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Dark Matter Searches at ATLAS

Dark Matter Searches at ATLAS. Dan Tovey University of Sheffield. ATLAS. Length: ~45 m Radius: ~12 m Weight: ~7000 tons. First Astroparticle Data…. Tower energies: ~ 2.5 GeV. First cosmic muons observed by ATLAS in the underground cavern on June 20 th 2005 (recorded by hadron

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Dark Matter Searches at ATLAS

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  1. Dark Matter Searchesat ATLAS Dan Tovey University of Sheffield 1

  2. ATLAS Length: ~45 m Radius: ~12 m Weight: ~7000 tons 2

  3. 3

  4. First Astroparticle Data… Tower energies: ~ 2.5 GeV First cosmic muons observed by ATLAS in the underground cavern on June 20th 2005 (recorded by hadron Tilecal calorimeter) 4

  5. Event Rates • Main asset of LHC: huge event statistics thanks to high s and L • Allows precision measurements/tests of SM • Searches for new particles with unprecedented precision. Process Events/s Events per year Total statistics collected at previous machines by 2008 W e 15 108 104 LEP / 107 Tevatron Z ee 1.5107107 LEP tt 1 107104 Tevatron bb 106 1012–1013109 Belle/BaBar? H m=130 GeV 0.02105 ? m= 1 TeV 0.001104--- Black holes 0.0001103--- m > 3 TeV (MD=3 TeV, n=4) L = 1033 cm-2 s-1 5

  6. Dark Matter @ ATLAS • Characteristic signature for Dark Matter production at ATLAS: Missing Transverse Energy (‘MET’) • Valid for any DM candidate (not just SUSY) • Observation of MET signal necessary but not sufficient to prove DM signal (DM particle could decay outside detector) ~ c01 MET Conclusive proof of both existence and identity of DM by combining LHC data with astroparticle data ~ c01 6

  7. Complementarity Polesello et al JHEP 0405 (2004) 071 No. MC Experiments scpsi Wch2 • Measurements at ATLAS complementary to direct and indirect astroparticle searches / measurements • Uncorrelated systematics • Measures different parameters EDELWEISS Wch2 log10 (scpsi / 1pb) CDMS scpsi (cm2) DAMA • Aim to test compatibility of e.g. SUSY signal with DM hypothesis (data from astroparticle experiments) • Fit SUSY model parameters to ATLAS measurements • Use to calculate DM parameters Wch2, mc, scpsietc. CDMS-II ZEPLIN-I CRESST-II scpsi ZEPLIN-2 ZEPLIN-4 EDELWEISS 2 GENIUS XENON ZEPLIN-MAX mc Provides strongest possible test of Dark Matter model DM particle mass mc (GeV) 7

  8. SUSY DM Strategy SUSY Dark Matter studies at ATLAS will proceed in four stages: • SUSY Discovery phase • Inclusive Studies (measurement of SUSY Mass Scale, comparison of significance in inclusive channels). • Exclusive studies and interpretation within specific model framework (e.g. Constrained MSSM / mSUGRA) • Specific model needed to calculate e.g. relic density g general SUSY studies will be less model dependent. • Less model-dependent interpretation • Relax model-dependent assumptions 8

  9. Stage 1: SUSY Discovery 9

  10. SUSY Signatures p p ~ c01 ~ ~ ~ q ~ c02 l g q q l l • Q: What do we expect SUSY events @ LHC to look like? • A: Look at typical decay chain: • Strongly interacting sparticles (squarks, gluinos) dominate production. • Heavier than sleptons, gauginos etc. g cascade decays to LSP. • Long decay chains and large mass differences between SUSY states • Many high pT objects observed (leptons, jets, b-jets). • If R-Parity conserved LSP (lightest neutralino in mSUGRA) stable and sparticles pair produced. • Large ETmiss signature (c.f. Wgln). • Closest equivalent SMsignature tgWb. 10

  11. Inclusive Searches • Use 'golden' Jets + n leptons + ETmiss discovery channel. • Map statistical discovery reach in mSUGRA m0-m1/2 parameter space. • Sensitivity only weakly dependent on A0, tan(b) and sign(m). • Syst.+ stat. reach harder to assess: focus of current & future work. 5s 5s ATLAS ATLAS 11

