| Minimal Supersymmetric Standard Model | |
|---|---|
| Name | Minimal Supersymmetric Standard Model |
| Introduced | 1970s–1980s |
| Creators | P. Fayet, H. Georgi (early supersymmetry work) |
| Discipline | Particle physics |
| Subdiscipline | Quantum field theory |
| Related | Standard Model, Supersymmetry |
Minimal Supersymmetric Standard Model
The Minimal Supersymmetric Standard Model (MSSM) is a theoretical extension of the Standard Model that implements Supersymmetry (SUSY) in its minimal field content. It pairs each known elementary particle with a superpartner differing by half a unit of spin and introduces a second Higgs doublet, aiming to stabilize the electroweak scale and provide candidates for dark matter. The MSSM is central to many phenomenological studies in particle physics and tests of beyond-Standard-Model ideas at colliders such as the Large Hadron Collider.
The MSSM arises from attempts to embed the Standard Model within a more symmetric framework of Quantum field theory, motivated by hierarchy and naturalness problems. By promoting spacetime symmetries to include transformations between bosons and fermions, supersymmetry alters radiative corrections to scalar masses and offers a technically natural solution to the gauge hierarchy problem. It connects to foundational quantum concepts developed in Quantum mechanics and relativistic field theory and is studied in the context of unified theories like Grand Unified Theory and string constructions such as Superstring theory.
The MSSM implements N=1 global supersymmetry with the minimal set of supermultiplets required to reproduce Standard Model interactions. It preserves gauge symmetry groups SU(3)×SU(2)×U(1) and introduces chiral superfields for quarks, leptons, and two Higgs doublets (Hu and Hd). Motivations include cancellation of quadratic divergences (addressing the hierarchy problem), gauge coupling unification observed in renormalization group analyses (as in studies by H. Georgi and S. Dimopoulos), and embedding in higher frameworks like Minimal supergravity (mSUGRA) or Gauge-mediated supersymmetry breaking models. The MSSM also maintains compatibility with precision electroweak data when superpartner masses are sufficiently heavy.
The particle content extends Standard Model fields to supermultiplets: quark and lepton chiral superfields, Higgs superfields Hu and Hd, and gauge superfields for the strong and electroweak interactions. The MSSM superpotential includes Yukawa couplings analogous to the Standard Model plus a supersymmetric Higgs mass parameter μ. The full Lagrangian combines the supersymmetric kinetic and interaction terms with soft supersymmetry-breaking terms; it is organized using superspace formalism and superfields. Key named structures include the neutralino and chargino mixing matrices, squark and slepton mass matrices, and the Higgs sector described by two CP-even, one CP-odd, and charged Higgs bosons. The model connects to computations performed in frameworks like Renormalization group evolution and loop calculations typical of Quantum chromodynamics (QCD) and electroweak theory.
Because no superpartners are observed at low energy, supersymmetry must be broken. The MSSM employs soft SUSY-breaking terms that avoid reintroducing quadratic divergences: gaugino masses, scalar mass-squared parameters, and trilinear A-terms. Common mediation schemes include gravity mediation (e.g., mSUGRA / CMSSM), Gauge-mediated supersymmetry breaking (GMSB), and Anomaly-mediated supersymmetry breaking (AMSB). Each mechanism specifies boundary conditions at high scales (e.g., the Planck scale or GUT scale) and affects low-energy spectra via renormalization group running, impacting collider phenomenology and cosmological abundances.
The MSSM predicts superpartners: squarks, sleptons, gluinos, charginos, and neutralinos, and an extended Higgs sector including a light CP-even Higgs whose mass receives large radiative corrections primarily from top squark loops. Typical collider signatures include missing transverse energy from stable lightest supersymmetric particles (LSPs), multi-jet and multi-lepton final states from cascade decays, and resonances from heavy Higgs bosons. Searches at the Large Hadron Collider (ATLAS and CMS) have constrained large regions of the parameter space; dedicated analyses use tools like PYTHIA, MadGraph, and spectrum calculators such as SoftSUSY and SPheno.
A compelling feature is the natural dark matter candidate: a stable, neutral LSP such as the lightest neutralino in R-parity conserving versions of the MSSM. Thermal relic calculations connect to Big Bang nucleosynthesis and cosmic microwave background constraints measured by experiments like WMAP and Planck. Alternatives include gravitino or axino LSP scenarios in specific mediation schemes. The MSSM also affects baryogenesis mechanisms and may interplay with electroweak baryogenesis if the Higgs sector and stop spectrum allow a sufficiently strong first-order electroweak phase transition.
Experimental results from LEP, the Tevatron, and especially Run 1–3 of the Large Hadron Collider have placed stringent lower bounds on superpartner masses and constrained Higgs sector parameters; the observed 125 GeV Higgs boson at ATLAS and CMS is compatible with MSSM expectations but requires significant radiative corrections. Direct detection experiments like XENON, LUX-ZEPLIN and indirect searches by Fermi constrain neutralino dark matter. Precision measurements (e.g., muon g−2 at BNL and FNAL g−2) and flavour observables from LHCb further restrict viable MSSM parameter space. Ongoing theoretical work, experimental upgrades, and proposed colliders (e.g., the International Linear Collider or future circular colliders) continue to probe remaining regions consistent with naturalness and grand-unification motivations.
Category:Supersymmetric quantum field theories Category:Beyond the Standard Model physics