Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Discovery potential for Supersymmetry with the
one-lepton channel in early ATLAS data
Gunn Kristine Holst Larsen
University of Oslo
13th of October 2008
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Contents
1 Introduction (The Standard Model-Supersymmetry-ATLAS)
2 One-Lepton Search
3 Result
4 Conclusion
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
The Standard Model
• Consistent renormalizable quantum field theory.
• U(1)Y xSU(2)L xSU(3)c
• Explains data from a fraction of an eV to a few hundred GeV.
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Supersymmetry
•
•
•
•
Symmetry between fermions and bosons, Qa |Ji = |J ± 1/2i.
Solution to the Hierarchy problem.
Possible Grand Unified Theory.
The Standard Model only explain ∼ 5% of the matter in the
universe, Supersymmetry provides candidate for dark matter
(∼ 25%).
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Supersymmetry
•
•
•
•
Symmetry between fermions and bosons, Qa |Ji = |J ± 1/2i.
Solution to the Hierarchy problem.
Possible Grand Unified Theory.
The Standard Model only explain ∼ 5% of the matter in the
universe, Supersymmetry provides candidate for dark matter
(∼ 25%).
• Quantum loop
corrections to Higgs
mass:
2
l|
• − |λ
Λ2 + ...
8π 2 cut
•
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
λB
Λ2 +
16π 2 cut
...
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Supersymmetry
•
•
•
•
Symmetry between fermions and bosons, Qa |Ji = |J ± 1/2i.
Solution to the Hierarchy problem.
Possible Grand Unified Theory.
The Standard Model only explain ∼ 5% of the matter in the
universe, Supersymmetry provides candidate for dark matter
(∼ 25%).
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
mSUGRA
• No supersymmetric particles observed.
• Supersymmetry broken in a hidden sector and communicated
to low energy approximation, MSSM (Minimal
Supersymmetric Standard Model) 125 free parameters
• Gravity is the messenger particle in mSUGRA, 5 free
parameters
• All scalars common mass m0 .
• All gauginos and Higgsinos common mass m1/2 .
• All trilinear Higgs-sfermion-sfermion couplings common value
A0 .
• tan β = vu /vd is the ratio of Higgs vacuum expectation values.
• The sign of the SUSY conserving Higgs mass can is ±1.
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
The Large Hadron Collider, LHC
•
•
•
•
Collide proton on proton
Luminosity: 1027 cm−2 s−1
Center of mass energy: 14 TeV
Observe phenomena at scale 10−18 m, conditions similar to
universe 1ps after the Big Bang
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Detecting particles with ATLAS
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
One Lepton
Except for SU2 gluons and squarks are dominant SUSY particles
produced at LHC. For Supersymmetry nearly every event contains
two χ̃02 , two χ̃±
1 or one of each, so lepton events are only
suppressed by the branching fraction for χ̃02 and χ̃±
1 to decay
leptonically. Real SM one lepton comes from W decay.
g̃
q̃
χ̃01
q′
q
χ̃±
1
W
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
ν
l
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Background and signal points (Monte Carlo simulated)
• QCD (strong interactions)
• di-boson (WW, ZZ, ZW)
• Z (Z → ll )
• W (W → l ν)
• t t̄ (top)
m0 [GeV]
m1/2 [GeV]
A0
tan(β)
sign(µ)
SU1 SU8.1
Coannihilation
70
210
350
360
0
0
10
40
+
+
SU2
Focus
3550
300
0
10
+
SU3
Bulk
100
300
-300
6
+
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
SU4
Low mass
200
160
-400
10
+
SU6
Funnel
320
375
0
50
+
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Cut And Count Strategy
• Robust method for search in early data.
• Cut on variables that discriminate signal and background.
• Significance √S or √ S 2 (one bin counting).
b
b+σsyst
q
q
2
2
2 .
• σtot = σstat
= b + σsyst
+ σsyst
• Systematic uncertainty in background estimate: Uncertainty in
W, Z and top estimated to 20 %, while 50% uncertainty is
expected for QCD.
• Multi-variable scan to deal with correlations.
