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SPS5 SUSY STUDIES AT ATLAS

SPS5 SUSY STUDIES AT ATLAS. Iris Borjanovic Institute of Physics, Belgrade. SUPERSYMMETRY. EXTENSION of the SM EVERY SM PARTICLE HAS ITS SUSY PARTNER UNIFICATION OF ALL INTERACTIONS SUSY IS BROKEN IF EXIST AT THE TeV SCALE IT IS LIKELY TO BE DISCOVERED AT LHC. MSSM. SPIN 0. SPIN 1/2.

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SPS5 SUSY STUDIES AT ATLAS

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  1. SPS5 SUSY STUDIES AT ATLAS Iris Borjanovic Institute of Physics, Belgrade

  2. SUPERSYMMETRY • EXTENSION of the SM • EVERY SM PARTICLE HAS ITS SUSY PARTNER • UNIFICATION OF ALL INTERACTIONS • SUSY IS BROKEN • IF EXIST AT THE TeV SCALE IT IS LIKELY TO BE DISCOVERED AT LHC

  3. MSSM SPIN 0 SPIN 1/2 SPIN 1 Left helicity matter Right helicity matter Higgs sector Electroweak gauge bosons Gluon Mixings: 128 free parameters

  4. R-parity R=(-1)3(B-L)+2S SM particles R=+1, SUSY particles R=-1 R-parity conservation (multiplicative QN): - SUSY particles are produced in pairs - LSP is absolutely stable and present in the final state of every SUSY decay LSP : colorless, uncharged, interacts weakly → SUSYevents withlarge ET missing

  5. SUSY breaking mechanism mass of SM particle ≠ mass of SUSY partner • SUGRA • GMSB • AMSB • BRANE MODELS Number of free parameters is significantly reduced !!! Which is the right one ?

  6. mSUGRA R-parity conserving, LSP is lightest neutralino • SCALAR MASS PARAMETER - m0 • GAUGINO MASS PARAMETER – m1/2 • TRILINEAR COUPLING – A0 • RATIO OF THE HIGGS VACUM EXPECTAION VALUES – tan(β) • HIGGS MASS PARAMETER – sign(μ) RGE Parameters at Planck scale Parameters at EW scale SUSY masses, BR, decays

  7. “SNOWMASS POINTS AND SLOPES”- new set of benchmark points - SPS 5

  8. SUSY masses heaviest light stop lightest

  9. SUSY decays squarks decay weakly gluinos decays strongly stop always decay to

  10. SUSY production Squark and gluino production dominate For L=30 fb-1 , N=1.2 x 106 SUSY events

  11. KINEMATIC ENDPOINTS • RPC models missing LSP , invariant mass formed from final state particles has no peaks • Relativistic kinematics invariant masses have maximums and minimums (kinematic endpoints ) which are function of SUSY masses. From endpoints measuremnets SUSY masses can be extracted.

  12. Example C Q + B, B P + A Q B P A C Maximum PQ invariant mass P and Q are back to back in the B rest frame

  13. Characteristics of SUSY events • Large missing transverse energy ET miss • High pT jets • Large multiplicity of jets • Large Meff= ET miss + pT (j1) + pT (j2) + pT (j3) + pT (j4) - SM background reduced by hard kinematics - large SUSY background

  14. MONTE CARLO • ISAJET 7.64 • HERWIG 6.5 • ATLFAST • Generated: - 300 fb-1 of SUSY events - 10fb-1 of top-antitop pairs

  15. LEFT SQUARK CASCADE DECAY • Left squark production: directly or from gluino decay • Event signature: - 2 SFOS leptons (natural trigger) high pT jet from left squark decay - large missing transverse energy - one more high high pT jet from the decay of squark/gluino produced with left squark 644 226 191 119

  16. ENDPOINTS Final state particles: l+ , l- , q Invariant masses: l+l- , l+l- q , l±q 5 ENDPOINTS - l+l- maximum - l +l- q maximum - l +l- q minimum - maximum for larger l± q mass - maximum for smaller l± q mass

  17. ENDPOINTS ARE FUNCTION OF 4 SUSY MASSES GeV 93.4 GeV 511 GeV 320 GeV 470 GeV 225 GeV lepton and quark masses were neglected while deriving formulas

  18. EVENT SELECTION • ET miss > 100 Gev • n(jet) ≥ 4 • pT (j1) > 150 GeV, pT (j2) > 100 GeV, pT (j3 , j4) > 50 GeV • 2 SFOS leptons (e+e- , μ+μ- ) with pT > 10 GeV • + additional cuts for every distributions SM background is negligible !!!

