Inclusive dijet production at low Bjorken-x in deep inelastic scattering.

Dijet production in deep inelastic ep scattering is investigated in the region of low values of the Bjorken-variable x (10-4 < x < 10-2) and low photon virtualities Q2 (5 < Q2 < 100 GeV2). The measured dijet cross sections are compared with perturbative QCD calculations in next-to-leading order. For most dijet variables studied, these calculations can provide a reasonable description of the data over the full phase space region covered, including the region of very low x. However, large discrepancies at low x and low Q2 are observed for events with small separation in azimuth between the two highest transverse momentum jets. This region of phase space is described better by predictions based on the CCFM evolution equation, which incorporates kt factorized unintegrated parton distributions. A reasonable description can be also obtained using the Color Dipole Model or models incorporating virtual photon structure

Forward pi^0 production and associated transverse energy flow in deep-inelastic scattering at HERA.

Deep-inelastic positron-proton interactions at low values of Bjorken-x down to which give rise to high transverse momentum -mesons are studied with the H1 experiment at HERA. The inclusive cross section for -mesons produced at small angles with respect to the proton remnant (the forward region) is presented as a function of the transverse momentum and energy of the and of the four-momentum transfer Q 2 and Bjorken-x. Measurements are also presented of the transverse energy flow in events containing a forward -meson. Hadronic final state calculations based on QCD models implementing different parton evolution schemes are confronted with the data

A general search for new phenomena in ep scattering at HERA

A model-independent search for deviations from the Standard Model prediction is performed in e^+ p and e^- p collisions at HERA using H1 data corresponding to an integrated luminosity of 117 pb^-1. For the first time all event topologies involving isolated electrons, photons, muons, neutrinos and jets with high transverse momenta are investigated in a single analysis. Events are assigned to exclusive classes according to their final state. A statistical algorithm is developed to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta or invariant mass of final state particles and to quantify their significance. A good agreement with the Standard Model prediction is observed in most of the event classes. The most significant deviation is found for a topology containing an isolated muon, missing transverse momentum and a jet, consistent with a previously reported observation.Comment: 23 pages, 6 figures and 4 table

Measurement of prompt photon cross sections in photoproduction at HERA

Results are presented on the photoproduction of isolated prompt photons, inclusively and associated with jets, in the γp center of mass energy range 142 4.5 GeV. They are measured differentially as a function of Eγ T,Ejet T , the pseudorapidities ηγ and ηjet and estimators of the momentum fractions xγ and xp of the incident photon and proton carried by the constituents participating in the hard process. In order to further investigate the underlying dynamics, the angular correlation between the prompt photon and the jet in the transverse plane is studied. Predictions by perturbative QCD calculations in next to leading order are about 30% below the inclusive prompt photon data after corrections for hadronisation and multiple interactions, but are in reasonable agreement with the results for prompt photons associated with jets. Comparisons with the predictions of the event generators PYTHIA and HERWIG are also presented

Search for squark production in R-parity violating supersyymety at HERA.

A search for squarks in R-parity violating supersymmetry is performed in collisions at HERA using the H1 detector. The data were taken at a centre-of-mass energy of 319 GeV and correspond to an integrated luminosity of 64.3 pb-1 for e + p collisions and 13.5 pb-1 for e - p collisions. The resonant production of squarks via a Yukawa coupling is considered, taking into account direct and indirect R-parity violating decay modes. No evidence for squark production is found in the multi-lepton and multi-jet final state topologies investigated. Mass dependent limits on are obtained in the framework of the Minimal Supersymmetric Standard Model. In addition, the results are interpreted in terms of constraints on the parameters of the minimal Supergravity model. At the confidence level squarks of all flavours with masses up to 275 GeV are excluded in a large part of the parameter space for a Yukawa coupling of electromagnetic strength. For a coupling strength 100 times smaller, masses up to 220 GeV can be ruled out

Inclusive production of D+,D0,D+s and D∗+ mesons in deep inelastic scattering at HERA

Inclusive production cross sections are measured in deep inelastic scattering at HERA for meson states composed of a charm quark and a light antiquark or the charge conjugate. The measurements cover the kinematic region of photon virtuality 2 < Q2 < 100 GeV2, inelasticity 0.05 < y < 0.7, D meson transverse momenta pt(D) ≥ 2.5 GeV and pseudorapidity |η(D)| ≤ 1.5. The identification of the D-meson decays and the reduction of the combinatorial background profit from the reconstruction of displaced secondary vertices by means of the H1 silicon vertex detector. The production of charmed mesons containing the light quarks u, d and s is found to be compatible with a description in which the hard scattering is followed by a factorisable and universal hadronisation process

Measurement of Dijet Production at Low Q2Q^{2} at HERA

The analyses of jet production in various high energy processes have become a major field for testing perturbative QCD. In the case of electron-proton collisions, investigated in the thesis, the jet cross sections are successfully described in most of the HERA kinematic range by next-to-leading (NLO) QCD calculations. However, regions of phase space have previously been identified where NLO predictions do not reproduce the data satisfactorily. The presented analysis describes a new measurement of the dijet cross sections together with detailed comparisons of the data with available perturbative QCD calculations in order to identify which of them describe the data in which region. Doing that we use NLO QCD calculations as well as LO calculations supplemented with parton showers, which take into account leading logarithmic contributions to all orders.Available from STL Prague, CZ / NTK - National Technical LibrarySIGLECZCzech Republi

Measurement of anti-deutron production and a search for heavy stable charged particles at HERA.

The cross section for anti-deuteron photoproduction is measured at HERA at a mean centre-of-mass energy of GeV in the range 0.2 < p T /M < 0.7 and |y| < 0.4, where M, p T and y are the mass, transverse momentum and rapidity of the anti-deuteron in the HERA laboratory frame, respectively. The numbers of anti-deuterons per event are found to be similar in photoproduction to those in central proton-proton collisions at the CERN ISR but much lower than those in central Au-Au collisions at RHIC. The coalescence parameter B 2, which characterizes the likelihood of anti-deuteron production, is measured in photoproduction to be , which is much higher than in Au-Au collisions at a similar nucleon-nucleon centre-of-mass energy. No significant production of particles heavier than deuterons is observed and upper limits are set on the photoproduction cross sections for such particles