  12. Background Systematics (Zll) ATLAS • Z gnn + n jets, W g ln + n jets, W gtn + (n-1) jets (t fakes jet) • Estimate from Z g l+l- + n jets (e or m) • Tag leptonic Z and use to validate MC / estimate ETmiss from pT(Z) & pT(l) • Alternatively tag W g ln + n jets and replace lepton with n (0l): • higher stats • biased by presence of SUSY Preliminary (Wln) ATLAS Preliminary 12

  13. Stage 2: Inclusive Studies 13

  14. Inclusive Studies LHC Point 5 (Physics TDR) • Following any discovery of SUSY next task will be to test broad features of potential Dark Matter candidate. • Question1: Is R-Parity Conserved? • If YES possible DM candidate • LHC experiments sensitive only to LSP lifetimes < 1 ms (<< tU ~ 13.7 Gyr) R-Parity Conserved R-Parity Violated ATLAS ~ Non-pointing photons from c01gGg ~ ~ • Question 2: Is the LSP the lightest neutralino? • Natural in many MSSM models • If YES then test for consistency with astrophysics • If NO then what is it? • e.g. Light Gravitino DM from GMSB models (not considered here) GMSB Point 1b (Physics TDR) ATLAS 14

  15. Measuring Parameters Point m0 m1/2 A0 tan(b) sign(m) LHC Point 5 100 300 300 2 +1 SPS1a 100 250 -100 10 +1 LHC Point 5 (A0 =300 GeV, tan(b)=2, m>0) Sparticle Mass (LHC Point 5)Mass (SPS1a) qL~690 GeV ~530 GeV 02233 GeV 177 GeV lR157 GeV 143 GeV 01122 GeV 96 GeV Point SPS1a (A0 =-100 GeV, tan(b)=10, m>0) ~ ~ ~ ~ ATLAS • First indication of mSUGRA parameters from inclusive channels • Compare significance in jets + ETmiss + n leptons channels • Detailed measurements from exclusive channels when accessible. • Consider here two specific example points studied previously: 15

  16. Stage 3: Exclusive studies and interpretation within specific model framework 16

  17. Exclusive Studies p p ~ c01 ~ ~ ~ ~ q c02 g lR q q l l • With more data will attempt to measure weak scale SUSY parameters (masses etc.) using exclusive channels. • Different philosophy to TeV Run II (better S/B, longer decay chains) g aim to use model-independent measures. • Two neutral LSPs escape from each event • Impossible to measure mass of each sparticle using one channel alone • Use kinematic end-points to measure combinations of masses. • Old technique used many times before (n mass from b decay spectrum, W (transverse) mass in Wgln). • Difference here is we don't know mass of neutral final state particles. 17

  18. Dilepton Edge Measurements ~ ~ c01 c02 ~ l l l • When kinematically accessible c02 canundergo sequential two-body decay to c01 via a right-slepton (e.g. LHC Point 5). • Results in sharp OS SF dilepton invariant mass edge sensitive to combination of masses of sparticles. • Can perform SM & SUSY background subtraction using OF distribution e+e- + m+m- - e+m- - m+e- • Position of edge measured with precision ~ 0.5% (30 fb-1). ~ ~ • m0 = 100 GeV • m1/2 = 300 GeV • A0 = -300 GeV • tan(b) = 6 • sgn(m) = +1 e+e- + m+m- - e+m- - m+e- e+e- + m+m- Point 5 ATLAS ATLAS 30 fb-1 atlfast 5 fb-1 SU3 Physics TDR 18

  19. Measurements With Squarks ~ qL ~ ~ c02 ~ c01 q l l l ~ qL ~ ~ c02 c01 q h lq edge llq edge b b 1% error (100 fb-1) 1% error (100 fb-1) • Dilepton edge starting point for reconstruction of decay chain. • Make invariant mass combinations of leptons and jets. • Gives multiple constraints on combinations of four masses. • Sensitivity to individual sparticle masses. bbq edge llq threshold 1% error (100 fb-1) 2% error (100 fb-1) TDR, Point 5 TDR, Point 5 TDR, Point 5 TDR, Point 5 ATLAS ATLAS ATLAS ATLAS 19

