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Dark Matter at LHC

Dark Matter at LHC. Introduction Hierarchy problem & motivation for dark matter. SUSY. Alternative BSM LHC & ATLAS Performance Outlook & Summary. Introduction. Hierarchy problem Why does the SM Higgs remain light?

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Dark Matter at LHC

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  1. Dark Matter at LHC • Introduction • Hierarchy problem & motivation for dark matter. • SUSY. • Alternative BSM • LHC & ATLAS Performance • Outlook & Summary Tony Weidberg

  2. Introduction • Hierarchy problem • Why does the SM Higgs remain light? • Expect radiative corrections  mass to highest scale in the theory (eg M_planck). • Requires improbable fine tuning • Solution requires new physics @ TeV scale. • eg SUSY • If we assume R parity conservation  LSP is stable  candidate for dark matter. • Other BSM theories also provide dark matter candidates • Eg UED: lightest particle has negative KK parity and would therefore be stable  dark matter candidate. Tony Weidberg

  3. SUSY • If squarks or gluinos <~ 1 TeV  large s  high rates at LHC. • Cascade decays to LSP. • Assume R parity  LSP stable  Missing transverse energy (MET) in detector • Very generic SUSY search: • Multi jets + MET in excess of SM background • Details are model dependent

  4. SUGRA • Can’t explore 105 dimensional parameter space of MSSM so need some unified model. • Look at SUGRA as an example. • 5 parameters, m0, m1/2, tanb, A0, sign(m). • LSP is dark matter candidate but don’t want too much! • Restricts regions in parameter space so that there is efficient annihilation of LSP.

  5. SUGRA • SU1 m0=70 GeV m1/2=350 GeV tanb=10 • Coannihilation: near degenerate • SU2 m0=3550 GeV m1/2 = 300 GeV tanb=10 • high higgsino  • SU3 m0=100 GeV m1/2=300 GeV tanb=6 • Bulk: LSP annihilation exchange of sleptons. • SU4: low mass point close to TeV limits. • SU6 m0=320 GeV, m1/2=375 GeV tanb=50 • enhances annihilation. • SU8.1 m0=210 GeV m1/2=360 GeV tanb=40. • Coannhiliation with small • SU9 m0=300 GeV, m1/2=425 GeV tanb=20 • Enhanced Higgs production.

  6. Useful Definitions • MET: • Requires good 4p calorimeters + muons • Infer presence of LSP from large MET • Can’t reconstruct mass event by event because of MET is only in transverse plane. • Define Transverse Mass • For 2 body decays  end point at mass of parent (eg MW).

  7. Definitions (2) • Effective Mass • Meff discriminates between SUSY and SM. • Peak in Meff gives first crude estimate of SUSY mass scale. • More complicated variables required for mass determinations in events with two invisible particles eg “stranverse” mass

  8. Backgrounds • Cosmics, beam halo, beam gas. • Fake MET from QCD jets • Real MET from SM backgrounds eg • W -> l n • t tabr, t b W, Wl n • Z + jets, Z nn • Reliable estimates essential for all backgrounds before SUSY discovery can be claimed! Time difference between scintillators on two sides ATLAS

  9. SM Backgrounds Background is cocktail of different SM processes S/B high at large Meff but still need data driven estimates.

  10. Need data driven approach. Use pT balance in photon jet events Photon well measured  resolution due to jets. Gaussian fits give s vs pt QCD Background (1) Resolutions vs photon pT

  11. QCD Background (2) • Estimate non-Gaussian tails from 3 jet events • Badly measured jet direction close to MET • Combine Gaussian & non-Gaussian tails  Jet Transfer Function (JTF) • Estimate QCD background from data & JTF

  12. W & ttbar Backgrounds • Use MT<100 GeV to define control region W/ttabr SUSY Count events in control region  predict SM background in signal region More sophisticated variations to allow for signal contamination of control region.

  13. No Lepton Mode Background is cocktail of different SM processes Z nn irreducible W/top from lost leptons S/B high at large Meff but still need data driven estimates.

