Leading neutron production in e(+)p collisions at HERA

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(1) Xn, is measured relative to the inclusive cross section, ep --> e(1)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 gammap 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(2)(LN(3)) (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 gammapi cross section and the structure function of the pion, F(2)(pi) (x(pi), Q(2)) where x(pi) = 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(2)(pi) has approximately the same x dependence as F(2) of the proton. (C) 2002 Published by Elsevier Science B.V.