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The electroproduction of and mesons is studied in elastic, quasi-elastic and inclusive reactions for four momentum transfers and photon-proton centre of mass energies GeV. The data were taken with the H1 detector at the electron proton collider HERA in the years 1995 to 1997. The total virtual photon-proton cross section for elastic production is measured as a function of and W. The dependence of the production rates on the square of the momentum transfer from the proton (t) is extracted. Decay angular distributions are analysed and the ratio of the longitudinal and transverse cross sections is derived. The ratio of the cross sections for quasi-elastic and meson production is measured as a function of . The results are discussed in terms of theoretical models based upon perturbative QCD. Differential cross sections for inclusive and inelastic production of mesons are determined and predictions within two theoretical frameworks are compared with the data, the non-relativistic QCD factorization approach including colour octet and colour singlet contributions, and the model of Soft Colour Interactions.

Deep-inelastic scattering events with a leading baryon have been detected by the H1 experiment at HERA using a forward proton spectrometer and a forward neutron calorimeter. Semi-inclusive cross sections have been measured in the kinematic region \\(2 \\le Q^2 \\le 50\\,{\\mathrm GeV}^2\\), \\(6\\times 10^{-5}\\le x\\le 6\\times10^{-3}\\) and baryon \\(p_{\\rm T} \\le 200\\) MeV, for events with a final-state proton with energy \\(580 \\le E^{\\prime} \\le 740\\) GeV, or a neutron with energy \\(E^{\\prime} \\ge 160\\) GeV. The measurements are used to test production models and factorization hypotheses. A Regge model of leading baryon production which consists of pion, pomeron and secondary reggeon exchanges gives an acceptable description of both semi-inclusive cross sections in the region \\(0.7 \\le E^{\\prime} / E_{\\rm p} \\le 0.9\\), where \\(E_{\\rm p}\\) is the proton beam energy. The leading neutron data are used to estimate for the first time the structure function of the pion at small Bjorken-x.

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.

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 \\({\\rm d}\\sigma/{\\rm d}p_T^2\\), for \\(|\\eta| < 1\\) in the HERA laboratory frame, and \\({\\rm d}\\sigma/{\\rm d}\\eta\\) for \\(p_T> 2\\) GeV/c and \\(p_T> 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 \\(p_T\\) charged particles. The gluon content of the photon is extracted and found to rise with decreasing \\(x_{\\gamma}\\).

The multiplicity structure of the hadronic system X produced in deep-inelastic processes at HERA of the type , where Y is a hadronic system with mass GeV and where the squared momentum transfer at the pY vertex, t, is limited to , is studied as a function of the invariant mass of the system X. Results are presented on multiplicity distributions and multiplicity moments, rapidity spectra and forward-backward correlations in the centre-of-mass system of X. The data are compared to results in annihilation, fixed-target lepton-nucleon collisions, hadro-produced diffractive final states and to non-diffractive hadron-hadron collisions. The comparison suggests a production mechanism of virtual photon dissociation which involves a mixture of partonic states and a significant gluon content. The data are well described by a model, based on a QCD-Regge analysis of the diffractive structure function, which assumes a large hard gluonic component of the colourless exchange at low . A model with soft colour interactions is also successful.

Jet event rates in deep inelastic ep scattering at HERA are investigated applying the modified JADE jet algorithm. The data are corrected for detector and hadronization effects and then compared with perturbative QCD predictions using next-to-leading order calculations. The strong coupling constant \\(\\alpha_s (M^2_Z)\\) is determined evaluating the jet event rates. Values of \\(\\alpha_s (Q^2)\\) are extracted in four different bins of the negative squared momentum transfer \\(Q^2\\) in the range from 40 GeV\\(^2\\) to 4000 GeV\\(^2\\). A combined fit of the renormalization group equation to these several \\(\\alpha_s (Q^2)\\) values results in \\(\\alpha_s (M^2_Z) = 0.117 \\pm 0.003 (stat)^{+0.009}_{0.013} (sys) + 0.006 (jet algorithm)\\)