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

TL;DR

This study investigates the production of energetic neutrons in ep collisions at HERA, analyzing their energy and momentum transfer distributions to understand pion-exchange mechanisms in photoproduction.

Contribution

It provides the first detailed measurement of neutron energy distributions and their t-dependence, supporting pion-exchange models in ep collisions at HERA.

Findings

Neutron energy fraction distribution follows a power law in (1-x_L).

The power-law exponent a(t) varies linearly with t.

Results support pion-exchange as the dominant mechanism.

Abstract

Energetic neutrons produced in $ep$ 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 $dN/dx_L \propto (1-x_L)^{a(t)}$, with the powers $a(t)$ following a linear function of $t$: \mbox{$a(t)=0.88\pm 0.09 ({\rm stat.})^{+0.34}_{-0.39}({\rm syst.})-(2.81\pm 0.42({\rm stat.})^{+1.13}_{-0.62}({\rm syst.}) {\rm Ge V}^{-2})t$.} 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.