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SUSY Search at CMS
Anwar A BhattiThe Rockefeller University
On behalf of CMS CollaborationLHC Dark Matter Workshop
Michigan Center of Theoretical PhysicsJanuary 6-10, 2009
• Jet+MET+0 lepton analysis• Jet+MET+leptons analysis• MET independent analysis• Conclusions
SuperSymmetry
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 2
Avoids fine-tuning of SM, can lead to GUTs Assume LSP is stable ⇒ possible dark matter candidate SUSY breaking mechanism is unknown ⇒ many parameters
A symmetry between fermions and bosons |S=0 or S=1⟩ ↔ |S=½⟩
mSUGRA:supergravity inspired model5 free parameters: m0, m½, A0, tan(β) and sign(µ)
Supersymmetry at CMS
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 3
Final state typically has multiple jets and large missing transverse energy.Cross sections depend on the SUSY parameters, specially masses of squarks and gluinos.
LM160,250GeV
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 4
Benchmark Point LM1
m0=60 GeV, m½=250 GeV, tan(β)=10,A0=0,sign(µ)>0Gluino mass = 600 GeV, squark mass =320 GeV
Inclusive Jet+MET Analysis
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 5
Standrad Model Processes
SUSY LM1 ~50 pb• Require large MET and multi-jets to suppress the SM backgrounds.• Use data-driven techniques to estimate backgrounds.Event Selection:• Cleanup• No leptons (no e,µ, isolated tracks, EM rich jets)• Three leading jets with Pt>180, 110, 30 GeV• HT = PtJet2+PtJet3+PtJet4+MET >500 GeV• MET> 200 GeV
QCD 1010 pb
W+jet (leptons) 7x104 pb
Z+jets (leptons) 7x103 pb
800 pbtt
Missing Transverse Energy
Event Cleanup ≥ 1 primary vertex Activity in Ecal, Hcal and tracker
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 6
Energy: 250 GeV (ECAL)
Halo Muons in CSCs and HB
Not a big issue as long as such events do not overlap with real pp collision.
Cosmic Ray Muon Air Shower
Cosmic Muon
>0.175
> 0.1
Lepton Veto (remove W/Z/tt background)
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QCD Background Events
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 8
MET in QCD is due mis-measured jets and is in the same direction as the jet.
Even after these cuts, QCD is the largest background at LM1.
QCD
SUSY@LM1
1,2 METjet1,jet2
2 21,2 1,2
|1,2
|where ( ) 0.5R
ϕ ϕ
ϕ π ϕϕ = −
= ∆ + − >
Data-driven technique: Matrix Method
BBackground
CSignal+BG
ABackground
DBackground
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Variable X
Varia
ble Y
• For uncorrelated X,Y, background in C: C= D (B/A)
• Signal contamination in A,B,D should be small.• Additional corrections if X and Y are correlated.• Possible pair: MET and Δφ(Jet,Met) min• Working on optimizing the procedure.
Data Driven Technique: Jet Smearing
MET in QCD events arises from the fluctuations in detector response to jets.
The high MET tail in multi-jet events can be determined by smearing the jets in the low MET region using detector response functions.
Response can be measured in well-understood data:• multijet events• dijet events• photon-jet events
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 10
Use response function to predict high MET region
Response Function
Based on earlier ATLAS work
MET (GeV)
PtCaloJet/PtParticleJet
Top Background
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 11
• MET Template: Events passing SUSY cuts but require a lepton and
MT(lepton, MET) <100 GeV• MET distribution same as the background except lepton.
• Normalize the MET template to the data in low MET region 100< MET<200 GeV.• Works quite well if no SUSY contamination in normalization region.• Effect of SUSY contamination in normalization region on the background estimate is under study.
not-identifiedNon-isolated
Semi-leptonic decays are dominant.
Based on earlier ATLAS work Template Data
Norm. Region
MET MET
Irreducible Background: Zνν
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 12
More details in talk by James Lamb, later in the session.
Data-driven Estimation I (Z+jets)Standard Candle: use Z→µµ
Replace leptons by neutrinosTotal uncertainty ~20% for 1 fb-1
Statistically limited • Br(Z→µµ)= 1/6 Br(Z→νν)
Data-driven Estimation II (W,γ+jets)Gain in statistics:σ(Z+2jet)=σ(W+2jet)/3=σ(γ+2jets)/0.8
• Only Z→µµ (ee) are usable. 3.3%Approximation: V+jets events at high pT have similar event shapes.
Z→νν background estimate (100 pb-1)
MC-truth 35
From γ+jet 29±3(stat)±5(sys)
From W+jets 35±10(stat)±8(sys)±3(theoy)
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 13
Discovery Potential
SUSY LM1 discovery possible with < 10 pb-1 of data at √s= 14 TeV.
600GeV 520GeV,g qm m= =
SUSY LM1
Acceptance ~13%
Signal QCD Top pair Z(νν) W/Z Single top6319 107 54 48 33 3
Search for SUSY using leptonic decays
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Concentrating on muons: Relatively easy to identify Good momentum resolution
Small SM backgrounds Low susy cross section but still
good S/B and S/√B
0 02 1g qq qq qqll qqllχ χ→ → → →
Sources of muons:(no particular order)•b/c quarks (semi-leptonic decays)•W/Z/top quark decays•decay-in-flight•τ-leptons• cosmic rays• mis-identified hadrons (punch through)• new physics
Muon identification:•Good track •Good matching muon stub•No additional track around the tracks•No large energy deposit around muontrack in calorimeter.
