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Supersymmetry Phenomenology using Hadron Colliders

Supersymmetry Phenomenology using Hadron Colliders. Where to start looking?. Many different SUSY models available. Differ in symmetry breaking mechanisms. R-parity Consequences: SUSY particles produced in pairs. Lightest SUSY particle (LSP) must be neutral

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Supersymmetry Phenomenology using Hadron Colliders

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  1. SupersymmetryPhenomenology using Hadron Colliders SUSY, Phenomenology using Hadron Colliders

  2. Where to start looking? • Many different SUSY models available. • Differ in symmetry breaking mechanisms. • R-parity • Consequences: • SUSY particles produced in pairs. • Lightest SUSY particle (LSP) must be neutral and colourless (from cosmological constraints). • LSP is stable. • No theoretical argument requires R-parity. • Some models conserve, others violate R-parity. • Concentrate on simplest ones, try to find signatures which are universal. • But: limits usually only valid in context of specific model. large missing ET SUSY, Phenomenology using Hadron Colliders

  3. Supersymmetric Decay Cascades • Heavier supersymmetric particles decay in cascades ending in LSP. • Neutralinos & charginos: Typically 2 body decays when kinematically allowed, otherwise 3 body decay ( ) through virtual gauge bosons or sleptons/squarks. • Charginos (for example from ) can decay through with an isolated lepton in the final state. • Long decay chains → several high pT daughters. • Spherical events. • Gluino is Majorana fermion → can decay to either ℓ+ or ℓ-. Possibility to have same-charge decay chains on both sides. • Simplest signatures for SUSY: • Multiple jets (some of them hard) + missing ET. • Several leptons + missing ET. SUSY, Phenomenology using Hadron Colliders

  4. How to shape our expectation? Masses in SUGRA: • Predictions very dependent on SUSY models and parameters used. • Use different Monte Carlo generators (ISAJET, SPYTHIA). • Different approximations in the generators require careful tuning and comparison. • Slight variations can have dramatic change in behaviour (channels open up or close). • Typically multi-dimensional parameter space, hard to cover everything by simulation. → Select benchmark parameter sets (e.g. ‘ATLAS 1-5’) to allow estimate of the search capacity of future experiments. Different parameter sets SUSY, Phenomenology using Hadron Colliders

  5. minimal SUper GRAvity • SUSY breaking communicated through flavour-blind gravitational interactions. • 5 Parameters assuming unified masses & couplings at GUT scale: • Scalars have mass m0, • gauginos and higgsinos m1/2, • trilinear terms A0, • ratio of vacuum expectation values of Higgs doublets β (yields bilinear couplings and higgsino mass parameter μ2), • sign of the higgs mass term sign(μ). • Non-minimal: > 100 parameters. • LSP is neutralino or sneutrino. SUSY, Phenomenology using Hadron Colliders

  6. Chargino and Neutralino Production at Hadron Colliders • Indirectly: • Result of decay chain of heavier sparticles. • Directly: • Through EW couplings to squarks, g, W, Z. • s-channel gauge boson production • t-channel squark exchange • interference SUSY, Phenomenology using Hadron Colliders

  7. tt W→ℓ Z→, QCD jets How to establish signal? • ATLAS: First step: Take four hardest jets and calculate + cuts on minimum ET and pT, no isolated leptons. • 5-10 larger than SM. • Gives first indication of SUSY mass scale (where SUSY data exceeds SM backgrounds). • Simple. SUSY signal SM backgrounds SUSY, Phenomenology using Hadron Colliders

  8. Precision Measurements • Measurements of sparticle masses. • . • Select bb with mbbaround h mass, add hard jet in event → mbbj, depends on mq. • Endpoint of dilepton (same flavour) mass spectrum: measurement of mass difference. • Combination allows model independent way to establish sparticle masses. • After 1y ATLAS (10 fb-1) expect: ~ SUSY, Phenomenology using Hadron Colliders

