Professor Chris Hays

Abstract:

We search for direct pair production of supersymmetric top quarks and supersymmetric bottom quarks in proton-antiproton collisions at s=1.96TeV, using 295pb-1 of data recorded by the Collider Detector at Fermilab (CDF II) experiment. The supersymmetric top (supersymmetric bottom) quarks are selected by reconstructing their decay into a charm (bottom) quark and a neutralino, which is assumed to be the lightest supersymmetric particle. The signature of such processes is two energetic heavy-flavor jets and missing transverse energy. The number of events that pass our selection for each search process is consistent with the expected standard model background. By comparing our results to the theoretical production cross sections of the supersymmetric top and supersymmetric bottom quarks in the minimal supersymmetric standard model, we exclude, at a 95% confidence level in the frame of that model, a supersymmetric top quark mass up to 132GeV/c2 for a neutralino mass of 48GeV/c2, and a supersymmetric bottom quark mass up to 193GeV/c2 for a neutralino mass of 40GeV/c2. © 2007 The American Physical Society.

Abstract:

We report the measurements of the tt̄ production cross section and of the top quark mass using 1.02fb-1 of pp̄ data collected with the CDF II detector at the Fermilab Tevatron. We select events with six or more jets on which a number of kinematical requirements are imposed by means of a neural network algorithm. At least one of these jets must be identified as initiated by a b-quark candidate by the reconstruction of a secondary vertex. The cross section is measured to be σtt̄=8.3±1.0(stat)-1.5+2.0(syst) ±0.5(lumi)pb, which is consistent with the standard model prediction. The top quark mass of 174.0±2.2(stat)±4.8(syst)GeV/c2 is derived from a likelihood fit incorporating reconstructed mass distributions representative of signal and background. © 2007 The American Physical Society.

Abstract:

We have performed a search for new particles which decay to two photons using 1.2fb-1 of integrated luminosity from pp̄ collisions at s=1.96TeV collected using the CDF II detector at the Fermilab Tevatron. We find the diphoton mass spectrum to be in agreement with the standard model expectation, and set limits on the cross section times branching ratio for the Randall-Sundrum graviton, as a function of diphoton mass. We subsequently derive lower limits for the graviton mass of 230GeV/c2 and 850GeV/c2, at the 95% confidence level, for coupling parameters (k/M̄Pl) of 0.01 and 0.1, respectively. © 2007 The American Physical Society.

Abstract:

We report the results of a search for a narrow resonance in electron-positron events in the invariant mass range of 150-950GeV/c2 using 1.3fb-1 of pp̄ collision data at s=1.96TeV collected by the CDF II detector at Fermilab. No significant evidence of such a resonance is observed and we interpret the results to exclude the standard-model-like Z′ with a mass below 923GeV/c2 and the Randall-Sundrum graviton with a mass below 807GeV/c2 for k/M̄pl=0.1, both at the 95% confidence level. Combining with diphoton data excludes the Randall-Sundrum graviton for masses below 889GeV/c2 for k/M̄pl=0.1. © 2007 The American Physical Society.

Abstract:

We present the results of a search for new particles that lead to a Z boson plus jets in pp̄ collisions at s=1.96TeV using the Collider Detector at Fermilab (CDF II). A data sample with a luminosity of 1.06fb-1 collected using Z boson decays to ee and μμ is used. We describe a completely data-based method to predict the dominant background from standard model Z+jet events. This method can be similarly applied to other analyses requiring background predictions in multijet environments, as shown when validating the method by predicting the background from W+jets in tt̄ production. No significant excess above the background prediction is observed, and a limit is set using a fourth generation quark model to quantify the acceptance. Assuming BR(b′→bZ)=100% and using a leading-order calculation of the b′ cross section, b′ quark masses below 268GeV/c2 are excluded at 95% confidence level. © 2007 The American Physical Society.