  19. SUSY BACKGROUND N(SFOS)=N(OFOS), identical shape of mass distributions background : SFOS leptons from two independent decays SFOS-OFOS subtraction removes background, applied on all mass distributions ll invariant mass SFOS-OFOS SFOS OFOS Mll (GeV) Mll (GeV)

  20. FITTED ENDPOINTS llq llq ll M(GeV) M(GeV) lq low lq high M(GeV) M(GeV) M(GeV) L=300 fb-1

  21. FIT RESULTS Energy scale error Fit error up to 2% 1% for jets, 0.1% for leptons Good agreement between theory and fit

  22. MASS RECONSTRUCTION 5 endpoint measurements and 4 unknown masses over constrained system of equations is solved numerically by minimising χ2 Only one set of endpoint measurement, one set of SUSY masses ansambl of endpoint set of measurements is modeled, mass distributions Endpoints are function of SUSY masses Modeled experimental endpoint value Random numbers

  23. RECONSTRUCTED MASSES NEUTRALINO 1 LEFT SQUARK ENTRIES ENTRIES M(GeV) M(GeV)

  24. RESULTS reconstructed mean value calculated value ANSAMBL OF 1000 EXPERIMENTS

  25. LIGHT STOP SQUARK • Mixture of left and right stop • Mass: 236 GeV • Higgs mass depends on stop mass • Production: • Decay:

  26. STOP PAIRS • 50% of all SUSY production • 2 low pT b quarks • No detectable signal !!! < pT(b) > =15 GeV Entries 236 226 pT (GeV)

  27. tb + ETmiss channel 38% (1) tb edge structure 38% (2) 2% 14% 14% (3) Kinematically equivalent to decay (1) if M(bW)~M(t) Hadronic top(W) decay →bjj Decay (1) can be isolated !!!

  28. TOP-BOTTOM ENDPOINT Decay (1) 2552GeV2 Maximum occurs for bottom and top back to back in the stop rest fr. Decay (2) 4622 GeV2 Maximum occurs for bottom and top back to back in the sbottom rest frame

  29. M(tb) from HERWIG Events/4 GeV (1) (3) (2) M (GeV)

  30. Event signature • low pT b quark from stop • b quark from top decay • two light (u,d,s,c,) quarks from W decay • high pT quark from left/right squark produced with gluino • LSP from chargino decay and large missing energy

  31. EVENT SELECTION • ET miss > 200 GeV • njet ≥ 3 ( ≠ b, τ jets), pT > 30 GeV, |η|<3 , pT (j1) > 300 GeV • n(b jet)=2 30< pT (b1) <150 GeV, 30< pT (b2) <50 GeV • no leptons

  32. TOP-BOTTOM MASS RECONSTRUCTION • Excluding j1 , jj →:|mjj - mW |< 15GeV • bjj minimizing | mbjj - mt| + pT(b’ ) < 50 GeV • Scaling mjj = mW ,mbjj recalculated, |mbjj - mt |< 30GeV • mbjj + b jet → mtb • ΔR(tb) < 2 • ‘Sideband’ subtraction

  33. SIDEBAND METHOD Some jj pairs accidentally have masses in W zone W :|mjj - mW |< 15GeV A :| mjj – (mW -30)| <15GeV B :| mjj – (mW +30)| <15GeV W-0.5(A+B) subtraction jj mass Events/5 GeV/300 fb-1 W B A M(GeV)

  34. TOP MASS L=300 fb-1

  35. TOP-BOTTOM MASS W 0.5(A+B) W-0.5(A+B) signal L=300 fb-1

  36. M(tb) shape linear part for background gaus smeared triangular shape

  37. M(tb) FIT L=300 fb-1 Good agreement between fit and theory M (GeV) M(tb)fit = 258.7 ± 0.3(stat.) ± 2.6(syst.) GeV M(tb)th = 255 GeV This measurement puts contraints on the stop, gluino and chargino masses !!!

  38. CONCLUSION If SUSY particles with SPS5 characteristics exist, it will be possible to: - identify SUSY particles - reduce SM and SUSY background - measure SUSY masses with ATLAS detector at LHC

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