1 Top activities in Milano

2 Overview Study of W+jets background to semileptonic tt –I. Besana (1 st year PhD student, continuing work of diploma thesis) Study of Z(  ) background to double leptonic tt –M.C.Conidi (diploma student) and L. Dell’Asta (2 nd year PhD student) Study of b-tagging in ttbar events –S. Montesano (3 rd year PhD student) –See next talk One of the main goals of Milano is the search for supersymmetry Requires energy/luminosity, and well understood detector/backgrounds For 2009, main effort will be on ttbar and relative backgrounds

3 W+jets analysis goals In order of increasing luminosity we would like to Select event with one lepton and MET, see W over QCD. Need a QCD estimate - for example, from jet stream with non isolated leptons, normalizing at low MT or MET Measure W/Z ratio for low jet multeplicity (0, 1, 2? jets) and compare with MC predictions From measured Z+jets (high jet multeplicity as in ttbar selection), measured W/Z ratio at low jet multiplicity, and (small) MC corrections, predict W+jets background. c is a coefficient taking into account mass difference between W and Z Compare with estimate using W+jets MC normalized at low jet multiplicity. Contribute to first ttbar cross section measurement in semileptonic channel The first point is doable even with very low luminosity/energy Need to decide wether to focus on electron or muon channel

4 W+jets release 12 studies some ambiguity in the normalization of alpgen data (samples had a jet filter) All numbers for 100 pb -1 Ilaria’s presentations at SM and top meetings: Other studies: LBNL, ATLAS-COM , Generator-level studies with several MC Marisa Sandhorf (Wuppertal) I. Besana, diploma thesis

5 Ongoing work Use mc08 data –Migrate analysis code from custom ntuples produced from rel12 AOD to Athena-based analysis in release 14. Repeat W/Z ratio studies done with CSC data, then go beyond -Systematics (at generator-level, comparing different programs and tunings, and at detector-level) -Offline and trigger selection -Select W and remove/estimate QCD with very early data -Can we measure W+1jet/Z+1jet and constrain generator predictions? Top contamination with 2 jets? -How does it compare with just using W+jets MC predictions normalized to low jet multiplicity bins ?

6 Z(  ) and top Events rates after di-leptonic top selection (Maria Chiara thesis, 1000 pb -1 at 14 TeV) Z(  ) with di-leptonic tau decay is an important background to dilepton top searches Idea: estimate it from a Z(  ) control sample, replacing the two muons with a Z->  ->  simulated decay - jets, underlying event and Z momentum is taken from the data - Z decay simulation is MC (reliable) Similar to the technique used to estimate the Z background to H->  ->ll signal (see CSC book). 2 jets pt > 35 GeV EtMiss > 30 GeV (50 GeV for e  ) 2 isolated leptons pt>15 GeV Invariant mass outside [80 GeV,100 GeV] CSC trigger menu M.C. Conidi, diploma thesis

7 Z(  ) estimate - technique a) Select Z(  ) from data (98% purity) b) Simulate a Z decay in its rest frame - generate the two muons energy and relative angle according to 3-dimensional reference distributions from Z(  ) MC sample - 3 other angles assumed isotropic (need to check effect of Z polarization, if any) c) Use the measured Z velocity to go back to ATLAS frame d) Replace the two original muons with the simulated decay, recompute EtMiss, simulate muon trigger+reco efficiency 2 opposite charge muons 80 GeV < m inv < 100 GeV 2 or more jets with p T >20GeV No requirment on E T miss Single Muon Trigger (mu20)

8 Massa invariante dei μ in eventi Z  ττ MC (in blu) e in eventi Z  ττ  μμ+4ν simulati (in verde), dopo aver applicato i criteri di selezione su p T dei getti, missing E T ed energia di isolamento. Energia del μ + in eventi Z  ττ MC (in blu) e in eventi Z  ττ  μμ + 4ν simulati (in verde). Cut on MC truth info saved in TopView ntuples Technique seems to work. Need much more MC events to assess precision (Alpgen statistics with loose filter very limited in release 12) Some aproximations (constant muon efficiency, isotropic muon decay) need to be improved. Z(  ) estimate - results

9 Z(  ) future developments Maria Chiara gets her degree next week. Lidia may continue her work –Move code from ROOT macros to ATHENA. Replacement of part of a real event with a simulated Z decay should become a public tool –Repeat analysis with mc08 and mc09 data (MC statistics much better by the way). Improve analysis as mentioned; evaluate systematics We are however discussing if the Z(  ) background is the best usage of this tool with early data –With ~2 pb -1 at 10 TeV, 10 signal events and ~0.2 Z(  ) background events – do we need a data-driven background estimate? –An other possible application of the tool would be replace an hadronic tau instead, generate W(  ) from W(l ) control sample, compare with W(  ) signal and measure tau trigger and reconstruction efficiency