LHCb

Abstract:

We present the first observation of exclusive e+e- production in hadron-hadron collisions, using pp̄ collision data at s=1.96TeV taken by the run II Collider Detector at Fermilab, and corresponding to an integrated luminosity of 532pb-1. We require the absence of any particle signatures in the detector except for an electron and a positron candidate, each with transverse energy ET>5GeV and pseudorapidity |η|<2. With these criteria, 16 events are observed compared to a background expectation of 1.9±0.3 events. These events are consistent in cross section and properties with the QED process pp̄→p+e+e-+p̄ through two-photon exchange. The measured cross section is 1.6-0.3+0.5(stat)±0.3(syst)pb. This agrees with the theoretical prediction of 1.71±0.01pb. © 2007 The American Physical Society.

Abstract:

We report a measurement of the top-quark mass, Mt, in the dilepton decay channel of tt̄→b ′+ν ′b̄ -ν̄ using an integrated luminosity of 1.0fb-1 of pp̄ collisions collected with the CDF II detector. We apply a method that convolutes a leading-order matrix element with detector resolution functions to form event-by-event likelihoods; we have enhanced the leading-order description to describe the effects of initial-state radiation. The joint likelihood is the product of the likelihoods from 78 candidate events in this sample, which yields a measurement of Mt=164.5±3.9(stat.)±3.9(syst.)GeV/c2, the most precise measurement of Mt in the dilepton channel. © 2007 The American Physical Society.

Abstract:

We report the observation of decays B0→Ds(*)+π- and B0→Ds(*)-K+ in a sample of 230×106 Υ(4S)→BB̄ events recorded with the BABAR detector at the SLAC PEP-II asymmetric-energy e+e- storage ring. We measure the branching fractions B(B0→Ds+π-)=(1. 3±0.3(stat)±0.2(syst))×10-5, B(B0→Ds-K+)=(2.5±0. 4(stat)±0.4(syst))×10-5, B(B0→Ds*+π-)=(2.8±0. 6(stat)±0.5(syst))×10-5, and B(B0→Ds*-K+)=(2.0±0. 5(stat)±0.4(syst))×10-5. The significances of the measurements to differ from zero are 5, 9, 6, and 5 standard deviations, respectively. This is the first observation of B0→Ds+π-, B0→Ds*+π-, and B0→Ds*-K+ decays. © 2007 The American Physical Society.

Abstract:

We report an upper limit on the fraction of V+A current, fV+A, in top-quark decays, using approximately 700pb-1 of pp̄ collisions at s=1.96TeV acquired by the upgraded Collider Detector at Fermilab. For the decay t→Wb→ νb (where =e or μ), the invariant mass of the charged lepton and the bottom quark jet is sensitive to the polarization of the W boson. We determine fV+A=-0.06±0.25 given a top-quark mass of 175GeV/c2. We set an upper limit on fV+A of 0.29 at the 95% confidence level, an improvement by a factor of 2 on the previous best direct limit. © 2007 The American Physical Society.