are reconstructed in their decay to ${\rm D}^0\pi^+$ with the ${\rm D}^0$ observed in the two decay modes ${\rm K}^-\pi^+$ and ${\rm K}^-\pi^+\pi^-\pi^+$ . After background subtraction, $100.4\pm12.6 (\rm stat)$ ${\rm D}^{\ast\pm}$ mesons have been selected in events without observed scattered beam electron (“anti-tagged”) and $29.8 \pm 5.9 ({\rm stat}){{\rm D}^{\ast\pm}}$ mesons in events where one beam electron is scattered into the detector (“single-tagged”). Direct and single-resolved events are studied separately. Differential cross-sections ${\rm d}\sigma/{\rm d}p_{\rm T}^{{\rm D}^{\ast}}$ and ${\rm d} \sigma/{\rm d} \vert \eta^{{\rm D^{\ast}}}\vert$ as functions of the ${{\rm D}^{\ast\pm}}$ transverse momentum $p^{\rm D^{\ast}}_{\rm T}$ and pseudorapidity $\eta^{{\rm D}^{\ast}}$ are presented in the kinematic region $2{\rm GeV} < p^{\rm D^{\ast}}_{\rm T} < 12{\rm GeV}$ and $|\eta^{{\rm D}^{\ast}}|< 1.5$ . They are compared to next-to-leading order (NLO) perturbative QCD calculations. The total cross-section for the process $\mboxe^+\mboxe^- \to \mboxe^+\mboxe^- \mbox c\overline{\mbox c}$ where the charm quarks are produced in the collision of two quasi-real photons is measured to be $\sigma({\rm e}^+{\rm e}^- \to{\rm e}^+{\rm e}^-{\rm c}\bar{{\rm c}}) = 963\pm 110 ({\rm stat})\pm 86({\rm sys})\pm 224 ({\rm extrapolation}){\rm pb}$ . A first measurement of the charm structure function $F^\gamma _{2,{\rm c}}$ of the photon is performed in the kinematic range $0.0014 < x < 0.87$ and $5 {\rm GeV}^2 < Q^2 < 100{\rm GeV}^2$ , and the result is compared to a NLO perturbative QCD calculation.