Inclusive photoproduction of neutral pions in the photon hemisphere at HERA

Abstract.

The inclusive cross section for the photoproduction of neutral pions has been measured as a function of the transverse momentum, rapidity, and Feynman x of the $\pi^0$ mesons at an average photon–proton centre-of-mass energy of 208 GeV and for photon virtualities below $Q^2=0.01$ GeV$^2$. The $\pi^0$ measurement extends the range covered by previous charged particle measurements at HERA by two units of rapidity in the photon direction down to a value of $-5.5$ in the $\gamma p$ centre-of-mass frame. The $\pi^0$ transverse momentum distribution is well described over the whole measured range by a power law ansatz, while an exponential fit falls below the data at transverse momentum values above 1.5 GeV/c. Good agreement with the predictions of the Monte Carlo models PYTHIA and PHOJET is found. In the context of the PYTHIA model the data are inconsistent with large intrinsic transverse momentum values in the photon.