paper

Study of the pion trajectory in the photoproduction of leading neutrons at HERA

arXiv:hep-ex/0404002 · doi:10.1016/j.physletb.2005.01.101

Abstract

Energetic neutrons produced in $e p$ collisions at HERA have been studied with the ZEUS detector in the photoproduction regime at a mean photon-proton center-of-mass energy of 220 GeV. The neutrons carry a large fraction $0.64 < x_{L} < 0.925$ of the incoming proton energy, and the four-momentum-transfer squared at the proton-neutron vertex is small, $∣ t ∣ < 0.425$ GeV $^{2}$ . The $x_{L}$ distribution of the neutrons is measured in bins of $t$ . The $(1 - x_{L})$ distributions in the $t$ bins studied satisfy a power law $d N / d x_{L} \propto (1 - x_{L})^{a (t)}$ , with the powers $a (t)$ following a linear function of $t$ : \mbox{ $a (t) = 0.88 \pm 0.09 (stat.)_{- 0.39}^{+ 0.34} (syst.) - (2.81 \pm 0.42 (stat.)_{- 0.62}^{+ 1.13} (syst.) GeV^{- 2}) t$ .} This result is consistent with the expectations of pion-exchange models, in which the incoming proton fluctuates to a neutron-pion state, and the electron interacts with the pion.

11 pages, 3 figures; revised analysis