A measurement of spin correlation in top anti-top pairs and a search for top squarks at $\sqrt{s}=8$ TeV using the ATLAS detector

  • Thomas Neep

Student thesis: Phd

Abstract

The first measurement of the spin correlation strength in pairs of top quarks at$\sqrt{s}=8$ TeV is presented, using data collected using the ATLAS detector,corresponding to an integrated luminosity of $20.3 \mathrm{~fb^{-1}}$.Dileptonically decaying \ttbar\ events are selected with two high-\pt\ leptonsand at least two jets, one of which is required to be $b$-tagged.The azimuthal angle between the two charged leptons, $\Delta\phi$,is used to measure the degree of spin correlation of top quark pairs.The extracted value of \fsm,which is a measure of the spin-correlation strength,is $\fsm=1.20 \pm0.14$.This is the most precise spin-correlation measurement to date.Using the $\Delta\phi$ distribution,a search for Supersymmetric top squarks is performed,with the assumption that the top squarks decay to a top quark and a neutralino.No excess of events is observed and top squarks with massesbetween the top quark mass and 191 GeV are excluded at 95\% confidence level,extending previous limits.
Date of Award1 Aug 2015
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorFrederick Loebinger (Supervisor) & Christian Schwanenberger (Supervisor)

Keywords

  • LHC
  • ATLAS
  • spin correlation
  • top quark
  • SUSY

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