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Mass Reconstruction Methods in ATLAS

Mass Reconstruction Methods in ATLAS. S. Laplace On behalf of the ATLAS collaboration. Physics at LHC – Cracow, Poland SUSY Session, July 4 th 2006. Outline. Introduction: ATLAS Activities in SUSY SUSY Phenomenology and Meas. Strategies Discovering SUSY Mass Measurements:

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Mass Reconstruction Methods in ATLAS

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  1. Mass Reconstruction Methodsin ATLAS S. Laplace On behalf of the ATLAS collaboration Physics at LHC – Cracow, Poland SUSY Session, July 4th 2006 S. Laplace, "Mass Reconstruction Methods"

  2. Outline • Introduction: ATLAS Activities in SUSY • SUSY Phenomenology and Meas. Strategies • Discovering SUSY • Mass Measurements: • Masses: Endpoint Method • Masses near Dilepton Endpoint and Mass Relation Method • From Measurements to Model Parameters • Conclusion (note: no time to talk about stop mass measurement and other methods than endpoints like Mass Relation Method…) S. Laplace, "Mass Reconstruction Methods"

  3. Introduction • ATLAS activities in SUSY: • TDR (1998): fast simulation studies  discovery potential • Currently: • Full simulation studies (preliminary results) • Commissioning, systematics • Background estimation (from latest MC and plans to • measure it from data) • New measurement techniques • Note: in this talk, • MET = Missing Transverse Energy • Sleptons = selectrons and smuons (will explicitly call a stau a stau) S. Laplace, "Mass Reconstruction Methods"

  4. SUSY Phenomenology and Mass Measurement Strategies • If R-parity ( ) is conserved, then: • Lightest Supersymetric Particle (LSP) is stable • LSP not detected thus large MET (few x 100 GeV) • Event is not fully reconstructed: no mass peak • Sparticles produced in pairs: both sides of event are not reconstructed ! • Mass measurement strategy: exploit kinematics of long decay chains • Production of SUSY at LHC: strong interactions dominates:  decay chain starts from a gluino or a squark: S. Laplace, "Mass Reconstruction Methods"

  5. (RPC) SUSY Models SUSY Parameters (SM = 19): M.S.S.M. 105 (note: if RPV + 48) Constrained models: mSUGRA m0, m1/2, A0, tan β, sgn μ 5 G.M.S.B. λ, Mmes, N5, tan β, sgn μ, Cgrav 6 A.M.S.B. m0, m3/2, tan β, sgn μ 4 Simple benchmark: mSUGRA Focus point (m0 3 TeV) + funnel region at large tan g-2 bs WMAP Bulk (SPS1a) Stau coannihilation Ellis et al., Phys. B565 (2003) 176 S. Laplace, "Mass Reconstruction Methods"

  6. Discovering SUSYand Evaluating MSUSY RPC models signature: MET + several high-pT jets  Build discriminating variable Meff: where Coannihilation point Full sim 20.6fb−1 SUSY signal SM Bkg (Herwig) S. Laplace, "Mass Reconstruction Methods"

  7. Mass Measurement:Endpoint Method • Example: dilepton endpoint • mll has a kinematic endpoint that • depends on the masses of the • sparticles in the chain • Does not need a-priori knowledge • of any sparticle mass • Backgrounds: • SM & uncorrelated (not Z) SUSY: • use Same Flavour (SF) – • Different Flavour (DF) • Edge fit: stat. error = 0.05%, syst. error • dominated by lepton energy scale (0.1%) SPS1a Fast sim 300 fb−1 B.K. Gjelsten et al, J. High Energy Phys. JHEP12(2004)003  S. Laplace, "Mass Reconstruction Methods"

  8. A Variety of Endpoint Measurements Sequential: Branched: SPS1a Fast sim 300 fb−1 Bulk Full sim 4.20fb−1 S. Laplace, "Mass Reconstruction Methods"

  9. Di-lepton Endpoint inVarious mSUGRA Scenarii Depending on point: different shape, number of edges, 2-body vs 3-body decay, … Focus Point Coannihilation ATLAS MC truth lL MC truth lR Full sim 6.9fb−1 signal Full Sim 20.6fb−1 • 2 edges for left and right slepton • m0 large,heavy scalars •  no sleptons in  decays • direct 3-body decay: • small BR • at least 1 lepton with • small pT S. Laplace, "Mass Reconstruction Methods"

  10. Extraction of Sparticle Masses from Endpoints 100 fb-1 MC toy of 10000 ATLAS experiments, use inversion formulae to get masses from edges: SPS1a All masses are strongly correlated with B.K. Gjelsten et al, J. High Energy Phys. JHEP12(2004)003  S. Laplace, "Mass Reconstruction Methods"

