Photoproduction of $D^{*\pm}$ Mesons in $ep$ Collisions at HERA

Abstract

At the electron-proton collider HERA the inclusive $D^{*\pm}$ meson photoproduction cross section has been measured with the H1 detector in two different, but partly overlapping, kinematical regions. For the first, where $<W_{\gamma p}> \approx 200$ GeV and $Q^2 < 0.01 \gev^2$, the result is $\sigma(\gamma p \to c \bar{c} X) = (13.2 \pm 2.2 ^{+2.1}_{-1.7} ^{+9.9}_{-4.8}) \mu b$. The second measurement for $Q^2 < 4 \gev^2$ yields $\sigma(\gamma p \to c \bar{c} X) = (9.3 \pm 2.1 ^{+1.9}_{-1.8} ^{+6.9}_ {-3.2}) \mu b$ at $<W_{\gamma p}> \approx 142$ GeV and $\sigma(\gamma p \to c \bar{c} X) = (20.6 \pm 5.5 ^ {+4.3}_{-3.9} ^{+15.4}_{-7.2}) \mu b$ at $<W_{\gamma p} > \approx 230$ GeV, respectively. The third error accounts for an additional uncertainty due to the proton and photon parton density parametrizations. Differential cross sections are presented as a function of the $D^{*\pm}$ transverse momentum and rapidity. The results compare reasonably well with next-to-leading order QCD calculations. Evidence for diffractive photoproduction of charm quark is presented.