Charged particle cross sections in photoproduction and extraction of the gluon density in the photon

Photoproduction data collected with the H1 detector at HERA in 1994 are used to study the cross-sections for inclusive charged particle production and the structure of the photon. The differential cross-sections \({\rm d}\sigma/{\rm d}p_T^2\), for \(|\eta| < 1\) in the HERA laboratory frame, and \({\rm d}\sigma/{\rm d}\eta\) for \(p_T> 2\) GeV/c and \(p_T> 3\) GeV/c have been measured. Model calculations of these cross-sections, based on perturbative QCD, indicate that the results are sensitive to the parton densities of the photon as well as to higher order effects, which are phenomenologically treated by multiple interactions. This sensitivity is exploited to determine the leading order \(x_{\gamma}\) distribution of partons in the photon using a new method based on high \(p_T\) charged particles. The gluon content of the photon is extracted and found to rise with decreasing \(x_{\gamma}\).