  11. Right-Handed Squark Mass q q • mSUGRA: 1 essentially a bino: Br( )  100% • If both gluino decay to right-handed squarks: • require 2 high-pT jets, MET • Discriminant: Cambridge variable MT2 endpoint gives • the right squark mass: (low pT) (high pT) Coannihilation Full sim 20.6 fb−1 SPS1a Fast sim 30 fb−1 True Mass 520 GeV True: 735 Fit: 7115 Fitted edge: 512 GeV Lower than true because of SUSY bkg SM bkg S. Laplace, "Mass Reconstruction Methods"

  12. Staus Signatures • SPS1a: dominant decay is • (because of relatively high tan value) • Look at hadronic  decays (dedicated algorithms for -jets) • Background (QCD jets misidentified as  ) evaluated from • same signs events: Same sign substracted: All: (Z+j, tt) (signal) SPS1a Fast sim 30 fb-1 (background) B.K. Gjelsten et al, ATL-PHYS-2004-007 S. Laplace, "Mass Reconstruction Methods"

  13. Sbottom and Gluino Masses:Near The l+l- Endpoint • Near l+l- endpoint: LSP and l+l- are at rest in frame, • thus can evaluate momentum (approximation): where and are known from endpoints b b • Add 1 or 2 b-jet to get sbottom and gluino masses: and SPS1a Fast sim 300 fb-1 Correlation between and =2.2 GeV Wrong associated b-jet SUSY bkg Spread from p(2)approximation is common to both masses Gluino – sbottom masses Gluino mass B.K. Gjelsten et al, ATL-PHYS-2004-007 S. Laplace, "Mass Reconstruction Methods"

  14. Sbottom and Gluino Masses:Mass Relation Method Alternative method to previous one using ALL data set (not only near endpoint) • Each event = 4D surface in 5D space • In principle: 5 events to determine • the 4 unknowns ! • In practice: know • so have following constraint: 5 parameters 4 unknowns (4-momentum) Endpoint only: Not obvious to resolve the 2 peaks ! SPS1a Fast sim 300 fb-1 Two possible solutions (2 lepton assignments) b1 b2 b1 b2  The two b-peaks are well resolved Mass Relation Method Kawagoe et al, hep-ph/0410160 S. Laplace, "Mass Reconstruction Methods"

  15. Obtaining the Fundamental Model Parameters LHC Measurements SUSY Model Ex: mSUGRA m0, m1/2, A0, tan, sgn() Spectrum Generator (Ex: SUSPECT, SoftSUSY, …) Ex: endpoints Fit: 2 Mes. Note: better to exploit edges than masses (correlations) S. Laplace, "Mass Reconstruction Methods"

  16. An Example SFITTER program: List of measurements (300 fb-1) mSUGRA Parameter determination Sign(μ) fixed Note: m(ll) most powerful input (m0 driven by 1st and 2nd generation slepton sector) R. Lafaye, T. Plehn, D. Zerwas, hep-ph/0512028 S. Laplace, "Mass Reconstruction Methods"

  17. Conclusion • New era for SUSY studies in ATLAS is currently starting: • large scale productions to prepare for real data analysis • study detector systematics • SM background: latest MC and plans to measure it from data • new models studied • new techniques developed • Discovery potential: in most models, a few fb-1 are sufficient to: • observe squarks and gluons below 1-2 TeV and sleptons below 300 GeV • accurately measure squark, slepton and neutralino masses using cascades S. Laplace, "Mass Reconstruction Methods"

  18. Backup S. Laplace, "Mass Reconstruction Methods"

  19. mSUGRA Excluded by b s (CLEO,BELLE) Favored by gμ−2 at the 2σ level Muon g−2 coll. Focus point WMAP: 0.094<Ωχh2<0.129 Stau1=LSP Funnel region s-channel Higgs-exchange. Stau coannihilation Bulk region t-channel slepton exchange. (ATL-PHYS-2004-011) (Ellis et al., Phys. B565 (2003) 176) S. Laplace, "Mass Reconstruction Methods"

  20. SPS1a Point • Mass spectrum : • mSUGRA fundamental parameters : • Main branching ratios : (note: ) S. Laplace, "Mass Reconstruction Methods"

  21. Meff: Parton Shower vs Matrix Element for Bkg Simulation TDR: LHC Point 5 Isajet (PS) Fast sim 10 fb-1 Recently: Alpgen (ME) Fast sim 10 fb-1 Parton Shower (only good in collinear region) Matrix Element (more correct)  Background increases by factor 2 to 5 ! S. Laplace, "Mass Reconstruction Methods"

  22. Stop Mass Measurement • SPS5: light stop • Reconstruct stop mass via • Signature: 2 b-jets, MET, 3 light-quark jets • Fit m(tb) distribution endpoint: Fast sim 300 fb-1 M(tb)fit= 258.7 ± 0.3(stat.) ± 2.6(syst.) S. Laplace, "Mass Reconstruction Methods"

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