Jonathan Nistor Purdue University 1
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) super string 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) 2
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 m h ≤ 135 GeV 3
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 4
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! 5
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 ! Production of pair of neutralinos R=(+1)(-1)(-1) 6
7 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
Experiment performed within the framework of mSUGRA Assumed R-Parity consv. 8 At the Fermilab Tevatron Collider Gluino production squark production
Squark/gluino production: 9 At the Fermilab Tevatron Collider
Multijet-plus-E T Signature If squarks much lighter than gluinos squark-squark production enhanced squark decay: dijet signature with missing E T If gluinos lighter than squarks gluino-gluino process dominates Gluino decay: Large number of jets missing E T 10 At the Fermilab Tevatron Collider
11 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
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 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… 12