Supersymmetry at the LHC: Searches, Discovery Windows, and Expected Signatures

Transcription

1 Supersymmetry at the LHC: Searches, Discovery Windows, and Expected Signatures Darin Acosta representing ATLAS &

2 Outline Introduction to SUSY, LHC, and the Detectors Non-Higgs sparticle searches: Trigger Strategies msugra Inclusive squark/gluino searches Exclusive sparticle mass reconstruction neutralino sbottom, gluino GMSB stau (heavy lepton) search Radiative neutralino decay and lifetime Cascade reconstruction Summary 2

3 Minimal SuperSymmetry SUSY Symmetry between bosons and fermions Squarks/sleptons: q, l scalar counterparts to the fermions ± Charginos/neutralinos/gluinos:, χ, 12, χ1234,,, g fermion counterparts to SM gauge bosons At least two Higgs doublets (5 scalars): Avoids fine-tuning of SM, can lead to GUTs MSSM Usually consider R P (-1) 3(B-L)+2S conserved LSP is stable 15 new parameters msugra: Require SUSY to be a local symmetry Universal gravitational interactions break SUSY at scale F (1 11 GeV) 2 5 free parameters m : Common scalar mass m 1/2 : Common gaugino mass A : Common scalar trilinear coupling tan β : Ratio of v.e.v. of Higgs doublets Sign(µ) : sign of Higgsino mixing parameter χ χ 1 ± 2 2 χ1 Typically: M M M M g > M q > M χ d i d i d i af af af 3 ± hh,, AH,

4 Large Hadron Collider (LHC) CMS ATLAS R = 4.5 km E = 7 TeV Two proton rings housed in same tunnel as LEP Design luminosity: L = 1 34 cm 2 s 1 = 1 fb -1 /year (Pile up: 2 collisions/crossing) Start-up luminosity: L 1 33 cm 2 s 1 = 1 fb -1 /year Completion: mid 27 4

5 msugra Cross LHC Total cross section, q g m 1/2 (GeV) fb 1 TH 1 fb 8 1 fb 6 1 pb 4 1 pb 2 EX A =, tan β = 35, µ > m (GeV) Squark/gluino production dominates total SUSY cross-section for low to moderate m 1/2 Cross sections don t vary much with µ, tanβ 5

6 Tile CAL Detectors LAr CAL ATLAS toroids 4T solenoid 2T solenoid TRT and Si tracker muon Cu/Scin HCAL PbWO 4 ECAL Full Si tracker Compact Muon Solenoid (CMS) 6

7 SUSY Signatures Complex squark/gluino decay chains Many high-e T jets Heavy-flavor (τ and b, especially at large tanβ) Leptons From sleptons, charginos, W/Z, and b-jets Missing transverse energy (MET) From LSP and neutrinos from taus, sneutrinos Example: m = 1 GeV m 1/2 = 5 GeV tan β = 35 µ > A = CMS event simulation 7

8 Trigger Challenge Reduce 4 MHz bx rate (1 GHz pp) O(1 Hz) Inclusive Jet Rate (cone algorithm, R=.5): L= Full GEANT-based detector simulation on QCD background (used in preparation of CMS DAQ Technical Design Report) Expected MET Rate: Recon. MET (hi lumi) Recon. MET (low lumi) Gen. MET (hi lumi) Gen MET (low lumi) Requiring a rate to tape of a few Hz implies an inclusive single jet threshold of 4 6GeV, and an inclusive MET threshold of 1 2 GeV Reconstructed MET rate below 1 GeV mainly from calorimeter energy resolution 8

9 SUSY Trigger Exercise (CMS) Consider several points in the m -m 1/2 plane near the Tevatron reach (most difficult for LHC) Consider points with and without R p conservation For R p choose most difficult case: χ 1 3 j Run full GEANT-based detector simulation on SUSY signals and SM backgrounds to evaluate trigger performance Optimize efficiency for a rate to tape of 3 Hz 9

