search for direct gaugino production decaying to two leptons...

Search for Direct Gaugino Production Decaying to TwoLeptons and Missing Transverse Momentum at ATLAS

with√s = 7 TeV

Brokk ToggersonpNut Lunch Club Seminar

18 February 2013

1 / 48

Overview

1. The Standard Model

SuccessesChallenges

2. Supersymmetry

Particle contentMotivationSearch paradigmsDirect gaugino searches

MotivationpMSSMSimplified models

3. The ATLAS experiment

The Large Hadron ColliderThe ATLAS detector

4. Searches for direct gauginoproduction

Object definitionsSignal regions

5. Background estimations

OverviewEstimation techniques

Fakestt and single-tZ + XWW data drivenMC diboson

Uncertainties

6. Results

2 / 48

The Standard Model


SuccessesChallenges

2. Supersymmetry










Uncertainties

6. Results

3 / 48

The Standard Model (SM)

Description of known particles

Describes three interactions

Strong interactionElectromagnetic interactionWeak interaction

Chiral: left- and right-handed chiral fields have different interactions

Only left-handed experience weak interaction

4 / 48

Higgs mechanism in the SM

Cannot add explicit mass terms to the SM Lagrangian!LSM ⊃ mq qLqR terms break gauge invarianceSimilarly, adding mass terms for bosons breaks gauge invariance

Add a complex scalar doublet φ and its conjugate φ,with potential V (φ) = −µφ2 + λφ4

φ =

(φ+

φ0

)φ = iσ2φ

† =

(φ0†

−φ−)

Neutral components can get a vacuum expectation value (VEV)

Yukawa couplings → fermion massesmd

[uLφ

+dR + dLφ0dR

]mu

[uLφ

0†uR + dLφ−uR

] Massless gauge bosons “eat” Higgsd.o.f. to become massive

massless︷︸︸︷W 0

1 , W02 , W

03 , B

0, φ+, φ0 SSB−−−−→mixing

massive︷︸︸︷W±, Z 0,

massless︷︸︸︷γ0 , h0

5 / 48

Successes and problems in SM

Sample Successes

W Z tt t WW WZ Wt ZZ

[pb]

tota

lσ

1

10

210

310

410

510

-15.8 fb

-14.7 fb

-12.1 fb-14.6 fb

-14.7 fb

-11.0 fb

-11.0 fb

-135 pb

-135 pb = 7 TeVsLHC pp

Theory

)-1Data 2010 (L = 35 pb

)-1Data 2011 (L = 1.0 - 4.7 fb

= 8 TeVsLHC pp

Theory

)-1Data 2012 (L = 5.8 fb

ATLAS PreliminaryATLAS PreliminaryATLAS Preliminary

Many predictions confirmedto impressive accuracy

Some problems

1. Dark matter

Known to comprise ∼ 20%of the UniverseNo candidate inStandard Model

2. Hierarchy problem

Quadratic divergence in mh0 due toloop corrections → push mh0 to scaleof new physics Λ

To preserve perturbation theory inWW scattering, mh0 ∼ mZ

∆mlooph0 = mbare

h0to O(10−17) if

Λ = GUT

6 / 48More on dark matter

Supersymmetry


SuccessesChallenges

2. Supersymmetry










Uncertainties

6. Results

7 / 48

What is supersymmetry (SUSY)?

SUSY is a symmetry relating fermions and bosonsFor each fermion there is a bosonic partner

Quarks and Squarks Leptons and SleptonsSM SUSY SM SUSY

spin 12

spin 0 spin 12

spin 0gauge mass gauge mass

eigenstates eigenstates eigenstates eigenstates

(uL, dL) (uL, dL) (νe , eL) (νe , eL)uR uR same eR eR same

dR dR νe νe

(cL, sL) (cL, sL) (νµ, µL) (νµ, µL)cR cR same µR µR samesR sR νµ νµ

(tL, bL) (tL, bL) (ντ , τL) (ντ , τL)tR tR tL tR → t1 t2 τR τR τLτR → τ1τ2

bR bR bLbR → b1b2 ντ ντ ντ

Since there is no 511 keV e, SUSY must be broken in nature

8 / 48

The bosons are a bit more complicated. . .

Cannot construct a supersymmetric theory containing φ and φ

Need two Higgs doublets at minimumOne for up-type quarks Hu = (H+

u ,H0u )

One for down-type quarks and leptons Hd = (H0d ,H

−d )

Each has a separate VEV, tanβ =<H0

u>

<H0d>

After electroweak breaking: pseudo-scalar A0 and scalars (H±,H0, h0)

Mass eigenstates are mixes of gauge eigenstates

B

Wi∈{1,2,3} Hd

Hu

γ

W±

Z0

h0H0

H±

A0

Electroweak

Symmetry Breaking

B

Wi∈{1,2,3} Hd

Hu

χ±1

χ±2

χ01

χ02

χ03

χ04

Electroweak

Symmetry Breaking

9 / 48

Why would we want to add all these particles?

1. Dark matter

A neutral and stable lightest SUSY particle is a viable dark matter candidate

Many SUSY models achieve such stability through R parity, R = (−1)3B+L+2S

2. Hierarchy problem

A difference in sign between scalar and fermion loops → t-loops naturally cancel t-loops

Converts quadratic divergence to logarithmic

3. Coleman-Mandula Theorem

Supersymmetry is the last symmetry allowed by Special Relativity.

10 / 48More on R-parity More on Coleman-Mandula Theorem

Search paradigms

The MSSM† has O(100) parametersImpossible to scan such a large space

pMSSM

Effective Lagrangians for SUSYbreaking → the 19 parameter pMSSM

Pro: Based on actual SUSYmodel

Con: Assumes MSSM

Simplified models

Considers a specific processPlace limit on σ × BR for each process

Pro: Limits applicable to anymodel containing this process

Pro: No assumptions about theSUSY breaking sector

Pro: Useful for optimization

Con: Must decide what diagramsto investigate

Here, focus is on direct gaugino searches

11 / 48†Minimally Supersymmetric Standard Model

Why search for direct gaugino production?

