NLSP

NLSP stands for next-to-lightest supersymmetric particle, a term used in supersymmetric theories of particle physics. In models that conserve R-parity, the lightest supersymmetric particle (LSP) is stable and commonly serves as a dark matter candidate. The NLSP is the second-lightest superpartner and sits just above the LSP in the mass spectrum. The NLSP thus acts as an intermediate state between heavier superpartners produced in high-energy collisions and the LSP, which escapes detectors as missing energy.

The identity of the NLSP is model-dependent. Common possibilities include the lightest neutralino, the lighter stau (tau slepton), the lighter chargino, and, less commonly, a sneutrino or a stop, depending on the SUSY-breaking parameters and resulting mass hierarchy.

Decay modes and lifetimes of the NLSP depend on how supersymmetry is broken and on which particle is the LSP. In gauge-mediated SUSY breaking (GMSB), the gravitino is typically the LSP, and the NLSP decays to a gravitino plus a Standard Model particle. Examples include neutralino NLSP decaying to gravitino and photon or Z boson, and stau NLSP decaying to gravitino and tau. The decay length can range from prompt to macroscopic, or even collider-stable, depending on the gravitino mass and coupling strengths. In gravity-mediated scenarios, the LSP is often the lightest neutralino, and the NLSP decays to this neutralino plus Standard Model particles, with corresponding collider signatures.

Phenomenologically, NLSP decays yield events with missing energy accompanied by photons or leptons, and may produce displaced vertices or long-lived charged tracks if the NLSP is long-lived. Studying NLSP properties helps connect collider data to the underlying SUSY-breaking mechanism and the nature of the LSP.