SUSY searches in CMSlss.fnal.gov/conf2/C110529/widl.pdf · SUSY searches in CMS Backgrounds for SUSY searches at the LHC • SUSY cross sections are expected to be orders of magnitude
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1 June 2011
Edmund Widl
SUSY searches in CMS
Particle Physics and Cosmology, Blois, France
SUSY searches in CMS
Early SUSY signatures • The theoretical framework of supersymmetry
allows for a large amount of diverse phenomenological features: – various mass spectra with distinct decay channels – different cross sections and branching ratios
• For early SUSY searches at CMS the focus lies on signatures that have two features in common: – high-pT jets
• due to strong production of SUSY particles • strong production has high cross sections, hence
suitable for small luminosities – a large excess of missing energy
• decay chains contain (or even end with) massive weakly-interacting sparticles (e.g. LSP in mSUGRA)
• natural dark matter candidate 2 Edmund Widl 1 June 2011
SUSY searches in CMS
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CMS strategy for SUSY searches • Search for inclusive signatures of the kind MET + jets + X
– overlaps of signatures as small as possible
• So far, results using the data recorded in 2010 (Lint ~ 36 pb-1), have been made public for these signatures: – MET + jets – MET + jets + b-tag – MET + jets + multi-leptons – MET + jets + same-sign di-leptons – MET + jets + opposite-sign di-leptons – MET + jets + single lepton – MET + jets + single lepton + photon – MET + jets + di-photons – MET + jets + Z
• Recent efforts to interpret results in a generalized, less model-dependent manner by means of phenomenologically simplified models
3 Edmund Widl 1 June 2011
SUSY searches in CMS
Backgrounds for SUSY searches at the LHC • SUSY cross sections are expected to be orders of magnitude smaller
than typical QCD cross sections: – backgrounds from QCD processes have to be understood and controlled
4 Edmund Widl 1 June 2011
• Modeling and controlling the high-energy tails of QCD processes is very difficult: – QCD cross sections are not predicted with
high enough precision – key topological and kinematic distributions
such as the number of jets and their pT spectra are difficult to predict
• Determination of backgrounds using data-driven methods: – use dedicated control samples – use multiple methods for cross-checks
whenever possible
SUSY searches in CMS
Hadronic signatures • Highest potential for early discoveries due to large cross-sections for
strong processes at the LHC • Search signature
– events with high-pT jets – large missing transverse enery – veto on isolated, high-pT leptons
5 Edmund Widl 1 June 2011
• Three complementary approaches: ‒ αT method (with & without b-tagging):
• reduces QCD background drastically by exploiting kinematical constraints
– razor method: • tests kinematic consistency of events against
the hypothesis of heavy particle pair-production – inclusive method:
• estimates all backgrounds with high precision without applying kinematical constraints
SUSY searches in CMS
The razor method • Divide events in hemispheres and combine all
jets in each hemisphere into a “mega-jet” – hemisphere algorithm selects jet combinations
minimizing the invariant masses – resulting kinematics correspond to pair production
of two heavy squarks (with q~ → jet + χ~0)
• Characterize resulting system kinematically: – transverse variable MT
R (similar to mT):
– event-by-event estimator MR:
– background suppressing razor variable R≡MTR/MR
6 Edmund Widl 1 June 2011
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SUSY searches in CMS
The razor method • SUSY decay-chains arise from pair production
of heavy sparticles and end with an LSP: – MR peaks around MΔ ≡ (Mq~
2 – Mχ~2 )2/Mq~
– signal region: MR ≥ 500 GeV and R≥ 0.5
• QCD background estimation: – only relevant scale for backgrounds is √s
• distribution of MR is falling exponentially – slopes of background distribution shapes can be
parameterized as a function of R2
• W/Z+jets and t+X background estimation: – similar to QCD estimate, but use control samples
including leptons into the hemisphere algorithm – relative and absolute normalizations of back-
ground distributions shapes • cross-section measurements • data-driven corrections from simulation
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SUSY searches in CMS
Results for hadronic signatures
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• Results show great improvement with respect to previous studies
• Results from all hadronic analyses give a coherent picture
