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The Quest for Supersymmetry Sabine Kraml (CERN, ÖAW APART)

Explore the Standard Model, the hierarchy problem, and the need for new physics. Learn about Supersymmetry (SUSY) and its role in understanding particle physics, dark matter, and the origin of the universe.

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The Quest for Supersymmetry Sabine Kraml (CERN, ÖAW APART)

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  1. The Quest for SupersymmetrySabine Kraml (CERN, ÖAW APART) Habilitationskolloquium 8 May 2007

  2. Outline • Introduction • The Standard Model of particle physics • The hierarchy problem and need for New Physics • Supersymmetry (SUSY) • What is SUSY • The minimal supersymmetic model • SUSY @ LHC • SUSY dark matter • Conclusions The Quest for Supersymmetry

  3. What is the world made of…. …. and what holds it together? The Quest for Supersymmetry

  4. The Standard Model (SM) of elementary particle physics Masses O(MeV) 175 GeV • Matter: 3 families of quarks and leptons, spin ½ fermions. • Forces mediated by spin 1 gauge bosons: g, Z0, W±, g • Gauge group: • SU(3)c x SU(2)L x U(1)Y Interactions described as local gauge theories mass- less ~100 GeV c.f. mass of proton ~ 1 GeV strong int., weak int., hypercharge Q=T3-Y/2 Arbitrary inclusion of masses spoils renormalizability  Generate masses through gauge-invariant dynamics  The Quest for Supersymmetry

  5. <Φ>≠0 The Standard Model (SM) of elementary particle physics • Matter: 3 families of quarks and leptons, spin ½ fermions. • Forces mediated by spin 1 gauge bosons: g, Z0, W± • Gauge group: • SU(3)c x SU(2)L x U(1)Y Higgs field Interaction with scalar background “Higgs” field breaks the symmetry at ~100 GeV to SU(3)c x U(1)em → generation of particle masses The Quest for Supersymmetry

  6. < 1973: theoretical foundations of the SM • renormalizability of SU(2)xU(1) with Higgs mech. for EWSB • asymptotic freedom, QCD as gauge theory of strong force • KM description of CP violation • Followed by [more than] 30 years of consolidation • experimental verification via discovery of • gauge bosons: gluon, W, Z (Europe) • matter fermions: charm, 3rd family (USA) • experimental precision measurements of • EW radiative corrections • running of the strong coupling as • CP violation in the 3rd generation • technical theoretical advances (higher-order calculations, ....) The Quest for Supersymmetry

  7. The development of particle physics has also led to • significant progress in astrophysics and cosmology, • in particular in our description of the Early Universe. E ~ 100 GeV ↔ T ~ 1015 K ↔ t ~ 10−10 s E~1MeV ↔ T~1010K ↔ t~1s after the Big Bang (Nucleosynthesis) The Quest for Supersymmetry

  8. The SM is tremendously successful; • it continues to survive all experimental tests The Quest for Supersymmetry

  9. Only missing piece: the Higgs!the particle most sought after … c2 fit of the Higgs boson mass from EW precision data as of Summer 2006 The Quest for Supersymmetry

  10. Only missing piece: the Higgs!the particle most sought after … • LEP Higgs search • e+e−→ ZH @ √s = 180-208 GeV • 2s evidence of a 115 GeV Higgs until 2000, but then LEP had to stop operation • Transformed into lower limit of mH > 114.4 GeV ALEPH Aleph, Delphi, L3 and Opal collaborations and the LEP Higgs Working Group The Quest for Supersymmetry

  11. The SM is tremendously successful. • Nevertheless it can’t be the ultimate theory! The Quest for Supersymmetry

  12. Grand Unified Theory ? • GUTs attempt to embed the SM gauge group SU(3)xSU(2)xU(1) into a larger simple group G with only one single gauge coupling constant g. • Moreover, the matter particles (quarks & leptons) should be combined into common multiplet representations of G. • Prediction: Unification of the strong, weak and electro-magnetic interactions into one single force g at MGUT. • NB: If MGUT is too low → problems with proton decay The Quest for Supersymmetry

