Di-Lepton Channel in ATLAS

S. Tokár, Comenius Univ. Bratislava

04/19/23 S. Tokár, Cz-Sk meeting, Košice 1

Di-lepton Channel A combination of single lepton and di-lepton

triggers – the overall trigger efficiency is: ee (97 (971)%1)%, , ee (98ee (981)%, 1)%, (96(961)% 1)%

Preselection:Preselection: two high p two high pTT opposite signed leptons opposite signed leptons

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ee: Isolation (Calo R=0.2) < 6 GeV: R(-jet) > 0.2

L=100pb-1

Selection criteria: to maximizeSignificance: s S S Bp

exactly two leptons, pT > 20 GeV at least two jets, pT > 20 GeV ET-mis > 35 GeV (ee,), >30 GeV (e)

tt-bar cross section in DIL

Cross section determined as

Efficiency = geom. acceptance recon. efficiency

04/19/23 S. Tokár, Cz-Sk meeting, Košice 3

obs bkgN N

L

From MC

From exp.

Expected x-section statistical error

Inclusive template method 3 dominant sources of isol. leptons: tttt, WWWW and ZZThe proceses can be separated in the plane ETmis vs #jets

Normalised 2D templates are created for each channel and relative contribution of tttt, WWWW and ZZ determined (binned likelihood fit to pseudoexp -10 params –inc. tt Xsec)

Inclusive template cross section results

Total composition of the inclusive di-lepton selection versus numberof jets

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Projected significance from pseudo-experiments for the inclusive template fit versus integrated luminosity including all channels and all systematic uncertainties.

Likelihood approachThe log-likelihood function used to extract NNsigsig and NNbkgbkg given the

fixed total number of events NNtottot and the measurements (xxii):

04/19/23 S. Tokár, Cz-Sk meeting, Košice 5

The Signal (S) and background (B) functions: by fitting MC

with 1

ln | , ( ) ( ) ( )totN

i sig bkg to sit bkgi

gL G x G x N S x NN B xN N

Distribution of between the highest ppTT lepton and the EEtt

missmiss momentum vector for (a) signalsignal, (b) bkgdbkgd and (c) signal/bkgdsignal/bkgd composition according to the ratio obtained from the cut analysis.

Expected statistical error evaluated on X-section measurement for the likelihood method for different luminosities.

Top quark mass in dilepton channel

04/19/23 S. Tokár, Cz-Sk meeting, Košice 6

No top quark mass determination in DiL channel

is in the ATLAS book

Presented in in the top quark mass group:

Matrix element method (P. Cavalleri, LPNHE Paris)

Modified Dalitz-Goldstein method (K.Sliwa et al., Tufts Univ./CDF)

Top mass using J/ - lepton correlations (S. Behar and Y. Rozen, Technion)

DIL top mass: Matrix Element Method

04/19/23 S. Tokár, Cz-Sk meeting, Košice 7

2

1 2

1( | ) ( , ; ) ( , ) ( ) ( )

( )S t i i t PDF PDFt tt

P x M d M q p M W p x f gl f glM

Tuning of the methods continues – Transfer function, etc

TOP MASS : Modified DALITZ-GOLDSTEIN Method

04/19/23 S. Tokár, Cz-Sk meeting, Košice 8

• Unknowns are four-momenta of the two ’s• 8 equations of constraint

Assuming top quark mass, MMtt, first 6 eq.’s constrain t and tbar t and tbar three-momentum vectors to point somewhere on respective ellipses in three momentumellipses in three momentum

(lepton, b-quark and assumed Mt suffices to

find such an ellipse for each combination) The last 2 eq.’s - projection of an ellipse from 3-dim momentum space onto a transverse momentum plane is also an ellipse, the t and tbar transverse momenta are equal and opposite - ellipses should then cross each other, intersection intersection points points are the solutions for the t and t-bar momenta satisfying all the constraints

PLAN FOR ATLAS STUDIES

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• Monte Carlo studies with events with 10 TeV LHC energy10 TeV LHC energy

