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SUPERSYMMETRY (SUSY) (< .1%)

SUPERSYMMETRY (SUSY) (< .1%). Jonathan Nistor Purdue University. Who’s SUSY? – The basics. A symmetry relating elementary particles together in pairs whose respective spins differ by half a unit  superpartners Provides a pairing between fermions and bosons

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SUPERSYMMETRY (SUSY) (< .1%)

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  1. SUPERSYMMETRY (SUSY) (< .1%) JonathanNistor Purdue University

  2. Who’s SUSY? –The basics • A symmetry relating elementary particles together in pairs whose respective spins differ by half a unit  superpartners • Provides a pairing between fermions and bosons • A quantum symmetry of space-time (No classical analog!) • Supersymmetry algebra first discovered in late 1960s (most general extension of Poincare group) • Subsequently applied to “bosonic” string theory to incorporate fermionic patterns of vibration (1971) •  superstring theory is born • First applied to the field of Particle Physics by Julius Wess and Bruno Zumino (1973) • By early 1980’s, several supersymmetric SM had been proposed (MSSM)

  3. Motivation for SUSY in the SM • Allows for unification of the • couplings strengths at grand unification scale • Offers a good candidate for cold dark • matter (a bit more on this one later…) • Predicts light Higgs Boson • MSSM  mh≤ 135 GeV

  4. Motivation for SUSY - Higgs • SUSY stabilizes the quadratic divergences in the Higgs mass • Fermion/boson pairing leads to “cancellation” of similar Feynman loop diagrams • Same vertices • Same coupling constants • Amplitudes have “equal” magnitude • Opposite sign • SUSY is a broken symmetry – How broken? • sparticle masses must be < ~1 TeV to maintain • cancellations Higgs boson dissociating into a virtual fermion-antifermion pair Higgs boson dissociating into a virtual sfermion-antisfermion pair

  5. Minimal Supersymmetric SM (MSSM) • Double the number of particles? • Five Higgs bosons: • Postulate superpartner for each SM particle with identical coupling strengths • Must also distinguish between left-handed and right- handed fermions, why? • Drastically increases the parameter space! • 124 parameters • Solutions? Work with constrained models • cMSSM • mSUGRA! Down to only 5 parameters!

  6. R-Parity Production of pair of neutralinos • R-Parity – a multiplicative quantum number • R= +1 (SM particles) • R= –1 (SUSY particles) • R-Parity conservation – At every vertex the R-product must be + 1 • Implications of R-Parity conservation • Every SUSY interaction must involve two SUSY particles • SUSY particles created in pairs • a SUSY particle decays into another SUSY particle and SM particles • Lightest sparticle (LSP) cannot decay  WIMP • Good dark matter candidate ! R=(+1)(-1)(-1)

  7. Searches for SUSY • SUSY provides compelling arguments for investigations of the TeV scale • No evidence for sparticles has been found so far •  constraints on various models •  establishes lower bounds on the masses • The Large Hadron Collider (LHC) promises to explore directly TeV energy range. •  Low–Energy SUSY may be as risk • CDF detector in Tevatron Run II • Recent results on a search for gluino and squark production • New limits on the gluino and squark masses were established

  8. Search for Gluino and Squark production At the Fermilab Tevatron Collider • Experiment performed within the framework of mSUGRA • Assumed R-Parity consv. squark production Gluino production

  9. Search for Gluino and Squark production At the Fermilab Tevatron Collider Squark/gluino production:

  10. Search for Gluino and Squark production At the Fermilab Tevatron Collider Multijet-plus-ET Signature • If squarks much lighter than gluinos  squark-squark production enhanced  squark decay: dijet signature with missing ET • If gluinos lighter than squarks  gluino-gluino process dominates  Gluino decay: Large number of jets missing ET

  11. Search for Gluino and Squark production At the Fermilab Tevatron Collider • Results: • Observed events matched SM expected • events •  No significant deviation • Data provided exclusion limits on • gluino/squark production •  eg. Excluded gluino masses up to 280 • GeV for every squark mass

  12. Conclusions • SUSY, “the best motivated scenario today for physics beyond the SM?” • Many motivations for recasting of the SM into a SUSY framework • Currently no experimental evidence that nature obeys SUSY • Future prospects • LHC’s discovery potential extends up to squark/gluino masses of 2.5 -3 TeV • If nothing is found at LHC  Low-energy SUSY will lose most of its motivation No longer able to stabilize Higgs mass • On the other hand…

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