  13. The Quest for Supersymmetry

  14. The hierarchy problem • To break the electroweak symmetry and give masses to the SM particles, some scalar background field must acquire a non-zero VEV. •  Elementary scalar “Higgs” boson of mass mH. However, • mH=O(mW) • dmH2≤ mH2 where L is the scale (=cut-off) up to which the theory is valid.  MGUT? MPlanck? The Quest for Supersymmetry

  15. Beyond the SM (BSM) • The need to stabilize the electroweak scale, dmH2 < mH2, lets us expect new physics at TeV energies • Besides, neutrino masses as well as the dark matter and the baryon asymmetry of the Universe provide concreteexperimental evidence for BSM physics. The search for this new physics is the genuine motivation to build the LHC The Quest for Supersymmetry

  16. Supersymmetry (SUSY) The Quest for Supersymmetry

  17. What is SUSY? • Supersymmetry (SUSY) is a symmetry between fermions and bosons. • The SUSY generator Q changes a fermion into a boson & vice versa • Extension of space-time to include anticommuting coordinates • xm→ (xm, qa)with{qa,qb} = eab • This combines the relativistic “external” symmetries (such as Lorentz invariance) with the “internal” symmetries such as weak isospin. • Actually the unique extension of the Poincare algebra * • * (the algebra of space-time translations, rotations and boosts) The Quest for Supersymmetry

  18. ... predicts a partner particle for every SM state The Quest for Supersymmetry

  19. ... predicts a partner particle for every SM state Minimal supersymmetric standard model (MSSM) gauge structure SU(3)xSU(2)xU(1) The Quest for Supersymmetry

  20. Compare: Antiparticles space-time symmetry (special relativity) doubling of the spectrum Superpartners space-time supersymmetry The Quest for Supersymmetry

  21. If SUSY were an exact symmetry, SM particles and their superpartners would have equal mass. This is obviously not the case (no superpartners found so far), so SUSY must be broken SUSY as a local gauge theory includes a spin-2 state, the graviton (!) and its superpartner the gravitino. The Quest for Supersymmetry

  22. Back to the hierarchy problem ... • In SUSY, every fermion has a bosonic partner (and vice versa) The Quest for Supersymmetry

  23. ... the SUSY solution + − XX XX solves the hierachy problem provided MSUSY < O(1) TeV ! XX The Quest for Supersymmetry

  24. Gauge coupling unification SM SUSY Again requires SUSY masses of < O(1) TeV! The Quest for Supersymmetry

  25. MSSM particle spectrum gauginos + higgsinos mix to 2 charginos c± 4 neutralinos c0 Lightest neutralino c01 = lightest SUSY particle (LSP) 2 Higgs doublets → 5 physical Higgs bosons: 3 neutral states: scalar h, H; pseudoscalar A 2 charged states: H+, H− R parity: symmetry under which SM particles are even while SUSY particles are odd. If RP is conserved, superpartners can only be produced in pairs and every spuperpartner will cascade-decay to the LSP, which is stable  dark matter candidate! The Quest for Supersymmetry

  26. SUSY breaking The Quest for Supersymmetry

  27. The Quest for Supersymmetry

  28. gluinos, squarks charginos, neutralinos, sleptons Minimal supergravity (mSUGRA) Universal boundary conditions @ GUT scale Heavy top effect, drives mH2 < 0 Radiative electroweak symmetry breaking! univ. gaugino mass univ. scalar mass The Quest for Supersymmetry

  29. A light Higgs tanb = v2/v1 The Quest for Supersymmetry

  30. The beauties of SUSY • Unique extension of relativistic symmetries • Solution to gauge hierarchy problem • Radiative EW symmetry breaking, light Higgs • Gauge coupling unification • Ingredient of string theories • Very rich collider phenomenology • Cold dark matter candidate • .... • .... The Quest for Supersymmetry

  31. SUSY @ the LHC The Quest for Supersymmetry

  32. Large Hadron Collider • New accelerator currently built at CERN, scheduled to go in operation this year • pp collisions at 14 TeV • Searches for Higgs and new physics beyond the Standard Model • „discovery machine“, • typ. precisions O(few%) The Quest for Supersymmetry

  33. The LHC machine and experiments 100m underground 27km circumference pp collisions at 14 TeV High Energy factor 7 increase w.r.t. present accelerators High Luminosity (#events/cross section/time)  factor 100 increase 108 pp collisions per second, bunch spacing 24.95 ns event size 1 MB, storage rate 1 Hz, data to tape: 106 GB/yr The Quest for Supersymmetry

