Supersymmetry Search Supersymmetry Search in Trilepton in - - PowerPoint PPT Presentation

Supersymmetry Search Supersymmetry Search in Trilepton in Trilepton Final States Final States at ATLAS: at ATLAS: Backgrounds from Secondary Leptons Backgrounds from Secondary Leptons from from b-Decays

• Decays

and Systematic Uncertainties and Systematic Uncertainties ATLAS CSC 7 / 5 ATLAS CSC 7 / 5

Oleg Brandt Oleg Brandt

supervisors: Pa Pawe wel Brückma l Brückman Ala Alan B Barr rr Ton Tony W Weidberg idberg

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 2

Today‘s Menu Today‘s Menu

Lepton isolation:

Track isolation Calorimeter isolation Impact Parameter

First thoughts on measuring the

rate of isolated leptons from b-jets

Estimation of Systematic Uncertainties Conclusion

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 3

Leading Lepton Leading Lepton Pt for t for SU2 U2

10 fb-1 10 fb-1

SUSY Signal Need high lepton purity! Need high lepton purity! Need primary leptons! Need primary leptons!

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 4

Friend and Foe Friend and Foe

Very few SM backgrounds with 3 primary leptons! Our most important backgrounds:

Z+jets ttbar

Only 2 primary leptons Third lepton from:

light jet faking an electron punch-through and in-flight decays to muons secondary leptons from c- and b-decays

From MC: O(10) higher rate than the two above!!!

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 5

Preselection Outlined Preselection Outlined

Preselection (ATLAS SUSY WG cuts in blue):

Muons:

reconstructed by the MuID algorithm, | η | < 2.5 Calorimeter isolation in ∆R = 0.2 cone: < 10 GeV bestMatch(), isCombinedMuon() 0 < chi²(track match) < 100 Jet isolation: no jets in ∆R = 0.4 cone (against b) Isolation w/r/t each other in: ∆R = 0.1 (against J/Psi & Y) Pt > 10 GeV

Electrons:

reconstructed by the eGamma algorithm (isEM() & 0x3FF) == 0 exclude crack region: 0 < | η | < 1.37; 1.52 < | η | < 2.5 Jet isolation: no jets in ∆R = 0.4 cone (against b) Isolation w/r/t each other in: ∆R = 0.1 Pt > 10 GeV

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 6

Track Isolation: Track Isolation: After After 3rd Lepton Requirement rd Lepton Requirement

µ

e

n

• r

m a l i s e d t

• 1

n

• r

m a l i s e d t

• 1

Define track isolation as:

(inspired by H->4l)

where

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 7

Calorimeter Isolation: Calorimeter Isolation: After After 3rd Lepton rd Lepton Requirement equirement

µ

e

n

• r

m a l i s e d t

• 1

n

• r

m a l i s e d t

• 1

Additional discrimination power? „Conventional“ ATLAS calorimeter isolation from AOD, rel. 12.0.7

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 8

Track vs. Calorimeter Isolation Track vs. Calorimeter Isolation

µ

e

f u l l M C s t a t . f u l l M C s t a t .

Calo isolation potentially

easier to understand

Use the discrimination

variable first understood!

not 100% correlated :) for tight track isolation

calo isolation not efficient :(

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 9

µ

e

f u l l M C s t a t . f u l l M C s t a t .

keep it for cross-checks Impact Parameter rather

weak constraint against secondary leptons from b‘s

Normalised Impact Parameter Normalised Impact Parameter

• vs. Track Isolation
• vs. Track Isolation

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 10

Study Secondary Leptons Study Secondary Leptons from from b-Jets b-Jets in tt n tt Events Events

• Study isolated lepton fake rate from b-jets:
• Need a reasonably pure sample of b-jets:
• bb
• tt
• bb:
• difficult due to high backgrounds, e.g. W+j
• tt:
• Several handles to tag tt events

without using one of the b-jets

• Use semileptonic tt channel
• Orthogonal sample
• approx. 10x more statistics than dileptonic tt

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 11

Study Secondary Leptons Study Secondary Leptons from from b-Jets b-Jets in tt n tt Events: Ansatz Events: Ansatz

• same sign!

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 12

Study Secondary Leptons Study Secondary Leptons from from b-Jets b-Jets in tt n tt Events: Ansatz Events: Ansatz

• same sign!

“Top“-Tags “Top“-Tags

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 13

Study Secondary Leptons Study Secondary Leptons from from b-Jets b-Jets in tt n tt Events: Ansatz Events: Ansatz

• Define the rate as:
• Denominator given by ”top-tags“ from previous page:
• 1 b-tagged jet
• 2 jets with
• >0 lepton
• This should give reasonably high S/B
• With typical semileptonic tt backgrounds
• Numerator given by:
• 2x rate of same sign events passing ”top“-tag
• Interested in events with 1 primary, 1 secondary lepton!
• ”Background“: events with 2 primary leptons!

