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Potential to Discover Supersymmetry in Events with Same-sign Dimuon, Jets and Missing Energy

Potential to Discover Supersymmetry in Events with Same-sign Dimuon, Jets and Missing Energy at LHC Yu riy Pakhotin (pakhotin@ufl.edu) for CMS and ATLAS Collaboration. SUSY08, COEX, Seoul, Korea June 19 th , 2008. LHC: Large Hadron Collider. LHC is pp-collider nominal energy: E=14 TeV

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Potential to Discover Supersymmetry in Events with Same-sign Dimuon, Jets and Missing Energy

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  1. Potential to Discover Supersymmetry in Events with Same-sign Dimuon, Jets and Missing Energy at LHC Yuriy Pakhotin (pakhotin@ufl.edu) for CMS and ATLAS Collaboration SUSY08, COEX, Seoul, Korea June 19th, 2008 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  2. LHC: Large Hadron Collider LHC is pp-collider nominal energy: E=14 TeV design luminosity: L=1034 cm-2s-1 first physics run: (Fall 2008) E = 10 TeV, L=2·1033 cm-2s-1 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  3. CMS: Compact Muon Solenoid SUPERCONDUCTING CALORIMETERS COIL ECAL HCAL Scintillating PbWO4 crystals Plastic scintillator/brass sandwich IRON YOKE TRACKER MUON ENDCAPS Silicon Microstrips Pixels MUON BARREL Cathode Strip Chambers (CSC ) Drift Tube Resistive Plate Chambers ( DT ) Chambers ( RPC ) Resistive Plate Chambers (RPC) Total weight: 12,500 t Diameter: 15 m Overall Length: 22 m Magnetic field: 4 Tesla Muons: • muon system acceptance: |η|<2.4 • muons momentum resolution: dpT/pT~1% (pT~25 GeV) Calorimetry: • HCAL |η|<5.0, δE/E ~ 70% / √E + 8% • ECAL |η|<3.0, δE/E ~ 2.8% / √E + 0.3% + 12% / E Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  4. ATLAS: A Toroidal LHC ApparatuS Total weight: 7000 t Diameter: 25 m Overall Length: 46 m Magnetic field: 2 Tesla Muons: • muon system acceptance: |η|<3.0 • muons momentum resolution: δpT/pT~2%(pTµ~30 GeV) Calorimetry: • HCAL |η|<4.9, δE/E ~ 45% / √E + 3% • ECAL |η|<3.2, δE/E ~ 10% / √E + 0.6% + 0.5% / E Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  5. Physics Motivation • Inclusive topological search for physics beyond SM • Despite many successes of Standard Model, there are strong indications that this theory is only an effective low-energy model and new physics must be present at a higher energy scale • An excellent signature to search for deviations from the SM is production of two leptons with both leptons of the same electric charge • This signature occurs naturally in many extensions to the SM and occurs rather rarely in SM interactions • Different theory models can be used as templates for searches. Don’t need to believe in any of them, though well motivated examples are to be preferred: • SUSY (used in this analysis) • Majorana neutrino • etc. Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  6. Same-sign Dimuon Signature The key signature for SUSY in analyses presented here • MET - Missing Transverse Energy (R-parity conservation, neutral LSP) • + number of jets with high ET • + muon • clean trigger • + second like-sign muon • even cleaner signature with low background due to the same-sign muon requirement • complementary to trilepton searches: more diagrams, for example • two same-sign muons analysis is able to distinguish SUSY diagrams with good efficiency and purity by applying muon isolation & tight quality cuts Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  7. Brief History of the Analyses • Theoretical studies • H. Baer et al. Phys. Rev. D41, #3 (1990) • R.Barnett et al. Phys. Lett. B315 (1993), 349 • K. Matchev, D. Pierce hep-ph/9904282 (1999) • and others • Experimental study at Tevatron • CDF Phys. Rev. Lett. 98, 221803 (2007) • D0 Phys. Rev. Lett. 97, 151804 (2006) Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  8. Strategy of the Inclusive Analyses • The main goal of the presented analysis is to prepare for the start-up running of LHC by searching for extensions of Standard Model using nSUGRA model for optimization • The strategy is to search for excess in number of events over expected number from SM background events (counting experiment) • Apply muon triggers: which are expected to be