ATLAS

Abstract:

The azimuthal distribution of jets produced in the Breit frame in high-Q² deep inelastic e⁺p scattering has been studied with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb-¹. The measured azimuthal distribution shows a structure that is well described by next-to-leading-order QCD predictions over the Q² range considered, Q² > 125 GeV². © 2002 Elsevier Science B.V. All rights reserved.

Abstract:

Diffractive photoproduction of vector mesons, γp → VY, where Y is a proton-dissociative system, has been measured in e⁺p interactions with the ZEUS detector at HERA using an integrated luminosity of 25 pb^-1. The differential cross section, dσ/dt, is presented for 1.2 < -t < 12 GeV², where t is the square of the four-momentum transferred to the vector meson. The data span the range in photon-proton centre-of-mass energy, W, from 80 GeV to 120 GeV. The t distributions are well fit by a power law, dσ/dt ∝ (-t)^-n. The slope of the effective Pomeron trajectory, measured from the W dependence of the ρ⁰ and φ cross sections in bins of t, is consistent with zero. The ratios dσγp→φY/dt to dσγp→ρ0Y/dt and dσγp→J/ψY/dt to dσγp→ρ0Y/dt increase with increasing -t. Decay-angle analyses for ρ⁰, φ and J/ψ mesons have been carried out. For the ρ⁰ and φ mesons, contributions from single and double helicity flip are observed. The results are compared to expectations of theoretical models.

Abstract:

Inclusive momentum distributions of charged particles in restricted cones around jet axes were measured in dijet events with invariant dijet masses in the range 80 to 600 GeV/c2. Events were produced at the Fermilab Tevatron in pp collisions with a center of mass energy of 1.8 TeV and recorded by the Collider Detector at Fermilab. The results were compared to perturbative QCD calculations carried out in the framework of the modified leading log approximation (MLLA) and assuming local parton-hadron duality. It was shown that the data follow theoretical predictions quite well over the whole range of the jet energies included in this analysis. We extracted the MLLA cutoff scale Qe f f and found a value of 230±40 MeV. The theoretical prediction of Ejetsin θc scaling, where u c is the cone opening angle, was experimentally observed for the first time. From the MLLA fits to the data, two more parameters were extracted: the ratio of parton multiplicities in gluon and quark jets, r = Npartonsg-jet /Npartonsq-jet = 1.9±0.5, and the ratio of the number of charged hadrons to the number of predicted partons in a jet, KLPHDcharged=Nhadronscharged/Npartons=0.56±0.10.

Abstract:

A search for narrow-width resonances that decay into electron+jet or neutrino+jet has been performed with the ZEUS detector at the DESY ep collider HERA operating at center-of-mass energies of 300 and 318 GeV. An integrated e⁺p luminosity of 114.8 pb^-1 and e^-p luminosity of 16.7 pb^-1 were used. No evidence for any resonance was found. Limits were derived on the Yukawa coupling λ as a function of the mass of a hypothetical resonance that has arbitrary decay branching ratios into eq or vq. These limits also apply to squarks predicted by R-parity-violating supersymmetry. Limits for the production of leptoquarks described by the Buchmüller-Rückl-Wyler model were also derived for masses up to 400 GeV. For λ = 0.1, leptoquark masses up to 290 GeV are excluded.

Abstract:

We report on a search for the flavor-changing neutral current decay D ⁰→µ⁺µ^- in pp^- collisions at √s = 1.96 TeV using 65 pb^-1 of data collected by the CDF II experiment at the Fermilab Tevatron Collider. A displaced-track trigger selects long-lived D⁰ candidates in the D⁰→µ ⁺µ^- search channel, the kinematically similar D ⁰→П⁺p^- channel used for normalization, the Cabibbo-favored D⁰K^-П^- channel used to optimize the selection criteria in an unbiased manner, and their charge conjugates. Finding no signal events in the D⁰→µ⁺µ^- search window, we set an upper limit on the branching fraction B(D⁰→µ⁺µ^-)≤2.5×10^-6 (3.3×10^-6) at the 90% (95%) confidence level.