  14. Reach in SUGRA space 500 pb-1 @ 7 TeV Aim 1000 pb-1 by end 2011 Gluino 0.5 TeV Squark 0.5 TeV jet PT > [100,40,40,40] GeV ETmiss > 80 GeV

  15. Reach for squarks & gluinos • With 500 pb-1 @ 7TeV: • Could exclude up to • - msquark ~ 700 GeV • - mgluino ~ 600 GeV • Improve limits of Tevatron Tony Weidberg

  16. Mass Fitting • Can’t easily determine mass because don’t know how much MET carried away by each LSP. • Can determine mass differences by fitting end points of spectra. • eg squark decay chain:

  17. End Point Analysis • End point for Tony Weidberg

  18. llq End Points M(lqq) end point gives mass difference Tony Weidberg

  19. SUSY Higgs • BR( ) can be large. • Clean signal for b bar because SM suppressed by MET cut. M(b bar)

  20. SUSY Mass determination • Can use measured end points in global fit  SUSY masses: • Results for 1 fb-1 SU3:

  21. Mass Determinations • More sophisticated tools being developed. • Use event by event information • M3c variable: lower and upper bounds for mass of LSP by varying fraction of Etmiss given to two LSPs • event by event subject to constraints: • Momentum conservation • MET • Mass differences (already measured from end point analysis). • Might get much higher precision on mass LSP • E.g. Barr, Pinder & Serna, arXiv:0811.2138v1 claim precision ~ 1 GeV for 100 fb-1 for SPS1a.

  22. LHC Tony Weidberg

  23. LHC Performance • Number of bunches increasing • 36 colliding bunches Lmax ~ 10 31 cm-2 s-1 Tony Weidberg

  24. ATLAS & CMS • Will show results from ATLAS • Similar quality results from CMS • ICHEP 2010: http://indico.cern.ch/contributionDisplay.py?contribId=75&confId=73513

  25. ATLAS

  26. ATLAS • SUSY search needs • good missing Et • Jets • Leptons • b-tagging • tau id

  27. MET Large tail at high MET removed by cleaning cuts Rates agree with MC Tony Weidberg

  28. MET Resolution • Fit resolution in x/y in slices of SET. • Data and MC in excellent agreement.

  29. QCD Jets • Spectrum for di-jet mass agrees well with LO QCD calculations. • Extends beyond 2 TeV! Tony Weidberg

  30. Dijet Mass Spectrum • Already allowed best limits on q* production • Mq*> 1.26 TeV. Tony Weidberg

  31. W m n • Low MET background dominated • Fit shape of QCD background to control region at low MET • Very clean signal after MET>25 GeV cut. Tony Weidberg

  32. W e n • Very similar story to muon channel • Data driven background estimates  very clean signal after MET>25 GeV cut Tony Weidberg

  33. Z ee and Z  m m Tony Weidberg

  34. b-tagging • B lifetime  separate b jets from light quarks using displaced vertices. Tony Weidberg

  35. b-tagging • Use signed transverse impact parameter of 3 tracks in jet to define jet probability for light quark jets. • Clear signal for b jets at low jet probability. Tony Weidberg

  36. Top Physics • Signals in e/m + jets, reasonable S/B in lepton+4 jets after b-tagging. • Better S/B but smaller BR for di-lepton channels

  37. Very early look at a SUSY search … • After SUSY cuts: • jet PT > [70,30,30,30] GeV • ETmiss > 40 GeV • Df(jet,ETmiss) [0.2,0.2,0.2] • ETmiss/Meff cut > 0.2

  38. Outlook & Summary • Large cross section for strongly interacting particles at LHC  high rates for squarks and gluons. • Significant improvement in Tevatron limits after 2010/11 run. • Large mass reach for SUSY discovery after upgrade to 14 TeV and full luminosity. • Can do much more than just discover SUSY: can determine many parameters, eventually pin down mass of dark matter candidate. • Alternative BSM that can also provide dark matter candidates will also be observable through MET. • LHC ramp up going well. • ATLAS & CMS working well and producing physics results

  39. Backup Slides

  40. Fitted SUSY Masses • Fits for SU3 1 fb-1 and SU4 0.5 fb-1 Tony Weidberg

  41. mSUGRA Parameters • SU3 1 fb-1 • M0 and M1/2 well determined. • Some constraint on tan beta • Little constraint on A0 • Sign (mu) not fixed. Tony Weidberg

  42. 7 TeV vs 14 TeV

  43. More Info • LHC: Steve Myers talk at ICHEP • http://indico.cern.ch/contributionDisplay.py?contribId=73&confId=73513 • ATLAS MC studies CSC book • http://cdsweb.cern.ch/record/1125884?ln=en • ATLAS tau-id see note and event display • https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2010-086/ATLAS-CONF-2010-086.pdf

  44. Determining Masses of Invisible Particles (1)

  45. Determining Masses of Invisible Particles (2) • Vary fraction of MET assigned to two LSPs & find lower bound (upper bounds) subject to constraints

  46. Example Fits MY=200 GeV 250 GeV

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