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 14

Study Secondary Leptons Study Secondary Leptons from from b-Jets b-Jets in tt n tt Events Events

• Important processes for the numerator of
• Dileptonic tt
• lepton charge mismeasurement
• Single top
• associated W+t production:
• hadronic W: contribution to the desired ”signal“ sample
• leptonic W: with additional contribution to ”background“
• most of other single top: contribution to the ”signal“
• Z + QCD
• lepton charge mismeasurement + jets to pass ”top“-tag
• WZ, ZZ
• leptons from W / Z + additional jets
• bb + jets, W + jets
• not strictly „background“ or ”signal”
• More detailed discussion:
• http://www-pnp.physics.ox.ac.uk/~obrandt/TrileptonAnalysis/INT/int/Int.pdf or CSC 7

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 15

Systematic Uncertainties: Systematic Uncertainties: Classif Classification cation

• Systematic Uncertainties can be classified:
• Instrumental:
• Modelling of the detector and its response;
• Pile-up;
• Secondary effects like cosmics, cavern background,

beam-gas and beam halo interactions;

• .
• Physics:
• Total cross sections;
• PDF’s;
• Differential distributions like , ‘s;
• Underlying event.

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 16

Systematic Uncertainties: Systematic Uncertainties: Instrumental Instrumental

• Missing Et:
• cut around 20-30 GeV -> do not bother about tails!
• Study e.g. in or ttbar events
• Lepton ID + trigger:
• Use “Tag and Probe” for efficiency and fake rates
• Lepton Isolation (reject secondary leptons, slides 10ff.)
• Luminosity:
• include the uncertainty on as systematics
• Pile up:
• study isolation etc. in blocks of constant

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 17

Systematic Uncertainties: Systematic Uncertainties: Physics Physics

• Uncertainties on differential distributions like , ‘s:
• Beyond the scope of this analysis, more needed
• Underlying event:
• To be tuned in an LHC-wide effort
• PDF uncertainties:
• Vary PDF sets within their 1σ uncertainties
• But: PDF sets from various groups differ by > 1σ!
• Cross section:
• This analysis is a counting experiment
• affected by cross section uncertainties
• Use latest calculations available
• Use control regions!

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 18

Use of Control Regions to Evaluate Use of Control Regions to Evaluate Systematics from Xsec and Lumi Systematics from Xsec and Lumi

• Control regions:
• Estimate background cross sections from data
• Minimise dependence on:
• uncertainties
• PDF’s
• Define control regions as:
• will isolate ,
• will isolate

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 19

Use of Control Regions to Evaluate Use of Control Regions to Evaluate Systematics from Xsec and Lumi Systematics from Xsec and Lumi

• Estimate the contribution:
• Major background with production before the cut
• Compare areas under the fit to the peak for:
• Take from MC only the fraction of those 2 fits!
• Take into account other backgrounds like
• Estimate the ZZ contribution:
• Count events with:
• 2 OSSF lepton pairs
• Correct for lepton ID efficiency
• Similarly:

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 20

Conclusion Conclusion

Studied lepton isolation:

Calorimeter isolation

probably easier to understand

Track isolation

more powerful

Systematic uncertainties:

Instrumental Physics

Resulting table of statistical significancies for SUx:

10 fb-1

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 21

Backup & further discussion Backup & further discussion

Backup slides following

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 22

Efficiencies Efficiencies and Fake Rates: and Fake Rates: Object Definitions Object Definitions

Lepton efficiency defined as: Lepton fake rate defined as: Reconstructed electrons:

Pt > 6 GeV cut for efficiency Pt > 10 GeV for fake rates

MC level electrons:

Pt > 10 GeV cut for efficiency Pt > 6 GeV cut for fake rate no Geantinos, final state particles only

Jets: reco level jets like in analysis:

after overlap removal w/ electrons in dR=0.2, Pt > 10 GeV

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 23

Efficiency Efficiency & Fake Fake Rate: SU3 ate: SU3

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 24

Efficiency Efficiency & Fake Fake Rate: SU3 ate: SU3

etcone20 (dR=0.2) etcone (dR=0.45) „Pt_max“ trk. isol. „Pt_sum“ trk. isol.

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

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Efficiency Efficiency & Fake Fake Rate: ttbar ate: ttbar

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 26

Isolation in Pile-Up Conditions Isolation in Pile-Up Conditions

Pile Up affects:

Track Isolation Calorimeter Isolation

No optimisation

No optimisation for pile up so far

?

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 27

Efficiency Efficiency with ith Pile-Up: SU2 ile-Up: SU2 n n

• p

p i i l l e e

• u

u p p w w i i t t h h p p i i l l e e

• u

u p p

Very similar degradation for

track and calo isolation

In central region: “Pt_max” track isolation

slightly better than “Pt_sum”

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 28

Fake Rate with Fake Rate with Pile-Up: SU2 Pile-Up: SU2 no pile-up with pile-up

Slight improvement for

track and calo isolation

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

• ns from b-decays

b-decays + systematics + systematics 29

Study Study Secondary Leptons econdary Leptons from from b-Jets b-Jets in ttbar Events: Ansatz n ttbar Events: Ansatz

• pposite sign

Trilepton SUSY: secondary lept Trilepton SUSY: secondary leptons from

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b-decays + systematics + systematics 30

Study Secondary Leptons Study Secondary Leptons from from b-Jets b-Jets in ttbar Events: Ansatz n ttbar Events: Ansatz

N N

• t

t f f e e a a s s i i b b l l e e d d u u e e t t

• d

d i i l l e e p p t t

• n

n i i c c t t t t b b a a r r d d e e c c a a y y s s ! !