1. Simon Dean UCL Christmas Meeting 15/12/2008 "A bit of fast simulation and ttH"

2. 15/12/2008 "A bit of fast simulation and ttH" 2 Simon Dean UCL Christmas Meeting The Atlas Detector

3. 15/12/2008 "A bit of fast simulation and ttH" 3 Simon Dean UCL Christmas Meeting Atlas Simulation Options Geant4 simulation –1990s per ttbar event, full detail –~80% of time spent simulating particles in calorimeter –~75% of time spent simulating EM particles Fast Geant4 –Frozen showers replace low energy EM particles in calorimeter –757s per ttbar event Fast calorimeter simulation (Atlfast-2A) –Parameterised shower for each particle hitting the calorimeter face –101s per ttbar event Fast calorimeter + inner detector simulation (Atlfast-2F) –Track hit distribution parameterised –7.41s per ttbar event Fully parameterized fast simulation (Atlfast-1) –Particle kinematics modelled, most detector effects not modelled –0.097s per ttbar event

4. 15/12/2008 "A bit of fast simulation and ttH" 4 Simon Dean UCL Christmas Meeting Atlfast-1 Collaborators Tom Doherty Sarah Allwood-Spiers Chris Collins-Tooth Catherine Wright Samir Ferrag Tony Doyle Peter Sherwood Simon Dean Alex Richards Jonathan Butterworth

5. 15/12/2008 "A bit of fast simulation and ttH" 5 Simon Dean UCL Christmas Meeting Detector Effects in Early Data (Alex) Hadronic Endcap sector problem (electronic readout) E T bias = (E T (reco) - E T (truth))/E T (truth)

6. 15/12/2008 "A bit of fast simulation and ttH" 6 Simon Dean UCL Christmas Meeting Cell Scaling in Atlfast-1 Crude granularity in Atlfast-1 calorimeter –  x  = 0.1x0.1 in central region, no sample layers 75% cell energy 50% cell energy 25% cell energy

7. 15/12/2008 "A bit of fast simulation and ttH" 7 Simon Dean UCL Christmas Meeting Detector Effects in Early Data Muon Spectrometer is design to have 10% resolution on the momentum for 1 TeV muons with 40 μm alignment At high momentum, RT calibration of the drift tubes and alignment dominates the resolution –Resolution becomes  pT –When increasing the misalignment, the aligment contribution will dominate the total resolution

8. 15/12/2008 "A bit of fast simulation and ttH" 8 Simon Dean UCL Christmas Meeting 7 misalignment scenarios fully simulated with translation, rotation in 3 dimensions –(40 μm, 20 μrad) nominal –(100 μm, 50 μrad) –(200 μm, 100 μrad) –(300 μm, 150 μrad) –(500 μm, 250 μrad) –(700 μm, 350 μrad) –(1000 μm, 500 μrad) Determine the alignment contribution to the resolution in bin (pT, ,  ) using the full simulation 0 mrad 50 mrad

9. 15/12/2008 "A bit of fast simulation and ttH" 9 Simon Dean UCL Christmas Meeting p T resolution contribution shown to be proportional to misalignment Applied as correction to muon p T smearing resolution in Atlfast-1

10. 15/12/2008 "A bit of fast simulation and ttH" 10 Simon Dean UCL Christmas Meeting ttH(H  bb) One of various channels important for Higgs search in low mass region (110-150 GeV) –Also WH(H  bb), H  γγ, qqH  qq ττ LEP search limit –114 GeV < m H (SM) < 237 GeV (95% C.L.) H  bb is the dominant decay channel for m H <135 GeV Direct production difficult to extract from the huge QCD 2-jet background Associated production.

11. 15/12/2008 "A bit of fast simulation and ttH" 11 Simon Dean UCL Christmas Meeting ttH(H  bb) Collaborators Chris Collins-Tooth Lily Asquith Catrin Bernius Simon Dean Nikos Konstantinidis Bill Murray Michael Nash

12. 15/12/2008 "A bit of fast simulation and ttH" 12 Simon Dean UCL Christmas Meeting Jet Charge Studies (Lily) Weighted jet charge defined as Already used in top charge analysis (  =0.5) Use in ttH analysis to improve fraction of correct jet combinations where: qi = charge of ith track n = jet direction pi = ith track direction  = tuned exponent

13. 15/12/2008 "A bit of fast simulation and ttH" 13 Simon Dean UCL Christmas Meeting b-bbar jet discrimination b-jets split into 3 samples according to fraction of total jet pT in leading track (f track L) –f track L  15% –15% < f track L < 30% –30%  f track L

14. 15/12/2008 "A bit of fast simulation and ttH" 14 Simon Dean UCL Christmas Meeting

15. 15/12/2008 "A bit of fast simulation and ttH" 15 Simon Dean UCL Christmas Meeting For f track L  30%, leading track has very high charge efficiency –Cutoff for using weighted jet charge: |Q| > 0.6 –Otherwise take leading track charge  = 0.6 chosen as best value over all samples

16. 15/12/2008 "A bit of fast simulation and ttH" 16 Simon Dean UCL Christmas Meeting Jet Charge as a Likelihood Parameter Correct combination likelihood for ttbar system Correct (p S ) and incorrect (p B ) probability for each likelihood parameter Likelihood definition:

17. 15/12/2008 "A bit of fast simulation and ttH" 17 Simon Dean UCL Christmas Meeting Know charge of lepton with 99.2% efficiency Sets charge of remaining particles in event Charge products shown have same value in positive / negative lepton events More results to come... -2/3 +1/3 +2/3 -1/3 +1

18. 15/12/2008 "A bit of fast simulation and ttH" 18 Simon Dean UCL Christmas Meeting That's it from me Merry Christmas!