• Example: – mSUGRA exclusions with tan β = 3, µ > 0, A0 = 0
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SUSY searches in CMS
Leptonic signatures • Isolated high-pT lepton:
– in conjunction with large amounts of missing transverse energy an indication of a weak decay of a heavy object
– reduced QCD background
9 Edmund Widl 1 June 2011
• Search signature: – events with hight-pT jets – large missing transverse energy – well isolated, high-pT leptons
• Four complementary approaches: – multi-lepton analysis – opposite-sign di-lepton analysis – same-sign di-lepton analysis – single lepton analysis
• Small topological overlap between leptonic searches allows for clear phenomenological interpretations
SUSY searches in CMS
The single lepton analysis • Search for excesses in region with HT > 500 GeV and E/T > 250 GeV • Background estimation:
– lepton-spectrum method: • pT distributions of lepton and neutrinos from W-two body decays are closely related • determine spectrum of E/ T from lepton pT spectrum from tt–- and W-backgrounds • robust against signal contamination from typical SUSY topologies
– cross-checked with ABCD-method (matrix method): • uses (almost) uncorrelated observables HT and y ≡ E/ T/√HT
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same-sign
Results for leptonic signatures
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• Results show improvements with respect to previous studies – even though much smaller cross-section than for
fully hadronic processes • Consistent with hadronic analyses • Example:
– mSUGRA exclusions with tan β = 3, µ > 0 and A0 = 0
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CMS LO observed
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multi-lepton
SUSY searches in CMS
Simplified models • Simplified models allow for more generalized
interpretations – reduced SUSY-like particle content – masses are generic, not model dependant – assume constant cross sections, branching
ratios are described by phase space – broadens reach of kinematically accessible
regions of parameter space – describe features of data in a way that is
useful for further model-building
• Results can be used by theorists outside of the LHC collaborations – good interface between experimentalists and
theorists
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SUSY searches in CMS
6 4 Interpretation in the context of Simplified Model Spectra
Figure 5: Diagram of simplified models. Top left: gluino pair production; top right: squarkpair production. Both the gluon and the quark initiated production modes are used in theevent generation.
4 Interpretation in the context of Simplified Model SpectraThe following description of the Simplified Model Spectra is based on Refs. [3–5] where theyare described in detail.
In particular we consider:
• pair-produced gluinos where each gluino directly decays to two light quarks andthe LSP;
• pair-produced squarks where each squark decays to one jet and the LSP.
Figure 5 shows the respective diagrams for the simplified topologies. Events samples producedwith fast simulation [12] were generated for different combinations of squark (gluino) and LSPmasses.
Figures 6 and 7 show the variation of the analysis efficiency (selection efficiency times accep-tance) dependent on the gluino (squark) and LSP mass. The sample size is 10000 events foreach point in the (mq̃, mLSP) and (mg̃, mLSP) parameter space. It can be seen that the efficiencyof the analysis is much reduced in regions of parameter space where the squark (gluino) andLSP masses are similar, as in this case the production of hard jets is suppressed. The efficiencyis highest for heavy squarks (gluinos) and LSP mass roughly half the squark (gluino) masswhich leads to hard jets and sizeable missing transverse momentum.
In addition, Figures 6 and 7 show the experimental and theoretical uncertainties on the analysisefficiency, respectively. The experimental uncertainties include the same components as listedin Section 3.2, however, the uncertainty on the integrated luminosity of 11% is not includedin the figure 2. The only difference with respect to the treatment in Section 3.2 is that theuncertainty on the jet energy scale and resolution have been evaluated separately for everypoint in parameter space. This uncertainty is generally 2-3% where mg̃(mq̃) ! mLSP. However,for points along the diagonal where mg̃(mq̃)" mLSP < 200 GeV it can increase to # 10-15%. Asthe production of high pT jets is suppressed in this region of parameter space, threshold effectson the jet pT requirements play a larger role.