ATLAS 4.7 fb−1 0-lepton search

No strongly produced SUSY yet!

No excess seen in 0-lepton, 1-lepton, 2-lepton,or multijet channels → (mq = mg ) > 1.4 TeV

Possible that gluinos and first two generationsquarks are too heavy for direct production

While easiest SUSY to search for, light squarksof the first two generations are not necessaryfor Higgs mass stabilization

Naturalness considerations

One χ±

, χ03, and χ

04 have m ∼ µ

|µ|2 ∼ mZ for naturalness

→ Gauginos should be light

If SUSY is discovered, gaugino searcheswill be critical for exploring SUSY structure

12 / 48

https://cdsweb.cern.ch/record/1432199

pMSSM

Assuming heavy colored sparticles leaves four parameters:(M1, M2, µ, tanβ)

Assume tanβ = 6

Insert m˜R

midway between mχ02

and mχ01

Enhances BR→`±`∓No sensitivity to no-˜ case

M1 = 100 GeV M1 = 140 GeV M1 = 250 GeV

13 / 48

Simplified models

σ assuming 100% W

Focus of this work

From the the 2-lepton 1.04fb−1 support note

[GeV]02!", ±

1!"

m100 200 300 400 500

NLO

Cro

ss S

ectio

n [p

b]

-610

-510

-410

-310

-210

-110

1

10

210 )02!"±

1!"Mode A (

)-/+1!"±

1!"Mode C (

)02!"0

2!"Mode D (

Cross-section for χ02χ

02 production

<< than χ±1 χ

02 or χ

+1 χ

−1

Neglect χ02χ

02 for optimization

Sarah is investigating the slepton case

Also only look at those with ≤ 3

leptons in truth

≥ 4 in truth → 3 lepif one missing

From Christophe’s table, we are left with (All can be with or without intermediate �’s)

Signal region # of leps Signal ≥ 2 lep signals

OSSF + jet veto =2 (A1) χ02χ

±1 → (�+rec�

−rec χ

01) + (�±νχ

01)

=2 (A5) χ+1 χ

−1 → (�+recνχ

01) + (�−recνχ

01)

SS + jet veto =2 (A2) χ02χ

+1 → (�+rec�

−χ01) + (�+recνχ

01)

(A3) χ02χ

−1 → (�+�−rec χ

01) + (�−recνχ

01)

OSSF + ≥ 2 jets =2 (A4) χ02χ

+1 → (�+rec�

−rec χ

01) + (qqχ

01)

+ bjet veto

3 / 22

Use these cross sections select highest-σ diagrams

χ±1 χ02 χ+

1 χ−1

Allowing ˜ and ν increases BR→ `±`∓

χ±1 → ˜±ν (ν`±) χ02 → ˜±`∓(νν)

No sensitivity to no-˜ case

BR(Z → ``) and BR(W → `ν) too small

Look at two and three lepton processes

2-lepton can supplement dedicated 3-lepton

χ±1

χ∓1

ℓ, ν ν, ℓ

ν, ℓ χ01

ℓ, ν

ν, ℓ ℓ, ν

χ01

χ±1

χ02

ℓ/ν

ν/ℓ

ν/ℓ

χ01

ℓ

ℓ

ℓ

χ01

14 / 48

The ATLAS experiment


SuccessesChallenges

2. Supersymmetry










Uncertainties

6. Results

15 / 48

The Large Hadron Collider (LHC)

Explores high-energy frontier

27 km circumference

Collisions at√s = 7 TeV

Four major experiments

ATLAS, CMS,ALICE, LHCb

Period Date Date Peak interactions∫

Ldt

start end per bunch-crossing [pb−1]B 22 - Mar 23 - Mar 8.48 16.98D 15 - Apr 28 - Apr 7.30 178.8E 30 - Apr 11 - May 7.60 50.18F 15 - May 25 - May 8.07 152.2G 27 - May 13 - Jun 8.02 560.8H 18 - Jun 27 - Jun 6.89 278.3I 17 - Jul 29 - Jul 9.14 399.2J 2 - Aug 4 - Aug 9.75 232.9K 7 - Aug 22 - Aug 11.34 660.2L 10 - Sep 4 - Oct 15.83 1568.8M 7 - Oct 30 - Oct 32.08 1121.82011 22 - Mar 30 - Oct 32.08 5228.5

16 / 48

The ATLAS Experiment

17 / 48

Searches for direct gauginos


SuccessesChallenges

2. Supersymmetry










Uncertainties

6. Results

18 / 48

Targeted processes

Investigating SUSY produced through the weak interaction

Direct gaugino productionSleptons are always accessible for gaugino decaysBR(Z → ``,W → `ν) limit event yield for no-˜ decays

χ+1 χ−1 pair production

χ±1

χ∓1

ℓ, ν ν, ℓ

ν, ℓ χ01

ℓ, ν

ν, ℓ ℓ, ν

χ01

OS dilepton

No flavor correlation

EmissT from χ0

1’s

No jets

No m`` peak

χ±1 χ02 production

χ±1

χ02

ℓ/ν

ν/ℓ

ν/ℓ

χ01

ℓ

ℓ

ℓ

χ01

If one lepton failsselection → dilepton

SS or OS

EmissT from χ0

1’s

No jets

No m`` peak

DG pMSSM

χ±1

χ02

χ01

χ01

W

" "

q

q

"

OS dilepton

Same flavor

2 jets

Jets are generally lightflavor

EmissT from χ0

1’s

First search for direct χ+1 χ−1 production at LHC! 19 / 48

Baseline selections

SM processes resulting in two leptons + EmissT :

Z → ``, tt, single-t, WW WZ ZZ , and events with a fake lepton

Tools to reduce these backgrounds1. Require good, isolated leptons down to 10 GeV and an object-based Emiss

T

High quality isolated leptons reduce fakesObject-based Emiss

T improves EmissT resolution

2. Baseline quality cutsEnsure good detector performanceTrigger on leptons as soft as 12 GeVRemove low-mass resonances

3. Z veto: |m`` −mZ | > 10 GeVsuppresses Z → ``, WZ and ZZRemoves 84.5% of Z → `` events with negligible signal impact

4. Jet veto: suppresses tt and single-t5. Emiss,rel

T : suppresses events with fake EmissT

6. mT2: suppresses WW and tt7. mCT: suppresses tt

20 / 48

Jet veto

Signal processes have OS dilepton and EmissT

tt → (W+b) (W−b)→ (¯νb) (`νb) is a background

tt has jets, many signal processes do not → jet veto

Can still get jets from:

