Update on Invisible Higgs analysis in the tth channel Review of previous results Cut optimisation Conclusion Ricardo Goncalo Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Analysis overview Cut-based analysis Only signal and tt backg so far Difficulties: Two components of missing momentum: can’t reconstruct tbl ttbar is the most significant background and is very similar to signal Signal/Background ~10-3 Very much “ongoing work”!…. And talk cobbled together in a hurry (sorry!) Process xBR tth 330 (*) fb tt 490 000 fb bbW, W  l 73 000 fb bbZ, Z  l+l- 61 400 fb ttW, W  l 420 fb ttZ, Z   190 fb (*) =520 fb in reference analysis, with SM couplings Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Cuts  Reference analysis cuts: 1 electron (pT>25GeV; ||<2.5) or 1 muon (pT>20GeV; ||<2.5) Veto on additional electron (pT>10GeV) or additional muon (pT>6GeV) 2 b-tagged jets 2 or more un-tagged jets tbjj reconstruction: |mjj-mW|<15GeV; ||<2.0 for jets in Wjj |mbjj-mt|<25GeV mT > 120 GeV Missing ET > 150GeV Scalar sum of pT of reconstructed l j j b b (+ missing ET): ET>250GeV In reconstructed W  jj: Rjj =(2jj+2jj) < 2.2 (to reject lep-tau decays) Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Simulation Channel: Higgs (mh = 120 GeV) decaying to neutralinos in MSSM (tan= 5, mA= 1 TeV; M1= 44 GeV, M2= 220 GeV, M3= 1 TeV) PYTHIA 6.203 for signal and background Generated 142 M tt (L~25fb-1)+ 1 M tth (L~2pb-1) Atlfast simulation, ATLAS release 7.0.2 Low luminosity setting Cone jets (Rcone = 0.4) Jet tagging: b jets 60%; c mistag 10%; u,d,s, mistag 1% CTEQ5L PDFs mtop = 175 GeV (..historical) Interfaced code to Atlfast within Athena to produce dedicated ntuple Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Results so far… Most background comes from lep-tau and lep-lep decays of tt, as concluded in reference paper  decays increase the missing ET Wjj reconstructed from ISR/FSR jets in lep-tau and lep-lep events lep-had lep-lep lep-tau tau-tau had-had Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Results so far Accepted events for 30 fb-1 tth x-section scaled to SM value as used in reference analysis tth: x-section = 520 fb-1 Signal = 44.3 events tt: x-section = 490000.0 fb-1 Background = 812 events S/B = 1.55 Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Kinematic fit Tried something different: Assume pTmiss comes from  and h only Build grid of points in pT and (l,) and calculate pZ pZ can be found from pT and plep assuming W on mass-shell From pb , p and plep, calculate mt for each point Propagate errors in mt from grid spacing to obtain mt and calculate 2 mW W b t e/  mt Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Kinematic fit results Kinematic fit ~works for signal: (pTh)~85GeV Will try to use fit results for discrimination against tt background The hope is that this allows other cuts to be relaxed Correlations to mT and ETmiss may be important Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Variables used in event selection Applying cuts as in standard analysis: Missing ET: Signal …and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Transverse W mass Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Rjj (distance in eta-phi between jets from W decay) Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Scalar sum of event ET Problem found: cutting on ET of lepton, 2 jets, 2 b jets AND missing ET…means that truth histo will be wrong (no ETmiss), but preselection ok Signal …tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Lepton and jet variables Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

New variables to use in selection? Higgs pT from the kinematic fit Signal tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Cut on l,Etmiss Naively expect lepton to be mainly away from ETmiss direction in signal and close to ETmiss direction in background (with lepton and  from W decay) In fact, most tt background which passes standard cuts comes from lep-tau with ETmiss from both  and W decay and one detected lepton Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Cut optimisation Varied several cuts over a few threshold values to optimise significance = S/(S+B) All cut combinations considered Cuts varied: mT (standard cut: > 120 GeV) ETmiss (standard cut: > 150GeV) Scalar sum of pT of reconstructed l j j b b: ET (standard cut: >250GeV Added pTH obtained from kinematic fit Added  between reconstructed lepton and ETmiss Also tried varying electron and muon pT cuts; still trying things out Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Cut optimisation Significance for 30 fb-1 increases from ~ 1.6 to ~ 3 Neglecting the cut on pTH,fit doesn’t seem to make much difference other thresholds must vary to achieve equal significance Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Cut optimisation Some cuts have more impact than others Results point to optimal thresholds: mTW cut ~ 115GeV ETmiss cut ~ 220 GeV l,Etmiss cut < 2.8 rad No clear solution for ET or pTH,fit Large correlations between variables should be expected Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Optimised cuts Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Sum ET Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Transverse W mass Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Lepton and jet variables Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Not using Rjj as selection variable Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Higgs pT from fit Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL l,Etmiss Signal… … and tt background Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL

Atlas Higgs UK meeting @ RHUL Conclusions Studying major background to tth  inv (other backgrounds much less significant) Improvement in significance may be achieved through cut on l,Etmiss Kinematic fit to semileptonic top decay: still assessing usefulness as discriminating variable Simple optimisation of cut thresholds done (ongoing work) Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL