ATLAS

Abstract:

This paper reports a measurement of the cross section for the pair production of top quarks in pp̄ collisions at s=1.96TeV at the Fermilab Tevatron. The data were collected from the CDF run II detector in a set of runs with a total integrated luminosity of 1.1fb-1. The cross section is measured in the dilepton channel, the subset of tt̄ events in which both top quarks decay through t→Wb→ νb, where =e, μ, or τ. The lepton pair is reconstructed as one identified electron or muon and one isolated track. The use of an isolated track to identify the second lepton increases the tt̄ acceptance, particularly for the case in which one W decays as W→τν. The purity of the sample may be further improved at the cost of a reduction in the number of signal events, by requiring an identified b jet. We present the results of measurements performed with and without the request of an identified b jet. The former is the first published CDF result for which a b-jet requirement is added to the dilepton selection. In the CDF data there are 129 pretag lepton+track candidate events, of which 69 are tagged. With the tagging information, the sample is divided into tagged and untagged subsamples, and a combined cross section is calculated by maximizing a likelihood. The result is σtt̄=9.6±1.2(stat)-0.5+0.6(sys)±0.6(lum)pb, assuming a branching ratio of BR(W→ ν)=10.8% and a top mass of mt=175GeV/c2. © 2009 The American Physical Society.

Abstract:

We report a measurement of the ratio of branching fractions of the decays B±→J/ψπ± and B±→J/ψK± using the CDF II detector at the Fermilab Tevatron Collider. The signal from the Cabbibo-suppressed B±→J/ψπ± decay is separated from B±→J/ψK± using the B±→J/ψK± invariant mass distribution and the kinematical differences of the hadron track in the two decay modes. From a sample of 220pb-1 of pp̄ collisions at s=1.96TeV, we observe 91±15 B±→J/ψπ± events together with 1883±34 B±→J/ψK± events. The ratio of branching fractions is found to be B(B±→J/ψπ±)/ B(B±→J/ψK±)=(4.86±0.82(stat)±0.15(syst))%. © 2009 The American Physical Society.

Abstract:

We report on a study of the relative rates of B meson decays into ψ(2S) and J/ψ mesons using 1.3fb-1 of pp̄ collisions at s=1.96TeV recorded by the D0 detector operating at the Fermilab Tevatron Collider. We observe the channels Bs0→ψ(2S), Bs0→J/ψ, B±→ψ(2S) K±, and B±→J/ψK± and we measure the relative branching fractions for these channels to be B(Bs0→ψ(2S))B(Bs0→J/ ψ)=0.53±0.10(stat)±0.07(syst)±0.06(B), B(B±→ψ(2S)K±)B(B±→J/ψK±)=0. 63±0.05(stat)±0.03(syst)±0.07(B),where the final error corresponds to the uncertainty in the J/ψ and ψ(2S) branching ratio into two muons. © 2009 The American Physical Society.