SUSY Phenomenology & Experimental searches

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1 SUSY Phenomenology & Experimental searches Alex Tapper Slides available at:

2 Reminder Supersymmetry is a theory which postulates a new symmetry between fermions and bosons Best studied extension to the Standard Model with vast literature Has the potential to solve some of the most serious problems in the Standard Model quite naturally Now how we search for Supersymmetry at colliders? Graduate lectures, December Page 2

3 Outline What s the strategy? Detailed example of hadronic search What if we find something? Next steps Graduate lectures, December Page 3

4 Reading list Vast literature on Supersymmetry Latest results from the LHC ATLAS SUSY group SupersymmetryPublicResults CMS SUSY group PhysicsResultsSUS Check any day on hep-ex to see latest papers Graduate lectures, December Page 4

5 SUSY search strategy Be as model independent as possible But the MSSM has > 100 parameters Need more constrained models Choose a set of benchmark points that are representative of a range of topologies and areas of phase space m 1/2 (GeV) MSUGRA, tanβ = 10, A 0 = 0, µ > τ 1 LSP 7 HM1 Br( χ 2 0 l l) > 0.15 HM2 LM6 LM5 LM2 LM4 LM1 LM LM8 m(e L )<m(χ 2 0 ) HM3 8 m χ = 103 GeV m(u L ) > m(g ) Br( χ 2 0 h 0 χ 1 0 ) > 0.5 Br( χ 2 0 Z 0 χ 1 0 ) > 0.5 m h = 122 GeV m h = 120 GeV HM4 m(t 1 ) < m(g ) m h = 114 GeV l Teva tron NO EWSB LM LM10 LM7 200 Points usually used CMSSM at low masses, just above the (LMx) m 0 (GeV) J. Phys. G: Nucl. Part. Phys. 34 (2006) Graduate lectures, December Page 5

6 SUSY search strategy Production Squark and gluino expected to dominate Strong production so high cross section Cross section depends only on masses Approx. independent of SUSY model Graduate lectures, December Page 6

SUSY search strategy Production Squark and gluino expected to dominate Strong production so high cross section Cross section depends only on masses Approx.

7 SUSY search strategy Production Squark and gluino expected to dominate Strong production so high cross section Cross section depends only on masses Approx. independent of SUSY model Decay Details of decay chain depend on SUSY model (mass spectra, branching ratios, etc.) Assume RP conserved decay to lightest SUSY particle (LSP) Assume squarks and gluinos are heavy long decay chains Signatures MET from LSPs, high-et jets and leptons from long decay chain Focus on robust and simple signatures Common to wide variety of models Background and detector performance define searches not models Graduate lectures, December Page 7

8 Backgrounds Physics Standard Model processes that give the same signatures as SUSY Rely on Monte Carlo predictions? measure in data Detector effects Detector noise, mis-measurements etc. that generate MET or extra jets Commissioning and calibration Beam related Beam-halo muons (and cosmic-ray muons), beam-gas events Data and simulation already measure in situ too Graduate lectures, December Page 8

9 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET Generic missing energy signatures Categorised by numbers of leptons and photons Many include jets strong production Graduate lectures, December Page 9

10 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET Very challenging due to large amount and wide range of backgrounds However most sensitive search for strongly produced SUSY CMS pursues several complementary strategies based on kinematics and detector understanding Extend to b, τ and top-tagged final states Graduate lectures, December Page 10

11 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET Lepton (electron or muon) requirement reduces background considerably Only ttbar and W+jets left Graduate lectures, December Page 11

12 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET Adding a second lepton (electron or muon) reduced W background Several techniques including opposite-sign opposite-flavour subtraction Shape information and mass edges Graduate lectures, December Page 12

13 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET A natural SUSY signature Very small Standard Model backgrounds Include all three generations of leptons and all cross channels Graduate lectures, December Page 13

14 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET Very clean events with very low Standard Model background Include all three generations of leptons and all combinations Search inclusively, on the Z peak, with and without MET Some striking Standard Model events observed Graduate lectures, December Page 14

15 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET Many gauge-mediated models predict photons in final state Di-photon searches dominated by QCD multijet and γ+jet backgrounds Graduate lectures, December Page 15

16 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET Many gauge mediated models predict photons in final state Lepton reduces QCD multijet and γ+jet backgrounds Graduate lectures, December Page 16

