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The Search for Supersymmetry

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The Search for Supersymmetry The Search for Supersymmetry • Introduction the Standard Model of Particle Physics • Introduction to Collider Physics • Successes of the Standard Model • What the Standard Model Does Not Do • Physics Beyond the Standard Model • Introduction to Supersymmetry • Searching for Supersymmetry • Dark Matter Searches • Future Prospects • Other Scenarios • Summary Peter Krieger, Carleton University, August 2000 Force Unifications Standard Model does NOT account magnetism for gravitational interactions Maxwell electromagnetism electricity electroweak S T A Planck Scale (or Planck Mass) weak interactions N D A is defined as the energy scale at R GUT D which gravitational interactions M O become of the same strength as D E SM interactions strong interactions L TOE celestial movement gravitation Newton terrestrial movement - - - MEW MGUT Mplanck The Standard Model Describes the FUNDAMENTAL PARTICLES and their INTERACTIONS All known FORCES are mediated by PARTICLE EXCHANGE a a Effective strength of an interaction depends on X • the coupling strength at the vertex αα • the mass of the exchanged particle MX a a Force Effective Strength Process Strong 100 Nuclear binding Electromagnetic 10-2 Electron-nucleus binding Weak 10-5 Radioactive β decay The Standard Model SPIN-½ MATTER PARTICLES interact via the exchange of SPIN-1 BOSONS MATTER PARTICLES – three generations of quarks and leptons |Q| m < m < m ⎛ e ⎞ ⎛ µ ⎞ ⎛ τ ⎞ 1 e µ τ Mass increases with generation: ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ m = 0 ⎜ ⎟ ⎜ν ⎟ ⎜ ⎟ ν ⎝ν e ⎠ ⎝ µ ⎠ ⎝ν τ ⎠ 0 Mu,d ~ 0.3 GeV ⎛u⎞ ⎛c⎞ ⎛ t ⎞ 2 ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ 3 Each quark comes in 1 three ‘colour’ changes Mt ~ 170 GeV ⎝d⎠ ⎝s⎠ ⎝b⎠ 3 GAUGE BOSONS – mediate the interaction of the fundamental fermions γ 1 Gauge particle of electromagnetism (carries no electric charge) W ± ,Z 0 3 Gauge particles of the weak interaction (each carries weak charge) g 8 Gauge particles of the strong interaction (each gluon carries a colour and an anti-colour charge charge) All Standard Model fermions and gauge bosons have been experimentally observed There is one more particle in the SM – the Higgs Boson. This is vital to the SM but remains experimentally undetected Spontaneous Symmetry Breaking Initial Attempts to unify the weak and electromagnetic interactions failed because while M( γ ) = 0 , the relative strength of the weak interaction requires M(weak gauge boson) ~ 100 GeV The SM is formulated in terms of MASSLESS particles Introduction of explicit mass terms destroys a vital property of the theory In the SM, masses for the weak interaction gauge bosons are generated via SPONTANEOUS SYMMETRY BREAKING SSB may occur in systems where the equations describing the system have > < a SYMMETRY which is NOT (T-Tc) 0 (T-Tc) 0 OBEYED by the GROUND STATE Consider a ferromagnet at temperatures above and below the Curie point: Spins randomly oriented Spins aligned Rotational Symmetry Rotational symmetry broken Electroweak Unification In the Standard Model SSB is introduced ‘by hand’ by introducing new fields (4 degrees of freedom) 2 2 And and interaction potential V ( µ , λ ) with µ analogous to T-Tc Choosing µ 2 < 0 ⇒ Spontaneous Symmetry Breaking The 4 degrees of freedom • three masses (one each for W+, W-, Z0) • One physical spin-0 (scalar) particle (the Higgs Boson) The mass of the Higgs boson is NOT predicted by the theory M 2 The masses of the W and Z are related by the weak-mixing angle sin 2 ϑ = 1− Z W 2 M W The W and Z bosons were both discovered at CERN in 1984 (at the predicted masses) ! Design and construction of the Large Electron Positron Collider at CERN The LEP e + e − Collider at CERN LAKE GENEVA GENEVA POINT 8. CERN POINT 2. Two phase experimental CERN Prévessin program planned: POINT 6. POINT 4. DELPHI L3 SPS e - Electron e + Positron OPAL ALEPH LEP s R. Lewi jan. 1990 0 Phase 1 - precision measurements of the Z (Ecm= MZ) completed Phase 2 – precision measurements of the W (Ecm > 2MW) ends soon (Sept/2000) Most recent running has been at energies up to 209 GeV The OPAL Detector at LEP Electromagnetic calorimeters Run : even t 4093 : 1000 Date 930527 T ime 20716 C t rk( N = 3 9 S u m p = 7 3.3 ) E c a l ( N = 2 5 SumE= 32.6) Hcal (N=22 SumE= 22.6) Muon detectors Ebeam 45.658 Evis 99.9 Emiss -8.6 Vtx ( -0.07, 0.06, -0.80) M u o n ( N = 0 ) Sec Vtx(N= 3) Fdet(N= 0 SumE= 0.0) Bz=4 . 350 Thrus t=0 . 9873 Ap l an=0 . 0017 Ob l a t=0 . 0248 Spher=0 . 0073 Hadron calorimeters and return yoke Jet chamber Vertex detector Microvertex detector y Y Z chambers θ ϕ X Z Solenoid and Pressure Vessel z x 200. cm. 5 10 20 50 GeV Presamplers Cen t re o f sc reen i s ( 0 . 0000 , 0 . 0000 , 0 . 0000) Original forward detector Time of flight detector Silicon-tungsten forward detector e +e − → Z 0 → qq → hadron jets Some Basic Collider Physics How does one calculate the rate for some physics process at a collider ? M = sum of all contributing processes (diagrams) – here for e +e − → W + W − e- W- e- W- e- W- γ 0 Z ν + ..... ++e e+ W+ e+ W+ e+ W+ Define CROSS-SECTION σ ∝ | M | 2 (units of length2) ~ cross-sectional + − f •n • N e • N e size of the beams Define LUMINOSITY L ∝ bunch A • Instantaneous production rate N = L σ • Size of data samples typically quoted in ∫ L dt (pb-1) • Number of events in data sample given by N = σ ∫L dt Standard Model Electroweak Summary The Standard Model is very healthy (unfortunately ?) 80.6 6 ν theory uncertainty LEP1, SLD, N Data (5) − ∆α = Higgs mass from EW fit LEP2, pp Data had 0.02804±0.00065 % 80.5 68 CL 0.02784±0.00026 SMfit ] 4 M Higgs < 170 GeV @95% CL 2 GeV [ 80.4 ∆χ W m 2 LEP direct search limit 80.3 [ ] SM mH GeV M Higgs > 113.3 GeV @95% CL 113 300 1000 Preliminary Excluded Preliminary 80.2 0 2 3 130 150 170 190 210 10 10 10 [ ] [ ] mt GeV mH GeV Electroweak quantities affected by virtual particle loops e- f e- f t Z M 2 ZZ ZZ sin 2 1 Z ϑW = − 2 M W t- - H - e+ f e+ f 2 (Mt/MW) ln(MH/MW) The SM Does Not Do Everything Some examples: Quarks and leptons are unrelated fundamental What causes the hierarchy in the fermion fermions, yet their electric charges are related masses ? (these are free parameters in the by simple ratios. Why should this be the case ? SM) particle ν d u e Mu,d ~ 0.3 GeV Æ Mt ~ 170 GeV | Q | 