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Missing E nergy Searches at the LHC (in the context of SUSY) Henning Flaecher

Missing E nergy Searches at the LHC (in the context of SUSY) Henning Flaecher University of Rochester with University of Bristol from Oct. 1 st IoP Dark Matter Meeting. Overview. ATLAS and CMS have established their N ew Physics searches with missing energy

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Missing E nergy Searches at the LHC (in the context of SUSY) Henning Flaecher

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  1. Missing Energy Searches at the LHC (in the context of SUSY) Henning Flaecher University of Rochester with University of Bristolfrom Oct. 1st IoP Dark Matter Meeting King's College London

  2. Overview • ATLAS and CMS have established their New Physics searches with missing energy • mostly presented in context of Supersymmetry • “2nd round” of results presented at this years summer conferences • EPS, LP, SUSY11 • All results discussed today are based on~1fb-1 • twice as much data on tape by now • In this talk: • SUSY missing energy searches • Jets + missing energy • Jets + missing energy + 1 lepton • Jets + missing energy + 2 OS/SS leptons King's College London

  3. There’s more… • Unfortunately no time to cover many analyses, e.g., • Jets + missing energy + (1 or 2) photons • Jets + missing energy + photon + lepton • analyses with b-tags • analyses with taus • search with >=6 jets • search with >=3 leptons • Please see: • ATLAS • https://twiki.cern.ch/twiki/bin/view/AtlasPublic/SupersymmetryPublicResults • CMS • https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults King's College London

  4. SUSY Searches: Overview • Search for heavy pair-produced particles that decay to SM particles and LSP • strong production of squarks and gluinos •  large cross-section • heavy squarks and gluinos •  possibility of long decay chains • direct decay of squarks or gluinos to quarks (jets) + LSP • cascade decays via charginos resulting in leptons • R-parity conservation: LSP •  missing energy • Signature: high-pT jets, leptons and missing energy • Focus on simple signatures/topologies • common to many models Rochester Seminar

  5. Search Strategy • 2 lepton (same sign) • A natural SUSY signature • Very small Standard Model backgrounds • Include all three generations of leptons and all cross channels • Multi-lepton • Very clean events, very low SM background • Include all three generations of leptons and all combinations • Search inclusively, on the Z peak, with and without MET • Photons • Many gauge-mediated models predict photons in final state • Di-photon searches dominated by QCD multijet and γ+jet backgrounds • 0 lepton • Very challenging due to large amount and wide range of backgrounds • However most sensitive search for strongly produced SUSY • pursue complementary strategies based on kinematics and detector understanding • 1 lepton • Lepton (electron or muon) requirement reduces background considerably • Only ttbar and W+jets left ➔ topological handles • 2 lepton (opposite sign) • Adding a second lepton (electron or muon) reduced W background • Several techniques including opposite-sign opposite-flavour subtraction • Shape information and mass edges From A. Tapper, CERN LPCC seminar

  6. Unfortunately, no signs of new physics so far… • Focus on complementary methods King's College London

  7. Jets + missing energy • CMS • “inclusive” HT & MHT search • Complementary approaches, using kinematic variables • αT, MT2, “razor” • Different selections/variables result in different SM backgrounds • ATLAS • Selection on jet pT and ETmiss • split by jet multiplicity • Meff = HT + ETmiss as discriminating variable King's College London

  8. Jets + missing energy • Analysis strategy • Define control regions or control samples for different types of background • W, Z, tt, QCD multijet events • Obtained by reversing a signal region cut, or requiring leptons or photons • defined in a kinematically similar environment to signal region • MC transfer factors used to extrapolate from control region/sample to signal region • takes care of efficiency, acceptance etc. King's College London

  9. ATLAS: Jets + missing energy • Example • Zνν + jets: use Zμμ + jets and photon+jets control samples • for photon pT >> mZ, process is kinematically similar • W and tt background: require well reconstructed lepton • use b-tag (veto) to separate W’s from tt • QCD background • reverse cut on min(ΔΦ) between ETmiss and 3 leading jets • select badly measured jets that align with ETmiss QCD tt King's College London

  10. ATLAS: control distributions photon +jets Zμμ W+jets King's College London

  11. ATLAS: signal sample King's College London

  12. CMS: αT jet jet LSP LSP jet jet • Kinematic variable αT • Exploits QCD di-jet properties • jets are balanced in pT • back-to-back in φ • Define: HT = ΣpT(ji), MHT = |-ΣpT(ji)|, ΔHT=ET(pj1)-ET(pj2) aT for n jets: aT for dijets: Expectation for QCD: αT = 0.5 Jet mis-measurements: αT < 0.5 (form two pseudo-jets – defined by balance in “pseudo-jet” HT = SET) inspired by L. Randall & D. Tucker-Smith, Phys.Rev. Lett. 101 (2008) 221803 12 Rochester Seminar March 22nd, 2011

  13. CMS: αT • αT as “QCD killer” • exploit RαT = N(αT>0.55)/N(αT<0.55) independence of HT • photon+jets control sample for Zνν + jets • muon control sample for W/tt • combined fit to photon, muon and hadronic signal sample mu control sample (possible “signal contamination” considered) King's College London

  14. CMS: αT SUS-11-003 arXiv:1109.2352 “Hadronic” signal sample King's College London

  15. Classic HT & MHT • Search regions in HT & MHT • Baseline (HT>350 & MHT > 200) • loose event selection used for validation • Medium (HT>500 & MHT > 350) • generic high multiplicity and missing momentum search • High HT (HT>800 & MHT > 200) • long cascades, high particle multiplicity • High MHT (HT>800 & MHT > 500) • generic search for weakly interacting neutralparticle, good background rejection SUS-11-004 King's College London

