Susy

SUSY, short for supersymmetry, is a proposed extension of spacetime symmetry that relates bosons and fermions. In a supersymmetric theory, each Standard Model particle has a superpartner whose spin differs by 1/2: fermions acquire bosonic partners and bosons acquire fermionic partners. If exact, their masses would be degenerate, but in phenomenologically viable models supersymmetry is broken, so superpartners are heavier and have not yet been observed.

Superpartners are typically named by adding suffixes or prefixes to the original particles: squarks, sleptons, gluinos, neutralinos, charginos, and gravitinos, among others. The minimal realistic realization is the Minimal Supersymmetric Standard Model (MSSM), which doubles the particle content and introduces soft supersymmetry-breaking terms to parameterize the breaking without reintroducing large quantum corrections. In many models, the lightest supersymmetric particle (LSP) is stable due to a conserved quantum number called R-parity, making it a dark matter candidate, commonly the lightest neutralino. The gravitino is another possible LSP in certain mediation scenarios.

Motivations for SUSY include addressing the hierarchy problem by stabilizing the Higgs mass against large quantum corrections, enabling more precise gauge coupling unification at high energy scales, and providing natural dark matter candidates. The mu problem and flavor/CP issues are active areas of model-building, leading to various extensions such as the NMSSM, which introduces an additional singlet field to help generate the Higgsino mass parameter dynamically.

Experimental status: searches at colliders, particularly the Large Hadron Collider, have not observed superpartners. Results place lower limits on masses and constrain production cross-sections, especially for colored superpartners like squarks and gluinos. Signatures often involve events with missing transverse energy and cascades ending in an LSP, which escapes detection.

SUSY remains a major framework for beyond-Standard Model physics, but despite extensive searches, its realization in nature has not been demonstrated. Ongoing and future experiments continue to probe higher mass scales and alternative mediation mechanisms.