The theoretical uncertainties on the analysis efficiency include variations of the parton distribu-tion functions (PDFs). These have been evaluated following the prescription given in [13], us-
2While the uncertainty on the luminosity measurement has since reduced to 4% [11], the results presented herecontinue to use 11% for consistency with Ref. [1]
Simplified models • Extended interpretation of all hadronic analysis
– two simplified models based on proposals from the LHC New Physics Working Group • group of theorists addressing questions regarding
characterization of BSM at the LHC • initiated by a workshop at a joint CMS, ATLAS and
theory meeting in June 2010 – Considered signatures:
• pair-produced gluinos, gluinos decay to 2 light quarks and an LSP
• pair-produced squarks, squark decay to 1 jet and an LSP
– Generated with Pythia and CMS fast simulation
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SUSY searches in CMS
Conclusions and outlook
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• The CMS effort to discover supersymmetry covers a broad range of signatures
• All analyses have pushed the previously known exclusion limits further using the LHC data recorded in 2010
• Characterization of new results through simplified models allows for generalized, model-independent BSM searches inspired by SUSY
• The CMS analysis effort is not focused on optimized exclusions, but rather on identifying possible new signals
• With the excellent performance of the LHC in mind, we hope for new physics to be just around the corner …
SUSY searches in CMS
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Martin, S., “A Supersymmetry Primer”, arXiv:hep-ph/9709356v5
“The CMS Experiment at the CERN LHC”, JINST 3, S08004
“Search for Supersymmetry in pp Collisions at √s = 7 TeV in Events with Two Photons and Missing Transverse Energy”, CMS Physics Analysis Summary: SUS-10-002
“Search for Supersymmetry in pp Collisions at 7 TeV in Events with Jets and Missing Transverse Energy”, CMS Physics Analysis Summary: SUS-10-003
“Search for new physics with same-sign isolated dilepton events with jets and missing transverse energy at the LHC”, CMS Physics Analysis Summary: SUS-10-004
“Search for new physics at CMS with jets and missing momentum”, CMS Physics Analysis Summary: SUS-10-005
“Search for Physics Beyond the Standard Model in Opposite-sign Dilepton Events in pp Collisions at √s = 7 TeV”, CMS Physics Analysis Summary: SUS-10-007
“Inclusive search for squarks and gluinos at √s = 7 TeV”, CMS Physics Analysis Summary: SUS-10-009
“Search for Supersymmetry in Final States with b Jets and Missing Energy at the LHC”, CMS Physics Analysis Summary: SUS-10-011
“Further interpretation of the search for supersymmetry based on αT”, CMS Physics Analysis Summary: SUS-11-001
LHC New Physics Working Group Collaboration, “Simplified Models for LHC New Physics Searches”, to be published (June, 2010), http://www.lhcnewphysics.org.
