update on invisible higgs analysis in the tth channel

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


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


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


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


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


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


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


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


Variables used in event selection

Applying cuts as in standard analysis: Missing ET: Signal …and tt background


Transverse W mass

Signal… … and tt background


Rjj (distance in eta-phi between jets from W decay)



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


Lepton and jet variablesSignal… … and tt background


New variables to use in selection?

Higgs pT from the kinematic fit

Signal tt background


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


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


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


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


Optimised cuts



Sum ET



Transverse W mass



Lepton and jet variables



Not using Rjj as selection variable



Higgs pT from fit



l,Etmiss



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)

update on invisible higgs analysis in the tth channel

Documents

higgs pt

higgs mh

invisible higgs analysis

leplep decays of tt

events tt

b jets

tt backgroundthe hope

leplep eventslepleplep