  34. The Quest for Supersymmetry

  35. q jet q jet 02 jets, l+l− Z 01 missing energy SUSY searches at LHC Largecross sections ~100 events/day for M ~ 1 TeV Spectacular signatures  SUSY could be found early on Cascade decays into LSP lead to typical signature: multi-jets / multi-leptons plus large missing energy From Meff peak first+fast measurement of SUSY mass scale to  20% (ca 10 fb-1) The Quest for Supersymmetry

  36. Compare with Higgs search The Quest for Supersymmetry

  37. Mass measurements: cascade decays ETmiss → no peaks → mass reconstruction through kinematic endpoints Typical precisions: a few % [ATLAS, G. Polesello] The Quest for Supersymmetry

  38. If TeV-scale SUSY is realized in Nature, • the LHC will discover a wealth of new states: • the superpartner world! would also revolutionize our understanding of space-time The Quest for Supersymmetry

  39. SUSY dark matter The Quest for Supersymmetry

  40. strong evidence for DM: large-scale structures rotation curves CMB dark matter from BSM? WMAP+SDSS: WCDMh2 = 0.105 ± 0.004 [astro-ph/0608632] multipole moment (l) 0.094 < WCDMh2 < 0.136 (95% CL) [astro-ph/0611582] The Quest for Supersymmetry

  41. WIMPs (weakly interacting massive particles) • Dark matter (DM) should be stable, electrically neutral, • weakly and gravitationally interacting • WIMPs are predicted by most BSM theories • Stable as result of new discrete symmetries • Thermal relic of the Big Bang • Testable at colliders! Neutralino, gravitino, axino, lightest KK state, T-odd little Higgs, etc., ... A neutralino LSP would indeed be an excellent dark matter candidate BSM-DM The Quest for Supersymmetry

  42. Relic density of WIMPs (weakly interacting massive particles) • Early Universe dense and hot; WIMPs in thermal equilibrium • Universe expands and cools; WIMP density is reduced through pair annihilation; Boltzmann suppression: n~e-m/T • Temperature and density too low for WIMP annihilation to keep up with expansion rate → freeze out Final dark matter density: Wh2 ~ sv−1 Thermally avaraged annihilation cross section The Quest for Supersymmetry

  43. Neutralino relic density c0 LSP as thermal relic: relic density computed as thermally avaraged cross section of all annihilation channels → Wh2 ~ sv −1 0.094 < Wh2 < 0.135 puts strong bounds on the parameter space The Quest for Supersymmetry

  44. Neutralino relic density c0 LSP as thermal relic: relic density computed as thermally avaraged cross section of all annihilation channels → Wh2 ~ sv −1 mSUGRA 0.094 < Wh2 < 0.135 puts strong bounds on the parameter space The Quest for Supersymmetry

  45. q Prediction of sv  from colliders ~~ ~~ Recall LHC: large cross sections, ~100 gg, gq,... events/day jet q jet 02 jets, l+l− Z 01 Abundant production of our DM candidate missing energy  LHC as „DM factory“ The Quest for Supersymmetry

  46. Prediction of sv  from colliders: What do we need to measure? • LSP mass and decomposition • bino, wino, higgsino admixture • Sfermion masses (bulk, coannhilation) • or at least lower limits on them • Higgs masses and widths: h,H,A • tanb Required precisions investigated in, e.g. • Allanach et al, 2004; • Belanger, SK, Pukhov, 2005; • Baltz et al., 2006 LHC precision most likely not sufficient to match WMAP/PLANCK accuracies Need precision measuremants of O(‰) !  ILC: international e+e− linear collider  NB: determination of sv  also gives a prediction of (in)direct detection rates The Quest for Supersymmetry

  47. Direct detection ratesCheck by direct detection is indispensible to pin down the dark matter [H.Baer et al, hep-ph/0611387] The Quest for Supersymmetry

  48. Indirect searches:high energetic positrons or gamma rays from cc annihilation [P. Gondolo, hep-ph/0501134] The Quest for Supersymmetry

  49. There are exciting times ahead of us ! Higgs? SUSY? LHC 1 GeV ~ 1.3 * 1013 K The Quest for Supersymmetry

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