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Supersymmetry at ATLAS

Supersymmetry at ATLAS. Dan Tovey University of Sheffield. ATLAS. Length: ~45 m Radius: ~12 m Weight: ~7000 tons. SUSY @ ATLAS. ~. c 0 1. Many motivations for low-scale SUSY: Gauge hierarchy problem Higgs mass Coupling constant unification Dark Matter candidate

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Supersymmetry at ATLAS

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  1. Supersymmetryat ATLAS Dan Tovey University of Sheffield 1

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

  3. 3

  4. SUSY @ ATLAS ~ c01 • Many motivations for low-scale SUSY: • Gauge hierarchy problem • Higgs mass • Coupling constant unification • Dark Matter candidate • Focus here on R-Parity Conserving models • Dark Matter Candidate  ETmiss signature • NB ETmiss signature valid for any DM candidate (not just SUSY) MET ~ c01 4

  5. ATLAS SUSY Strategy SUSY studies at ATLAS will proceed in three stages: • SUSY Discovery phase • Inclusive Studies • comparison of significance in inclusive channels, • measurement of SUSY Mass Scale. • Exclusive studies • model-dependent interpretation (e.g. mSUGRA DM), • less model-dependent DM, • universalities / flavour physics, • spin measurement (is it SUSY?), • …. 5

  6. Stage 1: SUSY Discovery 6

  7. 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. 7

  8. 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 8

  9. Monte Carlo Studies Jets + ETmiss + 0 leptons Meff=S|pTi| + ETmiss ATLAS 10 fb-1 • Monte Carlo background estimates subject to significant systematics • Original studies used Parton Shower • Recent studies use ALPGEN MPFS generator + MLM matching to PS • Re-assessment of significances Meff=S|pTi| + ETmiss 9

  10. Background Estimation One lepton mode BG @ 1fb-1 • Blue: tt(lnln) • Green: tt(lnqq) • Red: w • Black: sum BG • Pink:SU3 Preliminary • 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 background calibration samples (control region); • Extrapolate into high ETmiss signal region. • Used by CDF / D0 • Extrapolation is non-trivial. • Must find variables uncorrelated with ETmiss • Several approaches being developed. ATLAS Meff=S|pTi| + ETmiss Effective mass (GeV) Jets + ETmiss + 1 lepton 10 fb-1 Meff=S|pTi| + ETmiss 10

  11. W/Z+Jets Background (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 11

  12. Top Background • Jets + ETmiss + 1 lepton: cut hard on mT(ETmiss, l) • Rejects bulk of ttbar (semi-leptonic decays), W g ln + n jets • Remaining background mostly fully leptonic top decays (ttbblnln, ttbbtnln, ttbbtntn) with one top missed (e.g. hadronic tau) ATLAS Preliminary Edge at W Mass • tt(lnln) • tt(lnqq) • w+jet • sum of all BG • Pink:SU3 12

  13. Top Background Control region Signal region Signal region ATLAS ATLAS Preliminary Preliminary • One possibility  use mT(W) • Divide mT into signal region + control region (mT<100GeV) • Use shape of control distribution for ETmiss or Effective Mass • normalization factor determined in the low ETmiss region (100 - 200GeV) Missing ET (GeV) Transverse Mass (GeV) Control region ATLAS Preliminary • tt(lnln) • tt(lnqq) • w • sum BG • SU3 Missing ET (GeV) normalization region 13

  14. mT Method @1fb-1 @1fb-1 real BG estimated BG real BG estimated BG >300GeV 9.5±1.1 8.6±0.6 >800GeV 24.8±1.6 22.0±0.9 • With cut mT > 100 GeV, BG well reproduced in absence of SUSY signal (statistical error ~ 10%) • With SUSY signal estimated BG increased byfactor ~2.5. • Rejection of SUSY contamination next issue ATLAS ATLAS Preliminary Preliminary Meff=S|pTi| + ETmiss Effective Mass (GeV) Missing ET (GeV) With SU3 signal >800GeV 24.8±1.6 60.8±2.5 ATLAS Preliminary Effective Mass (GeV) 14

