1. Search for a 4 th generation t’ quark in p-p collision at ATLAS -Suyog Shrestha, Iowa State University

2. Intro t’t’bar -> WqWq – Same as ttbar decay (assuming (m t’ -m b’ ) < m W ) Final state signature-> leptons+jets – 1 charged lepton (el, mu), 4 jets, MET Using TopCommon object definitions Slim top D3PDs and work on slimmed ntuples

3. Need for Kinematic Fit 17 measured parameters –pT, η, ϕ of 4 jets, 1 charged lepton –Px, Py of MET 2 unmeasured parameters – Pz of MET and t’ mass Use W and t’ = t’bar mass constraints Use Kinematic Fit to: –Find Pz of MET (w mass constraint) 12/01/10Suyog Shrestha Oral Preliminary Exam3

4. Kinematic Fit Get (detector – truth) resolutions (σ) using Monte Carlo Create a log likelihood: Above: i runs over e, mu, jets Constrained by t & W masses Use TMinuit to find the parameters 12/01/10Suyog Shrestha Oral Preliminary Exam4

5. Plots from Kinematic Fit I Using ttbar top D3PD – mc10_7TeV.105200.T1_McAtNlo_Jimmy.merge.NTUP_TOP.e598_s933_s946_r2215_r2260_p542_tid314699_00 No pre-selection cuts But pT > 20 GeV and eta < 2.1 required of el, mu, and jets All jet permutations allowed, keep the one with highest likelihood

6. Black = Reco. Red = Fit

8. Comparing the fit masses Fit mass for hadronic and leptonic top Fit mass for hadronic and leptonic W

9. Plots from Kinematic Fit II Using the slimmed ntuple Pre-selection cuts applied Note the peak at 115 GeV after the fit Better top mass distribution with recon. Values compared to fit parameters seems to suggest a problem with the fit (more bugs in the fit code?)

10. Black = Reco. Red = Fit