SUSY searches in CMS
Backup Slides
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SUSY searches in CMS
Low mass benchmark scenarios • All benchmark points are
mSUGRA scenarios
• Parameterized by only five different parameters (m1/2, m0, tan β, A0, sign µ)
• Pros: – beyond the exclusion reaches
of SPS, LEP, Tevatron, etc. – cover a large variety of distinct
signatures – very well understood
• Cons: – restrictive (mgluino ~ 6 mLSP)
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SUSY searches in CMS
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• Using the kinematic variables HT and αT to suppress SM backgrounds: - characterize the overall pT-balance of the event
- motivated by di-jet analyses, generalized for multi-jet events
• Properties extensively validated on data: - SM backgrounds mostly confined to αT<0.55 - suppression improves with increasing HT
- ratio of background events failing an αT-cut decreases exponentially
- allows for background estimates in higher HT-bins
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Validating αT on data
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• Jet-triggered QCD events (blue) decrease exponentially with increasing HT – behavior can be used to calculate a limit for higher HT-bins
• Artificially degraded data shows a consistent exponential trend – emulation of jet-loss with a 5-10 times increased removal probability (red) – photon-triggered events, dominated by misidentified jets (green) – the momentum of 10% of all jets is smeared using a one-sided Gaussian
with σ=0.5.pT (violet)
SUSY searches in CMS
Validating αT on data
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• Distribution of leading jets w.r.t. η is uniform for QCD events failing αT<0.55 – robustness: even when introducing artificial MET by randomly
removing jets the uniformity remains
• SUSY events are produced more centrally – aim: use η-sidebands of leading jets to estimate background
SUSY searches in CMS
The inclusive method • search for deviations from expected SM
distributions in HT and H/ T • event selection aims to be as inclusive as
possible – avoid topological restrictions due to kinematical
constraints • all sources of backgrounds are measured from
data, including the complete QCD background – invisible Z→νν:
• directly from Z→l+l– (small sample) • via Z/γ-correspondence at large pT from γ+jets
– W and top: • estimate events with lost leptons from µ+jets data
sample using efficiency information from tag&probe • mimic effect from hadronic τ’s using by replacing µ’s
with τ-jet templates in µ+jets data samples
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0 G
eV
-210
-110
1
10
210
(GeV)TH0 200 400 600 800 1000
Even
ts / 1
0 G
eV
-210
-110
1
10
210
310
410
510
610
CMS Preliminary
= 7 TeVs-1
L = 36 pb Data
QCD
! ! "Z
W
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otherEWK
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(GeV)TH0 200 400 600 800 1000
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ts / 1
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-210
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10
210
310
410
510
610
(GeV)TH0 200 400 600 800 1000
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-210
-110
1
10
210
310
410
510
610
The inclusive method • QCD background estimation:
– “Rebalance and Smear” method • predicts full kinematics in multi-jet events • unaffected by events with true missing momentum • produce so-called seed events in “Rebalance” step:
– adjust jet momenta such that H/ T ≈ 0 • use seed events in “Smear” step:
– scale jets with values drawn from jet resolution distribution
– apply search cut to the resulting events to predict complete jet kinematics and correlations
– cross-check using factorization method • uses relation between H/ T and Δφmin (minimum
azimuthal angle between H/ T and three leading jets) • H/ T and Δφmin are not uncorrelated: functional form of
the correlation is estimated at low H/ T • use relation to estimate background in signal region
(high H/ T, low Δφmin) from sideband (high H/ T, low Δφmin) 22 Edmund Widl 1 June 2011
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SUSY searches in CMS
The opposite-sign di-lepton analysis • Search for signals in region with:
– high hadronic activity: HT > 300 GeV – significant amount of missing transverse
energy: y ≡ E/ T/√HT > 8.5 GeV1/2
• Require isolated opposite-sign lepton pairs – Drell-Yan suppression via invariant mass
exclusion interval: Mll < 76 GeV and Mll > 106 GeV
• Background estimation: – apply ABCD method
• HT and y are (almost) uncorrelated • estimate ND = NA×NC/Nb
– cross-check using pT(ll)-method • pT distributions of lepton and neutrinos from W-
two body decays are closely related • correct lepton spectrum for differences in pre-