Initial/final state radiationPileup

Requiring a jet vertex fractionJVF > 0.75 cut prevents veto on pileupjets

∼ 20% of χ+1 χ−1 processes have a jet

with JVF > 0.75 and 20 < pT < 30 GeV

Rejects ∼ 98% of tt ) [GeV]JVF

0(j

Tp

0 50 100 150 200 250 300 350 400

even

ts /

10 G

eV

-210

-110

1

10

210

310

410

510

610

710

810-1Ldt = 4.7 fb∫

fakes ) [GeV]0

1χ∼

, m±

1χ∼

(m

ll MC→Z (150, 50)

MCtt (250, 100)

single-t MC (350, 0)

diboson MC

Jets passing: pT > 30 GeV, |η| < 2.5, JVF > 0.75are called signal jets and are used for jet counting in all signal regions

21 / 48

Emiss,relT

EmissT can come from two sources

True EmissT : ν or χ

01

Fake EmissT : mismeasured objects in the event, hard to model

Due to R-parity all processes have stable χ01 → true Emiss

T

Backgrounds (i.e. Drell-Yan) do not have true EmissT

Could contribute to signal regions due to fake EmissT

Emiss,relT =

{EmissT if ∆φ`,j ≥ π/2

EmissT × sin ∆φ`,j if ∆φ`,j < π/2

Reduces the impact of:Emiss fakeT = Emiss

T +∑

pνT

[GeV]missTE

0 50 100 150 200 250 300 350 400

even

ts /

5 G

eV

-210

-110

1

10

210

310

410

510-1Ldt = 4.7 fb∫

MCtPowHeg + Pythia t OS dilepton

and single top MC = 2jN

1≥ bjN

Z-vetomissTmeasured E miss

Tfake E

[GeV]miss, relTE

0 50 100 150 200 250 300 350 400

even

ts /

5 G

eV

-210

-110

1

10

210

310

410

510-1Ldt = 4.7 fb∫

MCtPowHeg + Pythia t OS dilepton

and single top MC = 2jN

1≥ bjN

Z-vetomiss, relTmeasured E miss, rel

Tfake E

22 / 48EmissT vs. E

miss,relT for all backgrounds

mT2

For pair produced heavy ψ → `+ χ with χ invisible

mT2 = minpmissT,1 +pmiss

T,2 =pmissT

[max{m2

T(pT`1 , pmissT,1 ),m2

T(pT`2 , pmissT,2 )}

]This distribution has theoretical endpoint

(mendpointT2 )2 =

m2ψ−m2

χ

2mψ

√(m2ψ

+m2χ

2mψ

)2

−m2χ −

m2ψ+m2

χ

2mψ

+ (m2ψ −m2

χ)

For tt and WW mT2 ≤ mW

χ+1 χ−1 pair-production can be

much larger

Can cut at mT2 > 90 GeV toreduce background

[GeV]1

±χ∼m

100 150 200 250 300 350 400 450 500

[G

eV

]10

χ∼m

0

50

100

150

200

250

300

350

400

450

500

edge [G

eV

]T

2E

xpecte

d m

0

50

100

150

200

250

300

350

400

450

7083

100

91.7111

102125

155

111137

172

121151

191

250

138172

220

294

350

158197

253

344

415

500

186230

296

405

203

distributionT2

Expected edge of m

23 / 48mT2 in a top-enriched control region

mCT-based top tag

For pair produced ψ → φ+ χ with χ invisible

mCT(φ1, φ2) = [E 2T(φ1) + E 2

T(φ2)]1/2 − [p2T(φ1) + p2

T(φ2)]1/2

If mφ1 = mφ2 = mφ and mχ1 = mχ2 = mχ, then mCT distribution is bounded

from above by mCT <m2(φ)m(ψ) + m2(ψ)−m2(χ)

m(ψ)

There are 3 pairs of (ψ, χ) that can be used in tt

1. Only leptons

(ψ, χ) = (W , ν)mCT(`, `) < mW

2. Leading three jets

Calculate mCT(jj) for all combinations(ψ, χ) = (t,W )

3. Leptons and leading three jets

Calculate mCT(jj``) for all combinations(ψ, χ) = (t, ν)

4. If mCT(`, `), mCT(jj), and mCT(jj``) are consistent with tt for one pair of jets →reject event

24 / 48

Signal regions

Optimization

Use Zn =√

2 erf−1(1− 2p) where p ∝∫∞

0 db G (b;Nb, δb)∑∞

i=Ndata

e−bbi

i!Assume 20% background uncertainty

Scan for optimal Emiss,relT cut

χ+1 χ−1 production and pMSSM

SR-mT2 (OS)Jet Veto Z veto

Emiss,relT > 40 GeV mT2 > 90 GeV

[GeV]1

±χ∼m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

350

400

nZ

0

1

2

3

4

5

6

7

8

= 7 TeVs, -1Ldt = 4.7 fb∫0

1χ∼ν l× 2 →l) ν∼(νl~ × 2 → -

1χ∼ +

1χ∼

= 0.5 0

1χ∼

- m±1

χ∼m

0

1χ∼

- ml~,ν∼

m

> 90 GeV, 20% systT2SR-mT2: m

SR-OSjvetoJet Veto Z veto

Emiss,relT > 100 GeV

[GeV]1

±χ∼m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

350

400

nZ

0

1

2

3

4

5

6

7

8-1Ldt = 4.7 fb∫

pair prod.±1

χ∼

= 0.5 1

0χ∼

- m±1

χ∼m

1

0χ∼ - m

l~,ν∼

m

> 100 GeV, 20% systmiss, relTJVF 30 GeV Veto E

25 / 48

Signal regions II

χ±1 χ02 and pMSSMSR-SSjveto

Jet Veto Emiss,relT > 100 GeV

pMSSMSR-2jets (OS)