17 Search strategy (what and how?) 0-leptons 1-lepton OSDL SSDL 3 leptons 2-photons γ+lepton Jets + MET Single lepton + Jets + MET Opposite-sign di-lepton + jets + MET Same-sign dilepton + jets + MET Multi-lepton Di-photon + jet + MET Photon + lepton + MET RPV Exotic R-Parity violating searches Long-lived particles etc. Non-MET based searches R-parity conserving and exotic SUSY Examples are long-lived particles Graduate lectures, December Page 17

18 All-hadronic SUSY search SUSY particles produced strongly and decay through long cascade Search for excess of events with large MET (from LSP) and several hadronic jets Veto events with leptons Graduate lectures, December Page 18

19 All-hadronic SUSY search Simple (pre)selection At least two jets with ET>50 GeV and η <3.0 Veto events with an electron or muon PT>10 GeV Use energy sums based on jets More robust since you can put minimum ET cut HT scalar sum of jet ET MHT vector sum of jet ET Enhance SUSY-like processes ET of two highest ET jets > 100 GeV ηj1 <2.0 Look at simulation to see what processes form backgrounds to your signal Graduate lectures, December Page 19

All-hadronic SUSY search CMS-PAS-SUS-08-005 CMS-PAS-SUS-09-001 CMS-PAS-SUS-11-001 Phys. Lett. B698:196 (2011) arxiv:1109.

20 All-hadronic SUSY search CMS-PAS-SUS CMS-PAS-SUS CMS-PAS-SUS Phys. Lett. B698:196 (2011) arxiv: QCD is by far largest background Z-boson decays to neutrinos Top-pair production and W-boson decays Graduate lectures, December Page 20

21 Background from QCD QCD processes lead to di-jet events Gluon radiation gives >2 jets When perfectly measured no MET but... Not a perfect detector Semi-leptonic decays in jets (b and c quarks) Graduate lectures, December Page 21

energy measurements α T = E T j 2 M T j1 j 2 = E T j 2 / E T j1 2(1 cosδϕ)

22 All-hadronic search Phys. Rev. Lett. 101: (2008) jet LSP LSP jet jet jet A novel approach combining angular and energy measurements α T = E T j 2 M T j1 j 2 = E T j 2 / E T j1 2(1 cosδϕ) Perfectly balanced events have αt=0.5 Mis-measurement of either jet leads to lower values Graduate lectures, December Page 22

23 All-hadronic search Originally proposed for di-jet events generalised up to six jets Perfectly balanced events have αt=0.5 (cut at αt>0.55) Mis-measurement of either jet leads to lower values Graduate lectures, December Page 23

All-hadronic SUSY search Events 10 4 3 10 2 10 CMS Preliminary 2011-1 L dt = 1.

24 All-hadronic SUSY search Events CMS Preliminary L dt = 1.1 fb, s = 7 TeV Data Standard Model QCD MultiJet tt, W, Z + Jets LM4 LM6 Events / 25 GeV CMS Preliminary L dt = 1.1 fb, s = 7 TeV Data Standard Model QCD MultiJet tt, W, Z + Jets LM4 LM N Jets (GeV) H T Make cut on αt>0.55 QCD under control Look at other backgrounds Graduate lectures, December Page 24

25 Z-boson background Data-driven background estimate Find a control region in phase space where SM background dominates ν MET ν Use measurements in this region to infer SM background in signal region Z Example Z νν + jets irreducible background Replacement technique µ µ µ ν Z W γ Z ll + jets Strength: very clean Weakness: low statistics W lν + jets Strength: larger statistics Weakness: background from SM and SUSY γ + jets Strength: large statistics and clean at high ET Weakness: background at low ET, theoretical errors Graduate lectures, December Page 25

26 Z-boson background Select γ + 3 jets with Eγ>150 GeV Clean sample S/B>20 Remove photon from the event Recalculate MET Normalise with σ(z+jets)/σ(γ+jets) from MC or measurements CMS-PAS-SUS Graduate lectures, December Page 26

27 Results Graduate lectures, December Page 27

28 Candidate event Graduate lectures, December Page 28

29 Limit in the CMSSM Graduate lectures, December Page 29

Simplified Models (GeV) m χ 0 pp q q, q q χ 0 ; m( g)>>m( q ) 1200 1000 800 600 400 200 CMS Preliminary -1 s = 7 TeV L=1.1 fb α T 400 600 800 1000 1200 m q (GeV) 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.

