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Update on Invisible Higgs analysis in the tth channel. Review of previous results Cut optimisation Conclusion. Ricardo Goncalo. Analysis overview. Cut-based analysis Only signal and tt backg so far Difficulties: Two components of missing momentum: can’t reconstruct t b l - PowerPoint PPT Presentation
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Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 1
Update on Invisible Higgs analysis in the tth channel
Review of previous resultsCut optimisation
Conclusion
Ricardo Goncalo
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 2
Analysis overviewCut-based analysisOnly signal and tt backg so farDifficulties: Two components of missing
momentum: can’t reconstruct tbl 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 3
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 tbjj reconstruction:
|mjj-mW|<15GeV; ||<2.0 for jets in Wjj |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 =(2
jj+2jj) < 2.2 (to reject lep-tau decays)
2
2
lepton
T
miss
TleptonT
missTT ppEEm
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 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 (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 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
Wjj reconstructed from ISR/FSR jets in lep-tau and lep-lep events
lep-
lep
lep-
tau
tau-
tau
lep-
had
had-
had
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 6
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 7
mWW
bt
e/
mt
Kinematic fit
Tried something different: Assume pT
miss 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
T
HiggsT
totalT ppp
2
2
2
mt
PDGt
rect mm
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 8
Kinematic fit results
Kinematic fit ~works for signal: (pT
h)~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 9
Variables used in event selection
Applying cuts as in standard analysis: Missing ET: Signal …and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 10
Transverse W mass
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 11
Rjj (distance in eta-phi between jets from W decay)
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 12
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 ET
miss), but preselection okSignal …tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 13
Lepton and jet variablesSignal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 14
New variables to use in selection?
Higgs pT from the kinematic fit
Signal tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 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
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 16
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) ET
miss (standard cut: > 150GeV) Scalar sum of pT of reconstructed l j j b b: ET (standard
cut: >250GeV Added pT
H obtained from kinematic fit Added between reconstructed lepton and ET
miss Also tried varying electron and muon pT cuts; still trying
things out
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 17
Cut optimisation
Neglecting the cut on pT
H,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
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 18
Cut optimisation Some cuts have more impact
than others Results point to optimal
thresholds: mT
W cut ~ 115GeV ET
miss cut ~ 220 GeV l,Etmiss cut < 2.8 rad No clear solution for ET or pT
H,fit
Large correlations between variables should be expected
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 19
Optimised cuts
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 20
Sum ET
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 21
Transverse W mass
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 22
Lepton and jet variables
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 23
Not using Rjj as selection variable
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 24
Higgs pT from fit
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 25
l,Etmiss
Signal… … and tt background
Ricardo Gonçalo Atlas Higgs UK meeting @ RHUL 26
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)
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