selection efficiency and W polarization 23 Edmund Widl 1 June 2011
CM
S P
AS
SU
S-1
0-00
7
(GeV)TH0 50 100 150 200 250 300 350 400 450 500
Events
0
5
10
15
20
25
30 CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
)1/2
y (GeV0 2 4 6 8 10 12 14 16 18 20
Events
0
5
10
15
20
25
30
35CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
) (GeV)llM(0 50 100 150 200 250 300
Events
0
5
10
15
20
25CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
) (GeV/c)ll(T
p0 50 100 150 200 250 300
Events
0
5
10
15
20
25
30 CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
data
-l
+l !tt
other!tt
DY
single top
VV
+ jetsW
LM1
(GeV)TH0 50 100 150 200 250 300 350 400 450 500
Eve
nts
0
5
10
15
20
25
30 CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
)1/2
y (GeV0 2 4 6 8 10 12 14 16 18 20
Eve
nts
0
5
10
15
20
25
30
35CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
) (GeV)llM(0 50 100 150 200 250 300
Eve
nts
0
5
10
15
20
25CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
) (GeV/c)ll(T
p0 50 100 150 200 250 300
Eve
nts
0
5
10
15
20
25
30 CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
data
-l
+l !tt
other!tt
DY
single top
VV
+ jetsW
LM1
(GeV)TH0 50 100 150 200 250 300 350 400 450 500
Events
0
5
10
15
20
25
30 CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
)1/2
y (GeV0 2 4 6 8 10 12 14 16 18 20
Events
0
5
10
15
20
25
30
35CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
) (GeV)llM(0 50 100 150 200 250 300
Events
0
5
10
15
20
25CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
) (GeV/c)ll(T
p0 50 100 150 200 250 300
Events
0
5
10
15
20
25
30 CMS
= 7 TeVs at -1
34 pb
µ/eµµEvents with ee/
data
-l
+l !tt
other!tt
DY
single top
VV
+ jetsW
LM1
(GeV)TH0 200 400 600 800 1000 1200 1400
)1
/2y (
GeV
0
5
10
15
20
25
30
SM MC
Data
B
A
C
D
CMS
= 7 TeVs at -1
34.0 pb
µ/eµµEvents with ee/
SUSY searches in CMS
The same-sign di-lepton analysis • Search for excesses in regions with HT > 200 GeV and E/T > 80 GeV
• Same-sign isolated lepton pairs from hadron collisions are very rare
• Analysis includes hadronically decayed τ’s – use neural network, trained to identify the hadronic τ-decay modes using the
kinematics of the reconstructed charged and neutral pions
24 Edmund Widl 1 June 2011
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4
• Background estimation with tight-loose method: – determine probability εTL for leptons
passing a loose isolation selection to also pass the tight analysis selection
– measure in a QCD multi-jet sample – applying εTL to a sample of di-lepton
events, where one of the leptons fails the tight selection but passes the loose one
(GeV)T
muon p10 15 20 25 30 35
TL
pro
ba
bili
ty
0
0.1
0.2
0.3
0.4
> 20 GeVT
jet p
> 40 GeVT
jet p
> 60 GeVT
jet p
= 7 TeVsCMS,
-1 = 35 pbintL
SUSY searches in CMS
Franklin Hall a.k.a. “Graviton”
The di-photon channel • Aims for general gauge-mediation scenarios:
– decay chains include one or several quarks/gluons plus a neutralino, which in turn decays to a photon and a gravitino
– the gravitino escapes detection, leading to a significant amount of missing transverse energy
• Search signature: – two or more isolated high-pT photons (pT ≥ 30 GeV) – at least one high-pT jet (pT ≥ 30 GeV) – large missing transverse energy (E/ T ≥ 30 GeV)
25 Edmund Widl 1 June 2011
• QCD Background: – direct photons – quarks/gluons hadronizing predominantly to π0s decaying into photons – estimated from two control samples:
• events containing two fake photons • events containing 2 electrons with invariant mass 70 - 110 GeV (mostly
Z → ee)
CM
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SU
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SUSY searches in CMS
The di-photon channel
26 Edmund Widl 1 June 2011
• Electroweak background: – events with a genuine or fake photon and a W that decays into a neutrino
and an electron, with the latter mis-identified as a photon – estimated from events containing 1 electron plus 1 photon
(GeV)missTE
0 20 40 60 80 100 120 140
Num
ber
of E
vents
-110
1
10
210
310
410 CMS -136 pb
= 7 TeVs
(With Jet Requirement)!!Data:
Total BG (QCD shape from Z)
Electroweak Background
(With Jet Requirement)!!GGM:
>1
00
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Vm
iss
TIn
teg
ral E
) (GeV)q~M(
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) (
GeV
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400
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800
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1600
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-136 pb
= 7 TeVs
) = 50 GeV1
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M(
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Expected for) = 150 GeV
1
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SUSY searches in CMS
27 Edmund Widl 1 June 2011
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