Physics – High Energy Physics – High Energy Physics - Experiment
Scientific paper
1995-02-02
Phys.Lett.B349:225-237,1995
Physics
High Energy Physics
High Energy Physics - Experiment
19 pages, latex, 3 figures appended as uuencoded file
Scientific paper
10.1016/0370-2693(95)00253-H
We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel ${\rm D}^{*+}\rightarrow (\Do \rightarrow {\rm K}^-\pi^+)\pi^+$ (+ c.c.). Clear signals in the mass difference $\Delta M$=$M$(D$^*$)--$M$(D$^0)$ as well as in the $M(K\pi)$ distribution at the D$^0$ mass are found. The $ep$ cross section for inclusive \DSpm\ production with $Q^2<4~\GeV^2$ in the $\gamma p$ centre-of-mass energy range $115 < W < 275$ \GeV\ has been determined to be $(32 \pm 7^{+4}_{-7} )$ nb in the kinematic region \mbox{\{$p_T(\DS)\geq $ 1.7 \,\GeV, $|\eta(\DS)| < 1.5 $\}}. Ex\-tra\-po\-la\-ting outside this region, assuming a mass of the charm quark of 1.5 \GeV, we estimate the $ep$ charm cross section to be $\sigma(e p \rightarrow c \bar{c}X ) = (0.45 \pm 0.11^{+0.37}_{-0.22}) \, \mu {\rm b} $ at \mbox{$\sqrt{s} = 296$}~\GeV\ and $\langle W \rangle = 198$ \GeV. The average $\gamma p$ charm cross section \mbox{$\sigma(\gamma p \rightarrow c \bar{c}X )$} is found to be \mbox{$(6.3 \pm 2.2^{+6.3}_{-3.0}) \, \mu {\rm b} $} at $\langle W \rangle = 163$ \GeV\ and \mbox{$(16.9 \pm 5.2^{+13.9}_{-8.5}) \, \mu {\rm b} $} at $\langle W \rangle = 243$ \GeV. The increase of the total charm photoproduction cross section by one order of magnitude with respect to low energy data experiments is well described by QCD NLO calculations using singular gluon distributions in the proton.
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