b-jet Veto Z veto

Emiss,relT > 50 GeV same-flavor

mCT top-tag

[GeV]1,2

,0±χ∼

m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

nZ

0

0.5

1

1.5

2

2.5

3-1Ldt = 4.7 fb∫

assoc. prod.0

2χ∼ ±

1χ∼

= 0.5 1

0χ∼

- m,0±

1,2χ∼

m

1

0χ∼ - m

l~,ν∼

m

> 100 GeV, 20% uncert.miss, relTJVF 30 GeV Veto E

[GeV]0

1χ∼

m40 50 60 70 80 90

[GeV

]10 χ∼

- m

1± χ∼m

10

20

30

40

50

60

70

80

90

nZ

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5-1

Ldt = 4.7 fb∫pMSSM grid

M1 = 100 GeV

26 / 48

Background estimations


SuccessesChallenges

2. Supersymmetry










Uncertainties

6. Results

27 / 48

Overview of background estimates

The following standard model processes are relevant

Events with one fake leptonW , Z , or SUSY, including from intermediate τ → trueLeptonic b decays or, for e, conversions and miss id π’s → fakeEstimate with matrix method

tt and single-tCan enter if jets are pT < 30 GeV or |η| > 2.5Estimate together by normalizing to a control region

Z + XIncludes: ZW , ZZ , and Z → `` + fake Emiss

T

Can enter if Z boson is off shellSame flavor: Estimate by normalizing to a control regioneµ: Monte Carlo expectation

WWVery similar to χ

+1 χ−1

SR-OSjveto: estimate by normalizing to a control regionOther SR’s: Monte Carlo expectation

For same sign: charge flipe∓hard → γhard e

∓soft → e∓soft e

∓soft e

±hard

Estimate rate in data and weight Monte Carlo opposite sign events 28 / 48

Events with a fake lepton

The matrix method employs a set of linear equations relating kinematic properties

of the leptons to the real (R) and fake (F ) lepton composition

Use isolation to define “loose” and “tight” leptons“tight” (T ) are the signal leptons“loose” (L) are the baseline leptons (more relaxed isolation)

The real efficiency r = (NT/NL)|m``−mZ |<5 GeV

The fake rate f is the probability that a loose fake lepton passes tightTwo sources for electrons, conversions and QCD; only QCD for muonsMeasure as a function of pT for 40 < Emiss,rel

T < 100 GeV in MCCompared with rates in fake-rich control regions → si

For electrons, use a weighted average:

f (e(pT)) =∑

i∈QCD, conv

fi (pT)wi (pT)si (pT)

For muons, only one component: f (µ(pT)) = fQCD(pT)sQCD(pT)NTT

NTL

NLT

NLL

=

rr rf fr ff

r(1− r) r(1− f ) f (1− r) f (1− f )(1− r)r (1− r)f (1− f )r (1− f )f

(1− r)(1− r) (1− r)(1− f ) (1− f )(1− r) (1− f )(1− f )

NRR

NRF

NFR

NFF

NRF

TT = r1f2NRF , NFR

TT = f1r2NFR , NFF

TT = f1f2NFF

29 / 48Why are the fakes measured in Monte Carlo

Top background (tt and single-t) control regions

Njets Nbjets Emiss,rel

T cut

pT > 30 GeV pT > 30 GeV GeVJVF > 0.75 JVF > 0.75

CombNN> −1.25SR-OSjveto =0 =0 100SR-OSjveto CR ≥ 2 ≥ 1 100SR-2jets ≥ 2 =0 50SR-2jets CR ≥ 2 ≥ 1 50SR-mT2 =0 =0 40SR-pre-mT2 CR ≥ 2 ≥ 1 40

e+e−

[GeV]miss, relTE

0 50 100 150 200 250 300

data

/ pr

edic

tion

00.5

11.5

2

even

ts /

20 G

eV

-110

1

10

210

310

410=7TeV)sData 2011 (

Prediction

fakes

PowHeg+Pythia MCttsingle top MC@NLO MC

diboson Herwig MC

ATLAS Thesis

-1Ldt = 4.7 fb∫

| > 10 GeV, [ee]Z - mll

1, |m≥ b

j = 2, Nj

CR: OS, Ntt

µ+µ−

[GeV]miss, relTE

0 50 100 150 200 250 300

data

/ pr

edic

tion

00.5

11.5

2

even

ts /

20 G

eV

-110

1

10

210

310

410=7TeV)sData 2011 (

Prediction

fakes


diboson Herwig MC

ATLAS Thesis

-1Ldt = 4.7 fb∫

]µµ| > 10 GeV, [Z - mll

1, |m≥ b

j = 2, Nj

top CR: OS, N

e±µ∓

[GeV]miss, relTE

0 50 100 150 200 250 300

data

/ pr

edic

tion

00.5

11.5

2

even

ts /

20 G

eV

-110

1

10

210

310

410=7TeV)sData 2011 (

Prediction

fakes


diboson Herwig MC

ATLAS Thesis

-1Ldt = 4.7 fb∫

]µ| > 10 GeV, [eZ - mll

1, |m≥ b

j = 2, Nj

top CR: OS, N

30 / 48

Top background estimate

NSRtop =

(NCR

data − Nnon−top MC

)×(

NSRtop

NCRtop

)MC× SFT

Fake contamination estimated using matrix method, diboson from Herwig

For SR-mT2 estimate to before the mT2 cut (SR-pre-mT2) due to limited stats,then apply εmT2 as measured from MC

SFT covers differences between jet veto efficiency between data and MC

From dedicated control region

Z veto and Emiss,relT > 40 GeV

A “tag” jet: pT > 25 GeV, |η| < 2.5, and b-tagged

εjet−veto =N0 signal jets

CR jet−veto

NallCR jet−veto

SFT = 1.0± 0.06

Variation between generators used to assess systematics

Central generators PowHeg+Pythia for tt and MC@NLO for single-tPowHeg+Jimmy and MC@NLO → generator systematicAcer samples used to assess ISR/FSR systematics