• evaluate error on top mass achievable within 1 year, 2 years etc. of low luminosity running

• dominant error dominant error will be systematic (statistical scales with √N), most likely the most important factor will still be jet jet energy uncertaintyenergy uncertainty

• backgrounds could be different than in CDF, on the other hand much larger statistics should allow MUCH more stringent selection cuts and most likely in background reduction to less than 10% level, or better

• use support vector machine (Ben Whitehouse) and tagging information to enrich the sample of top-like events with just the kinematics cuts to bring S/B to 10/1 or better

04/19/23 S. Tokár, Cz-Sk meeting, Košice 10

Top Mass via J/Top Mass via J/ – lepton correlation – lepton correlationTop Mass via J/Top Mass via J/ – lepton correlation – lepton correlation

The invariant mass of the J/-lepton (from the W) is highly

correlated with the top mass. Create the correlation graphMeasure the InvMass in dataRead the Top mass from the

correlation graph

W-

W+

bb

t t

l+

J/→l+l-

W→W→, J/, J/→2→2 - first channel to use - first channel to use

Main advantages of this channel:Major systematic effects

(from jet energy scale) are avoided.Very clean signature

04/19/23 S. Tokár, Cz-Sk meeting, Košice 11

Signal and Background in 20fbSignal and Background in 20fb-1-1

SourceCross

section (pb)Expected

Events% of Total

Signal 0.12 792±28 36%

Wrong Pairing 0.12 792±28 36%

W+Jets 3160 86±9 5%

Z+Jets 243 <120 (Zero found) <7%

bbar 6140 (b) <1 <0.2%

ttbar ( not from W) 1.8 5±5 <0.5%

Combinatorial ~300±17 ~16% (?)

Fake Muon <15 <1%

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Unbinned Maximum Likelihood MethodUnbinned Maximum Likelihood MethodUnbinned Maximum Likelihood MethodUnbinned Maximum Likelihood Method

Top Mass: 175GeV (~ 20 fb-1)

Top Mass (GeV)

Result from the fit (GeV)

160 160.5

175 174.7

190 192.6

Statistical Error for 20 fb-1: 2.8GeV

Single top cross section in DILSingle top cross section in DIL

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studies: ATL-PHYS-INT-2009-013, ATL-COM-PHYS-2009-058 (PUB draft)

Signal: Wt-channel dilepton→ Final state: 1 b-jet, 2 high-PT leptons, large missing ET

→ Small cross section, but clean signal→ Privately produced 50k fast simulation events in release 12● Used premade AcerMC input files (acermc.006220.Wtlep)● Ran through Pythia -> Atlfast -> TopView121403

Backgrounds:→ Top pair (Dilepton mode + l+jets mode with fake leptons)→ Diboson production (WW, ZZ, WZ)→ Z+jets and W+jets with fake leptons● Cross section is huge, so might be an issue...→ Other single top channels with fake leptons

Event Selection / results

04/19/23 S. Tokár, Cz-Sk meeting, Košice 14

Selection cutsSelection cuts

• 2 OS leptons with PT > 30 GeV (no 3rd one)

● DIL inv-mass out [81,101] GeV (ee/ only)

● Pt(jet1) > 30 GeV and Pt(jet2) < 20 GeV

● Missing ET > 20 GeV (e), 30 GeV (ee/)

VetoesVetoes ee is in 1.37 < || < 1.52 is in 1.1 < || < 1.7 (mis-ET,) <0.5 or >2.62 muons in 1.1 < || < 1.7

Using multivariate analysis (BDT) to improve discriminating power vs ttbar, Z events – different variable are used: MET, m(ll), (MET,j1), (ll,j1),

HT, R(l1,l2), pT(j1), …

Conclusions

04/19/23 S. Tokár, Cz-Sk meeting, Košice 15

DIL analyses are present at ATLAS, but are less advanced than lepton + jets one

Good perspectives are in the top mass determination, spin correlations,…