  20. ‘Model-Independent’ Masses Sparticle Expected precision (100 fb-1) qL 3% 02 6% lR 9% 01 12% ~ ~ ~ ~ ~ ~ c01 lR ATLAS ATLAS • Combine measurements from edges from different jet/lepton combinations to obtain ‘model-independent’ mass measurements. Mass (GeV) Mass (GeV) ~ ~ c02 qL ATLAS ATLAS LHCC Point 5 Mass (GeV) Mass (GeV) 20

  21. Measuring Model Parameters Point m0 m1/2 A0 tan(b) sign(m) LHC Point 5 100 300 300 2 +1 SPS1a 100 250 -100 10 +1 Parameter Expected precision (300 fb-1) m0 2% m1/2 0.6% tan(b)  9% A0 16% • Alternative use for SUSY observables (invariant mass end-points, thresholds etc.). • Here assume mSUGRA/CMSSM model and perform global fit of model parameters to observables • So far mostly private codes but e.g. SFITTER, FITTINO now on the market; • c.f. global EW fits at LEP, ZFITTER, TOPAZ0 etc. 21

  22. Dark Matter Parameters • Can use parameter measurements for many purposes, e.g. estimate LSP Dark Matter properties (e.g. for 300 fb-1, SPS1a) • Wch2 = 0.1921  0.0053 • log10(scp/pb) = -8.17  0.04 Baer et al. hep-ph/0305191 LHC Point 5: >5s error (300 fb-1) SPS1a: >5s error (300 fb-1) Micromegas 1.1 (Belanger et al.) + ISASUGRA 7.69 DarkSUSY 3.14.02 (Gondolo et al.) + ISASUGRA 7.69 scp=10-11 pb scp=10-10 pb Wch2 scp scp=10-9 pb 300 fb-1 300 fb-1 No REWSB LEP 2 ATLAS ATLAS 22

  23. Target Models 'Focus point' region: significant h component to LSP enhances annihilation to gauge bosons ~ ~ c01 t ~ c01 l ~ t1 ~ ~ t1 g/Z/h lR ~ c01 l mSUGRA A0=0, tan(b) = 10, m>0 Slepton Co-annihilation region: LSP ~ pure Bino. Small slepton-LSP mass difference makes measurements difficult. Ellis et al. hep-ph/0303043 • SUSY (e.g. mSUGRA) parameter space strongly constrained by cosmology (e.g. WMAP satellite) data. Disfavoured by BR (b  s) = (3.2  0.5)  10-4 (CLEO, BELLE) 'Bulk' region: t-channel slepton exchange - LSP mostly Bino. 'Bread and Butter' region for LHC Expts. Also 'rapid annihilation funnel' at Higgs pole at high tan(b), stop co-annihilation region at large A0 0.094    h2  0.129 (WMAP) 23

  24. Coannihilation Signatures • Small slepton-neutralino mass difference gives soft leptons • Low electron/muon/tau energy thresholds crucial. • Study point chosen within region: • m0=70 GeV; m1/2=350 GeV; A0=0; tanß=10 ; μ>0; • Decays of c02 to both lL and lR kinematically allowed. • Double dilepton invariant mass edge structure; • Edges expected at 57 / 101 GeV • Stau channels enhanced (tanb) • Soft tau signatures; • Edge expected at 79 GeV; • Less clear due to poor tau visible energy resolution. • ETmiss>300 GeV • 2 OSSF leptons PT>10 GeV • >1 jet with PT>150 GeV • OSSF-OSOF subtraction applied 100 fb-1 ATLAS Preliminary ~ ~ ~ • ETmiss>300 GeV • 1 tau PT>40 GeV;1 tau PT<25 GeV • >1 jet with PT>100 GeV • SS tau subtraction 100 fb-1 ATLAS Preliminary 24

  25. Focus Point Signatures ~ ~ ~ ~ c03 → c01 ll c02 → c01 ll Z0→ ll ATLAS 300 fb-1 Preliminary • Large m0 sfermions are heavy • Most useful signatures from heavy neutralino decay • Study point chosen within focus point region : • m0=3550 GeV; m1/2=300 GeV; A0=0; tanß=10 ; μ>0 • Direct three-body decaysc0n → c01 ll • Edges givem(c0n)-m(c01) : flavour subtraction applied ~ ~ ~ ~ M = mA+mB m = mA-mB 25