  16. Limits on CMSSM • Similar sensitivity of ATLAS and CMS searches and complementary approaches • CMSSM: 4 parameter model assuming common gaugino and scalar masses at GUT scale (m1/2, m0) King's College London

  17. Other searches: MT2 • MT2 is a generalization of transverse mass to a system with two semi-invisibly decaying particles • Two selections: • heavy sparticles large msq, small mgl King's College London

  18. Other searches: MT2 • High MT2 Low MT2 SUS-11-005 King's College London

  19. Leptonic Searches • Isolated lepton requirement reduces background considerably • strong suppression of QCD • mainly W’s and tt left • Analyses typically require leptons (el,mu) with pT>20-25 GeV and additional (>2) jets with pT >40 GeV + ETmiss King's College London

  20. CMS 1-lepton search: 2 Methods • 1) Lepton Projection Method • Exploit difference in correlations between lepton and ETmiss in SM (W, tt) and SUSY events • Lepton-projection variable: • Define signal and control regions and use MC to extrapolate SUS-11-015 Muons Electrons King's College London

  21. CMS 1-lepton search SUS-11-015 • 2) Lepton Spectrum Method • exploit similarity of charged lepton pT and neutrino pT (=ETmiss) spectra in W decays • correct for efficiency, acceptance and polarisation • smear lepton pT using QCD templates (ETmissresolution) • Etmissfrom LSP’s extends to much higher values • Selection: • HT>500 GeV • MHT>250 (loose) • MHT >350 (tight) King's College London

  22. ATLAS 1-lepton search • 1 lepton + 3 or 4 jets • 4 signal regions with different selections on MT, ETmiss and Meff • Low MT and ETmiss control regions for W and tt background • b-tag for tops King's College London

  23. Interpretation in CMSSM • Single lepton analyses of CMS and ATLAS King's College London

  24. OS dileptons • Selection of 2 isolated leptons • strong change in SM background composition • strong QCD suppression as well as W’s • main background: tt • Analyses inside & outside Z window • Require two isolated opposite sign leptons • >= two jets • requirements on HT and MHT • HT > 300 and MET > 275 GeV • HT > 600 and MET > 200 GeV King's College London

  25. CMS OS dilepton search • 2 methods for background estimation • 1) Lepton Spectrum method • use the observed pT(ll) distribution to model the pT(vv) distribution, which is identified with ETmiss • 2) “Matrix method” • two uncorrelated variables, each with signal and control region, HT and y=ETmiss/√HT SUS-11-011 King's College London

  26. ATLAS OS dilepton • 3 signal regions: • 1) ETmiss> 250 GeV • 2) ETmiss> 220 GeV, jet pT > 80,40,40 GeV • 3) ETmiss> 100 GeV, jet pT >100,70,70,70 GeV • tt and Z background from control regions • fake leptons from “matrix method” • WW,WZ,ZZ from MC King's College London

  27. SS dileptons • Basically no significant SM process • apart from “fake” leptons • Natural SUSY signature through charginos in gluino cascade decays King's College London

  28. ATLAS SS dilepton • 2 same sign dileptons with missing energy • two search regions: • ETmiss> 100 GeV • ETmiss> 100 GeV, 2 jets with pT > 50 GeV • Charge misidentification from Z events • Fake lepton with matrix method using “tight” and “loose” selection King's College London

  29. CMS SS dilepton search • require two isolated same sign dileptons • search in different HT & ETmiss regions • Main backgrounds from fakes, various data driven methods using “tight”-”loose” selections on the leptons SUS-11-010 King's College London

  30. CMS SS dilepton • Data yields compared with different background estimation metods SUS-11-011 inclusive dilepton high-pTdilepton tau dilepton King's College London

  31. CMS CMSSM Summary Similar for ATLAS King's College London

  32. Alternative Interpretation: Simplified Models • Working with theorists in context of LPCC • Models proposed at: http://www.lhcnewphysics.org • Agreed on reference topologies for early searches • Cover what one might see in the early running phase, all initiated by strong production • So far considered • pair produced squarks, where each squark decays to quark (jet) and LSP • gluino decoupled at high scale • pair produced gluinos, where each gluino decays directly to two light quarks (jets) and LSP • squarks decoupled at high scale • Study 2D parameter space in msq (mgl) and mLSP • Set limits on production cross section Rochester Seminar

  33. Variable mass splittings! • αT: efficiency maps cross-section limit King's College London

  34. Models with leptons: SS and OS • Same-sign dilepton Opposite sign dilepton King's College London

  35. ALTAS Simplified Models King's College London

  36. Simplified Model Summary Similar for ATLAS King's College London

  37. Conclusions & Outlook • New Physics searches well underway at both CMS and ATLAS • Investigating a wide variety of New Physics scenarios: • see https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults • http://twiki.cern.ch/twiki/bin/view/AtlasPublic/SupersymmetryPublicResults • Unfortunately no smoking gun seen so far, but • CMS and ATLAS have entered new territory • starting to “challenge” simple models • Need to design analyses that are sensitive to models with small mass splittings • simplified models give experimentalists good idea of coverage of their search • Much more data already on tape, 5fb-1 by end of year or Moriond • keep searching for New Physics in model independent way • diversification to third generation (b-tag, taus) King's College London

  38. BACKUP King's College London

  39. αT: CLS King's College London

  40. ATLAS Limits King's College London

  41. King's College London

  42. “Classic” MHT search King's College London

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