Update on Invisible Higgs analysis in the tth channel

1
Update on Invisible Higgs analysis in the tth
channel

• Review of previous results
• Cut optimisation
• Conclusion

Ricardo Goncalo
2
Analysis overview

• Cut-based analysis
• Only signal and tt backg so far
• Difficulties
• Two components of missing momentum cant
  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 ? ll- 61 400 fb
ttW, W ? l? 420 fb
ttZ, Z ? ? 190 fb
() ?520 fb in reference analysis, with SM
couplings
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Cuts

• Reference analysis cuts
• 1 electron (pTgt25GeV ?lt2.5)
• or 1 muon (pTgt20GeV ?lt2.5)
• Veto on additional electron (pTgt10GeV)
• or additional muon (pTgt6GeV)
• 2 b-tagged jets
• 2 or more un-tagged jets
• t?bjj reconstruction
• mjj-mWlt15GeV ?lt2.0 for jets in W?jj
• mbjj-mtlt25GeV
• mT gt 120 GeV
• Missing ET gt 150GeV
• Scalar sum of pT of reconstructed l j j b b (
  missing ET) ?ETgt250GeV
• In reconstructed W ? jj Rjj ?(?2jj?2jj) lt 2.2
  (to reject lep-tau decays)

4
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 (L25fb-1) 1 M tth (L2pb-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

5
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
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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
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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

8
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

9
Variables used in event selection
Applying cuts as in standard analysis Missing
ET Signal and tt background
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Transverse W mass

• Signal and tt background

11
Rjj (distance in eta-phi between jets from W
decay)
Signal and tt background
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Scalar sum of event ET

• Problem found cutting on ?ET of lepton, 2 jets,
  2 b jets AND missing ETmeans that truth histo
  will be wrong (no ETmiss), but preselection ok
• Signal tt background

13
Lepton and jet variables
Signal and tt background
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New variables to use in selection?

• Higgs pT from the kinematic fit
• Signal tt background

15
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

16
Cut optimisation

• Varied several cuts over a few threshold values
  to optimise significance S/?(SB)
• All cut combinations considered
• Cuts varied
• mT (standard cut gt 120 GeV)
• ETmiss (standard cut gt 150GeV)
• Scalar sum of pT of reconstructed l j j b b ?ET
  (standard cut gt250GeV
• Added pTH obtained from kinematic fit
• Added ?? between reconstructed lepton and ETmiss
• Also tried varying electron and muon pT cuts
  still trying things out

17
Cut optimisation

• Significance for 30 fb-1 increases from 1.6 to
  3

• Neglecting the cut on pTH,fit doesnt seem to
  make much difference
• other thresholds must vary to achieve equal
  significance

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Cut optimisation

• Some cuts have more impact than others
• Results point to optimal thresholds
• mTW cut 115GeV
• ETmiss cut 220 GeV
• ??l,Etmiss cut lt 2.8 rad
• No clear solution for ?ET or pTH,fit
• Large correlations between variables should be
  expected

19
Optimised cuts
Signal and tt background
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Sum ET
Signal and tt background
21
Transverse W mass
Signal and tt background
22
Lepton and jet variables
Signal and tt background
23
Not using Rjj as selection variable
Signal and tt background
24
Higgs pT from fit
Signal and tt background
25
??l,Etmiss
Signal and tt background
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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)