Future Colliders

Abstract:

A search for events containing two or more high-transverse-momentum isolated leptons has been performed in ep collisions with the ZEUS detector at HERA using the full collected data sample, corresponding to an integrated luminosity of 480 pb -1. The number of observed events has been compared with the prediction from the Standard Model, searching for possible deviations, especially for multi-lepton events with invariant mass larger than 100 GeV. Good agreement with the Standard Model has been observed. Total and differential cross sections for di-lepton production have been measured in a restricted phase space dominated by photon-photon collisions. © 2009 Elsevier B.V.

Abstract:

We present a signature-based search for the anomalous production of events containing a photon, two jets, of which at least one is identified as originating from a b quark, and missing transverse energy (E T). The search uses data corresponding to 2.0fb-1 of integrated luminosity from pp̄ collisions at a center-of-mass energy of s=1.96TeV, collected with the CDF II detector at the Fermilab Tevatron. From 6.69747×106 events with a photon candidate with transverse energy ET>25GeV, we find 617 events with E T>25GeV and two or more jets with ET>15GeV, at least one identified as originating from a b quark, versus an expectation of 607±113 events. Increasing the requirement on E T to 50 GeV, we find 28 events versus an expectation of 30±11 events. We find no indications of non-standard-model phenomena. © 2009 The American Physical Society.

Abstract:

We present the first measurement of the mass of the top quark in a sample of tt̄→ ν̄bb̄qq̄ events (where =e,μ) selected by identifying jets containing a muon candidate from the semileptonic decay of heavy-flavor hadrons (soft muon b tagging). The pp̄ collision data used correspond to an integrated luminosity of 2fb-1 and were collected by the CDF II detector at the Fermilab Tevatron Collider. The measurement is based on a novel technique exploiting the invariant mass of a subset of the decay particles, specifically the lepton from the W boson of the t→Wb decay and the muon from a semileptonic b decay. We fit template histograms, derived from simulation of tt̄ events and a modeling of the background, to the mass distribution observed in the data and measure a top quark mass of 180.5±12.0(stat) ±3.6(syst)GeV/c2, consistent with the current world average value. © 2009 The American Physical Society.

Abstract:

We have used the Collider Detector at Fermilab (CDF-II) to search for the flavor-changing neutral-current (FCNC) top-quark decay t→Zc using a technique employing ratios of W and Z production, measured in pp̄ data corresponding to an integrated luminosity of 1.52fb-1. The analysis uses a comparison of two decay chains, pp̄→tt̄→WbWb→νbjjb and pp̄→tt̄→ZcWb→cjjb, to cancel systematic uncertainties in acceptance, efficiency, and luminosity. We validate the modeling of acceptance and efficiency for lepton identification over the multiyear data set using another ratio of W and Z production, in this case the observed ratio of inclusive production of W to Z bosons. To improve the discrimination against standard model backgrounds to top-quark decays, we calculate the top-quark mass for each event with two leptons and four jets assuming it is a tt̄ event with one of the top quarks decaying to Zc. For additional background discrimination we require at least one jet to be identified as originating from a b quark. No significant signal is found and we set an upper limit on the FCNC branching ratio Br(t→Zc) using a likelihood constructed from the cjjb top-quark mass distribution and the number of νbjjb events. Limits are set as a function of the helicity of the Z boson produced in the FCNC decay. For 100% longitudinally-polarized Z bosons we find limits of 8.3% and 9.3% (95% C.L.) depending on the assumptions regarding the theoretical top-quark pair production cross section. © 2009 The American Physical Society.