1 fb α T σ σ σ prod prod prod NLO-QCD NLO-QCD σ NLO-QCD = σ = 3 = 1/3 σ 400 600 800 1000 1200 m q (GeV) 10 1-1 10-2 10 ) s 95% CL upper limit on σ (pb) (CL (GeV) m χ 0 pp g g, g q q χ 0 ; m( q)>>m( g

30 Simplified Models (GeV) m χ 0 pp q q, q q χ 0 ; m( g)>>m( q ) CMS Preliminary -1 s = 7 TeV L=1.1 fb α T m q (GeV) A ε (GeV) m LSP pp q q, q q + LSP; m( g)>>m( q ) CMS Preliminary -1 s = 7 TeV L=1.1 fb α T σ σ σ prod prod prod NLO-QCD NLO-QCD σ NLO-QCD = σ = 3 = 1/3 σ m q (GeV) ) s 95% CL upper limit on σ (pb) (CL (GeV) m χ 0 pp g g, g q q χ 0 ; m( q)>>m( g ) CMS Preliminary -1 s = 7 TeV L=1.1 fb α T A ε (GeV) m χ 0 pp g g, g q q χ 0 ; m( q)>>m( g ) CMS Preliminary -1 s = 7 TeV L=1.1 fb α T σ σ σ prod prod prod NLO-QCD NLO-QCD σ NLO-QCD = σ = 3 = 1/3 σ ) s σ (pb) (CL 95% CL upper limit on m g (GeV) m g (GeV) Graduate lectures, December Page 30

31 Summary of limits ) 2 (GeV/c m 1/ CMS Preliminary τ = LSP 2011 Limits q (1250)GeV 2010 Limits tanβ = 10, A 0 q (1000)GeV q (750)GeV 1 Lepton = 0, µ > 0 MT2 SS Dilepton s = 7 TeV, Ldt Jets+MHT α T CDF 1 fb -1 g, q, tanβ=5, µ<0 D0 g, q, tanβ=3, µ<0 ± LEP2 χ LEP2 1 ± l g (1250)GeV -1 ) g (1000)GeV Razor (0.8 fb 300 OS Dilepton g (750)GeV 200 q (500)GeV Multi-Lepton (2.1 fb -1 ) g (500)GeV m 0 (GeV/c ) Graduate lectures, December Page 31

Mass determination example Two undetected LSPs per event No mass peaks Constraints from edges and endpoints in kinematic distributions Two-body Three-body Simplest example - many

32 Mass determination example Two undetected LSPs per event No mass peaks Constraints from edges and endpoints in kinematic distributions Two-body Three-body Simplest example - many others with endpoints, thresholds and other variables (MT2 and friends) Vast literature recommended review Barr & Lester arxiv: Graduate lectures, December Page 32

33 Mass determination example CMS PAS-SUS Fit ee, µµ and eµ distributions simultaneously Monte Carlo study for 200 pb 10 TeV ( pb 7 TeV) Di-leptonic end-point mll,max=51.3 ± 1.5 (stat.) ± 0.9 (syst.) GeV [52.7 GeV] Can also determine spins given enough data need upgraded LHC Graduate lectures, December Page 33

34 Where next? What constrains SUSY masses? First lecture reminder Superpartners could 10 TeV (scalars anywhere) Graduate lectures, December Page 34

35 Where next? What constrains SUSY masses? First lecture reminder WIMP Dark Matter Wino 3 TeV Graduate lectures, December Page 35

36 Where next? What constrains SUSY masses? First lecture reminder For 120 GeV Higgs need coloured top partners 400 GeV Graduate lectures, December Page 36

37 Where next? N. Implications of LHC results for TeV-scale physics Graduate lectures, December Page 37

38 Where next? Use parton luminosities to illustrate gain from 8 TeV to 14 TeV Higgs pp à H, Hà WW, ZZ and γγ mainly gg: factor 2 SUSY 3 rd Generation Mass scale 500 GeV qq and gg: factor 2 to 4 SUSY squarks/gluino 2.0 TeV Z 5.0 TeV SUSY Squarks/Gluino Mass scale 2.0 TeV qq,gg,qg: factor 6 to 10 Higgs 125 GeV SUSY 3 rd Gen 500 GeV Z Mass scale 5 TeV qq: factor 200 Graduate lectures, December Page 38

39 Summary Wide range of searches underway at the LHC Unfortunately no signs of Supersymmetry yet Tools in place to measure masses and spins of any discovery The next few years should be very exciting! Graduate lectures, December Page 39

Collider Physics Analysis Procedures

Collider Physics Analysis Procedures Alex Tapper Slides available at: http://www.hep.ph.ic.ac.uk/~tapper/lecture.html Aim Overview of analysis techniques at CMS Contrast with Tevatron (see DØ lecture)