Dr Elizabeth Gallas

Abstract:

The results of a search for direct pair production of scalar top quarks (t̃), the supersymmetric partners of the top quark were presented. Events containing two or more jets and missing transverse energy, the signature of light scalar top quark decays to charm quarks and neutralinos were investigated. 27 events were observed after the selection, while expecting 31.1±6.4 events from known standard model processes. The results were compared to next-to-leading-order production cross sections, and a significant region of t̃ and neutralino phase space was excluded.

Abstract:

The standard model of particle physics contains parameters--such as particle masses--whose origins are still unknown and which cannot be predicted, but whose values are constrained through their interactions. In particular, the masses of the top quark (M(t)) and W boson (M(W)) constrain the mass of the long-hypothesized, but thus far not observed, Higgs boson. A precise measurement of M(t) can therefore indicate where to look for the Higgs, and indeed whether the hypothesis of a standard model Higgs is consistent with experimental data. As top quarks are produced in pairs and decay in only about 10(-24) s into various final states, reconstructing their masses from their decay products is very challenging. Here we report a technique that extracts more information from each top-quark event and yields a greatly improved precision (of +/- 5.3 GeV/c2) when compared to previous measurements. When our new result is combined with our published measurement in a complementary decay mode and with the only other measurements available, the new world average for M(t) becomes 178.0 +/- 4.3 GeV/c2. As a result, the most likely Higgs mass increases from the experimentally excluded value of 96 to 117 GeV/c2, which is beyond current experimental sensitivity. The upper limit on the Higgs mass at the 95% confidence level is raised from 219 to 251 GeV/c2.

Abstract:

We have searched for the signature of 3- and 4-body decays of pair-produced scalar top quarks (stop) in the inclusive final state containing an electron, a muon, and significant missing transverse energy using a sample of pp̄ events corresponding to 108.3 pb-1 of data collected with the DØ detector at Fermilab. The search is done in the framework of the minimal supersymmetric standard model assuming that the neutralino (χ̃10) is the lightest supersymmetric particle and is stable. No evidence for a signal is found and we derive cross-section upper limits as a function of stop (t̃) and neutralino masses in different decay scenarios leading to the bℓνχ̃10 final state. © 2004 Published by Elsevier B.V.

Abstract:

The results based on 1992-95 data (Run 1) from the CDF and D0 experiments on the measurements of the W boson mass and width are presented, along with the combined results. We report a Tevatron collider average MW = 80.456 ± 0.059 GeV. We also report the Tevatron collider average of the directly measured W boson width TW = 2.115 ± 0.105 GeV. We describe a new joint analysis of the direct W mass and width measurements. Assuming the validity of the standard model, we combine the directly measured W boson width with the width extracted from the ratio of W and Z boson leptonic partial cross sections. This combined result for the Tevatron is TW = 2.135 ± 0.050 GeV. Finally, we use the measurements of the direct total W width and the leptonic branching ratio to extract the leptonic partial width Γ(W → eν) = 224 ± 13 MeV.

Abstract:

We present the results of a search for the production of new particles decaying into two jets in [Formula Presented]collisions at [Formula Presented] using the DØ 1992–1995 data set corresponding to [Formula Presented] We exclude at the 95% confidence level the production of excited quarks [Formula Presented] with masses below 775 GeV/[Formula Presented] the most restrictive limit to date. We also exclude standard-model-like [Formula Presented][Formula Presented] bosons with masses between 300 and 800 GeV/[Formula Presented](400 and 640 GeV/[Formula Presented] A [Formula Presented]boson with mass [Formula Presented] has been excluded by previous measurements, and our lower limit is therefore the most stringent to date. © 2004 The American Physical Society.