Next-to-leading-order QCD analyses of the ZEUS data on deep inelastic scattering together with fixed-target data have been performed, from which the gluon and quark densities of the proton and the value of the strong coupling constant ␣ s (M Z ) were extracted. The study includes a full treatment of the experimental systematic uncertainties including point-to-point correlations. The resulting uncertainties in the parton density functions are presented. A combined fit for ␣ s (M Z ) and the gluon and quark densities yields a value for ␣ s (M Z ) in agreement with the world average. The parton density functions derived from ZEUS data alone indicate the importance of HERA data in determining the sea quark and gluon distributions at low x. The limits of applicability of the theoretical formalism have been explored by comparing the fit predictions to ZEUS data at very low Q 2 .

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. or 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

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Deep inelastic scattering and its diffractive component, ep -> e'y*p -> e'XN, have been studied at HERA with the ZEUS detector using an integrated luminosity of 4.2 pb(-1). The measurement covers a wide range in the y*p c.m. energy W (37-245 GeV), photon virtuality Q(2) (2.2-80 GeV(2)) and mass M(X) (0.28-35 GeV). The diffractive cross section for M(X) > 2 GeV rises strongly with W; the rise is steeper with increasing Q(2). The latter observation excludes the description of diffraclive deep inelastic scattering in terms of the exchange of a single pomeron. The ratio of diffractive to total cross section is constant as a function of W, in contradiction to the expectation of Regge phenomenology combined with a naive extension of the optical theorem to y*p scattering. Above M(X) of 8 GeV, the ratio is flat with Q(2), indicating a leading-twist behaviour of the diffractive cross section. The data are also presented in terms of the diffractive structure function, F(2)(D(3)) (beta, x(P), Q(2)) of the proton. For fixed beta, the Q(2) dependence of x(P)F2(D)((3)) changes with x(P) in violation of Regge factorisation. For fixed xp, x(P,) x(P)F(2)(D(3)) rises as beta -> 0, the rise accelerating with increasing Q(2). These positive scaling violations suggest substantial contributions of perturbative effects in the diffractive DIS cross section. (c) 2005 Elsevier B.V. All rights reserved