1. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 1 AOD on Single Top Marcello Barisonzi

2. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 2 Top Quark Largest fermion mass in Standard Model  large coupling to the Higgs boson Electroweak parameter with worst experimental resolution Constraint on the Higgs mass via loop diagrams Latest Tevatron average for the top mass: results in a Higgs mass prediction of: Better top mass resolution  more precise prediction of M H

3. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 3 Electroweak single top production Three different processes: s-channel W-t production W-gluon fusion Cross section measurement allows direct measurement of V tb in CKM Single top is 100% polarized: sensibility to anomalous couplings Background: ttbar and W+jets W-gluon fusion channel has a sizeable cross section: mass measurements are competitive with ttbar's Selection cuts W-gluon: 1 isolated lepton p T >20 GeV, |  |<2.5 1 b-jet p T >50 GeV, |  |<2.5 1 light jet p T >50 GeV, |  |>2.5 σ~11±1 pbσ~60±?? pb σ~247±8 pb

4. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 4 Single top mass measurement in W-gluon 100k events Fast Simulation M W reconstructed from lepton + missing E T No combinatorics (W-b assignment unique) Two solutions for p z : currently highest top p T chosen, other selections under study Single top mass resolution competitive with ttbar's Same data used for Full Simulation Method High p T top Δφ(W,b)Best Mass Top Mass174.0±11.4173.4±12.1175.7±8.9 ATLFAST

5. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 5 Full Simulation Data MC Data generated with TopReX (LO single top event gen.) Pythia used for decay and hadronisation 100k events generated and stored in GRID Detector simulation : GEANT4 Physics objects reconstructed with ATHENA v10 Data retrieved and stored locally at NIKHEF Analyses both on AOD and Root Ntuple

6. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 6 Single Top during Commissioning Phase The NIKHEF top group has a strong commitment to study the efficiency of analyses in the commissioning phase In this phase, the detector has to be properly understood Most evident effects: Jet Energy calibration Alignment of the Inner Detector Imperfect alignment can degrade the performance of b-tagging, which is an “ingredient” of all top-quark analyses. Talk from Rozanov (Atlas Overview Week 2004): Rejection factor R u =74 (ε b =60%) after 6 months of operation (91% of the nominal rejection)

7. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 7 B-tagging 101 B-tagging in ATLAS is based on secondary-vertex finding Discriminating variables: longitudinal impact parameter z 0 and transverse impact parameter a 0 Other variables: track multiplicity in secondary vertex a 0 <0 Secondary Vertex a 0 >0 x y Primary vertex Jet axis

8. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 8 B-tag Likelihood The secondary vertex parameters are combined to give a Likelihood value Which can be used as a quality cut for the b-jets The b-tagging algorithm returns a positive likelihood or a negative value (out of acceptance) for each jet

9. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 9 B-tag on AOD By default, AOD data contains b-tag for jets with Cone algorithm dR=0.7 ATLFAST studies were performed with dR=0.4 cones Luckily, b-tagging can be rerun on AOD data for comparison By changing the likelihood cut and the cone size, I can naively estimate the effect of a degraded b-tagging during commissioning

10. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 10 Efficiency and Purity W-gluon single top events have two b-jets: one from the top decay, one from a gluon splitting in the initial state To isolate the “hard” b-quark, I select b-quarks with Pt>50 GeV and |Eta|<2.5 For the selected b-quark, I look at the presence of at least one b-tagged jet (with Pt>50 GeV and |Eta|<2.5 and varying likelihood cuts). I define efficiency as the % of times I find such a jet I plot the dR between the highest Pt b-jet and the b-quark A “good” match is defined if the b-quark falls inside the jet cone (dR<0.4, dR<0.7) I define purity as the ratio between good and bad matches

11. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 11 Efficiency/Purity Plot Variation of the efficiency/purity for different likelihood cuts Straight lines = Efficiencies for 0.4 (blue) and 0.7 (red) cones Dashed lines = Purity

12. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 12 Effects of b-tagging for top mass reconstruction I applied my analysis to single top (signal) plus ttbar (background) data. b-tagging efficiency for ttbar larger (2 hard b-quarks) Requiring one and only one b-jet in the analysis then improves the rejection of single top w.r.t. ttbar This effect is evident for very loose cuts on b-likelihood, where the b-tag efficiency is maximum. Choose 0.7 cone. I will present the top mass reconstruction in two extreme cases: A cut on bLikelihood > 0 (all jets with positive likelihood are accepted as b-jets A cut on bLikelihood > 0.9 (perfect b-tagging)

13. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 13 bLikelihood > 0 Blue: single top Red: ttbar With bad b-tagging, single top peak is visible 1 fb -1 (~100 LHC days)

14. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 14 bLikelihood > 0.9 Blue: single top Red: ttbar With good b-tagging, S/B ~ 1 1 fb -1 (~100 LHC days)

15. Marcello Barisonzi First results of AOD analysis on t-channel Single Top DAD 30/05/2005 Page 15 What next? Before I can conclude that single top can be observed in the commissioning phase, I need to: Evaluate W+jets background (may be catastrophic!) Try a non-naïve approach to b-tagging (tweaking algorithm parameters) Study the effect of bad missing Et (necessary to reconstruct the W) If I am successful (timespan = 1 week!) the results will be presented by Ivo in Rome