Dr Elizabeth Gallas

Abstract:

The energy loss spectrum of 150 GeV muons has been measured with a prototype of the ATLAS hadron calorimeter in the H8 beam of the CERN SPS. The differential probability dP/dυ per radiation length of a fractional energy loss υ = ΔEμ/Eμ has been measured in the range υ = 0.01 ÷ 0.95; it is compared with the theoretical predictions for energy losses due to bremsstrahlung and production of electron-positron pairs or of energetic knock-on electrons. The integrated probability ∫0.010.95(dP/dυ)dυ is (1.610 ± 0.015stat ±0 .105syst)·10-3 in agreement with the theoretical predictions 1.556·10-3 and 1.619·10-3. Agreement with theory is also found in two intervals of υ where production of electron-positron pairs and knock-on electrons dominates. In the region of bremsstrahlung dominance (υ = 0.12 ÷ 0.95) the measured integrated probability (1.160 ± 0.040stat ± 0.075syst)·10-4 is in agreement with the theoretical value of 1.185·10-4, obtained using the Petrukhin and Shestakov description of the bremsstrahlung process. The same result is about 3.6 standard deviations (defined as the quadratic sum of statistical and systematic errors) lower than the theoretical prediction of 1.472·10-4, obtained using Tsai's description of bremsstrahlung. © Springer-Verlag 1997.

Abstract:

We measure the top quark mass mt t¯t using pairs produced in the D0 detector by √s = 1.8 TeV p ¯p collisions in a 125pb-1 exposure at the Fermilab Tevatron. We make a two constraint fit to mt in t¯t → b W+ ¯bW- final states with one W decaying to q¯q and the other to ev or μ v. Events are binned in fit mass versus a measure of probability for events to be signal rather than background. Likelihood fits to the data yield mt = 17.3 ± 5.6 (stat) ± 6.2(syst) GeV/c2. © 1998 The American Physical Society.

Abstract:

We present a measurement of the t¯t production cross section in p¯p collisions at √s=1.8TeV by the D0 experiment at the Fermilab Tevatron. The measurement is based on data from an integrated luminosity of approximately 125pb−1 accumulated during the 1992–1996 collider run. We observe 39 t¯t candidate events in the dilepton and lepton+jets decay channels with an expected background of 13.7 ± 2.2 events. For a top quark mass of 173.3 GeV/c2, we measure the t¯t production cross section to be 5.5 ± 1.8 pb. © 1997 The American Physical Society.

Abstract:

We report on a search for pair production of a fourth generation charge -1/3 quark (b’) in pp collisions at s = 1.8 TeV by the DØ experiment at the Fermilab Tevatron using an integrated luminosity of 93 pb-1. Both b’ quarks are assumed to decay via flavor changing neutral currents (FCNC). The search uses the signatures γ + 3 jets + μ-tag and 2γ + 2 jets. We see no significant excess of events over the expected background. We place an upper limit on the production cross section times branching fraction that is well below theoretical expectations for a b’ decaying exclusively via FCNC for b’ masses up to mz + mb. © 1996 American Physical Society.

Abstract:

A search for new physics has been carried out in the channel pp → γγ + ET, as expected in various supersymmetric models. We require two photons with transverse energy EγT > 12 GeV and pseudorapidity |ηγ < 1.1. The distribution of missing transverse energy (ET) is consistent with background and no events have ET > 25 GeV. We set limits on production cross sections for selectron, sneutrino, and neutralino pairs, decaying into photons, which range from about 400 fb to 1 pb depending on the sparticle masses. A general limit of 185 fb (95% C.L.) is set on σ(pp → γγET + X), where eγT > 12 GeV, |ηγ| < 1.1, and ET > 25 GeV. © 1997 The American Physical Society.