Photon gluon fusion cross sections at HERA energy

We present cross sections for heavy flavour production through photon gluon fusion in electron proton collisions at HERA energy. The electron photon vertex is taken into account explicitly, and theQ ² of the exchanged photon ranges from nearly zero (almost real photon) to the kinematically allowed maximum. In our approach the scale is set by the mass of the produced quarks. Our formalism is also applicable to the production of light quarks as long as the invariant mass of the pair is sufficiently high, so we also give cross sections for \(u\bar u,d\bar d\) and \(s\bar s\) production.     

Observation of events with an isolated high energy lepton and missing transverse momentum at HERA

A search for events with an imbalance in transverse momentum and with isolated high energy leptons has been carried out at the positron-proton collider HERA. One event with an and five events with a are found together with evidence for undetected particles carrying transverse momentum. Within the Standard Model the dominant origin of events with this kind of topology is the production of W bosons with subsequent leptonic decay. Three of the six events are within measurement errors found in a region of phase space likely to be populated by this process, while the remaining events show kinematic properties which are atypical of all Standard Model processes considered. Received: 9 June 1998 / Published online: 26 August 1998     

Expectations for two- and three-jet events at HERA

We calculate cross-sections for two- and three-jet events ine ^? p collisions at \(\sqrt s \) =314 GeV. We include contributions from point-like photon interactions, as well as contributions from when the photon is resolved into constituent quarks and gluons. It is shown that measurements of the angular distribution of the dijet system in two-jet events as a function of the dijet mass will assist in the separation of these two components. Aside from being a direct test of QCD photoproduction subprocesses, measurement of these events will yield information on the hadronic structure of the photon.     

Isolated lepton events at HERA: SUSY R-parity violation?

Events with an isolated high p_T lepton, a hadron jet and missing energy as observed in the H1 experiment at HERA, are potentially associated with R-parity violation in supersymmetric theories. However, stringent kinematic constraints must be fulfilled if the production of supersymmetric particles in R-parity violating scenarios were the correct path for explaining these puzzling events. A reference point ℝ/   is specified for which these constraints are illustrated and implications of the supersymmetric interpretation for new classes of multi-lepton events are indicated.     

Mapping the proton unintegrated gluon distribution in dijets correlations in real and virtual photoproduction at HERA

We discuss how the dijet azimuthal correlations in DIS and real photoproduction at HERA probe the differential (unintegrated) gluon distribution in the proton. We find a strong dependence of the azimuthal correlation pattern on Bjorken-x, photon virtuality and the cut on the jet transverse momenta. A rise of the azimuthal decorrelations is observed with decreasing Bjorken-x due to the interplay of perturbative and nonperturbative effects. We predict a strong rise of the same-side jet rate with photon energy for real photoproduction. We discuss conditions for the correlation function to be dominated by hard perturbative gluons and ways of constraining the size of the nonperturbative soft component. We make some predictions for the THERA energy range. The analysis of the energy dependence of the isolated jet and two-jet cross sections in photoproduction would be a new way to study the not yet well constrained unintegrated gluon distributions and to explore the onset of the pQCD regime.     

Measurement of diffractive photoproduction of vector mesons at large momentum transfer at HERA

Elastic and proton–dissociative photoproduction of , and vector mesons (, , respectively) have been measured in interactions at HERA up to GeV, where t is the four-momentum transfer squared at the photon–vector–meson vertex. The analysis is based on a data sample in which photoproduction reactions were tagged by detection of the scattered positron in a special-purpose calorimeter. This limits the photon virtuality, , to values less than 0.01 GeV, and selects a average center-of-mass energy of = 94 GeV. Results for the differential cross sections, , for , and mesons are presented and compared to the results of recent QCD calculations. Results are also presented for the t-dependence of the pion-pair invariant-mass distribution in the mass region and of the spin-density matrix elements determined from the decay-angle distributions. The Pomeron trajectory has been derived from measurements of the W dependence of the elastic differential cross sections for both and mesons. Received: 16 October 1999 / Revised version: 17 January 2000 / Published online: 6 April 2000     

Characteristics of radiative events in deep inelasticep scattering at HERA

A major contribution to the electroweak radiative corrections in deep inelastic electron proton scattering is due to hard photon bremsstrahlungep→eγX. The Monte Carlo event generator HERACLES is used to study this process. Predictions for event distributions are given and the observability of radiative events is discussed. As a result we find that cuts can be found that reduce the radiative corrections considerably.     

Measurement of proton-dissociative diffractive photoproduction of vector mesons at large momentum transfer at HERA

Diffractive photoproduction of vector mesons, , where Y is a proton-dissociative system, has been measured in e^+p interactions with the ZEUS detector at HERA using an integrated luminosity of 25. The differential cross section, , is presented for , where t is the square of the four-momentum transferred to the vector meson. The data span the range in photon-proton centre-of-mass energy, W, from 80 GeV to 120 GeV. The t distributions are well fit by a power law, . The slope of the effective Pomeron trajectory, measured from the W dependence of the and cross sections in bins of t, is consistent with zero. The ratios to and to increase with increasing -t. Decay-angle analyses for , and mesons have been carried out. For the and mesons, contributions from single and double helicity flip are observed. The results are compared to expectations of theoretical models. Received: 22 May 2002 / Published online: 9 December 2002     

Higher order radiative corrections to the QED Compton events at HERA

We estimate the higher order QED corrections to the QED Compton events at HERA employing the leading logarithmic approximation to all orders in α. The Compton events are events of the typee+p→e′+γ+ Hadrons, wheree′ and γ have nearly balancedp _T .     

