Leading neutron production in $\mathrm{e^+ p}$ collisions at HERA

Abstract

The production of neutrons carrying at least 20% of the proton beam energy ( x L > 0.2 ) in e + p collisions has been studied with the ZEUS detector at HERA for a wide range of Q 2 , the photon virtuality, from photoproduction to deep inelastic scattering. The neutron-tagged cross section, ep → e ′ Xn , is measured relative to the inclusive cross section, ep → e ′ X , thereby reducing the systematic uncertainties. For x L > 0.3, the rate of neutrons in photoproduction is about half of that measured in hadroproduction, which constitutes a clear breaking of factorisation. There is about a 20% rise in the neutron rate between photoproduction and deep inelastic scattering, which may be attributed to absorptive rescattering in the γp system. For 0.64< x L <0.82, the rate of neutrons is almost independent of the Bjorken scaling variable x and Q 2 . However, at lower and higher x L values, there is a clear but weak dependence on these variables, thus demonstrating the breaking of limiting fragmentation. The neutron-tagged structure function, F LN(3) 2 ( x , Q 2 , x L ), rises at low values of x in a way similar to that of the inclusive F 2 ( x , Q 2 ) of the proton. The total γπ cross section and the structure function of the pion, F π 2 ( x π , Q 2 ) where x π = x /(1− x L ), have been determined using a one-pion-exchange model, up to uncertainties in the normalisation due to the poorly understood pion flux. At fixed Q 2 , F π 2 has approximately the same x dependence as F 2 of the proton.