Searches for Supersymmetry with leptons in the final state

Searches for Supersymmetry with leptons in the final state

Didar DoburUniversity of Florida

Physics @ LHC, June 2011, Perugia Italy

On behalf of the CMS Collaboration

Lepton production in SUSY

–Many ways to produce isolated leptons at the LHC if SUSY exists

– Direct chargino or neutralino production (small )

– Direct squark production and subsequent decays to charginos and neutralinos

– Direct gluino production and subsequent decays to squark, which decay to charginos/neutralinos

Didar Dobur 2/21 Physics @ LHC, Perugia, 2011

Signal characteristics

q(q)

lepton( )n

mB

mC

mA

B

C

A

p

p

A: LSP [dark matter motivated; MET]B: gluino/squark [large x-sec; jets]C: chargino [leptons]

• mB drives the cross section

• DmBC drives hadronic activity : is arbitrary

• DmCA drives lepton’s pT

• MET is large as long as DmBA is large• tau’s can be “naturally” much more prolific than muons and electrons


SUSY Searches with CMS

1-lepton OS di-leptons (OSDL) SS di-leptons (SSDL) ≥3 leptons

Single lepton +

Jets+MET

e, m

Opposite-sign di-lepton +

Jets + MET,(Z veto)

Z + Jets + MET

e, m

Same-sign di-lepton +

Jets + MET

e, , m t

Multi-leptons+

Jets +MET

e, , m t

• All search topologies involve MET• Categorized by number of leptons• All counting experiments at this point• Major SM backgrounds are measured from data!

So far, no excess over expected SM yield is observed !

2011


Single Lepton + Jets & MET searchSignal Selection:

pT ( /m e) > 20 GeV , Nlep=1 , 4 jets, HT > 500 GeV, MET > 250 GeV

• Dominant bkg.: ttBar/Wjets• ABCD method •“lepton spectrum” method: Model the pT(ν) using pT(l)

• Corrections to account for differences (due to W helicity) taken from theory•Use MET templates measured in multi-jet events to

smear pT(l)


U.L. on the signal events at 95 C.L. is 4.1

OSDL: Search strategySignal Selection:

pT ( ,m e) > 10/20 GeV

|mll-mZ | > 20 GeV

HT > 300 GeV,

y > 8.5 GeV½

ee mm em

SM MC 16.6 ± 0.3 19.6 ± 0.3 45.3 ± 0.5

Data 15 22 45

Good agreement between Data/MC in the control region

Background prediction:•ttBar (dominant)

•ABCD method, (HT, y)

•PT(ll) method

• OF subtraction (not covered here)

• QCD (small) : Tight-to-loose method


OSDL: bkg prediction

NA x NC/NB ND(signal reg.)

MC 1.27 ± 0.05 1.27 ± 0.10

Data 1.30 ± 0.78 1

ABCD method :

PT(ll) method :

D: signal region

HT and y mostly uncorrelated in ttBar

ND = NA x NC/NB

• Exploits the fact that l and ν get the same boost in ttBar/Wjets• Model MET using pT spectrum of di-leptons• Corrections to account for differences

prediction: 2.1 ± 2.1 (stat) ± 0.6 (syst)


OSDL : results in 34 pb-1

MC expectation 1.27±0.10

pT(ll) estimate: 2.1 ± 2.1 (stat) ± 0.6 (syst)

ABCD estimate: 1.30 ± 0.8 (stat) ± 0.3 (syst)

Average 1.4 ± 0.8 (stat/syst )

observed 1

• All three methods give consistent estimation of SM backgrounds

•Upper limit on the non-SM event yield in the signal region at 95% CL is 4.0


Z + Jets+ MET search with “JZB”

control region

• Use JZB< 0 to predict Z+Jets bkg. in JZB > 0 • eμ pairs to predict ttBar in JZB > 0

Dominant bkgds: Z+jets, ttbar

signal region

Jet-Z-Balance (JZB): signal/background discrimination

Signal Selection: pT ( ,m e) > 20 GeV

|mll-mZ | < 20 GeV

> 1 Jet with pT > 30 GeV


“JZB”: closure testPredicted and observed JZB distribution in MC , no signal : very good agreement

Predicted and observed JZB distribution in MC in the presence of signal ( x 20) : excess of signal is well above predicted bkg.


