sneutrino

A sneutrino is the scalar superpartner of the neutrino in supersymmetric theories. In supersymmetry, each Standard Model fermion has a corresponding bosonic superpartner, and neutrinos have three sneutrino states, one for each flavor. Sneutrinos are spin-0, electrically neutral, and colorless particles. In the minimal supersymmetric Standard Model (MSSM) the relevant states are the left-handed sneutrinos, which belong to the SU(2) lepton doublets; in extended models with right-handed neutrinos additional right-handed (gauge-singlet) sneutrino states can exist, allowing mixing between left- and right-handed components.

Masses and interactions: Sneutrino masses arise from supersymmetry-breaking terms and typically lie above the masses of their Standard Model partners. The mass eigenstates can involve left-right mixing when both components are present. Sneutrinos interact with Z bosons through their SU(2) doublet component, and they couple to Higgs fields and to gauginos, influencing collider production and decay processes. In models with R-parity conservation, a sneutrino that is the lightest supersymmetric particle (LSP) would be stable and could serve as a dark matter candidate; however, in the MSSM a left-handed sneutrino LSP is strongly disfavored as thermal dark matter because it mediates sizeable elastic scattering with nuclei via Z exchange. Right-handed or singlet sneutrinos in extended models can evade these constraints and may constitute viable dark matter.

Phenomenology and searches: Sneutrinos can be produced in high-energy collisions and appear as missing-energy signals in supersymmetric decay cascades. Experimental searches and constraints on sneutrino masses and couplings depend on the broader SUSY spectrum and the specifics of the model. As of now, no sneutrino has been observed.

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