The diffractive photoproduction of rho mesons, e p \to e rho Y, with large momentum transfer squared at the proton vertex, |t|, is studied with the H1 detector at HERA using an integrated luminosity of 20.1 pb^{-1}. The photon-proton centre of mass energy spans the range 75 < W < 95 GeV, the photon virtuality is restricted to Q^2 < 0.01 GeV^2 and the mass M_Y of the proton remnant is below 5 GeV. The t dependence of the cross section is measured for the range 1.5 < |t| < 10.0 GeV^2 and is well described by a power law, dsigma/ d|t| \propto |t|^{-n}. The spin density matrix elements, which provide information on the helicity structure of the interaction, are extracted using measurements of angular distributions of the rho decay products. The data indicate a violation of s-channel helicity conservation, with contributions from both single and double helicity-flip being observed. The results are compared to the predictions of perturbative QCD models.
The normalized differential cross section as a function of T.
Normalised decay angular distribution w.r.t. the polar angle THETA.
Normalised decay angular distribution w.r.t. the polar angle THETA.
Deep-inelastic ep scattering data taken with the H1 detector at HERA and corresponding to an integrated luminosity of 106 pb^{-1} are used to study the differential distributions of event shape variables. These include thrust, jet broadening, jet mass and the C-parameter. The four-momentum transfer Q is taken to be the relevant energy scale and ranges between 14 GeV and 200 GeV. The event shape distributions are compared with perturbative QCD predictions, which include resummed contributions and analytical power law corrections, the latter accounting for non-perturbative hadronisation effects. The data clearly exhibit the running of the strong coupling alpha_s(Q) and are consistent with a universal power correction parameter alpha_0 for all event shape variables. A combined QCD fit using all event shape variables yields alpha_s(mZ) = 0.1198 \pm 0.0013 ^{+0.0056}_{-0.0043} and alpha_0 = 0.476 \pm 0.008 ^{+0.018} _{-0.059}.
Normalised distribution of (1-THRUST) where THRUST is w.r.t the axis which maximises the sum of the longitudinal momenta in the current hemisphere, for Q = 14.0 to 16.0 GeV and X = 0.00841 .
Normalised distribution of (1-THRUST) where THRUST is w.r.t the axis which maximises the sum of the longitudinal momenta in the current hemisphere, for Q = 16.0 to 20.0 GeV and X = 0.01180 .
Normalised distribution of (1-THRUST) where THRUST is w.r.t the axis which maximises the sum of the longitudinal momenta in the current hemisphere, for Q = 20.0 to 30.0 GeV and X = 0.02090 .
The multiplicity structure of the hadronic system X produced in deep-inelastic processes at HERA of the type ep -> eXY, where Y is a hadronic system with mass M_Y< 1.6 GeV and where the squared momentum transfer at the pY vertex, t, is limited to |t|<1 GeV^2, is studied as a function of the invariant mass M_X 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 e+e- 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 Q^2. A model with soft colour interactions is also successful.
The multiplicity moment MULT as a function of the mass of the charged hadron system in the full phase space and separately in the forward and backward hemispheres.
The multiplicity moment DISPERSION as a function of the mass of the charged hadron system in the full phase space and separately in the forward and backward hemispheres.
The multiplicity moment R2 as a function of the mass of the charged hadron system in the full phase space and separately in the forward and backward hemispheres.
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