Photoproduction of events with rapidity gaps between jets at HERA

The photoproduction of dijet events, where the two jets with the highest transverse energy are separated by a large gap in pseudorapidity, have been studied with the ZEUS detector using an integrated luminosity of 39 pb^-1. Rapidity-gap events are defined in terms of the energy flow between the jets, such that the total summed transverse energy in this region is less than some value E_T ^CUT. The data show a clear excess over the predictions of standard photoproduction models. This is interpreted as evidence for a strongly interacting exchange of a color-singlet object. Monte Carlo models which include such a color-singlet exchange are able to describe the data.     

Study of jet reconstruction algorithms for deep-inelastic events at HERA

Three commonly used jet algorithms and two new ones have been studied with respect to their performance in the new experimental situation of deep-inelastic scattering at the HERA electron-proton collider. Their ability to reconstruct properties of the underlying parton level subprocesses was investigated. This relates to first order QCD matrix elements and higher order parton emissions as described by parton showers. A new method was devised to determine suitable values for the resolution parameters of the algorithms and assess their reconstruction quality. The Jade algorithm, which is frequently used ine ⁺ e ^?-annihilation, is found to perform less well compared to other algorithms.

     

Neutral current cross-section measurement at low Q 2 and high y with the ZEUS detector at HERA

The measurements of the reduced cross-sections for e ^?+? p deep inelastic scattering at high inelasticities y for three different centre-of-mass energies, 318, 251 and 225 GeV have been extended to lower momentum transferred squared, Q ². The analysis of satellite vertex events allows one to extend the cross-section measurement at high y down to Q ² = 4.5 GeV², substantially lower that the previously published cross-section measurement from which the longitudinal structure function, F _L, was extracted.     

Some properties of the very high Q 2 events of HERA

The kinematic reconstruction of neutral current high Q ² events at HERA is discussed in detail using as an example the recently published events of the H1 and ZEUS collaborations at Q ² > 15000 GeV2 and M > 180 GeV, which are more numerous than expected from Standard Model predictions. Taking into account the complete information of these events, the mass reconstruction is improved and the difference between the average mass of the samples of the two experiments is reduced from 26±10 GeV to 17±7 GeV, but remains different enough to render unlikely an interpretation of the excess observed by the two collaborations as originating from the decay of a single narrow resonance.     

Dissociation of virtual photons in events with a leading proton at HERA

The ZEUS detector has been used to study dissociation of virtual photons in events with a leading proton, , in e ⁺ p collisions at HERA. The data cover photon virtualities in two ranges, 0.03 < Q ² < 0.60 GeV² and 2 < Q ² < 100 GeV², with M _X > 1.5 GeV, where M _X is the mass of the hadronic final state, X. Events were required to have a leading proton, detected in the ZEUS leading proton spectrometer, carrying at least 90% of the incoming proton energy. The cross section is presented as a function of t, the squared four-momentum transfer at the proton vertex, , the azimuthal angle between the positron scattering plane and the proton scattering plane, and Q ². The data are presented in terms of the diffractive structure function, . A next-to-leading-order QCD fit to the higher-Q ² data set and to previously published diffractive charm production data is presented.Received: 10 August 2004, Revised: 4 October 2004, Published online: 9 November 2004     

Study of the effective transverse momentum of partons in the proton using prompt photons in photoproduction at HERA

The photoproduction of prompt photons, together with an accompanying jet, has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb⁻¹. A study of the effective transverse momentum, 〈k_T〉, of partons in the proton, as modelled within the framework of the PYTHIA Monte Carlo, gives a value of 〈k_T〉=1.69±0.18^+0.18_−0.20 GeV for the γp centre-of-mass energy range 134<W<251 GeV. This result is in agreement with the previously observed trend in hadron–hadron scattering for 〈k_T〉 to rise with interaction energy.     

QCD corrections and the Leptoquark interpretation of the HERA high-Q 2 events

The excess of high-Q ² events found by H1 and ZEUS at HERA in e ⁺ p deep-inelastic scattering above the Standard Model prediction motivates us to calculate the NLO QCD corrections to the HERA scalar leptoquark (or squark) production cross sections. We find that the corrections are significant, of order 50% in the mass range of interest. We also calculate the leptoquark average transverse momentum squared and find it to be rather small. Various leptoquark production cross sections at the Tevatron pp? collider are also considered. We investigate in detail the leptoquark interpretation of the HERA data. First we assume a minimal leptoquark model with a single diagonal Yukawa coupling to first family lepton and quark mass eigenstates only. In this case constraints from atomic parity violating experiments allow only isodoublet scalar leptoquark production at HERA. This interpretation can be confirmed or ruled out in the near future by high luminosity data at the Tevatron. The Tevatron data already appear to rule out the vector leptoquark interpretation of the HERA data. We also consider a more general model which allows for all possible left-handed, right-handed, flavour and lepton number changing couplings. The allowed values of the Yukawa couplings of this general model offer several different interpretations of the data which are radically different from the minimal model solutions. However these somewhat ad hoc tuned solutions can easily be tested by future HERA experiments.