Charged particle cross-sections in photoproduction and extraction of the gluon density in the photon

Photoproduction data collected with the H1 detector at HERA in 1994 are used to study the cross-sections for inclusive charged particle production and the structure of the photon. The differential cross-sections dsigma/dpT2, for |eta| 2 GeV/c and pT > 3 GeV/c have been measured. Model calculations of these cross-sections, based on perturbative QCD, indicate that the results are sensitive to the parton densities of the photon as well as to higher order effects, which are phenomenologically treated by multiple interactions. This sensitivity is exploited to determine the leading order x_gamma distribution of partons in the photon using a new method based on high pT charged particles. The gluon content of the photon is extracted and found to rise with decreasing x_gamma

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 e^- and five events with a mu^+- 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.Comment: 18 pages, 5 figures, 2 tables, submitted to Eur. Phys.

Differential (2+1) jet event rates and determination of alpha(s) in deep inelastic scattering at HERA

Events with a (2+1) jet topology in deep-inelastic scattering at HERA are studied in the kinematic range 200 < Q2 < 10 000 GeV2. The rate of (2+1) jet events has been determined with the modified JADE jet algorithm as a function of the jet resolution parameter and is compared with the predictions of Monte Carlo models. In addition, the event rate is corrected for both hadronization and detector effects and is compared with next-to-leading order QCD calculations. A value of the strong coupling constant of αs(M2Z)=0.118±0.002 (stat.) +0.007-0.008 (syst.) +0.007-0.006 (theory) is extracted. The systematic error includes uncertainties in the calorimeter energy calibration, in the description of the data by current Monte Carlo models, and in the knowledge of the parton densities. The theoretical error is dominated by the renormalization scale ambiguity

Observation of events at very high Q**2 in e p collisions at HERA

Measurements of ep scattering with squared 4-momentum transfer Q^2 up to 35000 GeV^2 are compared with the expectation of the standard deep-inelastic model of lepton-nucleon scattering (DIS). For Q^2 > 15000 GeV^2, N_{obs} =12 neutral current candidate events are observed where the expectation is N_DIS = 4.71 \pm 0.76 events. In the same Q^2 range, N_{obs} =4 charged current candidates are observed where the expectation is N_{DIS} = 1.77 \pm 0.87 events. The probability P(N \geq N_{obs}) that the DIS model signal N fluctuates to N \geq N_{obs} in a random set of experiments is 6 10^{-3} 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^2/M^2.Comment: 30 pages, latex, 8 Figure