0 1 2 1 3 3 What causes electroweak symmetry breaking ? (Higgs mechanism for SSB put in by hand) GRAND UNIFIED THEORIES attempt to provide answers by postulating that the strong and electroweak forces unify to a single force at some high energy scale (MGUT) ¾ there is a single force with coupling α GUT ¾ quarks and leptons are related ⎛ e − ⎞ The structure of the theory requires that ⎜ ⎟ the charge in this multiplet sum to 0 ⎜ν e ⎟ e − ⎛ ⎞ Q(ν ) + Q(e ) + 3Q(d) = 0 instead of ⎜ ⎟ we get ⎜ d ⎟ e ⎜ν ⎟ ⎜ r ⎟ ⎝ e ⎠ SUCCESS ! But there ⎜ dg ⎟ ⎜ ⎟ are problems too Colour indices: red, green, blue ⎝db ⎠ Running of Coupling Strengths Imagine measuring the electromagetic coupling strength via ee scattering α ff q = + ff+ + .... ff α Charge Screening – the observed charge depends on the energy scale quark confinement e- + - Asymptotic freedom e- e e e+ e+ - e- e+ e+ e- electromagnetic interaction (charge screening) e f α strong coupling + + γγ e e - e- e+ e weak coupling - f e electromagnetic coupling - - e e strong interaction(colour anti-screening) momentum transfer Q q γ gg ∼ 1 distance scale q + + Q e e + g gg 2 g momentum transfer q Some GUT Problems For a single force with a single coupling at MGUT expect α1 = α 2 = α 3 60 µ ( ) U(1) E.M. Force 0 + -1 α τ (p → π e ) 50 α-1 ( µ ) 1 u 0 40 π SU(2) Weak Force u Grand Unified# 30 α-1 ( µ ) u 2 (GUT) Scale + 20 SU(3) Strong Force d e α-1 ( µ ) 3 10 No Supersymmetry (+ other diagrams) 0 Inverse coupling constant 103 105 107 109 1011 1013 1015 1017 Energy Scale, µ [GeV] Quark-lepton relationship proton decay Simplest models predict τ ( p → π 0 e + ) ~ 10 28 − 1031 years Inconsistent with experimental results τ (p → π 0e+ ) > 4.4×1033 years @ 90%CL ( from SuperKamiokande) The Naturalness Problem Masses in SM get radiative corrections Mphysical = M0 +δM Naturalness (aesthetic criterion) requires O(Mphysical ) ≈ O(M0 ) ≈ O(δM) i.e. no fine tuning 2 2 This is not true for fundamental scalars (Higgs) for which (δM) ∝ Λ where Λ is the highest energy to which the theory remains valid So for MH < 1 TeV (SM bias) require either Λ ≈1TeV i.e. SM breaks down at 1 TeV, OR Some symmetry exists which can produces O(δM) ≈ O(M H ) independent of Λ Such a symmetry exists Supersymmetry Supersymmetry Each SM boson (fermion) has a fermionic (bosonic) supersymmetric partner with IDENTICAL MASS and Standard Model COUPLINGS ~ W ± W ± leptons sleptons 0 ~ 0 gauginos ⎛ e ⎞ ⎛ µ ⎞ ⎛ τ ⎞ ⎛ ~e ⎞ ⎛ µ~ ⎞ ⎛ τ~ ⎞ Z Z ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ↔ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ~ ⎜ ⎟ ⎜ν ⎟ ⎜ ⎟ ⎜ ~ ⎟ ⎜ν~ ⎟ ⎜ ~ ⎟ γ γ ⎝ν e ⎠ ⎝ µ ⎠ ⎝ν τ ⎠ ⎝ν e ⎠ ⎝ µ ⎠ ⎝ν τ ⎠ ~ 0 ~ 0 u c t u~ ~c t h h ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ~ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ↔ ⎜~⎟ ⎜~⎟ ⎜~⎟ H 0 H 0 higgsinos ⎝d⎠ ⎝s⎠ ⎝b⎠ ⎝d⎠ ⎝ s ⎠ ⎝b⎠ 0 ~ 0 quarks squarks A A ~ H ± H ± g ~g gluinos This defines the particle content of the Mass eigenstates are mixtures of Minimal Supersymmetric Standard Model gauginos and higgsinos or MSSM ~ ± 2 Charginos χ i=1,2 ~ 0 4 Neutralinos χ j=1,4 Supersymmetry is a Broken Symmetry Supersymmetry requires a doubling of the particle spectrum.
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