sigma baryon

sigma baryon
Name	Sigma baryon
Composition	Quarks: Σ⁺ (u u s), Σ⁰ (u d s), Σ⁻ (d d s)
Statistics	Fermionic
Family	Baryon
Interaction	Strong interaction, Weak interaction, Electromagnetism, Gravity
Status	Confirmed
Theorized	Murray Gell-Mann, Kazuhiko Nishijima (c. 1956)
Discovered	Σ⁺: L. W. Alvarez et al. (1953), Σ⁻: L. W. Alvarez et al. (1954), Σ⁰: L. W. Alvarez et al. (1956)
Mass	Σ⁺: 1189.37 ± 0.07 MeV/c², Σ⁰: 1192.642 ± 0.024 MeV/c², Σ⁻: 1197.449 ± 0.030 MeV/c²
Decay time	Σ⁺: 8.018 ± 0.026 × 10⁻¹¹ s, Σ⁰: 7.4 ± 0.7 × 10⁻²⁰ s, Σ⁻: 1.479 ± 0.011 × 10⁻¹⁰ s
Electric charge	Σ⁺: +1 e, Σ⁰: 0 e, Σ⁻: –1 e
Spin	1/2
Parity	+1
Strangeness	–1

sigma baryon. The sigma baryon is a family of three subatomic particles in the baryon classification, each containing one strange quark and two lighter quarks. These particles, denoted Σ⁺, Σ⁰, and Σ⁻, are crucial for understanding the strong interaction and the organization of matter within the quark model. Their discovery and study provided pivotal evidence for the Eightfold Way and the subsequent development of quantum chromodynamics.

Overview

The sigma baryons are spin-½ particles that form an isospin triplet, a key multiplet in the SU(3) flavor symmetry scheme proposed by Murray Gell-Mann and Yuval Ne'eman. They are heavier than the proton and neutron due to the presence of the strange quark, which imparts the property of strangeness. As unstable particles, they decay via the weak interaction or, in the case of the Σ⁰, through the electromagnetic interaction, and are routinely produced in high-energy collisions at facilities like CERN and Fermilab.

Properties

The charged sigma baryons, Σ⁺ and Σ⁻, have masses near 1189 and 1197 MeV/c², respectively, while the neutral Σ⁰ is slightly heavier at about 1193 MeV/c². Their mean lifetimes differ significantly: the Σ⁺ and Σ⁻ decay weakly with lifetimes on the order of 10⁻¹⁰ seconds, whereas the Σ⁺ decays almost instantly via electromagnetic decay to the lambda baryon. Each carries a strangeness quantum number of –1 and possesses a magnetic moment that has been measured in experiments at Brookhaven National Laboratory.

Classification

Within the quark model, the sigma baryons are classified as members of the baryon octet, which also includes the proton, neutron, lambda baryon, and the Ξ baryons. Their specific quark compositions are Σ⁺ (u u s), Σ⁰ (u d s), and Σ⁻ (d d s). This grouping is a direct consequence of the SU(3) flavor symmetry underlying the Eightfold Way, a classification scheme that predated the full theory of quantum chromodynamics.

Discovery and experimental history

The first sigma baryon, the Σ⁰, was identified in 1953 from cosmic ray observations in a cloud chamber experiment led by Luis Walter Alvarez at the University of California, Berkeley. The Σ⁻ was discovered the following year by the same group, with the neutral Σ⁰ confirmed in 1956. These discoveries, made during the "particle zoo" era, were instrumental in validating the predictions of strangeness theory proposed by Murray Gell-Mann and Kazuhiko Nishijima. Subsequent precision measurements of their properties have been conducted at major accelerators including the Stanford Linear Accelerator Center and the Large Hadron Collider.

Theoretical importance

The existence and properties of the sigma baryons were a major triumph for the Eightfold Way, providing a clear experimental test of SU(3) flavor symmetry. Their mass splittings and decay patterns offered early insights into the nature of the strong interaction and the breaking of symmetry. They serve as a fundamental testing ground for lattice QCD calculations and theories of chiral symmetry breaking, linking the quark model to the low-energy dynamics described by effective field theories like chiral perturbation theory.

Decay modes

The primary decay channels for the sigma baryons are governed by the weak interaction for the charged states and the electromagnetic interaction for the neutral state. The Σ⁺ predominantly decays to a proton and a π⁰, or to a neutron and a π⁺. The Σ⁻ decays to a neutron and a π⁻. The Σ⁰, due to its extremely short lifetime, decays almost exclusively via the electromagnetic decay process to the lambda baryon and a photon, a key signature in particle detectors at laboratories like DESY and Jefferson Lab.

Category:Baryons Category:Subatomic particles