  26. Stage 4: Less model-dependent interpretation 26

  27. Dark Matter in the MSSM • Can relax mSUGRA constraints to obtain more ‘model-independent’ relic density estimate. • Much harder – needs more measurements • Not sufficient to measure relevant (co-) annihilation channels – must exclude all irrelevant ones also … • Stau, higgs, stop masses/mixings important as well as gaugino/higgsino parameters Nojiri, Polesello & Tovey, JHEP 0603 (2006) 063 σ(Wch2) vs σ(mtt) Wch2 σ(mtt)=5 GeV Wch2 σ(mtt)=0.5 GeV 300 fb-1 300 fb-1 SPA point 27

  28. Heavy Gaugino Measurements ATLAS • Potentially possible to identify dilepton edges from decays of heavy gauginos. • Requires high stats. • Crucial input to reconstruction of MSSM neutralino mass matrix (independent of SUSY breaking scenario). SPS1a ATLAS ATLAS ATLAS 100 fb-1 100 fb-1 100 fb-1 SPS1a 28

  29. Summary • Following a (SUSY) discovery ATLAS will aim to test the (SUSY) Dark Matter hypothesis. • Conclusive result only possible in conjunction with astroparticle experiments (constraints on LSP life-time). • Estimation of relic density and direct / indirect DM detection cross-sections in model-dependent scenario will be first goal. • Less model-dependent measurements will follow. • Ultimate goal: observation of neutralinos at LHC confirmed by observation of e.g. signal in (in)direct detection Dark Matter experiment at predicted mass and cross-section. • This would be major triumph for both • Particle Physics and Cosmology! 29

  30. BACK-UP SLIDES 30

  31. Supersymmetry • Supersymmetry (SUSY) fundamental continuous symmetry connecting fermions and bosons Qa|F> = |B>, Qa|B> = |F> • {Qa,Qb}=-2gmabpm: generators obey anti-commutation relations with 4-mom • Connection to space-time symmetry • SUSY stabilises Higgs mass against loop corrections (gauge hierarchy/fine-tuning problem) • Leads to Higgs mass £ 135 GeV • Good agreement with LEP constraints from EW global fits • SUSY modifies running of SM gauge couplings ‘just enough’ to give Grand Unification at single scale. LEPEWWG Winter 2006 mH<207 GeV (95%CL) 31

  32. SUSY Spectrum ~ ~ ~ ~ A H0 H± H0u H0d H+u H-d ~ ~ ~ ~ u d e ne ~ ~ ~ ~ c s m nm ~ ~ ~ ~ t b t nt ~ ~ ~ g Z W± g Z W± ~ g g ~ ~ ~ ~ ~ ~ ~ G c02 c03 c ±1 c04 c01 c ±2 • SUSY gives rise to partners of SM states with opposite spin-statistics but otherwise same Quantum Numbers. h • Expect SUSY partners to have same masses as SM states • Not observed (despite best efforts!) • SUSY must be a broken symmetry at low energy • Higgs sector also expanded ne u d e c s m nm t b t nt G 32

  33. Model Framework LHCC mSUGRA Points 3 1 2 5 4 • Minimal Supersymmetric Extension of the Standard Model (MSSM) contains > 105 free parameters, NMSSM etc. has more g difficult to map complete parameter space! • Assume specific well-motivated model framework in which generic signatures can be studied. • Often assume SUSY broken by gravitational interactions g mSUGRA/CMSSM framework : unified masses and couplings at the GUT scale g 5 free parameters • (m0, m1/2, A0, tan(b), sgn(m)). • R-Parity assumed to be conserved. • Exclusive studies use benchmark points in mSUGRA parameter space: • LHCC Points 1-6; • Post-LEP benchmarks (Battaglia et al.); • Snowmass Points and Slopes (SPS); • etc… 33

  34. SUSY & Dark Matter mSUGRA A0=0, tan(b) = 10, m>0 Ellis et al. hep-ph/0303043 Disfavoured by BR (b  s) = (3.2  0.5)  10-4 (CLEO, BELLE) Universe Over-Closed ~ c01 t ~ c01 l ~ t1 ~ ~ t1 g/Z/h lR ~ c01 l 0.094    h2  0.129 (WMAP-1year) • R-Parity Rp = (-1)3B+2S+L • Conservation of Rp (motivated e.g. by string models) attractive • e.g. protects proton from rapid decay via SUSY states • Causes Lightest SUSY Particle (LSP) to be absolutely stable • LSP neutral/weakly interacting to escape astroparticle bounds on anomalous heavy elements. • Naturally provides solution to dark matter problem • R-Parity violating models still possible  not covered here. 34

