Physics with Photons and Missing Energy at ATLAS
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Transcript of Physics with Photons and Missing Energy at ATLAS
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Physics with Photons and Missing Energy at ATLAS
DOE Site VisitWednesday July 27, 2011
Bangert*, Damiani, Kim, Kuhl, Litke, Mitrevski, Nielsen, Schumm (convener)
In collaboration with Liverpool, DESY, Annecy, Tokyo Tech, La Plata
*Ph.D. June 2011
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ATLAS MET-Based AnalysesAnalyses are signature-based:
• Monojets*
• Zero-Lepton• One-Lepton• Two-Lepton• Multi-Lepton• Photon(s)* (our focus)• Inclusive • B Jet(s)
* Co-analysis with Exotics group (that’s a different physics group altogether)
Red indicates some form of public result
available on full 2010 data sample (36 pb-1)
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Models Used (for inspiration and/or analysis)
• mSUGRA / Constrained MSSM
• “24-parameter” MSSM
• Generic MSSM parameter space (four degenerate light quarks, gluino octet, LSP)
• Generic GMSB (“GGM”) space (light gluino octet, light Bino NLSP, gravitino)
• Non-contextual SUSY partners (tau sneutrino, stop+sbottom, hadronizing scolored particles)
Largely redundant
model spaces
Independent; may appear
only in photons+MET
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Hidden gauge sector breaks SUSY via weakly-coupled messenger interactions
LSP is light (< 1 GeV) gravitino
NLSP is typically Bino-like neutralino 01,
which decays with ~75% BF to photon-gravitino
Two photons + missing transverse energy (MET) is effective signature.
Gauge Mediation
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Strong production: • gluino/gluino or squark/squark• dominates when colored sparticles are accessible
Weak production:• 1
20 or 1
1
• dominates when only EW-charged particles are accessible
In both cases, decay proceeds through NLSP 10
Little additional activity in limit that produced state is degenerate with NLSP.
Production Mechanisms
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Based on “minimal” SPS8 model designed for ‘Snowmass’-type comparitive studies
• Essentially one single phenomenological parameter, often cast as MBino
• Strong partner mass tied to 10 scale, and is
high (~ TeV)Luminosity (lower-mass EW states) trumps energy for SPS8 analyses
For 6.3 fb-1, D0 finds M > 170 GeV at 95% CL, out of reach of 36 fb-1 2010 ATLAS sample
Tevatron (2 TeV) Analyses
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The “minimal” SPS8 model is ad hoc, selected somewhat arbitrarily as a concrete model for which different facilities could be compared
In particular, no reason why EW and colored partner scales need be coupled
Generalize model so that one strong partner, one EW partner are light, all other partner masses are high (~1.5 TeV)
Free parameters are gluino, Bino-like 10 masses:
General Gauge Mediation (GGM)
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Production cross-section (7TeV)
Bino - like Neutralino: |M1| << and |M1| < |M2|; M of Neultralino NLSP ~ M1, Neultralino NLSP + Gravitino (76%)
For Bino-like neutralino, two photons + MET is most promisingbut lose coverage if hadronic activity is required (jets, HT, etc.)
No visible jet activity when
Mg ~ M
Thanks to Shih/Ruderman, ArXiv 0911.4130
SPS8 Trajectory
D0 Limit
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“Recast” D0 result in terms of GGM; compare to expected LHC reach (Ruderman & Shih, arXiv:1103.6083)
GGM Reach: Tevatron vs. LHC
2010 analysis (36 pb-1) breaks significant new ground
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Avoid inefficiencies with very simple selection:
• One photon with Et > 30 GeV
• Another photon with Et > 20 GeV
• MET > 125 GeV
2010 (36 pb-1) Analysis
Signal Region
MET
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Two major sources:
MET Distribution from QCD Sources • Loose photon control sample• Z e+e- to model events
MET from EW Sources (Ws, ttbar)• e control sample from data, MC• Scale to contribution with e fake rate from Z ee studies
Data-Driven Background Estimate
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Nothing there with 36 pb-1, so we set limits…
ISO: New Physics!
Signal region
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ATLAS has better photon reconstruction than CMS (conversions)
Observed, Expected Limits
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7/25/11: Accepted (pending small revisions)
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More selective trigger (2g20) forces higher Pt cuts
• Two photons with Et > 25 GeV
• MET > 125 GeV still
Estimated backgrounds above MET=125:
QCD 2.3 0.9
EW 2.4 1.3
Total 4.8 1.5
2011 (~1 fb-1) Analysis
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Observed in Signal Region vs.
Expected Background
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20101 (~1 fb-1) Analysis
2010
201130x increase in data sample size
12x improvement in cross-section limit
But: 1/M9 forMgluino
~ 900 GeV!
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1 fb-1 paper in preparation 3-4 fb-1 (full 2011 run) analysis will follow
But, progress with 2+MET to be incremental
• Background limited (re-optimization?)
• Steep mass dependence
Explore other channels• Photon + (b)jet + MET (Kuhl)• Photon + MET (Kim; dedicated trigger)• Photon + lepton + MET• Non-pointing photons
2011 Analysis: What Next?
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• SCIPP playing leadership (and major technical) role in central SUSY analysis
• One +MET paper in press (36 pb-1), one in preparation (1 fb-1), a third in planning (3-4 fb-1).
• Beginning to explore/develop complementary single-photon channels
• Talks by Bangert (Cargese 2010) Mitrevski (West-Cost Forum, SLAC) and Schumm (SUSY Recast, Davis)
Photon(s) + MET Summary