Observation of hard scattering in photoproduction events with a large rapidity gap at HERA

Events with a large rapidity gap and total transverse energy greater than 5 GeV have been observed in quasi-real photoproduction at HERA with the ZEUS detector. The distribution of these events as a function of the $\gamma p$ centre of mass energy is consistent with diffractive scattering. For total transverse energies above 12 GeV, the hadronic final states show predominantly a two-jet structure with each jet having a transverse energy greater than 4 GeV. For the two-jet events, little energy flow is found outside the jets. This observation is consistent with the hard scattering of a quasi-real photon with a colourless object in the proton.Comment: 19 pages, latex, 4 figures appended as uuencoded fil

Observation of Events with an Energetic Forward Neutron in Deep Inelastic Scattering at HERA

In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe, with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of Bjorken x and Q2 in the range 3 · 10-4 \u3c xBJ \u3c 6 · 10-3 and 10 \u3c Q2 \u3c 100 GeV2

Study of D*(2010)+- Production in ep Collisions at HERA

We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel D*(+) --> (D-0 --> K(-)pi(+)) pi(+) (+ c.c.). Clear signals in the mass difference Delta M=M(D*)-M(D-0) as well as in the M(K pi) distribution at the D-0 mass are found. The ep cross section for inclusive D*(+/-) production with Q(2) c $($) over bar cX)=(0.45+/-0.11(-0.22)(+0.37)) mu b at root s=296 GeV and [W]=198 GeV. The average gamma p charm cross section sigma(gamma p --> c $($) over bar cX) is found to be (6.3+/-2.2(-3.0)(+6.3)) mu b at [W]=163 GeV and (16.9+/-5.2(-8.5)(+13.9)) mu b at [W]=243 GeV. The increase of the total charm photoproduction cross section by one order of magnitude with respect to low energy data experiments is well described by QCD NLO calculations using singular gluon distributions in the proton

Diffractive hard photoproduction at HERA and evidence for the gluon content of the pomeron

Inclusive jet cross sections for events with a large rapidity gap with respect to the proton direction from the reaction $ep \rightarrow jet \; + \; X$ with quasi-real photons have been measured with the ZEUS detector. The cross sections refer to jets with transverse energies $E_T^{jet}>8$~GeV. The data show the characteristics of a diffractive process mediated by pomeron exchange. Assuming that the events are due to the exchange of a pomeron with partonic structure, the quark and gluon content of the pomeron is probed at a scale $\sim (E_T^{jet})^2$. A comparison of the measurements with model predictions based on QCD plus Regge phenomenology requires a contribution of partons with a hard momentum density in the pomeron. A combined analysis of the jet cross sections and recent ZEUS measurements of the diffractive structure function in deep inelastic scattering gives the first experimental evidence for the gluon content of the pomeron in diffractive hard scattering processes. The data indicate that between 30\% and 80\% of the momentum of the pomeron carried by partons is due to hard gluons.Comment: 25 pages, latex, 4 figures appended as uuencoded file. No change w.r.t. previous version (just avoiding warning latex messages while processing

Inclusive jet differential cross-sections in photoproduction at HERA

Inclusive jet differential cross sections for the reaction ep --> jet + X at Q(2) below 4 GeV2 have been measured with the ZEUS detector at HERA using an integrated luminosity of 0.55 pb(-1). These cross sections are given in the kinematic region 0.2 < y < 0.85, for jet pseudorapidities in the ep-laboratory range -1 < eta(jet) < 2 and refer to jets at the hadron level with a cone radius of one unit in the eta - phi plane. These results correspond to quasi-real photoproduction at centre-of-mass energies in the range 130-270 GeV and, approximately, for jet pseudorapidities in the interval -3 < eta(jet)(gamma p CMS) < 0. These measurements cover a new kinematic regime of the partonic structure of the photon, at typical scales up to similar to 300 GeV2 and photon fractional momenta down to x(y) similar to 10(-2). Leading logarithm parton shower Monte Carlo calculations, which include both resolved and direct processes and use the predictions of currently available parametrisations of the photon parton distributions, describe in general the shape and magnitude of the measured eta(jet) and E(T)(jet) distributions