  9. Tevatron tri-leptons • Final state: • Leptons are e, μ. • Low SM background: ‘Golden’ SUSY channel • Cuts: • 2e: pT> 15 GeV/c • 10 < Mee< 70 • MT(e,ET)>15 • Track isolation • ET> 15 GeV • D0 Run II (42pb-1): No events observed (0.0±1.4 expected). D0 SUSY, Phenomenology using Hadron Colliders

  10. Squarks and Gluinos • Produced through SU(3)C couplings to q and g. • Due to subsequent decays signatures like neutralinos and charginos, but with more jets. or or or • Final states depend on exact decay channels, but again typically involve ET and multiplicity of jets and/or leptons. • Cleanest: di-lepton (from chargino/neutralino decays), especially same-sign (possible in gluino decays as gluino is Majorana particle). D0 limits in m0/m1/2 plane for different SUGRA parameters SUSY, Phenomenology using Hadron Colliders

  11. Search for MSSM Stop • 3rd generation left-right mixing → stop can be light (accessible at Tevatron). • Production rate 10% of rate for t of same mass. • Signature: Di-lepton • Other possible stop decays: or with decay signatures bbℓ±jjET and bbjjjjET. SUSY, Phenomenology using Hadron Colliders

  12. High tanβ • For tanβ> 8 final state leptons dominated by t. • Large tanβis theoretically motivated & favoured by LEP2. • Tevatron standard trilepton search: • Improved t trigger and reconstruction in Run II. • ATLAS: reconstruct m (cuts on jetshape, isolation etc.), endpoint gives ∆m. SUSY, Phenomenology using Hadron Colliders

  13. Reach for SUSY signal at LHC • Final states: • Jets and missing ET (0l). • Missing ET and 1 lepton (1l). • Opposite sign leptons (OS). • Same charge leptons (SS). • Three leptons (3l). SUSY, Phenomenology using Hadron Colliders

  14. Gauge mediated symmetry breaking (GMSB) • Gauge interactions mediate SUSY breaking. • 6 fundamental parameters: • Number of equivalent messenger fields N5, • scale factor for gravitino mass CGrav, • tanβ, • sign(μ), • messenger mass Mm, • Ratio of SUSY breaking scale to messenger scale Λ. • LSP is gravitino with mass «1GeV (Unlike SUGRA, where ). • NLSP either neutralino (small N5) or slepton (large N5). • Small tanβ: slepton masses degenerate, large tanβ: lightest slepton. • Lifetime model-dependent (c from μm to km). SUSY, Phenomenology using Hadron Colliders

  15. GMSB with neutralino NLSP • Phenomenology as for SUGRA, but decay into lightest neutralino is followed by its subsequent decay yielding a photon and ET. • Production of pairs provides clear two-g signature (+ET). • SUSY masses can be determined from kinematics (combine same-flavour, opposite-charge leptons → mℓℓ, then pick smaller mℓℓ, and 2 mℓ distributions give 4 endpoints to determine 3 masses. • Decay length from Dalitz decays (2% of decays). Can be >1km for large Cgrav. SUSY, Phenomenology using Hadron Colliders

  16. GSMB search at Tevatron • Signature (for long lifetime): two non-pointing g + missing ET. • Backgrounds: jets and e faking photons. Messenger mass scale Run II: Run I: SUSY, Phenomenology using Hadron Colliders

  17. GSMB with slepton NLSP • Signature contains final state leptons & missing ET. • Dilepton mass spectrum has steps given by difference of slepton and neutralino mass. • N5>1, Cgrav = 5×103: NLSP is stau. Decay length 1km. Low velocity quasi-stable particles resemble muons: measure TOF in μ-detector. Study ATLAS slow SUSY, Phenomenology using Hadron Colliders

