13 research outputs found
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.
Measurement of D* meson cross-sections at HERA and determination of the gluon density in the proton using NLO QCD
Measurement of D* meson cross-sections at HERA and determination of the gluon density in the proton using NLO QCD
With the H1 detector at the ep collider HERA, D* meson production cross
sections have been measured in deep inelastic scattering with four-momentum
transfers Q^2>2 GeV2 and in photoproduction at energies around W(gamma p)~ 88
GeV and 194 GeV. Next-to-Leading Order QCD calculations are found to describe
the differential cross sections within theoretical and experimental
uncertainties. Using these calculations, the NLO gluon momentum distribution in
the proton, x_g g(x_g), has been extracted in the momentum fraction range
7.5x10^{-4}< x_g <4x10^{-2} at average scales mu^2 =25 to 50 GeV2. The gluon
momentum fraction x_g has been obtained from the measured kinematics of the
scattered electron and the D* meson in the final state. The results compare
well with the gluon distribution obtained from the analysis of scaling
violations of the proton structure function F_2.Comment: 27 pages, 9 figures, 2 tables, submitted to Nucl. Phys.
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
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