31 / 48

Same flavor Z + X (ZW , ZZ , Z → ``) control regions

SR-OSjveto jet-veto & Emiss,relT >100 GeV& Z -veto

Z + X CR for SR-OSjveto jet-veto & Emiss,relT >100 GeV& Z -window

SR-2jets 2 signal jets & b- and top-veto & Emiss,relT >50 GeV& Z -veto

Z + X CR for SR-2jets 2 signal jets & b- and top-veto & Emiss,relT >50 GeV& Z -window

SR-mT2 jet-veto & Emiss,relT >40 GeV& mT2 > 90 GeV & Z -veto

Z + X CR for SR-mT2 jet-veto & Emiss,relT >40 GeV& Z -window

Due to limited stats, estimate SR-mT2 before the mT2 cut then apply MC εmT2

SR-OSjveto

[GeV]llm

50 100 150 200 250 300 350 400

data

/ pr

edic

tion

00.5

11.5

2

even

ts /

10 G

eV

-110

1

10

210

310

410 = 7 TeV)sData 2011 (

PredictionFake leptons

+ jets*γZ/

ttSingle topDiboson

ATLAS Thesis

-1Ldt = 4.7 fb∫

]µµSR-OSjveto, no Z-veto [ee,

Z-CR

SRSR

SR-mT2

[GeV]llm

50 100 150 200 250 300 350 400

data

/ pr

edic

tion

00.5

11.5

2

even

ts /

10 G

eV

1

10

210

310

410

510

610 = 7 TeV)sData 2011 (


+ jets*γZ/

ttSingle topDiboson

ATLAS Thesis

-1Ldt = 4.7 fb∫

]µµ, no Z-veto [ee, T2SR-m

Z-CR

SR

SR

SR-2jets

[GeV]llm

50 100 150 200 250 300 350 400

data

/ pr

edic

tion

012345

even

ts /

10 G

eV

0

5

10

15

20

25

30

35 = 7 TeV)sData 2011 (PredictionFake leptons

+ jets*γZ/

ttSingle topDiboson

ATLAS Thesis

-1Ldt = 4.7 fb∫

]µµSR-2jets, no Z-veto [ee,

Z-CR

SRSR

32 / 48

Same flavor Z + X estimate

NoutZ+X→`` =

[N in

Z+X→`` − N innon−Z

]×(

NoutZ+X→``

NinZ+X→``

)MC

N innon−Z estimated from e±µ∓: Z → ττ → e±µ∓ is considered non-Z

SR-OSjveto

[GeV]llm

50 100 150 200 250 300 350 400

data

/ pr

edic

tion

00.5

11.5

2

even

ts /

10 G

eV

-110

1

10

210

310

410 = 7 TeV)sData 2011 (


+ jets*γZ/

ttSingle topDiboson

ATLAS Thesis

-1Ldt = 4.7 fb∫

]µSR-OSjveto, no Z-veto [e

Z-CR

SRSR

SR-mT2

[GeV]llm

50 100 150 200 250 300 350 400

data

/ pr

edic

tion

00.5

11.5

2

even

ts /

10 G

eV

1

10

210

310

410

510

610 = 7 TeV)sData 2011 (


+ jets*γZ/

ttSingle topDiboson

ATLAS Thesis

-1Ldt = 4.7 fb∫

]µ, no Z-veto [eT2SR-m

Z-CR

SR

SR

SR-2jets

[GeV]llm

50 100 150 200 250 300 350 400

data

/ pr

edic

tion

012345

even

ts /

10 G

eV

0

5

10

15

20

25

30


+ jets*γZ/

ttSingle topDiboson

ATLAS Thesis

-1Ldt = 4.7 fb∫

]µSR-2jets, no Z-veto [e

Z-CR

SRSR

N inee, non−Z = N in

eµ ×1

2× εe

εµ

εe

εµ=

(√N in

ee

N inµµ

)data

×

(εe

εµ

)MC

non−Z(εe

εµ

)MC

Z

33 / 48

Procedure for estimation of WW

WW is a significant background to allOS signal regions

The bulk of the WW distributionextends to ∼ 50 GeV

[GeV]miss, relTE

0 50 100 150 200 250 300 350

even

ts /

10 G

eV

-110

1

10

210

310

410WW Herwig MC

ATLAS Thesis-1

Ldt = 4.7 fb∫

]µ, eµµ cut [ee, miss, rel

TSR-OSjveto, no E

SR-mT2 and SR-2jets are in the bulk of the WW Emiss,relT distribution

Makes construction of an orthogonal SR challenging

Expect good agreement between data and MC in the bulk

Measure the WW contribution in MC

Compare generators to assess an uncertainty

SR-OSjveto is in the Emiss,relT tail

Possible to construct a control region: 70 < Emiss,relT < 100 GeV

Normalize to this CR to reduce lever arm34 / 48

Data-driven WW

Control region

OS dilepton

Standard jet-veto

Veto on b-jets with20 < pT < 30 GeV

|m`` −mZ | > 10 GeV

70 < Emiss,relT < 100 GeV

[GeV]miss, relTE

60 65 70 75 80 85 90 95 100 105 110

data

/ pr

edic

tion

00.5

11.5

2

evet

ns /

5 G

eV

1

10

210


+ jets*γZ/

ttSingle topDiboson

ATLAS Thesis

-1Ldt = 4.7 fb∫

>20GeV) = 0, [ee]T

(pb

j = 0, Nj

| < 10GeV, NZ-mll

WW CR: |m

NSRWW =

[NCRdata − NCR

MC non−WW

]× NSR

MC WW

NCRMC WW

Data-driven contamination estimates

tt and single-t normalized to top CR

Fakes from matrix method

35 / 48

MC diboson

Used for

All SR-SSjveto diboson contributions (in conjunction with charge flip)SR-OSjveto, SR-mT2, and SR-2jets e±µ∓ WZ and ZZSR-mT2 and SR-2jets WW for all channels

