| strange quark | |
|---|---|
| Name | Strange quark |
| Charge | −1/3 e |
| Spin | 1/2 |
| Color | color triplet |
| Mass | "≈ 95 MeV/c^2 (current quark mass)" |
| Discovered | 1947–1964 |
| Discovered by | Cecil Powell; Gell-Mann, Kazuhiko Nishijima |
strange quark
The strange quark is the second-generation down-type quark in the Standard Model of particle physics. It carries electric charge −1/3 e, participates in the strong interaction through Quantum chromodynamics (QCD), and is the defining constituent of many strange hadron states, making it central to understanding hadronic matter, CP violation, and the behavior of dense nuclear systems such as neutron star cores.
The existence of a new quantum number associated with what became known as "strangeness" was inferred in the late 1940s after the discovery of long-lived particles produced in cosmic ray interactions and accelerator experiments. The observation of unusual production and decay patterns of kaons and hyperons at Cambridge (Cavendish) and in early bubble chamber experiments motivated the proposal of strangeness by Murray Gell-Mann and independently by Kazuhiko Nishijima in the 1950s and early 1960s. The quark model of Gell-Mann and George Zweig (1964) incorporated the strange quark, explaining the classification of the baryon octet and meson nonet and influencing experiments at CERN, SLAC, and Brookhaven National Laboratory.
The strange quark is a spin-1/2 fermion carrying color charge under QCD and baryon number 1/3. Its defining quantum number is strangeness S = −1 for a single s quark. The strange quark participates in weak interactions via the CKM matrix with notable elements V_us and V_ts controlling flavor-changing processes. Mass parameters include the current quark mass and the larger constituent mass used in phenomenological models; lattice QCD calculations by collaborations such as HPQCD and RBC and UKQCD have refined these values. The s quark's role in CP violation emerges indirectly through strange meson systems like the neutral kaon complex and precision studies at experiments such as NA48 and KTeV.
Within the Standard Model, the strange quark is one of six flavors and participates in gluon-mediated interactions described by QCD. It contributes to the running of the strong coupling constant α_s and to hadron structure through sea-quark effects measured in deep inelastic scattering at facilities like DESY (HERA) and fixed-target programs. Chiral perturbation theory (χPT) and effective field theories incorporate the strange quark when describing low-energy hadron interactions; strange quark condensates are important order parameters in studies of spontaneous chiral symmetry breaking. The interplay between perturbative QCD at high momentum transfer and nonperturbative methods (lattice QCD, QCD sum rules) is crucial to computing observables involving strange quarks, such as form factors and hadronic matrix elements relevant for flavor physics.
Strange quarks form a rich spectrum of hadrons: strange mesons (kaons, K*, φ) and strange baryons (Λ, Σ, Ξ, Ω). The classification of these states follows SU(3) flavor symmetry introduced by Gell-Mann and refined via quark model potentials and relativistic quark models developed by groups at CERN, MIT, and Caltech. Spectroscopy programs at KEK, J-PARC, and Jefferson Lab map excited strange baryon resonances and search for exotic configurations such as pentaquarks with hidden or open strangeness reported in experiments like LHCb. The φ meson, with strong s s̄ content, provides a laboratory for studying OZI-suppressed processes and medium modifications in heavy-ion collisions at RHIC and the LHC.
Strange hadrons are produced copiously in strong-interaction processes: proton-proton and heavy-ion collisions (RHIC, LHC), fixed-target experiments, and in weak decays of heavier flavored hadrons. Weak decays of strange hadrons probe the CKM matrix (e.g., K → πℓν for V_us) and CP violation in the neutral kaon system provided historical insight into symmetry breaking investigated by Cronin and Fitch and later precision measurements by KLOE and NA62. Experimental detection uses tracking, calorimetry, and particle identification systems at detectors like ALICE, ATLAS, and CMS as well as hyperon studies in bubble chambers and modern spectrometers. Lifetime measurements, branching fractions, and Dalitz plot analyses constrain theoretical models and lattice QCD inputs.
Strangeness enters nuclear physics through hypernuclei—nuclei containing hyperons such as Λ—produced at KEK and J-PARC and studied to probe the hyperon–nucleon interaction and in-medium modifications. In astrophysics, the presence of strange degrees of freedom in dense matter affects the equation of state for neutron stars; hypotheses include hyperon-rich cores and deconfined strange quark matter discussed in the context of strange stars and the Tolman–Oppenheimer–Volkoff limit. Observations by NICER and gravitational-wave measurements by LIGO and VIRGO impose constraints on models that include strange quark contributions to high-density matter.
The strange quark remains pivotal in tests of the Standard Model and searches for new physics. Precision lattice QCD results for strange-quark matrix elements feed into determinations of CKM unitarity and searches for CP violation beyond the CKM paradigm. Strange quark content of the nucleon influences direct-detection rates in dark matter searches, while rare kaon decays (e.g., K → πνν̄) are sensitive to virtual contributions from heavy new particles predicted in supersymmetry, grand unified theories, and other extensions. Theoretical work by groups at CERN, Perimeter Institute, and major universities continues to refine hadronic inputs and propose experiments (NA62, KOTO, LHCb upgrades) aimed at exploiting strangeness as a precision window on fundamental interactions.
Category:Quarks Category:Particle physics