Measurement of the diffractive structure function in deep inelastic scattering at HERA

This paper presents an analysis of the inclusive properties of diffractive deep inelastic scattering events produced in $ep$ interactions at HERA. The events are characterised by a rapidity gap between the outgoing proton system and the remaining hadronic system. Inclusive distributions are presented and compared with Monte Carlo models for diffractive processes. The data are consistent with models where the pomeron structure function has a hard and a soft contribution. The diffractive structure function is measured as a function of \xpom, the momentum fraction lost by the proton, of $\beta$, the momentum fraction of the struck quark with respect to \xpom, and of $Q^2$. The \xpom dependence is consistent with the form \xpoma where $a~=~1.30~\pm~0.08~(stat)~^{+~0.08}_{-~0.14}~(sys)$ in all bins of $\beta$ and $Q^2$. In the measured $Q^2$ range, the diffractive structure function approximately scales with $Q^2$ at fixed $\beta$. In an Ingelman-Schlein type model, where commonly used pomeron flux factor normalisations are assumed, it is found that the quarks within the pomeron do not saturate the momentum sum rule.Comment: 36 pages, latex, 11 figures appended as uuencoded fil

Inclusive transverse momentum distributions of charged particles in diffractive and nondiffractive photoproduction at HERA

Inclusive transverse momentum spectra of charged particles in photoproduction events in the laboratory pseudorapidity range $-1.2<\eta<1.4$ have been measured up to p_{T}=8\GeV using the ZEUS detector. Diffractive and non--diffractive reactions have been selected with an average $\gamma p$ centre of mass (c.m.) energy of \langle W \rangle = 180\GeV. For diffractive reactions, the $p_{T}$ spectra of the photon dissociation events have been measured in two intervals of the dissociated photon mass with mean values $\langle M_{X} \rangle = 5$ GeV and $10$ GeV. The inclusive transverse momentum spectra fall exponentially in the low $p_{T}$ region. The non--diffractive data show a pronounced high $p_{T}$ tail departing from the exponential shape. The $p_{T}$ distributions are compared to lower energy photoproduction data and to hadron--hadron collisions at a similar c.m. energy. The data are also compared to the results of a next--to--leading order QCD calculation.Comment: 23 pages, latex, 4 figures appended as uuencoded fil

Hadronic energy distributions in deep inelastic electron - proton scattering

This paper presents energy distributions of the hadronic system produced in neutral-current electron-proton deep-inelastic scattering at a centre of mass energy of 296 GeV. Comparison of the results with QCD Monte Carlo models shows that QCD radiation has a strong influence on the characteristics of the final state. The data are reasonably reproduced by the Lund model based on a matrix element calculation in first order of alpha(s), followed by appropriate parton showers, as well as by the colour dipole model. The HERWIG parton shower model also gives a reasonable representation of the data. Neither the first order matrix elements alone nor the Lund parton shower model, without the matrix element calculation, reproduce the data

Measurement of multiplicity and momentum spectra in the current fragmentation region of the Breit frame at HERA

Charged particle production has been measured in Deep Inelastic Scattering (DIS) events using the ZEUS detector over a large range of Q^2 from 10 to 1280 {\rm\ GeV}^2. The evolution with Q of the charged multiplicity and scaled momentum has been investigated in the current fragmentation region of the Breit frame. The data are used to study QCD \linebreak coherence effects in DIS and are compared with corresponding \ee~data in order to test the universality of quark fragmentation

OBSERVATION OF JET PRODUCTION IN DEEP-INELASTIC SCATTERING WITH A LARGE RAPIDITY GAP AT HERA

Events with a large rapidity gap in deep inelastic scattering with Q2 ≥ 10 GeV2 have been studied in the ZEUS detector. The properties of these events with W &gt; 140 GeV are consistent with a leading twist diffractive production mechanism. In the laboratory frame, with ETjet ≥ 4 GeV, 15% of the events are of the 1-jet type with negligible 2-jet production. The single jet is back-to-back in azimuth with the scattered electron. No energy flow is observed between the jet and the proton direction. With a lower jet transverse energy cut 2-jet production is observed both in the laboratory and the γ*p centre-of-mass systems, demonstrating the presence of hard scattering in the virtual photon proton interactions that give rise to large rapidity gap events