10 Example Trigger Strategy (CMS) Possible jet and MET triggers (at Level-2): MET >17 GeV (for L= ) 3 jets > 6 GeV and MET > 11 GeV 4 jets > 12 GeV 1 jet > 19 GeV, MET>9 GeV, and φ(j1,j2) < π.5 2 jets>4 GeV, MET>1 GeV, and φ(j1,j2) < π.5 4 jets>8 GeV, MET>6 GeV, and φ(j1,j2) < π.5 Efficiency for SUSY points: ε=.78,.74,.54,.38,.27, R 5R 6R 2 nd jet With R P 1 st jet Background rate of 3Hz dominated by QCD For completeness, inclusive lepton triggers are: P T (electron)>25 3 GeV, P T (muon)>2 GeV (for L= ) 1

11 Fast Simulation Full simulation of signals and backgrounds like that shown for trigger exercise is too CPU intensive for complete SUSY reach determination Require O(1 8 ) events, but full GEANT simulation takes tens of minutes per event Use physics generators + parameterized detector performance ATLFAST: Tracks (µ): P T /P T =.4P T 1% (P T in TeV) EM resolution: σ/e 1%/ E.3% (E in GeV) Jet resolution: σ/e 6%/ E 2% (E in GeV) CMSJET: Tracks (µ): P T /P T =.15P T.5% (P T in TeV) EM resolution: σ/e 5%/ E.5% (E in GeV) Jet resolution: σ/e 1%/ E 5% (E in GeV) 11

12 Inclusive q, g Search Counting excess events over SM background Discovery mode SUSY search at LHC Explicit mass reconstruction not done 6 Analyses: E miss T : Ol: 1l: 2lOS: 2lSS: 3l: CMS Study: jets+met, no lepton requirements no leptons 1 lepton 2 leptons, opposite sign 2 leptons, same sign 3 leptons Lepton identification Electron: P T >2 GeV, isolated, η <2.4 Muon: P T >1 GeV, isolated or not, η <2.4 Vary cuts in 6 categories (1 4 combinations) #Jets, MET, Jet E T, φ(l,met), Circ., µ Iso. Common cuts: MET>2 GeV, 2 jets, E T jet > 4 GeV, η <3 Optimize S/ (S+B) in a counting experiment Probe 5 (m, m 1/2 ) points 1 6 signal events, 1 8 QCD, tt, W/Z+jets Plot 5σ sensitivity contours 12

13 CMS q, g Reach m 1/2 (GeV) g (3) l + 1l + 2l OS miss E T L dt = 1 fb -1 A =, tan β = 35, µ > miss E T (3 fb -1 ) h(123) ISAJET CMSJET 4.5 g (25) 1 1l l 8 TH q (2) 2l OS 2l SS 3l g (2) q (25) 6 g (15) q (15) h 2 =.4 h 2 = 1 4 g (1) h 2 =.15 q (1) 2 q (5) h(11) g (5) EX m ( GeV) Jets+MET search gives greatest sensitivity Nucl. Phys. B547 (1999) 6 13

14 Jets+MET Reach vs. Luminosity m 1/2 (GeV) g (3) miss E T (1 fb -1 ) L dt = 1, 1, 1, 3 fb -1 A =, tan β= 35, µ > miss E T (3 fb -1 ) h(123) CMS g (25) 1 TH q (2) g (2) q (25) 8 E T miss (1 fb -1 ) 6 g (15) h 2 =.4 h 2 = 1 q (15) miss E T (1 fb -1 ) 4 g (1) h 2 =.15 q (1) 2 q (5) h(11) g (5) EX m ( GeV) Squarks/gluinos probed to 1.5 TeV with 1 fb -1 Up to 2.5 TeV at design luminosity (1 fb -1 ) 14

15 Other Parameter Choices ATLAS TDR 15 CERN/LHCC l l tan β = 1, µ < L=1 fb 1 6 OS SS 3l 4 2 2l,j 3l,j 8 6 l 1l tan β = 1, µ > SS OS 3l 4 2 2l,j 3l,j m (GeV) Similar cuts and optimization as for CMS study Sensitivity for lower tanβ also derived, but lower mass Higgs inconsistent with present limits 15