Number of jets

Eve

nts

1

10

210

310

Sherpa WWHerwig WWAlpgen WW

Number of jets0 1 2 3 4 5 6 7 8

Rat

io -

1

-1-0.8-0.6-0.4-0.2

00.20.40.60.8

1

Herwig lacks the matrix element for

higher jet multiplicities

Use Herwig for jet veto SR’s

Use Sherpa for SR-2jets

Use comparison of generators to estimate uncertainty

σ(gen1, gen2) =Ngen1

−Ngen2

Ngen1

σ = max [σ(Herwig,Alpgen), σ(Herwig,Sherpa)]

36 / 48

Measure charge flip rates

e+e− and e±µ∓ can contribute to SR-SSjvetoe∓hard → γhard e

∓soft → e∓soft e

∓soft e

±hard

Rate ε differs in data and MC

Measure |η| dependence from Z → eedata using a likelihood method

Take softer pT dependence from MCtruth

|η|0 0.5 1 1.5 2 2.5

0.60.8

11.21.41.6

Ele

ctro

n ch

arge

flip

rat

e

0

0.01

0.02

0.03

0.04

0.05

ZeeNpX MC truth based

data likelihood technique

Compare rates using truth in tt and

Z → ee MC

As a function of pT for each |η| bin

Over whole (pT, |η|) plane

Can use rates measured in Z → ee for tt

Eta range χ2 ndf p-value0 - 0.4 0.43 2 0.81

0.4 - 1.37 1.20 2 0.541.37 - 1.52 0.44 2 0.801.52 - 2.3 0.56 2 0.76

2.3 - 2.5 1.54 2 0.46global 6.42 14 0.95

37 / 48

Get charge flip estimateWeight opposite sign events to get same sign estimate

w = ε1+ε2(1−ε1)(1−ε2)

Electrons undergoing charge flip loosea small amount of energy

Visible in position of Z peak

∼ 2 GeV lower in same sign

Correct for this in final estimates

[GeV]llm

50 100 150 200 250 300 350

data

/ pr

edic

tion

01234

even

ts /

10 G

eV

-110

1

10

210

310

410

510

610=7TeV)sData 2011 (

Prediction

fakes

ll→Z

diboson

tt

single top

ATLAS Thesis

-1Ldt = 4.7 fb∫

SS [ee]

[GeV]llm

0 50 100 150 200 250 300 350

even

ts /

10 G

eV

-110

1

10

210

310-1Ldt = 4.7 fb∫

SS ee

fakes

-1Ldt = 4.7 fb∫SS ee

fakes Peak in fake estimate indicates thatmatrix method is picking up somecharge flip

This overlap is 18.6% in e±e±

Charge flip is 11.4% of total e±e±

background → neglect this overlap38 / 48

Uncertainties

Uncertainty Size [%]Jet energy scale 2-4b-jets energy scale add. 0.7-2.5Pile up on Emiss

T add. 6.6Electron energy scale 0.3-1.6Muon energy scale 2

E miss, CellOutT energy scale 2.6

Jet energy resolution 12-20Electron energy resolution 1.2-1.8Muon pID

T resolution 2.5Muon pMS

T resolution 4b-tagging SF 6-17Electron identification SF 1.5Muon identification SF 2Luminosity 3.9Theoretical σ 2-11Monte Carlo generator and ISR/FSR 4-45

Emiss,relT dependence of fake rate 20-50

Process contributions to QCD fakes 5-50Fake control regions 10-25Real efficiencies and scale factors 7Statistical 4-13

39 / 48

Results


SuccessesChallenges

2. Supersymmetry










Uncertainties

6. Results

40 / 48

Final predictions

Sum all channels

Background SR-mT2 SR-OSjveto SR-2jetsZ+X 7.1± 1.7± 2.1 12.2± 1.8± 1.8 9.6± 2.0± 5.1WW 10.6± 0.6± 1.5 43.0± 3.7± 12.2 14.8± 0.7± 9.9

tt, single top 12.9± 2.4± 4.6 96.2± 4.8± 29.5 36.9± 2.9± 29.6Fake leptons 2.2± 0.9± 1.4 10.3± 2.2± 4.1 4.2± 1.8± 2.3

Total 32.8± 3.2± 6.3 161.7± 6.7± 30.8 65.5± 4.0± 31.8Data 24 139 78

Background SR-SSjvetoCharge flip 0.83± 0.04± 0.18Dibosons 3.50± 0.31± 0.54

Fake leptons 6.6± 1.4± 3.8Total 11.0± 1.5± 3.9Data 9

All agree to within approximately 1σProceed to set limits

41 / 48

Limit setting

Modified CLS method

E = µs + b

Systematics described bynuisance parameters θ

Method of maximum likelihood→ µ θ

ˆθµ → values of θ whichmaximize the likelihood for agiven µ

Test statistic: tµ = −2 ln

{L(µ, ˆθµ)

L(µ,θ)

}CLS = p1

1−pb

µ

tµ

µ µ = 1

q1

42 / 48

Model independent limits

σobs(exp)vis [fb]

Signal region ee eµ µµ all

SR-mT2 1.5 (1.8) 1.6 (2.0) 1.6 (1.9) 2.5 (3.3)SR-OSjveto 3.3 (3.8) 6.8 (7.8) 4.0 (1.6) 9.8 (11.9)SR-2jets 6.9 (5.3) - 7.7 (7.6) 13.6 (12.5)

SR-SSjveto 0.7 (1.1) 1.6 (1.6) 1.3 (0.9) 1.9 (2.1)

SR-SSjveto has tightest limits on new physicsSR-mT2 is the strongest opposite sign region

43 / 48

Limits on σ×BR for simplified models

χ+1 χ−1 simplified model

[GeV]1

±χ∼m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

350

400

BR

exc

lude

d at

95%

CL

[pb]

× σ-210

-110

1

10

210 = 7 TeVs, -1Ldt = 4.7 fb∫

0

1χ∼ν l× 2 →l) ν∼(νl~ × 2 → -

1χ∼ +

1χ∼

= 0.5 0

1χ∼

- m±1

χ∼m

0

1χ∼

- ml~,ν∼

m

)01χ∼

) = m

(

±1χ∼

m(

ATLAS Thesis

SR-mT2

[GeV]1

±χ∼m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

350

400

BR

exc

lude

d at

95%

CL

[pb]