Inclusive muon+MET+Jets analysis
≥ 1 muon with pT >30 GeV MET>130 GeV At least three jets:
• 440, 440, 50 GeV• optimized η cuts
cos[Δφ(jet1,jet2)]<0.2 Cuts on Δφ(jet1,2,3,MET)
Excellent sensitivity to all LM benchmark points.
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 15
5σ reach contour
Event Yield (1 fb-1)LM1: 300 events (efficiency=0.07%)BG: 3 events with uncertianty~20%
Same-sign Di-muon+MET+Jets Analysis
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 16
LEPTevatron
A0=0, tan(β)=10, sign(µ)=+1
5σ reach contours
Sample Events mSUGRA LM1 17
≤0.3
W/Z+jets ≤0.05
QCD ≤0.01
Expected events for 1 fb-1 @14 TeV
tt
♦ Same sign muon (PT>10 GeV) ≥ 2
♦ ETmiss ≥ 200 GeV
♦ 1st Jet ET ≥ 175 GeV♦ 2nd Jet ET ≥ 130 GeV♦ 3rd Jet ET ≥ 55 GeV
1 2
2 3, 13,10 GeV, 6 GeVIso Iso
nHitsE Eµ µ
µ µ
χ ≤ ≥≤ ≤
01
01
g qqq q
µ νχµ νχ
±
±
→→
Dilepton+Jets+MET, mass edge
Observation and measurement of
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 17
SUSY di-lepton analysis in the talk by Oliver GutscheFriday January 9th 2:00 pm
0 02 1 llχ χ→
MET Independent Search
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 18
Dijet Search
Signal: If , no cascade decays through gluinos• Two jets uncorrelated in Pt and direction• .
BackgroundsQCD dijets: back-to-back, similar pT’s
Z→νν: Irreducible background
W→(e,µ) ν: MET when (e,µ) out of acceptance.
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 19
01
02, 0
1Mainly sensitive to ,... q q q q qχ χ χνν→→ →
q gm m<
MET-independent Dijet Search
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 20
α(Randal-Tucker-Smith) arXiv:0806.1049
Analysis does not rely on missing transverse energy.
Transverse αT
QCD : back to back jets α(αT)≤0.5.SUSY: α (αT ) can be > 0.5.
Event Selection
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 21
HT>500 GeV Third jet veto pt< 50 GeVΔφ(MHT,jet1,2,3)>0.3 rad Lepton veto |η(jet1)|<2.5
Δφ(jet1,jet2)
α αT
1 2 1 2 | |JT
J J JT T TTH p p MHT pp= + = +
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Expected Event Yield
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 23
0 2.5 4
Data-Driven Background Estimation
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 24
Pre-selection (no η cut) + HT>500 GeV
B(Signal+BG) C(BG)
A(BG+some signal)~2 million QCD+~2K
tt/W/Z+~1k SUSYD(BG)
α T
0.55
|η| of lead jet
• Signal is in the central region.• Use the ratio
in forward region to predict thebackground in the central region.
• R= C/D (assumed to be independent of η.)
( 0.55)/ ( 0.55)T TR N Nα α= > <
background( )B A R= ×
Background Estimation
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 25
▲ Simulated signal+BG αT>0.55■ Estimated background αT>0.55
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 26
Signal Topology: Two squarks decaying into two quarks+ two LPS.• Squark-squark dominates• Squark-gluino contributes when
Background: QCD :0, Z→νν 57, W/Z 19, Total 86
g qq→
Summary
The results from global run data look very good. • Rate of events with large MET is low.
The dijet analysis looks very promising. Ready to analyze the real collider data.
Anwar Bhatti LHCDM@MCTPJan 6-10, 2009 27
BACKUP
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 29
CMS: Compact Muon Solenoid
Muons: • |η|<2.4 momentum resolution: dpT/pT~1% (pT~25 GeV) Calorimetry:• Hadron |η|<5.0, δE/E ~ 70% / √E + 8%• EM |η|<3.0, δE/E ~ 2.8% / √E + 0.3% + 12% / E
MUON BARRELDrift TubeChambers ( DT )
Resistive PlateChambers ( RPC )
SUPERCONDUCTINGCOIL
IRON YOKE
Silicon MicrostripsPixels
TRACKER
Cathode Strip Chambers (CSC )Resistive Plate Chambers (RPC)
MUONENDCAPS
CALORIMETERSECAL
ScintillatingPbWO4 crystals
HCALPlastic scintillator/brasssandwichTotal weight: 12,500 t
Diameter: 15 mOverall Length: 22 mMagnetic field: 3.8 Tesla
30Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP
CDF limits
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 31
tan(β)=5, A0=0 and μ<0.
Signal: If , no cascade decays through gluinos• Two jets uncorrelated in Pt and direction• .
BackgroundsQCD dijets: back-to-back, similar pT’s
Z→νν: Irreducible background
W→(e,µ) ν: MET when (e,µ) out of acceptance.
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 32
01
02, 0
1Mainly sensitive to ,... q q q q qχ χ χνν→→ →
Fraction of events passing selection cuts
Process Pt of third jet (GeV)<30 <50 <70
80 61 51
18 34 44
1 3 5
qq qg gg
q gm m<
Selection cuts described later.
Backgrounds
Sources of background QCD:
Seems to be under control but huge cross section Uncertainties due to higher order QCD effects missing in Monte Carlo
Third jet veto should minimize higher order effects.
W, Z, tops Zνν:
Represents an irreducible background Two jets+ real missing ET
Ideally, Zµµ events can be used but not enough statistics. Other control samples W+jets Photon+jets
Jan 6-10, 2009 Anwar Bhatti LHCDM@MCTP 33