16 Exclusive Di-Lepton Reconstruction Measure invariant mass distribution of OS same flavor leptons as evidence for χ χ l + l or χ l l χ l + l χ 2 can be produced via Drell-Yan χ 1 ± χ2, q, g but more prevalent in cascade decays of Endpoint in mass spectrum exhibits sharp edge: Events/4 GeV/3 fb 1 M(l + l ) (GeV) SUSY e + e + µ + µ SM background This point selected by: χ χ l l 2 1 ATLAS Point 4 : m =8GeV m 1/2 =2GeV tanβ=1 µ> A = L=3 fb 1 + Some Z from other gauginos SM background is small Two OS leptons, P T >(2,1) GeV, η <2.5 MET>2 GeV, 4 jets: P T1 >1 GeV, P T 234 >5 GeV d i d i 3-body decay endpoint: mll max = m χ m 2 χ1 2-body: mll max 2 m m l m l m / = χ χ m l ch e d 2 i 2 ie 2 ch 2 d 1 i i ch 16

17 Exclusive b, g Reconstruction Completely reconstruct a SUSY decay chain: m χ l p p ATLAS Study Point 3 of TDR CMS Study g b b χ 2 ± m =2 GeV, m 1/2 =1 GeV, tanβ=2, µ<, A = Investigate Point B of Proposed Post-LEP Benchmarks for SUSY Eur.Phys.J.C22 (21) 535 m =1 GeV, m 1/2 =25 GeV, tanβ=1, µ>, A = mg = 595 GeV, mb = 496,524 GeV af c h LR, md χ i 2 = 174 GeV b l l 1 L=1 fb 1 CMS ± Start with reconstructing : Two OS isolated leptons, P T >15 GeV, η <2.4 MET>5 GeV χ 2 SUSY bkgnd + + M(e e ) + M(µ µ - ) 17

18 b Reconstruction sbottom reconstruction: Select window around di-lepton endpoint (16 GeV) χ 2 rest frame with χ1 at rest F v χ χ 2 md ii 1 v p d i= G1+ l l mdl lij pd + i H + K Can estimate md χ1 i from di - lepton endpoint and d i d i m χ 2m χ 2 1 but analysis not too sensitive to details Add most energetic b-jet to reconstruct b E b-jet >25 GeV, η <2.4 b-jet: 2 tracks with IP significance > 3σ Require MET>15 GeV Require E(ll)>1 GeV Reconstructed mass in reasonable agreement with input (48 vs. 51 GeV) L=1 fb 1 CMS Resolution <1% with assumption on LSP mass (can t resolve L/R mass splitting, however) Mass (GeV) 18

19 g Reconstruction Add another b-jet closest in φ to reconstruct g Reconstructed mass in reasonable agreement with input (585 vs. 595 GeV) CMS Point B L=1 fb 1 af ch d i mg mb is independent of m χ 1 : Mass (GeV) -1 2 ATLAS Point 3 Expect 2 GeV CMS Point B Events/4 GeV/1 fb 15 1 Expect 85 GeV M(χ 2 bb)-m(χ 2 b) GeV Cut around g mass 19 Mass (GeV)

20 Minimal GMSB Gauge Mediated Symmetry Breaking Uses SM gauge interactions instead of gravity to break SUSY Solves FCNC problem SUSY breaking scale much less than msugra scale F << 1 11 GeV Particles get mass from SM gauge interactions at a messenger scale M m O(1 TeV) << M Pl n = number of SU(5) messenger fields Λ = F / M m 1 TeV G is LSP ( m<< 1 GeV) NLSP: χ1 Gγ ( n= 1, low tan β) l Gl ( n> 1, high tan β) NLSP lifetime: cτ 13. m F HG 1 GeV M NLSP I KJ F H G 5 4 F 1 TeV cτ >> detector size slepton ( τ ) is a long-lived heavy lepton (like µ) neutralino leads to MET, like MSSM cτ detector size Measure NLSP lifetime Estimate F cτ << detector size radiative decay with γ I K J 21

21 GMSB Heavy Lepton (τ) Search Use drift-tube muon systems of ATLAS and CMS to measure time-of-flight for heavy leptons (σ 1ns) 2 CMS: 1/β momentum (GeV) Measure 1/β and p reconstruct mass Require 2 muons with P T >45 GeV, M>97 GeV η <1 for CMS drift-tube system Can measure stau mass from 9 7 GeV: particles / 2 GeV GeV; L=1/fb; eff=5% 33GeV; L=1/fb; eff=15% 636GeV; L=1/fb; eff=26% GMSB scenario: n=3, tanβ=45, Λ=5-3 TeV, M/Λ= reconstructed mass (GeV) CMS CR 1999/19 22