× σ

-210

-110

1

10

210 = 7 TeVs, -1Ldt = 4.7 fb∫

0

1χ∼ν l× 2 →l) ν∼(νl~ × 2 → -

1χ∼ +

1χ∼

= 0.5 0

1χ∼

- m±1

χ∼m

0

1χ∼

- ml~,ν∼

m

)01χ∼

) = m

(

±1χ∼

m(

ATLAS Thesis

SR-OSjveto

[GeV]1

±χ∼m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

350

400

BR

exc

lude

d at

95%

CL

[pb]

× σ

-210

-110

1

10

210 = 7 TeVs, -1Ldt = 4.7 fb∫

0

1χ∼ν l× 2 →l) ν∼(νl~ × 2 → -

1χ∼ +

1χ∼

= 0.5 0

1χ∼

- m±1

χ∼m

0

1χ∼

- ml~,ν∼

m

)01χ∼

) = m

(

±1χ∼

m(

ATLAS Thesis

SR-2jets

χ±1 χ02 simplified model

[GeV]1,2

,0±χ∼

m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

350

400

BR

exc

lude

d at

95%

CL

[pb]

× σ

-110

1

10

= 7 TeVs, -1Ldt = 4.7 fb∫0

1χ∼ + ll

0

1χ∼ν l→l l

~) + ±lν∼(ν±

l~ → 0

2χ∼ ±

1χ∼

= 0.5 0

1χ∼

- m,0±

1,2χ∼

m

0

1χ∼

- ml~,ν∼

m

)01χ∼

) = m

(

,0±1,

2χ∼m

(

ATLAS Thesis

SR-mT2

[GeV]1,2

,0±χ∼

m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

350

400

BR

exc

lude

d at

95%

CL

[pb]

× σ

-110

1

10

= 7 TeVs, -1Ldt = 4.7 fb∫0

1χ∼ + ll

0

1χ∼ν l→l l

~) + ±lν∼(ν±

l~ → 0

2χ∼ ±

1χ∼

= 0.5 0

1χ∼

- m,0±

1,2χ∼

m

0

1χ∼

- ml~,ν∼

m

)01χ∼

) = m

(

,0±1,

2χ∼m

(

ATLAS Thesis

SR-SSjveto

[GeV]1,2

,0±χ∼

m100 150 200 250 300 350 400 450 500

[GeV

]10 χ∼

m

0

50

100

150

200

250

300

350

400

BR

exc

lude

d at

95%

CL

[pb]

× σ

-110

1

10

= 7 TeVs, -1Ldt = 4.7 fb∫0

1χ∼ + ll

0

1χ∼ν l→l l

~) + ±lν∼(ν±

l~ → 0

2χ∼ ±

1χ∼

= 0.5 0

1χ∼

- m,0±

1,2χ∼

m

0

1χ∼

- ml~,ν∼

m

)01χ∼

) = m

(

,0±1,

2χ∼m

(

ATLAS Thesis

SR-OSjveto

These are the primary results for simplified models44 / 48

Limits on simplified models

Need to assume a cross sectionAssume 100% W for χ

±1 and χ

02 to get a cross section

χ+1 χ−1

mass [GeV]1

±χ∼100 150 200 250 300 350 400 450 500

mass [G

eV

]10 χ∼

0

50

100

150

200

250

300

350

400

450

500

1 4 4 41

4

1

4 41

1

4

41

4

4

1 1

4

11

1

1

4

1 4 41

1 41

ATLAS Preliminary

=7TeVs-1

L dt = 4.7 fb∫0

1χ∼νl×2 →l) ν∼(νl

~ ×2 →

-

1χ∼

+

1χ∼

= 0.50

1χ∼

- m±

1χ∼

m

0

1χ∼

- ml~,ν∼

m

T21=SR-OSjveto, 2=SR-SSjveto, 3=SR-2jets, 4=SR-m

)0

1χ∼

)= m

(

±1χ∼

m(

)theory

SUSYσ1 ±Observed limit (

)expσ1 ±Expected limit (

χ±1 χ02

mass [GeV]1

±χ∼100 150 200 250 300 350 400 450 500

mass [GeV]

10 χ∼

0

50

100

150

200

250

300

350

400

450

500

1 4 4 41

4

4

4 44

1

4

44

4

4

1 4

4

44

3

4

4

4 4 44

4 44

ATLAS Preliminary

=7TeVs-1

L dt = 4.7 fb∫0

1χ∼+ ll

0

1χ∼νl→l l

~) +

±lν∼(ν

±l~ →

0

2χ∼±

1χ∼

= 0.5 01

χ∼- m,0±

1,2χ∼

m

0

1χ∼

- ml~,ν∼

m

T21=SR-OSjveto, 2=SR-SSjveto, 3=SR-2jets, 4=SR-m

)0

1χ∼

) = m(

,0±1,2χ∼

m(

)theory

SUSYσ1 ±Observed limit (

)expσ1 ±Expected limit (

Use the signal region with the best expected limit45 / 48

Limits on the pMSSM

Use the signal region with the best expected limit

M1 = 100 GeV M1 = 140 GeV M1 = 250 GeV

Combined with three-lepton results in arXiv:1208.3144v1 [hep-ex]

46 / 48

Conclusions

First search for direct gauginos with two lepton channel at LHC

Four different signal regionsThree opposite sign:

mT2, a jet veto, a Z boson veto, and Emiss,relT

A jet veto, Z boson veto, and high Emiss,relT

Two jets, Z boson veto, a mCT top veto, and Emiss,relT

One same sign: with a jet veto and high Emiss,relT

Many semi-data-driven estimatesFakestt + single-tOpposite sign same flavor Z + XWW in SR-OSjvetoCharge flip

No excesses → set limitsχ+

1 χ−1 simplified model with intermediate ˜’s ← First limit at LHC

χ±1 χ02 simplified model with intermediate ˜’s

pMSSM with gauginos and ˜’s

47 / 48

Thank you

48 / 48

BACKUPS

49 / 48

Dark matter

Bullet Cluster gives evidence that it is a kind of matter

Only potential SM candidates are the weakly interacting neutrinos

Neutrinos cannot comprise all of the dark matter as they are movingrelativistically and relativistic dark matter yields a Universe structureincompatible with observations

50 / 48Back to Successes and Problems in the SM

R-parity

The general form of thesuperpotential includes(all symbols are superfields):

λijkLiQj dk

λ′ijk ui dj dk

These terms allow for proton decay

ℓ−

u

d

u

dλ λ′

To prevent proton decay R-parity is enforced

Yields dark matter candidate

Unlike B and L in SM which arise spontaneously

51 / 48Back to SUSY motivation

The Coleman Mandula theorem

Noether’s theorem

For every conserved current jµ such that ∂µjµ = 0,

there is a conserved charge Q =∫j0 d

3x , i.e. dQ/dt = 0,

Coleman Mandula

Only conserved charges in Special Relativity are:

Scalars: electric charge etc.

