ATLAS

Abstract:

A resonance search has been made in the D*±p∓ invariant-mass spectrum with the ZEUS detector at HERA using an integrated luminosity of 126 pb-1. The decay channels D*+ -→ D0πs+ → (K -π+)πs+ and D*+ → D0πs+ → (K-π+π+π-) πs+ (and the corresponding antiparticle decays) were used to identify D*± mesons. No resonance structure was observed in the D*± p∓ mass spectrum from more than 60 000 reconstructed D*± mesons. The results are not compatible with a report of the H1 Collaboration of a charmed pentaquark, Θc0. © Springer-Verlag / Società Italiana di Fisica 2004.

Abstract:

The dependence of dijet production on the virtuality of the exchanged photon, Q2, has been studied by measuring dijet cross sections in the range 0 ≲ Q2 < 2000 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb-1. Dijet cross sections were measured for jets with transverse energy ETjet > 7.5 and 6.5 GeV and pseudorapidities in the photon-proton centre-of-mass frame in the range -3 < jet < 0. The variable xγobs, a measure of the photon momentum entering the hard process, was used to enhance the sensitivity of the measurement to the photon structure. The Q2 dependence of the ratio of low- to high-xγobs events was measured. Next-to-leading-order QCD predictions were found to generally underestimate the low-xγobs contribution relative to that at high xγobs. Monte Carlo models based on leading-logarithmic parton-showers, using a partonic structure for the photon which falls smoothly with increasing Q2, provide a qualitative description of the data. © Springer-Verlag / Società Italiana di Fisica 2004.