Update on Invisible Higgs analysis in the tth channel

Update on Invisible Higgs analysis in the tth channel Review of previous results Cut optimisation Conclusion Ricardo Goncalo 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!) (*) =520 fb in reference analysis, with SM couplings 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) 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 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jjreconstructed from ISR/FSR jets in lep-tau and lep-lep events lep-had lep-tau lep-lep tau-tau had-had 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 Atlas Higgs UK meeting @ RHUL

b t  mt W mW e/ 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 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 Atlas Higgs UK meeting @ RHUL

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

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

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

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

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

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

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

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

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

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

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

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

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

Update on Invisible Higgs analysis in the tth channel