  18. Anomaly mediated Supersymmetry (AMSB) • Conformal anomaly in the auxiliary field of the supergravity multiplet transmits SUSY breaking. • NLSB ( ) only marginally heavier than → large ET + soft tracks. • Parameters: m3/2 (gravitino m), m0 (scalar m), tanβ & sign(μ). • 3 ranges: • long lived (c≥1m) → track in μ detector, similar to GMSB. • isolated high pT tracks, which stop in tracker. • most difficult (short c and soft SM particle, which is hard to distinguish from background). SUSY, Phenomenology using Hadron Colliders

  19. R-parity violation (RPV) • Can be broken into 3 distinct interaction terms with strengths λ, λ’ and λ”: • λ≠ 0: Nl violation in • λ’≠ 0: Nl violation in and • λ”≠ 0:NB violation in • To be consistent with proton lifetime: either lepton or baryon number violated. • Dilutes ET signature but λ andλ’ give multi-jet, multi-lepton events, which are easy to isolate. • Strategy: completely reconstruct LSP decay. SUSY, Phenomenology using Hadron Colliders

  20. SM bounds on RPV opeators • (→en)/(→μn) • Br(D+→K0*μ+nμ)/ Br(D+→K0*e+ne) • nμ deep-inelastic scattering • Br(t→ nt) • Heavy nucleon decay • n - n oscillations • Charged-current universality • (t→enn)/(m→enn) • Bound on the mass of ne • Neutrino-less double-beta decay • Atomic parity violation • D0-D0 mixing • Rℓ→ had(Z0)/ℓ(Z0) All in remarkable agreement with SM predictions. SUSY, Phenomenology using Hadron Colliders

  21. RPV with λ≠ 0 • >4ℓ Signature easy to detect. • Mass of neutralino from dilepton mass spectrum end point (LHC: σm ≈ 180MeV). • Combining candidates at edge with events in h→bb peak allow reconstruction of (LHC: σm ≈ 5GeV). Correct combinations End point Wrong combinations SUSY, Phenomenology using Hadron Colliders

  22. RPV with λ’ ≠ 0 • Fully reconstructable with dilepton signature. • More diffcult. • Missing ET is less than in SUGRA. • Rely on additional leptons from cascade decays and large jet multiplicity. • di-gluinos produce like-sign fermions in 1/8 of time (+2j) CDF Run I: no events SUSY, Phenomenology using Hadron Colliders

  23. Baryon number violating RPV • Most challenging, as decays like provide no signatures like missing ET, or special lepton or quark flavour (b) tags. • Look for dilepton signature from • Signature: minimum 6 jets (3 jets from other neutralino) + 2 leptons, typically around 12 jets (from cascade involving squarks and gluinos). • Then: combine triplets of jets and require two combinations/event within 20 GeV. Then combine with leptons and reconstruct decay fully. SUSY, Phenomenology using Hadron Colliders

  24. An indirect evidence for SUSY: H± • If light enough: produced in t→ bH+ • For small tanβ: H+ → cs, large tanβ: H+ → t+nt • CDF: • Direct search: excess over SM of events with t leptons • Indirect search: ‘dissappearance’, depletion of SM decay t → bW (less di-lepton and ℓ+j). SUSY, Phenomenology using Hadron Colliders

  25. CDF eeggET event • One event with SUSY-like signature. • Probability that SM produced many orders < 1, but very difficult to determine ‘a posteriori’. • Possibly: or with subsequent decay to and • But: If model parameters get adjusted to explain event, then very large rate of multijet + multilepton + photon(s) expected. SUSY, Phenomenology using Hadron Colliders

  26. Summary of current SUSY analyses SUSY, Phenomenology using Hadron Colliders

  27. Summary • SUSY completely uncharted territory. • Still try to find signatures which we understand well, and which are untypical for SM • Large ET. • High jet multiplicities. • (Like-sign) multi-leptons. • All limits are highly model-dependent. Comparison of results difficult. • Almost all the searches introduced in this lecture establish SUSY signal through mass measurements. Beyond these measurements of branching rations, spins etc. will be needed. • Search so far unsuccessful, but so far luminosity limited. Just entering interesting regions (Tevatron Run II, LHC). SUSY, Phenomenology using Hadron Colliders

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