  35. Stop Mass mtbmax= (443.2 ± 7.4stat) GeV Expected = 459 GeV ~ ~ ~ ~ ~ ~ 120 fb-1 • Look at edge in tb mass distribution. • Contains contributions from • gtt1tbc+1 • gbb1btc+1 • SUSY backgrounds • Measures weighted mean of end-points • Require m(jj) ~ m(W), m(jjb) ~ m(t) ATLAS LHCC Pt 5 (tan(b)=10) mtbmax= (510.6 ± 5.4stat) GeV Expected = 543 GeV • Subtract sidebands from m(jj) distribution • Can use similar approach with gtt1ttc0i • Di-top selection with sideband subtraction • Also use ‘standard’ bbll analyses (previous slide) 120 fb-1 ~ ~ ~ ATLAS LHCC Pt 5 (tan(b)=10) 35

  36. Preparations for 1st Physics • Preparations needed to ensure efficient/reliable searches for/measurements of SUSY particles in timely manner: • Initial calibrations (energy scales, resolutions, efficiencies etc.); • Minimisation of poorly estimated SM backgrounds; • Estimation of remaining SM backgrounds; • Development of useful tools. • Definition of prescale (calibration) trigger strategy • Different situation to Run II (no previous s measurements at same Ös) • Will need convincing bckgrnd. estimate with little data as possible. • Background estimation techniques will change depending on integrated lumi. • Ditto optimum search channels & cuts. • Aim to use combination of • Fast-sim; • Full-sim; • Estimations from data. • Use comparison between different techniques to validate estimates and build confidence in (blind) analysis. 36

  37. Black Hole Signatures In large ED (ADD) scenario, when impact parameter smaller than Schwartzschild radius Black Hole produced with potentially large x-sec (~100 pb). Decays democratically through Black Body radiation of SM states – Boltzmann energy distribution. Mp=1TeV, n=2, MBH = 6.1TeV ATLAS w/o pile-up ATLAS w/o pile-up • Discovery potential (preliminary) • Mp < ~4 TeV  < ~ 1 day • Mp < ~6 TeV  < ~ 1 year • Studies continue … 37

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  40. Sbottom/Gluino Mass p p ~ c01 ~ ~ ~ ~ b c02 g lR b b l l ~ ~ m(g)-0.99m(c01) = (500.0 ± 6.4) GeV 300 fb-1 ATLAS • Following measurement of squark, slepton and neutralino masses move up decay chain and study alternative chains. • One possibility: require b-tagged jet in addition to dileptons. • Give sensitivity to sbottom mass (actually two peaks) and gluino mass. • Problem with large error on input c01 mass remains g reconstruct difference of gluino and sbottom masses. • Allows separation of b1 and b2 with 300 fb-1. SPS1a ~ ~ ~ m(g)-m(b1) = (103.3 ± 1.8) GeV ATLAS ~ ~ ~ ~ m(g)-m(b2) = (70.6 ± 2.6) GeV 300 fb-1 SPS1a 40

  41. RH Squark Mass ~ ~ c01 qR q ~ ~ ~ • Right handed squarks difficult as rarely decay via ‘standard’ c02 chain • Typically BR (qRgc01q) > 99%. • Instead search for events with 2 hard jets and lots of ETmiss. • Reconstruct mass using ‘stransverse mass’ (Allanach et al.): mT22 = min [max{mT2(pTj(1),qTc(1);mc),mT2(pTj(2),qTc(2);mc)}] • Needs c01 mass measurement as input. • Also works for sleptons. ~ qTc(1)+qTc(2)=ETmiss ATLAS ATLAS 30 fb-1 100 fb-1 30 fb-1 Right squark SPS1a ATLAS SPS1a SPS1a Right squark Left slepton Precision ~ 3% 41