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Effective Mass
• Meff = E
/T + ΣJet pT + Σlepton pT
Events/1fb-1/100GeV
• Peak correlated with MSUSY = min(mg̃ , mq̃ )
5
10
SU3
tt
W
Z
QCD
VV
104
3
10
sum bg
102
ATLAS Preliminary
10
1
0
500
1000 1500
2000
2500
3000 3500 4000
Meff [GeV]
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
104
10
SU3
tt
W
Z
QCD
VV
102
sum bg
3
Events/1fb-1/100GeV
Events/1fb-1/100GeV
Requiring One Isolated Lepton
104
SU1
SU2
SU3
SU4
SU6
SU8
3
10
102
SM bg
ATLAS Preliminary
ATLAS Preliminary
10
10
1
0
500
1000
1500
2000
2500
3000
3500 4000
Meff [GeV]
1
0
500
1000 1500
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
2000
2500
3000 3500 4000
Meff [GeV]
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Cut on MT
2 = 2p l p ν (1 − cos φ )
• mT
lν
T T
2 ≤ m2 (generates a lower bound for the W mass on an
• mT
W
2200
2000
Events/1fb-1/4GeV
Events/1fb-1/4GeV
event-by-event basis)
SU3
tt
W
Z
QCD
VV
1800
1600
1400
1200
1000
800
ATLAS Preliminary
600
SU3
tt
W
Z
QCD
VV
103
102
ATLAS Preliminary
10
400
200
0
50
100
150
200
250
300
1 lepton: M [GeV]
1
0
50
ll
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
100
150
200
250
300
1 lepton: M [GeV]
ll
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Events/1fb-1/20GeV
Cuts on E/T and pT of jet
104
SU1
SU2
SU3
SU4
SU6
SU8
3
10
102
SM bg
ATLAS Preliminary
10
1
0
100 200 300 400 500 600 700 800 900 1000
Missing E [GeV]
SU1
SU2
SU3
SU4
SU6
SU8
3
10
102
SM bg
Events/1fb-1/20GeV
Events/1fb-1/20GeV
T
104
SU1
SU2
SU3
SU4
SU6
SU8
3
10
102
SM bg
ATLAS Preliminary
10
1
0
ATLAS Preliminary
10
200
400
600
800
1000
p Jet1 [GeV]
T
1
0
200
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
400
600
800
1000
p Jet2 [GeV]
T
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Cuts on ST and δΦ (QCD cut)
• ST =
2λ2
(λ1 +λ2 )
with Sij = Σk pki pkj
• δφ1 = |φj1 − φ E
/ |
Events/1fb-1
Events/1fb-1
T
SU1
SU2
SU3
SU4
SU6
SU8
3
10
102
104
SU1
SU2
SU3
SU4
SU6
SU8
3
10
102
SM bg
SM bg
ATLAS Preliminary
10
1
ATLAS Preliminary
10
-0.4 -0.2
-0
0.2
0.4
0.6
0.8
1
1.2
1.4
ST
1
0
1
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
2
3
4
5
delφ
1
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
One base line cut + three cuts from matrix optimization
• matrix of 4374 cut combinations
• E
/T (100, 150, 200)GeV
• ST (0, 0.1, 0.2)
• jet1pT (100, 150, 200)GeV
• jet2pT (50, 100, 150)GeV
• jet3pT (20, 50, 100)GeV
• number of jets (>=2,>=3,>=4)
• δφ1 (0, 0.2, 0.4)
• E
/T /Meff (0, 0.2)
jet1
jet2
jet3
jet4
Cut
Meff
mT
ET
/
Jets
pT
pT
pT
pT
01
02
03
04
>800
>800
>800
>800
>100
>100
>100
>100
>100
>100
>200
>200
>=4
>=4
>=3
>=4
>100
>100
>200
>200
>50
>50
>150
>150
>50
>20
>20
>100
>50
>20
>20
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
ST
ET
/
Meff
δφ1
>0.2
>0.1
>0.1
>0.2
>0.2
-
>0.2
>0.4
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Events/1fb-1/100GeV
Effective Mass before Cut03
104
SU1
SU2
SU3
SU4
SU6
SU8
3
10
102
SM bg
ATLAS Preliminary
10
1
0
500
1000 1500
2000
2500
3000 3500 4000
Meff [GeV]
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Events/1fb-1/100GeV
Effective Mass for Cut03
103
SU1
SU2
SU3
SU4
SU6
SU8
102
SM bg
10
1
0
ATLAS Preliminary
500
1000
1500
2000
2500
3000
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
3500 4000
Meff [GeV]
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
Significance
CUT
S
B
Cut01
Cut02
Cut03
Cut04
241.8
729.8
492.4
250.5
42.0
354.7
50.3
26.7
Cut01
Cut02
Cut03
Cut04
41.3
95.7
46.8
39.8
42.0
354.7
50.4
26.7
Cut01
Cut02
Cut03
Cut04
380.1
1130.0
623.6
327.8
42.0
354.7
50.3
26.7
Cut01
Cut02
Cut03
Cut04
847.7
3703.7
1092.9
549.6
42.0
354.7
50.3
26.6
Cut01
Cut02
Cut03
Cut04
156.8
423.7
268.5
161.3
42.0
354.7
50.4
26.7
S/
√
b
SU1
37.3
38.7
69.3
48.4
SU2
6.3
5.0
6.6
7.7
SU3
58.6
60.0
87.8
63.3
SU4
130.7
196.6
153.9
106.3
SU6
24.2
22.5
37.8
31.1
S/
q
2
b + σsyst
S/
√
S+b
23.7
10.6
40.4
33.6
14.3
22.1
21.1
15.0
4.0
1.3
3.8
5.3
4.5
4.5
4.7
4.8
37.3
16.4
51.2
44.0
18.5
29.3
24.0
17.4
83.3
53.9
89.8
73.8
28.4
58.1
32.3
22.8
15.4
6.1
22.1
21.6
11.1
15.1
15.0
11.7
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo
Introduction (The Standard Model-Supersymmetry-ATLAS)
One-Lepton Search
Result
Conclusion
• If Supersymmetry is realized in nature in a region of
parameter space with similar phenomenology as any of the
studied mSUGRA points, ATLAS will be able to make a
discovery already with 1fb −1 of data.
• Assumed perfectly understood detector.
• Trigger must be taken into account.
• Now the work to alligne and calibrate the detector is about to
start.
• Soon we will have answers from the machine that make us
dream!
Discovery potential for Supersymmetry with the one-lepton channel in early ATLAS data
University of Oslo