“JZB”: results with 34 pb-1

signal region

Observed Data deriven Prediction MC

JZB > 50 GeV 4 8 ± 3 (stat) ± 1.0 (peak) ± 3.2 (sys) 5.5 ± 0.2


“JZB”: results with 191 pb-1

2011

signal region

Observed Data driven Prediction MC

JZB > 50 GeV 20 24 ± 6 (stat) ± 1.4 (peak) ± 2.4 (sys) 16 ± 1.2

JZB > 100 GeV 6 8 ± 4 (stat) ± 0.1 (peak) ± 0.8 (sys) 3.6 ± 0.4


U.L. on the signal yield 6.6 is set with 95% C.L. Bayesian interference & profile likelihood model for uncertainty

SSDL: Search strategy

(ee / / mm e )mpT (e, ) m > 20/10 GeV

HT > 60 (2 jets) , MET > 80

HT > 200 (2 jets) , MET > 30(20) GeV

(ee / / mm e / m e / t / mt tt)

pT ( , m e, t ) > 5, 10, 15 GeV

HT > 300 (350) (2 jets)

MET > 30 (50) GeV

• ğğ production would give SS:0S = 1:1• OS squark-squark can give SS di-leptons via multi-leptons• SS squark-squark production -> SS:0S = 2:1

SM background is very small for SSDL

Search regions to maximize the sensitivity to different mass splittings

• High pT leptons & lower hadronic activityLeptonicTriggers

• Low pT leptons & lower hadronic activityHadronic Triggers


SSDL: bgk. prediction strategy

• SS prompt leptons (WW,ZZ,WZ)

small contribution

reasonably well understood take from MC : < 0.1

• charge mis-identification (electrons only)

small contribution derive from data : 0.012 ± 0.006

Use SS/OS ee events within the Z mass window to estimate the charge mis-id rate : 0.0007

• non-prompt leptons from jets (ttBar/W+jets/QCD)

dominant contribution derive from data (next two slides)


b-tag-and-probe for ttBar/V+jets

• Measure Isolation templates in a bbBar enriched control sample

• Fix the normalization of the measured isolation templates with Sideband events

• Sideband: events passing all analysis cuts, except for one lepton failing the isolation cut

Prediction : 0.52±0.24 (stat) ~50% syst

Lepton isolation is the main handle for controlling the prompt+fake


Factorization method for QCD• Exploit the fact that some selection cuts are uncorrelated: Isolation of μ1 , Isolation of μ2 , MET

Npred = Npreselected . εIsoμ1 . εIsoμ2 . εMET

Prediction: 0.18±0.12 (stat) , ~65% syst uncertainty

εIsoμ1μ2 = ε2Isoμ1

• Npreselected: all cuts, except for isolation and MET


See the poster by: R.Remington

“Tight-to-loose” for fake tau bkg.

Prediction : 0.28±0.14 (stat) ~32% systematic uncertainty

• The dominant background is due to “fake” taus : “Tight-to-Loose” method

• eT/L : probability of loosely identified taus to pass the tight id-cuts

• Measure eT/L(pT,h) in multi-jet sample, <eT/L > ≈0.1

• Sideband: all analysis cuts but one or two taus pass the loose id and fail tight-id

• Reweight the observed events in the sideband events by efficiencies eT/L(pT,h)

Closure test of the method:


SSDL : results with 35 pb-1

Summary of the predicted SM background and observed events in each search region

We set 95 % CL upper limits on event yields in each search region : 3.1 , 4.3 , 4.4, 3.4


– Bayesian with a flat prior on signal strength– Lognormal pdf for systematic errors

Signal efficiency parameterization• Acceptance model is defined with respect to stable particles at generator level

•Lepton efficiencies:

•Isolation corrections:

• MET : smear by 7-25% (depending on HT)• HT : smear by 20-30% (depending on HT)

T

250.10 ,

15where is the average number of

stable charged particles with p >3 and | |<2.4

n

n


Summary

• CMS performed searches for SUSY in various leptonic final states using integrated luminosities 35- 191 pb-1