22 GMSB N 1 Lifetime Measurement Look for N 1 decays inside detectors: 1) Electromagnetic showers not pointing to vertex Use fine angular resolution from LAr EM calorimeter (ATLAS) and PbWO 4 crystals (CMS) ATLAS vertex resolution for H γγ ATLAS: if no non-pointing γ s in 3 fb -1 cτ > 1km (Λ=9 TeV, M = 5 TeV, n=1) Events/.2cm σ =1.33 cm z cal ō z true (cm) σ + (cτ)/cτ 2) Showers in muon system Identify showers with high hit multiplicity CMS: Overall sensitivity to measure cτ: 1 L=143/fb; eff kin =1% m(n 1 )=291GeV µ µ ECAL counting CAL counting µ ECAL/ CAL ECAL impact CAL slope COMBINED cτ (m) 23

23 GMSB Cascade Search (CMS) Consider the cascades N2 lr + l N1 + l + + l N1 lr + l G + l+ + l or (NLSP is neutralino) (NLSP is slepton) 1 Λ = 3 9 TeV M =12 3 TeV n=3,4,5 SM GMSB 1 GMSB 2 GMSB 3 GMSB 4 GMSB Λ=3-2 TeV, M=4-3 TeV, tanβ=1.5-55, signµ= ±1 25 Ñ1 Mass (GeV) Number of lepton pairs/1 fb-1 which will lead to a detectable sharp endpoint in the di-lepton invariant mass as in msugra d (ll) + (GeV/c) M e + e + µ + µ M µ e e+ µ ee+mm-em inv i edge>, M/ < 1.8 edge>, M/ > 1.8 eff=1 1 eff=.15 Î MET>2 GeV Î 2 OS same flavor leptons, PT>4 GeV 5 Î PT (jet4)>8 GeV ss s= Ñ1 1 Ma s Ma GMSB 1 GMSB 2 GMSB 3 Probe in 1 fb 1 : m τ1 < 35 GeV and a f mc N h < 5 GeV GMSB 4 GMSB τ 1 Mass (GeV)

24 LHC Summary Discovery of SUSY, if it exists, is almost assured at the LHC Inclusive msugra squark/gluino discovery reach to 1.5 TeV with 1 fb 1, 2.5 TeV with 1 fb 1 Difficult part will be untangling decay chains and measuring masses Possibility to reconstruct squark/gluino/neutralino decays in msugra and GMSB in several prototype analyses Trigger strategies identified for efficient coverage Ability exists to identify heavy leptons in GMSB scenarios, as well as NLSP lifetime in radiative decays Many more exhaustive SUSY studies at the LHC experiments are available: ATLAS TDR 15 CERN/LHCC CMS Note 1998/6 Looking forward to studying SUSY spectroscopy before the end of the decade! 25

25 Can msugra Escape LHC? M.Battaglia et al., Eur.Phys.J. C22 (21) 535 proposed several SUSY benchmark points in the post-lep era Two of them would lead to sparticles beyond the reach of the LHC except for a light Higgs squark/gluino masses > 2.5 TeV But most other points covered well If SUSY exists, prospects at LHC look favorable 2

26 R-Parity Violation Non-conservation of R P (-1) 3(B-L)+2S leads to 3 new terms in SUSY superpotential: W = λ L L E + λ QL D + λ U L D ijk i j k c ijk i j k c ijk i c j c k c Choose most challenging last case of baryon number violation: χ 1 3j ATLAS Study of Point 5 m =1 GeV, m 1/2 =3 GeV, tanβ=2, µ>, A =3 MET is reduced, but still substantial Number of jets increases Leptons from neutralino decays Should still be able to explore much of the parameter space as with msugra Events/1 GeV/3 fb Probability E miss (GeV) T N jet Figure 2-85 E miss T distribution for SUGRA Point 5 Figure 2-86 Total jet multiplicity ( p jet T > 15GeV ) in the case of R-parity conservation (shaded histogram) and R-parity violation (empty histogram). distribution for R-parity conservation (shaded) and R- parity violation at SUGRA Point 5. The jets are reconstructed using a topological algorithm based on joining neighbouring cells. 26