  15. QCD Background • Two main sources: • fake ETmiss (gaps in acceptance, dead/hot cells, non-gaussian tails etc.) • real ETmiss (neutrinos from b/c quark decays) • Much harder with MC : simulations require detailed understanding of detector performance • Strategy: • Initially choose channels which minimise contribution until well understood (e.g. jets + ETmiss + 1l) • Reject events where fake ETmiss likely: beam-gas and machine background, bad primary vertex, hot cells, CR muons, ETmiss phi correlations (jet fluctuations), jets pointing at regions of poor response … • Cut hard to minimise contribution to background (at expense of stats). • Estimate background using data Pythia dijets SUSY SU3 15

  16. QCD Background jets MET • Work in Progress: several ideas under study • Step 1: Measure jet smearing function from data • Select events: ETmiss > 100 GeV, Df(ETmiss, jet) < 0.1 • Estimate pT of jet closest to EtMiss as pTtrue-est = pTjet + ETMiss • Step 2: Smear low ETmiss multijet events with measured smearing function fluctuating jet Njets >= 4, pT(j1,j2) >100GeV, pT(j3,j4) > 50GeV 22pb-1 ATLAS ATLAS Preliminary Preliminary All QCD Znn SU3 Estimate (QCD)‏ ETmiss 16

  17. Stage 2: Inclusive Studies 17

  18. Inclusive Studies 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: 18

  19. Inclusive Studies Jets + ETmiss + 0 leptons • 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. • Typical measurement error (syst+stat) ~10% for mSUGRA models for 10 fb-1 (PS). ATLAS 10 fb-1 Jets + ETmiss + 1 lepton 19

  20. Stage 3: Exclusive studies 20

  21. 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. 21

  22. 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 22

  23. 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 23

  24. ‘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) 24

  25. 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. 25

  26. 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 26

  27. 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) 27

  28. 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 28

  29. 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 29

  30. 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 et al., 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 30

  31. 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 31

  32. 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 proposal – 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 32

  33. Supersummary The LHC will be THE PLACE to search for, and hopefully study, SUSY from next year onwards at colliders (at least until ILC). SUSY searches (preparations) will commence on Day 1 of LHC operation. Many studies of exclusive channels already performed. Lots of input from both theorists (new ideas) and experimentalists (new techniques). Big challenge for discovery will be understanding systematics. Big effort now to understand how to exploit first data in timely fashion 33

  34. BACK-UP SLIDES 34

  35. 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) 35

  36. 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 36

  37. 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. 37

  38. SUSY @ ATLAS • LHC will be a 14 TeV proton-proton collider located inside the LEP tunnel at CERN. • Luminosity goals: • 10 fb-1 / year (first ~3 years) • 100 fb-1/year (subsequently). • First data this year! • Higgs & SUSY main goals. • Much preparatory work carried out historically by ATLAS • Summarised in Detector and Physics Performance TDR (1998/9). • Work continuing to ensure ready to test new ideas with first data. • Concentrate here on more recent work. 38

  39. 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… 39

  40. 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% 40

  41. Inclusive Studies LHC Point 5 (Physics TDR) • Following any discovery of SUSY next task will be to test broad features of model. • 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 41

  42. Decay Resimulation • Second approach: Decay Resimulation • Goal is to model complex background events using samples of tagged SM events. • Initially we will know: • a lot about decays of SM particles (e.g. W, top) • a reasonable amount about the (gaussian) performance of the detector. • rather little about PDFs, the hard process and UE. • Philosophy • Tag ‘seed’ events containing W/top • Reconstruct 4-momentum of W/top (x2 if e.g. ttbar) • Decay/hadronise with e.g. Pythia • Simulate decay products with atlfast(here) or fullsim • Remove original decay products from seed event • Merge new decay products with seed event (inc. ETmiss) • Perform standard SUSY analysis on merged event • Technique applicable to wide range of channels (not just SUSY) • Involves measurement of (top) pT: useful for exotic-top searches? 42

  43. Decay Resimulation Estimate 5201 m(lb) Zmm • Select seed events: • 2 leptons with pT>25GeV • 2 jets with pT>50GeV • ETmiss < <pT(l1),pT(l2)> (SUSY) • Both m(lb) < 155GeV • Solve 6 constraints for p(n) • Resimulate + merge with seed • Apply 1l SUSY cuts cut ATLAS Preliminary GeV ETmiss Sample 5201, pT(top) > 200 GeV reco 11.0.4205, 450 pb-1 ATLAS Pull (top pT) Preliminary >=2 jets, Normalisation to data under study ATLAS Preliminary 43

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