Prof Roman Walczak

Abstract:

Bose-Einstein correlations of charged and neutral kaons have been measured in e ^± p deep inelastic scattering with an integrated luminosity of 121   pb ^-1 using the ZEUS detector at HERA. The two-particle correlation function was studied as a function of the four-momentum difference of the kaon pairs, Q 12 = sqrt(- (p 1 - p 2) ²), assuming a Gaussian shape for the particle source. The values of the radius of the production volume, r, and of the correlation strength, λ, were obtained for both neutral and charged kaons. The radii for charged and neutral kaons are similar and are consistent with those obtained at LEP. © 2007 Elsevier B.V. All rights reserved.

Abstract:

The production of energetic neutrons in ep collisions has been studied with the ZEUS detector at HERA. The neutron energy and pT² distributions were measured with a forward neutron calorimeter and tracker in a 40   pb^-1 sample of inclusive deep inelastic scattering (DIS) data and a 6   pb^-1 sample of photoproduction data. The neutron yield in photoproduction is suppressed relative to DIS for the lower neutron energies and the neutrons have a steeper pT² distribution, consistent with the expectation from absorption models. The distributions are compared to HERA measurements of leading protons. The neutron energy and transverse-momentum distributions in DIS are compared to Monte Carlo simulations and to the predictions of particle exchange models. Models of pion exchange incorporating absorption and additional secondary meson exchanges give a good description of the data. © 2007 Elsevier B.V. All rights reserved.

Abstract:

Diffractive photoproduction of D*^±(2010) mesons was measured with the ZEUS detector at the ep collider HERA, using an integrated luminosity of 78.6 pb^-1. The D* mesons were reconstructed in the kinematic range: transverse momentum pT(D*) > 1.9 GeV and pseudorapidity |η(D*)|<1.6, using the decay D*⁺→D⁰π⁺ s followed by D⁰→K^-π⁺(+c.c.). Diffractive events were identified by a large gap in pseudorapidity between the produced hadronic state and the outgoing proton. Cross sections are reported for photon-proton centre-of-mass energies in the range 130 < W < 300 GeV and for photon virtualities Q² < 1 GeV², in two ranges of the Pomeron fractional momentum xIP<0.035 and xIP<0.01. The relative contribution of diffractive events to the inclusive D*^±(2010) photoproduction cross section is about 6%. The data are in agreement with perturbative QCD calculations based on various parameterisations of diffractive parton distribution functions. The results are consistent with diffractive QCD factorisation. © Springer-Verlag Berlin Heidelberg 2007.