| neutralino | |
|---|---|
| Name | Neutralino |
| Caption | Conceptual depiction of a neutralino as a supersymmetric neutral fermion |
| Type | Hypothetical elementary particle |
| Interaction | Weak interaction, Gravity |
| Antiparticle | Self (Majorana) or distinct (Dirac) variants |
| Status | Predicted by Supersymmetry |
neutralino
A neutralino is a hypothetical electrically neutral fermion predicted by theories of supersymmetry that mixes the superpartners of neutral Standard Model gauge and Higgs bosons. Neutralinos are significant in Quantum Physics and theoretical particle physics because they exemplify supersymmetric extensions of the Standard Model and are leading candidates for particle dark matter in many cosmological models.
In quantum field theory neutralinos arise as mass eigenstates formed from linear combinations of the bino (superpartner of the U(1) gauge boson), the neutral wino (from SU(2)), and the neutral higgsinos (from the two Higgs doublets in the minimal supersymmetric Standard Model (MSSM)). Their existence would confirm global notions of symmetry between bosons and fermions introduced by theoretical work of Yuri Golfand, Efim Fradkin, Julius Wess, and Bruno Zumino. Neutralinos are of interest in quantum physics because they probe beyond-Standard-Model quantum field theory structure, test mechanisms of electroweak symmetry breaking, and illuminate the microscopic origin of cosmic matter–antimatter asymmetry and structure formation.
Neutralinos are predicted within frameworks such as the Minimal Supersymmetric Standard Model and its extensions (e.g., the NMSSM). In these models the supersymmetric partners—gauginos and higgsinos—mix through a mass matrix determined by soft SUSY-breaking parameters (e.g., M1, M2), the higgsino mass parameter μ, and the ratio of Higgs vacuum expectation values tanβ. Notable theoretical contributors to SUSY model-building include Howard Georgi, Savas Dimopoulos, and Hitoshi Murayama. Renormalization group running (studied by groups at CERN, Fermilab, and university theory departments such as Harvard University and Stanford University) ties neutralino phenomenology to high-scale physics like grand unified theories (GUTs) and mechanisms of supersymmetry breaking (e.g., gravity mediation, gauge mediation, anomaly mediation).
Neutralinos are electrically neutral Majorana fermions in the simplest constructions, though Dirac variants appear in some extended models. They are labeled χ̃0_1, χ̃0_2, χ̃0_3, χ̃0_4 in the MSSM, with χ̃0_1 typically the lightest supersymmetric particle (LSP) and thus stable under R-parity conservation. Masses and couplings depend on parameters constrained by experimental bounds from the Large Hadron Collider (ATLAS and CMS experiments), precision electroweak measurements, and flavor physics experiments at facilities like KEK and SLAC National Accelerator Laboratory. Key quantum properties include spin-1/2, self-conjugacy (for Majorana), weak-scale interactions, and suppressed electromagnetic couplings, making them difficult to detect directly.
As weakly interacting massive particles (WIMPs), neutralinos naturally produce relic abundances of cold dark matter through thermal freeze-out in the early universe, a mechanism quantitatively described by calculations following the Boltzmann equation and techniques developed by Edward W. Kolb and Michael S. Turner. In constrained frameworks like the CMSSM or mSUGRA, χ̃0_1 parameter space can satisfy relic density observations from missions such as Planck and WMAP. Alternative scenarios include nonthermal production, coannihilation with sfermions or charginos, and mixed-composition dark matter with axions (see work by Peccei–Quinn theory proponents and axion dark matter researchers). Neutralino dark matter links particle physics, astrophysical observations (e.g., galaxy rotation curves), and cosmological probes of large-scale structure.
Searches for neutralinos occur across three main channels: direct detection, indirect detection, and collider production. Direct detection experiments such as XENON, LUX-ZEPLIN, and PandaX seek nuclear recoils from χ̃0_1 scattering using noble-liquid detectors. Indirect searches by observatories like Fermi Gamma-ray Space Telescope, AMS-02, and IceCube look for annihilation or decay products (gamma rays, antimatter, neutrinos) in the Galactic halo, dwarf galaxies, and the Sun. Collider searches at CERN's Large Hadron Collider target missing transverse energy signatures from neutralino production in association with jets or leptons; dedicated analyses by the ATLAS and CMS collaborations set bounds on sparticle masses. Complementary constraints derive from LEP results, low-energy precision experiments at BESIII and LHCb, and planned facilities such as the proposed International Linear Collider.
Discovery of a neutralino would validate supersymmetry as a symmetry of nature, profoundly affecting our understanding of quantum gravity proposals (e.g., connections to string theory), gauge unification, and the stability of the electroweak scale. It would resolve the particle nature of most dark matter and guide construction of cosmological models for the early universe, including inflationary reheating and thermal history studied by cosmologists at institutions like Institute for Advanced Study and Princeton University. Conversely, increasingly stringent experimental limits challenge minimal SUSY scenarios, motivating alternative frameworks and emphasizing conservative scientific values of model economy, empirical adequacy, and institutional cooperation among laboratories such as CERN, Fermilab, and national funding agencies.
Category:Supersymmetric particles Category:Dark matter