SUSY searches in CMSlss.fnal.gov/conf2/C110529/widl.pdf · SUSY searches in CMS Backgrounds for...

1 June 2011

Edmund Widl

SUSY searches in CMS

Particle Physics and Cosmology, Blois, France


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


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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


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


•  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


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


•  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


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


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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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Results for hadronic signatures


•  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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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


•  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


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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•  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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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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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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Conclusions and outlook


•  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 …



  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.


Backup Slides



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)




•  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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HT= pT, jjets j∑ ΔHT= min

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Validating αT on data


•  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)


Validating αT on data


•  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


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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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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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

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(GeV)TH0 50 100 150 200 250 300 350 400 450 500

Events

0

5

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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

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15

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25

30

35CMS

= 7 TeVs at -1

34 pb


) (GeV)llM(0 50 100 150 200 250 300

Events

0

5

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15

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25CMS

= 7 TeVs at -1

34 pb


) (GeV/c)ll(T

p0 50 100 150 200 250 300

Events

0

5

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15

20

25

30 CMS

= 7 TeVs at -1

34 pb


data

-l

+l !tt

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single top

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0

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20

25

30 CMS

= 7 TeVs at -1

34 pb


)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


) (GeV)llM(0 50 100 150 200 250 300

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nts

0

5

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20

25CMS

= 7 TeVs at -1

34 pb


) (GeV/c)ll(T

p0 50 100 150 200 250 300

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0

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25

30 CMS

= 7 TeVs at -1

34 pb


data

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(GeV)TH0 50 100 150 200 250 300 350 400 450 500

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20

25

30 CMS

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34 pb


)1/2

y (GeV0 2 4 6 8 10 12 14 16 18 20

Events

0

5

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15

20

25

30

35CMS

= 7 TeVs at -1

34 pb


) (GeV)llM(0 50 100 150 200 250 300

Events

0

5

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20

25CMS

= 7 TeVs at -1

34 pb


) (GeV/c)ll(T

p0 50 100 150 200 250 300

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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


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•  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

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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)


•  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)

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The di-photon channel


•  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

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SUSY searches in CMSlss.fnal.gov/conf2/C110529/widl.pdf · SUSY searches in CMS Backgrounds for...

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