• Multiple data-driven techniques to measure SM bkg are developed

• No evidence beyond SM expectations found, set limits on new physics event yields

• 2011 will be ( the most ?) an exciting year for SUSY searches



Bibliography

see also:https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults

1. Search for new physics with opposite-sign di-leptons at the LHC, CMS PAS SUS-10-007 , Accepted by JHEP

2. Search for new physics with same-sign di-leptons at the LHC, CMS PAS SUS-10-004 , Accepted by JHEP

3. Search for new physics with single-leptons at the LHC, CMS PAS SUS-10-006

4. Search for new physics with multileptons at the LHC, CMS SUS-10-008

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults

http://cms.cern.ch/iCMS/analysisadmin/cadi?ancode=SUS-10-007











BackUp

Supersymmetry (SUSY)

gaugino/Higgsino mixing

• Theory introduces a spin-based symmetry between fermions and bossons ; result in each fermion having a bosonic super-partner and vica versa

• Provides solution to the Hierarchy Problem• Allows unification of the gauge couplings at high scales • Defines a new quantum number R-parity=(-1)3(B-L)+2S and if conserved, provides Dark Matter candidate, Lightest Supersymmetric Particle(LSP)

How to search for SUSY signature ?• Difficult without making assumptions, production x-sections and the details of

the decay chains depend on the SUSY model • Most popular ones R-parity conserving scenarios -> Missing Transverse

Energy(MET) , large squark/gluino masses -> long decay chains, jets, leptons


SS di-leptons: Trigger strategy

• HT* triggers• High HT (>300 GeV) • low-pT leptons including tau’s

– Efficiency in signal region: 94±5% (measured in an unbiased dataset)

• Lepton triggers – all un-prescaled lepton and di-lepton triggers not relying on isolation

• High-pT leptons (pT1>20/10 GeV)

• Can allow for low HT

– Trigger efficiency in signal region: 99±1% (T&P)

* HT: scalar sum of the pT of the jets with pT > 30 GeV


Same Sign prompt leptons

Contribution is very small and taken from simulation: ≤ 0.1 event for all search regions

Double W-strahlung Double-parton scatteringDi-boson production


Isolation templates

•Gray(QCD MC) and red(ttBar) form the closure of the method, black: data•Observe total 24 events in sidebands of ee, mm, e m channels:•Prediction (Signal Region C): 0.52±0.24 (stat) ~50% syst

isolation template for muons isolation template for muons


Charge mis-identification• Methodology– Map charge-flip probability e(pT,h) using e-gun MC– Direct SS Zee yield in Z MC: 7.9±0.9– Expected yield in Z MC using e(pT,h): 4.9±0.1– < >e data= N(SS Zee)/( OS Zee)= 5/3642=0.0007

Option 1: apply the MC-based e(pT,h) to the observed isolated OS ee/em events (+HT, +MET)

Option 2: use < >e data instead of MC

• Both options give self-consistent results:

• 50% : stat uncertainty of the data-driven validation

Sig. rgn A B C D

Bkg(CF)

0.012 0.04 0.012 negligible


Summary of bkg and observed yields

• Limits: – Bayesian with a flat prior on signal strength– Lognormal pdf for systematic errors– Errors on background rates and signal acceptance are not correlated

A

B

C

D



Signal acceptance uncertainties

Source of uncertainties

Error Comment

Muon reconstruction efficiency

1.5% (pT>20)

6% (pT~5)J/yμ μ and Zμ μ T&P

Electron reconstruction efficiency

3% (pT>20)

8% (pT~10)Zee T&P, pT(e)/pT(μ )-ratio in Z events

Isolation cut efficiency 5% Zμ μ LKT: data vs MC

Tau reconstruction efficiency 30% Ztt overall yield: data vs MC

HT+MET cut efficiency1% (HT=60)

7% (HT=350)

Standard JetMET prescription, applied to LM0 (5% JES uncertainties assumed)

Lepton triggers efficiency 1% Zee and Zμ μ T&P

HT triggers efficiency 5% Muon dataset

Luminosity 11% CMS Lumi uncertainties for 2010 run

Searches for Supersymmetry with leptons in the final state

Documents

Transcript of Searches for Supersymmetry with leptons in the final state