robust even at the LHC start-up • Apply quality cuts: pre-select well reconstructed quantities • Apply selection cuts: efficiently suppress SM background • Optimize cuts for benchmark mSUGRA sample to maximize significance • Estimate backgrounds from data • The main assumptions for the current analyses: • low luminosity (1 fb-1) – initial data collected by LHC • use mSUGRA as a guide to find optimized parameters region where the SM background is expected to be significantly small in comparison to signal. • optimized cuts applied to some other theories may also yield significant excess over SM backgrounds Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  9. mSUGRA Benchmark Test Points for CMS A0=0, tan(β)=10, sign(µ)=+1 • Full detector simulationwas performed in 10 mSUGRA test points for CMS • Some of these points are post-WMAP benchmark points • Positions of the test points in the m0-m1/2 plane are shown on the right plot (as stars) • Fast generation and simulationwas also performed in order to scan the plane of m0-m1/2 • Other mSUGRA parameters are fixed: A0 = 0, tan(β) = 10, sign(µ) = +1 • Points were generated on a coarse grid with: δm0 = 100 GeV and δm1/2 = 100 GeV • For validation purposes 7 benchmark points were also produced with fast simulation and compared with fully simulated data Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  10. mSUGRA Benchmark Test Points for Atlas 1. Set of SU benchmark points: fully simulated with Geant 2. LST1 Benchmark point (hep-ph/0512284): Atlas fast simulation • Light stop production from gluino pair production • gluino is Majorana, so following decays have equal probabilities: • g->t ~t1, g-> tbar ~t1 • pair-produced gluinos give the signature • 2b-jets + 2 same-sign leptons + jets + MET Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  11. Standard Model Backgrounds To prepare for Model Independent analysis it is imperativetounderstand the Standard Model background as well aspossible. Full detector (both, Atlas and CMS) simulation was performed for following SM backgrounds: • tt+jets sample • W+jets, Z+jets • WW+jets, WZ+jets, ZZ+jets • QCD jets Example of 2 same-sign muons production in t tbar event Data driven methods to estimate SM backgrounds are currently elaborating Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  12. Atlas: Selection Cuts • Effective Mass: • Meff = MET + ∑pTjets • Event Selection (not optimized): • 2 same-sign isolated leptons (muons + electrons) • lepton pT > 20 GeV • 4 jets • jet pT > 50 GeV • MET > 100GeV 30 fb-1 LST1 mSUGRA benchmark poin Easy to see excess events to SM Simple scaling to low luminosity still gives excess over SM Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  13. CMS: Selection Cuts pT of leading muons SUSY pT of leading muons QCD • Example: transverse momentum (pT) of leading muon • Signal (SUSY LM1) muons have muon pT spectrum: • harder than QCD • similar to W/Z+jets and TTbar • Pre-selection cut pTµ > 10 GeV to avoid problems with low-pT range where efficiency is not very good • Other cut variables: • muon trigger (di-muon High-Level Trigger) • transverse momentum of muons • combined (calorimeter + tracker) muons isolation • muons track parameters • jet multiplicity • pT of 3 leading jets • large missing transverse energy Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  14. CMS: Same-sign Dimuon Reach Contour A0=0, tan(β)=10, sign(µ)=+1 Optimized cuts for 10 fb-1 luminosity LEP Tevatron mSUGRA 5σ reach contours (Monte Carlo simulation) of the same-sign dimuon analysis, including systematic uncertainties, for different integrated luminosities and assuming no re-optimization of the selection cuts CMS Preliminary: Number of expected events after selection cuts applied for ∫L=1 fb-1 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  15. Atlas: In-situ Background Estimation • 2D Side Band (SB) is chosen: • pTjet2 [40 GeV ~ 80 GeV] • MET [50 GeV ~ 80 GeV] • Transverse mass (MT) of leading lepton and MET is a good candidate as a variable for Side Band to Signal Region (SR) normalization: ASR=ASB*BSR/BSB • Transverse mass distributions for SB and SR are in a very good agreement. True background events in 1 fb-1=14.8 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  