Vector: linear momentum

Anti-symmetric tensor: angular momentum and boosts

Loophole

Can fit a spinor “in between” scalar and vector

For scalar φ and Weyl fermion ψ

The current Jµα = ∂ρφ∗(γργmuψ)α is conserved

52 / 48Back to SUSY motivation

Object definitions

ElectronsCalorimeter cluster matched to a track

BaselinepT > 10 GeV|ηclus| < 2.47Track and shower shape cuts

SignalTighter cuts on track and shower shapepTcone20†/pT < 0.1

JetsTopological calo clusters

BaselineAnti-kT calibrated with EM-JESdR(e, j) > 0.2

b-jetsJetFitterCombNN > −1.2580% efficient

†pTcone20:pT sum of all tracks with pT > 1 GeV and dR < 0.2,

subtracting the pT of the e or µ track itself

MuonsCombination of ID and MS track

BaselinepT > 10 GeV|η| < 2.5

SignalpTcone20 < 1.8 GeV

EmissT

Object based

Emiss, RefFinalT

All cells with |η| < 4.9

Cells matched to objects and calibratedElectrons → electron calibrationsJets → add jet energy scaleRemaining cells → CellOut

at default EM scale

Add baseline muons

53 / 48Back to selection overview

Baseline event selection

Reject events with poorly reconstructed objects from detector effectsAt least one vertex with Ntracks > 4Overlap removal

If dR(e, e) < 0.1: remove e with lower Eclus

If dR(`, j) < 0.4: remove `If dR(e, µ) < 0.1: remove both

Veto events with cosmic muonsExactly 2 baseline leptons ← Ensures orthogonality with 3-lepton searchTrigger

Single and di-lepton triggersMC: trigger weights derived from data (no trigger simulation)Data: leptons must match on-line trigger objects

Exactly 2 signal leptonsm`` > 20 GeV

Removes low-mass resonances (J/ψ)Not modeled in diboson Monte Carlo samples

54 / 48Details on trigger logic, Monte Carlo samples, Back to selection overview

Trigger details

pT (e0)

pT (e1)

10 17 25

17

10Sin

gle−

etr

igge

r

di−e trigger

10 12 20

12

pT (µ0)

pT (µ1)

At least one single−µ

Both matching di−µ

10

pT (e)

pT (µ)

20

10

10 15 25

single−µ triggereµ

trig

ger

single − e

OR(single−µ &&

!single−e)

singl

e−e

trig

ger


Monte Carlo samples for backgrounds

Sample Default Theoretical cross Cross sectionGenerator section [pb] Uncert.

Z → ``+[0-5] jets Alpgen 1072.5 ±5%

tt PowHeg 166.8 +17.3−18.4 pb

WW Sherpa/Herwig 43.8 ±5%

WZ Sherpa/Herwig 19.1 ±7%

ZZ Sherpa/Herwig 6.21 ±5%

t-channel single-t MC@NLO 64.57 +2.63−1.74 pb

s-channel single-t MC@NLO 4.63 +0.20−0.18 pb

Wt-channel single-t MC@NLO 15.74 +1.17−1.21 pb

These are the default samples. Additional samples used to assess systematics.

56 / 48Back to baseline selection

Z boson veto

OS di-leptons → Z bosons are asource of standard modelbackground

Z → ``WZZZ

Targeted signal processes do nothave real Z bosons

Use a Z veto:|m`` −mZ | > 10 GeV [GeV]llm

50 100 150 200 250 300 350 400

even

ts /

2 G

eV

-110

1

10

210

310

410

510

610-1Ldt = 4.7 fb∫

fakes ) [GeV]0

1χ∼

, m±

1χ∼

(m

ll MC→Z (150, 50)

MCtt (250, 100)


diboson MC

Removes 84.5% of Z → `` eventsNegligible impact on signal


EmissT vs. Emiss,rel

T for all backgrounds

[GeV]missTE

0 50 100 150 200 250 300 350 400

even

ts /

10 G

eV

-110

1

10

210

310

410

510

610 -1Ldt = 4.7 fb∫JVF 30 GeV Veto

fakes ) [GeV]0

1χ∼

, m±

1χ∼

(m

ll MC→Z (150, 50)

MCtt (250, 100)


diboson MC

[GeV]miss, relTE

0 50 100 150 200 250 300 350 400

even

ts /

10 G

eV

-110

1

10

210

310

410

510

610 -1Ldt = 4.7 fb∫JVF 30 GeV Veto

fakes ) [GeV]0

1χ∼

, m±

1χ∼

(m

ll MC→Z (150, 50)

MCtt (250, 100)


diboson MC

Jet veto and Z veto already applied

58 / 48Back to E

miss,relT

mT2 in a top-enriched control region

e+e− µ+µ− e±µ∓

59 / 48Back to mT2

Why are the fake rates measured in Monte Carlo?

Generally a fake control region is bb or conversion enriched

In the signal regions, tt and W → `ν dominate the QCD component

The fake rates from the leptonic decaying b in tt are different fromthose in bb due to b pT

The fake rates in tt and W → `ν are intrinsically Emiss,relT dependent

Thus take them from Monte Carlo

Monte Carlo rates were also measured for bb enriched control regionsComparison to data yields the scale factors

60 / 48Back to the matrix method

search for direct gaugino production decaying to two leptons...

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