scalarisoscalar

A scalar isoscalar, in the context of hadron physics, refers to a meson or a meson-like state with total angular momentum J = 0, positive parity, positive charge conjugation (J^PC = 0^++), and zero isospin (I = 0). Such states are part of the scalar meson nonet and are commonly denoted as f0 states, distinguishing them from the scalar isotriplet a0 states (I = 1).

The lightest scalar isoscalar is the f0(500), also known as the sigma meson. It appears as a broad resonance in low-energy pion-pion scattering and plays a role in models of chiral symmetry breaking and spontaneous mass generation in quantum chromodynamics (QCD). Other well-known scalar isoscalars include f0(980), which lies near the KK̄ threshold, and higher-mass states such as f0(1370), f0(1500), and f0(1710).

The internal structure of scalar isoscalars is a subject of ongoing research. While some can be described as conventional quark-antiquark (qq̄) states, others are proposed to have more complex configurations, such as tetraquark states (qqqq̄), meson-meson molecular bound states, or mixtures with gluonic components (glueballs). In particular, lattice QCD predicts a scalar glueball with a mass in the vicinity of 1.5–1.7 GeV, and mixing among the f0 states is a common theme in trying to understand their observed properties.

Beyond hadron spectroscopy, the term scalar isoscalar can also describe a hypothetical scalar field that carries no isospin charge, i.e., a singlet under SU(2), which is a feature considered in various beyond-Standard-Model scenarios and dark-sector theories.