16. Conclusion • The preparation for topological search of new physics (excess over SM background) with same-sign dimuon in LHC experiments is presented • Assuming unknown signal, mSUGRA model for cut optimization is used • It is shown that SM background can be suppressed with optimized cuts almost to zero with luminosity less than 1 fb-1 (early running of LHC). Significant number of signal events are survived • Small number of expected SM background events leads to less dependency on the statistical and systematical uncertainties, which is crucial for initial experiment data. Then even relatively small excess (a few events) over expected SM background may be safely interpreted as a discovery of physics beyond SM • The same set of cuts may be applied to search for different theoretical predictions beyond SM • Data driven methods to estimate SM backgrounds for low luminosity (1 fb-1) are currently elaborating Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  17. Back-up slides Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  18. Early running of LHC • …it was decided to push for collisions at an energy of 10 TeV this year, as quickly as possible, with full commissioning to 14 TeV to follow over the winter shutdown. • Robert Aymar , CERN Director-general • CERN Bulletin, Issue No. 14-15/2008 - Monday 31 March 2008 • Reduction in all cross sections is expected • Consider two particular parton combinations, q qbar (e.g. for Z, Z', etc) and gg (for Higgs, ttbar, etc). • Thus, the reduction in cross section • for a 200 > GeV Higgs boson is almost exactly a factor of 2 • for W,Z the > reduction factor is less (70%) Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  19. SUSY: Supersymmetry A possible symmetry between fermions and bosons |S=0 or S=1〉 ↔ |S=½〉 • Avoids fine-tuning of SM, can lead to GUTs • Generally assume LSP is stable (R-parity conservation)  possible dark matter candidate • SUSY breaking mechanism is unknown  many parameters mSUGRA: • supergravity inspired model • 5 free parameters: m0, m½, A0, tan(β) and sign(µ) Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  20. mSUGRA • mSUGRA stands for minimal supergravity. The construction of a realistic model of interactions within N = 1 supergravity framework where supersymmetry breaking (Gravity Mediated Supersymmetry Breaking ) is communicated through the supergravity interactions. • Parameters of mSUGRA: • m0 – the universal scalar mass • m1/2 – the universal gaugino mass • tan(β) – the ration of the vacuum expectation values of the two Higgs fields • A0 – the Higgs-squark-squark trilinear coupling constant • sign(µ) – where µ is the unmixed Higgsino mass or the SUSY-conserving Higgs mass parameter Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  21. Physics motivation: other theories • Another model is one with a Majorana particle that decays through SM-like bosons into leptons: • pp -> νM l X • Heavy Majorana neutrinos (νM) can be produced in pp collisions in association with a lepton through a virtual W boson [T. Han and B. Zhang, Phys. Rev. Lett. 97, 171804 (2006)]. This new particle can subsequently decay to a W and another lepton: • νM -> W l • Given the Majorana nature of this neutrino, i.e., that it is its own antiparticle, more than half of such events will contain like-sign dileptons in the final state Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  22. Muons Eta distribution of leading muon (Pt>5 GeV) LM1 Eta distribution of leading muon (Pt>5 GeV) LM1 • Two different collections of muons are stored • STA - Standalone (muon system) • GMR - Global reconstructed (muon system + tracker) • Kinematics distributions for GMR: Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  23. Muons Pt of leading muons LM1 Pt of leading muons QCD • Signal (LM1) muons have muon Pt spectrum: • harder than QCD • similar to W/Z+jets and TTbar • Pre-selection cut PTµ > 10 GeV to avoid problems with low-PT range where efficiency is not very good Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  24. Muons Multiplicity Number of muons LM1 Number of muons Number of muons QCD W/Z + jets TTbar • SUSY events have a more of muons, hence we can require 2 same sign muons. • This cut efficiently kills QCD, W+jets and Z+jets, because these backgrounds typically have less than 2 muons in most of events • Nmuons = 2 same sign muons Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  25. Muons isolation 2SS prompt SUSY muons 2SS muons from hadron decay Efficiency • In order to distinguish SUSY diagrams combined isolation was used in the dimuon analysis: • Combined Isolation = Tracker_Iso + 0.75 * Calorimeter_Iso < 10 GeV prompt SUSY muons Isoµ < 10 GeV muons from hadrons decay 65% efficient at identifying SUSY diagrams, 90% pure w.r.t. SUSY hadrons decay and SM backgrounds Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  26. 