Professor Amanda Cooper-Sarkar

Abstract:

A search for the production of single top-quarks via flavour-changing neutral-currents is presented. Data collected with the ATLAS detector at a centre-of-mass energy of √s=7 TeV, corresponding to an integrated luminosity of 2.05 fb-1, are used. Candidate events with a semileptonic top-quark decay signature are classified as signal- or background-like events by using several kinematic variables as input to a neural network. No signal is observed in the neural network output distribution and a Bayesian upper limit is placed on the production cross-section. The observed upper limit at 95% confidence level on the cross-section multiplied by the t→Wb branching fraction is measured to be σqg→t×B(t→Wb)<3.9 pb. This upper limit is converted using a model-independent approach into upper limits on the coupling strengths κugt/Λ<6.9{dot operator}10-3 TeV-1 and κcgt/Λ<1.6{dot operator}10-2 TeV-1, where Λ is the new physics scale, and on the branching fractions B(t→ug)<5.7{dot operator}10-5 and B(t→cg)<2.7{dot operator}10-4. © 2012 CERN.

Abstract:

A search is presented for a narrow resonance decaying to a pair of Z bosons using data corresponding to 1.02fb-1 of integrated luminosity collected by the ATLAS experiment from pp collisions at √s=7TeV. Events containing either four charged leptons (ℓℓℓℓ) or two charged leptons and two jets (ℓℓjj) are analyzed and found to be consistent with the Standard Model background expectation. Lower limits on a resonance mass are set using the Randall-Sundrum (RS1) graviton model as a benchmark. Using both ℓℓℓℓ and ℓℓjj events, an RS1 graviton with k/m-pl=0.1 and mass between 325 and 845 GeV is excluded at 95% confidence level. In addition, the ℓℓℓℓ events are used to set a model-independent fiducial cross section limit of σfid(pp→X→ZZ)<0.92pb at 95% confidence level for any new sources of ZZ production with mZZ greater than 300 GeV. © 2012 CERN.