Search For Flavor Changing Neutral Current Top Quark Decays T Hq With H Bb In Pp Collisions At S 8 Tev With The Atlas Detector

This PhD thesis focuses on the search for flavor-changing neutral currents in the decay of a top quark to an up-type quark (q = u, c) and the Standard Model Higgs boson, where the Higgs boson decays to bb. Further, the thesis presents the combination of this search for top quark pair events with other ATLAS searches – in the course of which the most restrictive bounds to date on tqH interactions were obtained. Following on from the discovery of the Higgs boson, it is particularly important to measure the Yukawa couplings of the Standard Model fermions; these parameters may provide crucial insights to help solve the flavor puzzle and may help reveal the presence of new physics before it is directly observed in experiments.

Esta tesis presenta la búsqueda de decaimientos del quark top, via corrientes neutras que cambian el sabor, a quarks tipo u (q = u; c) y a el bosón de Higgs del modelo estándar. El análisis busca eventos donde un quark top decae a Wb y el W decae en leptones, mientras que el otro quark top decae a Hq. El estudio se basa en colisiones protón-protón a una energía en el centro de masa de √ s = 8 TeV obtenidas en 2012 con el detector ATLAS del Large Hadron Collider del CERN, usando una muestra de 20.3 fb-1. Los datos corresponden a eventos con un electrón o muon aislado y al menos cuatro jets en el estado final. La búsqueda explota la alta multiplicidad de quarks b que caracterizan la señal, empleando un discriminante basado en la verosimilitud que utiliza las diferencias cinemáticas entre la señal y el fondo (el cual está dominado por eventos tt—›WbWb). No se observan eventos en exceso del fondo esperado. Los límites de un 95% de confianza para el proceso observado (esperado) son de 0.56% (0.42%) para t —›Hc y de 0.61% (0.64%) para t—›Hu. Los límites de esta búsqueda han sido combinados con las búsquedas en los canales con modos de decaimiento H—› y H—›WW*; tt , logrando mejorar significativamente la sensibilidad. Los límites de un 95% de confianza para la combinación observada (esperada) es de 0.46% (0.25%) para t—› Hc y 0.45% (0.29%) para t ! Hu respectivamente. Los límites correspondientes en el acoplamiento.

A search for flavour changing neutral current (FCNC) processes in top-quark decays by the ATLAS Collaboration is presented. Data collected from pp collisions at the LHC at a centre-of-mass energy of √s = 7 TeV during 2011, corresponding to an integrated luminosity of 2.1 fb−1, were used. A search was performed for top-quark pair-production events, with one top quark decaying through the t → Zq FCNC (q = u, c) channel, and the other through the Standard Model dominant mode t → Wb. Only the decays of the Z boson to charged leptons and leptonic W-boson decays were considered as signal. Consequently, the final-state topology is characterised by the presence of three isolated charged leptons, at least two jets and missing transverse momentum from the undetected neutrino. No evidence for an FCNC signal was found. An upper limit on the t → Zq branching ratio of BR(t → Zq)

A search for flavour changing neutral current (FCNC) processes in top-quark decays by the ATLAS Collaboration is presented. Data collected from pp collisions at the LHC at a centre-of-mass energy of √s = 7 TeV during 2011, corresponding to an integrated luminosity of 2.1 fb−1, were used. A search was performed for top-quark pair-production events, with one top quark decaying through the t → Zq FCNC (q = u, c) channel, and the other through the Standard Model dominant mode t → Wb. Only the decays of the Z boson to charged leptons and leptonic W-boson decays were considered as signal. Consequently, the final-state topology is characterised by the presence of three isolated charged leptons, at least two jets and missing transverse momentum from the undetected neutrino. No evidence for an FCNC signal was found. An upper limit on the t → Zq branching ratio of BR(t → Zq)

Zeitungsausschnitte.

We report a search for the flavor changing neutral current (FCNC) decay of the top quark t → Zq (q = u, c) in p{bar p} collisions at √s = 1.96 TeV using a data sample corresponding to an integrated luminosity of 1.9 fb−1 collected by the CDF II detector. This decay is strongly suppressed in the standard model (SM) and a signal at the Tevatron would be an indication of physics beyond the SM. Using Z+ ≥ 4 jet final state candidate events, both with and without an identified bottom quark jet, we discriminate signal from background by exploring kinematic constraints present in FCNC events and obtain an upper limit of [Beta](t → Zq)

This dissertation presents a blind search for the flavor changing neutral current decay of the top quark t → qZ in pp collisions at [special characters omitted] = 1.96TeV using a data sample corresponding to an integrated luminosity of 1.12 fb-1 collected by the Collider Detector at Fermilab (CDF), This decay is extremely rare in the standard model, and a signal at the Tevatron would be an indication of new physics. Dividing candidate events with a Z boson and four or more jets into a sample of those which have a heavy flavor jet identified by a secondary vertex algorithm and those which do not, we observe data yields consistent with background expectations. We set a 95% C.L. upper limit on the branching fraction B (t → qZ) of 11.3%, consistent with an expected upper limit of 7.8% ± 3.3%.