Search

Interactions of the type ep → eXY are studied, where the component X of the hadronic final state contains two jets and is well separated in rapidity from a leading baryonic system Y. Analyses are performed of both resolved and direct photoproduction and of deep-inelastic scattering with photon virtualities in the range 7.5 < Q 2 < 80 GeV2. Cross sections are presented where Y has mass M Y < 1.6 GeV, the squared four-momentum transferred at the proton vertex satisfies |t| < 1 GeV2 and the two jets each have transverse momentum p > 5 GeV relative to the photon direction in the rest frame of X. Models based on a factorisable diffractive exchange with a gluon dominated structure, evolved to a scale set by the transverse momentum $$\hat p_T $$ of the outgoing partons from the hard interaction, give good descriptions of the data. Exclusive $$q\bar q$$ production, as calculated in perturbative QCD using the squared proton gluon density, represents at most a small fraction of the measured cross section. The compatibility of the data with a breaking of diffractive factorisation due to spectator interactions in resolved photoproduction is investigated. [ABSTRACT FROM AUTHOR]

A study is presented of the process γp → XY, where there is a large rapidity gap between the systems X and Y. Measurements are made of the differential cross section as a function of the invariant mass M of the system produced at the photon vertex. Results are presented at centre of mass energies of 〈W〉 = 187 GeV and 〈W〉 =231 GeV, both where the proton dominantly remains intact and, for the first time, where it dissociates. Both the centre of mass energy and the $M_X^2$ dependence of HERA data and those from a fixed target experiment may simultaneously be described in a triple-Regge model. The low mass photon dissociation process is found to be dominated by diffraction, though a sizable subleading contribution is present at larger masses. The pomeron intercept is extracted and found to be α(0) = 1.068 ± 0.016 (stat.) ± 0.022 (syst.) ± 0.041 (model), in good agreement with values obtained from total and elastic hadronic and photoproduction cross sections. The diffractive contribution to the process γp → Xp with $M_X^2/{W^2} < 0.05$ is measured to be 22.2 ± 0.6 (stat.) ± 2.6 (syst.) ± 1.7 (model)% of the total γp cross section at 〈W〉 = 187 GeV. [ABSTRACT FROM AUTHOR]

Measurements of ep scattering with squared 4-momentum transfer Q up to 35000 GeV are compared with the expectation of the standard deep-inelastic model of lepton-nucleon scattering (DIS). For Q > 15000 GeV, N = 12 neutral current candidate events are observed where the expectation is N = 4.71 ± 0.76 events. In the same Q range, N = 4 charged current candidates are observed where the expectation is N = 1.77 ± 0.87 events. The probability P( N ≥ N) that the DIS model signal N fluctuates to N ≥ N in a random set of experiments is 6 × 10 for neutral current and 0.14 for charged current. The difference in the observed and expected number of Neutral Current events is mostly due to events at large masses $M = \sqrt {xs}$ in which the positron is backscattered at large y = Q/M. [ABSTRACT FROM AUTHOR]

The double-differential inclusive di-jet cross section in photoproduction processes is measured with the H1 detector at HERA. The cross section is determined as a function of the average transverse jet energy E for ranges of the fractional energy ξ of the parton from the photon side. An effective leading order parton distribution in the photon is determined at large parton fractional energies for scale between 80 < p < 1250 GeV2. The measurement is compatible with the logarithmic scale dependence that is predicted by perturbative QCD. [ABSTRACT FROM AUTHOR]