Inclusive Photoproduction of Neutral Pions in the Photon Hemisphere at HERA

TL;DR

This paper reports measurements of neutral pion production in photon-proton collisions at HERA, extending previous rapidity coverage and comparing results with theoretical models, revealing insights into particle production mechanisms.

Contribution

It provides the first extensive measurement of neutral pion photoproduction at high rapidities at HERA, testing and constraining Monte Carlo models and photon structure assumptions.

Findings

Transverse momentum distribution follows a power law.

Good agreement with PYTHIA and PHOJET models.

Data inconsistent with large intrinsic transverse momentum in the photon.

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 pizero 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 pizero 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 pizero 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.