  42. Physics Strategy • December 2007(?): 900 GeV calibration run • commence tuning trigger menus / in situ calibration • Summer 2008: first 14 TeV physics run (L < 1032 cm-2 s-1,Lint ~ 1 fb-1) • commence tuning trigger menus / in situ calibration • First SM measurements: min bias, PDF constraints, Z / W / top / QCD • 2008/9: physics run (L ~ 2x1033 cm-2 s-1, Lint ~ 10 fb-1) • First B-physics measurements & rare decay searches (e.g. Bs→J/yf) • First searches: high mass dilepton / Z’, inclusive SUSY, Black Hole production, Higgs in ‘easier’ channels e.g. H→4l • 2009/10: physics run (L ~ 2x1033 cm-2 s-1, Lint = 10 fb-1/year) • First precision SM & B-physics measurements (systematics under control) • Improved searches sensitivity • Light Higgs searches (ttH, H→gg, VBF qqH(H→tt) etc.) • 2010/11: High luminosity running (L ~1034, Lint = 100 fb-1/year) • High precision measurements of New Physics (e.g. Higgs/SUSY/ED properties) 42

  43. Inclusive SUSY Jets + ETmiss + 0 leptons ATLAS • First SUSY parameter to be measured may be mass scale: • Defined as weighted mean of masses of initial sparticles. • Calculate distribution of 'effective mass' variable defined as scalar sum of masses of all jets (or four hardest) and ETmiss: Meff=S|pTi| + ETmiss. • Distribution peaked at ~ twice SUSY mass scale for signal events. • Pseudo 'model-independent' measurement. • With PS typical measurement error (syst+stat) ~10% for mSUGRA models for 10 fb-1 , errors much greater with ME calculation  an important lesson … 10 fb-1 43

  44. SUSY Spin Measurement Measure Angle Point 5 Spin-0 Spin-½ Straightline distn Polarise (phase-space) Spin-½, mostly wino Spin-0 Spin-½, mostly bino • Q: How do we know that a SUSY signal is really due to SUSY? • Other models (e.g. UED) can mimic SUSY mass spectrum • A: Measure spin of new particles. • One possibility – use ‘standard’ two-body slepton decay chain • charge asymmetry of lq pairs measures spin of c02 • relies on valence quark contribution to pdf of proton (C asymmetry) • shape of dilepton invariant mass spectrum measures slepton spin ~ Point 5 ATLAS 150 fb -1 mlq spin-0=flat 150 fb -1 ATLAS 44

  45. Processing the Data Detectors 16 Million channels 40 MHz 3 Gigacell COLLISION RATE buffers Charge Time Pattern 100 kHz LEVEL - 1 TRIGGER Energy Tracks 1 MegaByte EVENT DATA 1 Terabit/s 200 GigaByte BUFFERS (50000 DATA CHANNELS) 500 Readout memories EVENT BUILDER 500 Gigabit/s Networks 20 TeraIPS EVENT FILTER Gigabit/s PetaByte Grid Computing Service ARCHIVE SERVICE LAN 300 TeraIPS • Many events • ~109 events/experiment/year • ~3 MB/event raw data • several passes required • Worldwide LHC computing requirement (2007): • 100 Million SPECint2000 (=20,000 of today’s fastest processors) • 12-14 PetaBytes of data per year (=100,000 of today’s highest capacity HDD). • Understand/interpret data via numerically intensive simulations • e.g. 1 event (ATLAS Monte Carlo Simulation) ~20 mins/3 MB • Use worldwide computing Grid to solve problem 45

  46. ATLAS 25 ns Event rate in ATLAS : N = L x (pp)  109 interactions/s Mostly soft ( low pT ) events • Interesting hard (high-pT ) events are rare • Each accompanied by 20 soft events Also additional soft interactions (UE) 46

  47. Background Estimation Jets + ETmiss + 0 leptons ATLAS • Inclusive signature: jets + n leptons + ETmiss • Main backgrounds: • Z + n jets • W + n jets • QCD • ttbar • Greatest discrimination power from ETmiss (R-Parity conserving models) • Generic approach to background estimation: • Select low ETmiss background calibration samples; • Extrapolate into high ETmiss signal region. • Used by CDF / D0 • Extrapolation is non-trivial. • Must find variables uncorrelated with ETmiss • Several approaches being developed. 10 fb-1 47

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