2 Same Sign Muons Pt of leading muon in 2SS Pt of leading muon in 2SS LM1 LM1 Pt of second muon in 2SS Pt of second muon in 2SS W/Z+jets TTbar W/Z+jets TTbar • No remarkable difference in the PT distribution for muons between signal and background (see plots below) Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  27. Jets Et of leading jet LM1 Et of leading jet Et of leading jet QCD W/Z+jets TTbar • Because of large squark masses we expect high energy jets in SUSY events, hence we can cut on their ET • Transverse energy of jets: • ET1st > 175 GeV • ET2nd > 130 GeV • ET3rd > 55 GeV Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  28. Multiplicity of jets Number of jets LM1 Number of jets Number of jets W/Z+jets TTbar QCD • SUSY events have a lot of jets, hence we can cut that number at 3 • This cut efficiently kills QCD, W+jets and Z+jets, because these backgrounds typically have 2 jets or less in most of events • Njets >= 3 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  29. MET MET LM1 MET MET W/Z+jets TTbar QCD Missing energy is one of the most important cuts, because large value of missing energy is an inherent SUSY signature due to LSP which escape detection (R-Parity is conserved in mSUGRA). MET > 200 GeV Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  30. Gluino and squark isomass m1/2 no ewsb m0 m1/2 no ewsb m0 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  31. Cross section iso-contours no ewsb no ewsb no ewsb σ(p+ + p+→ ~q + ~q + X) σ(p+ + p+→ ~q + ~g + X) σ(p+ + p+→ ~g + ~g + X) Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  32. Same-Sign di-muons: Contour behavior m(~χ20) = m(~lL) m(~χ20) = m(~lR) reach contour for 10fb-1 no ewsb x-section isolines Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  33. Significance as a measure of merit • If for a particular set of cuts we observe ns (signal) and nb (background) MC events, is it better of worse than a different set of event numbers for a different set of cuts? • Common approach: • expected number of signal events in an experiment is s =ws ns … actually should be =ws (ns+1) • expected number of bkgd events in an experiment is b =wb nb … actually should be =wb (nb+1) • estimate significance S of observing (b+s) events, when ones expect b events for background Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  34. Significance: Sc (one bkgd) • number of MC events before cuts N • event weight w • number of MC events after cuts n • 1) pdf (probability distribution function) for the cut efficiency ε 2) probability to observe in experiment exactlyk events 3) calculate probability to observe k0 events and convert it to significance Sc we use this approximation (easy to integrate) very good for large N>>1 on a conservative side for smaller N side note: Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  35. Significance: Sc vs. ScL Significance Significance Observed events k0 Observed events k0 Significance Significance Observed events k0 Observed events k0 Red = Sc Blue = ScL ~ ScP b=w(n+1) w=0.1 n=0 w=0.5 n=0 w=0.1 n=10 w=0.5 n=10 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  36. Systematic uncertainties 36 Summary table: Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

  37. Results for fully simulated points 37 Summary table: total number of background and signal events which pass the optimized selection cuts for 10 fb-1, together with the corresponding significance (with and without systematic uncertainties) to discover different signal benchmark points. Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008

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