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In nuclear physics and particle physics, isospin (I) is a quantum number related to the up- and down quark content of the particle. Isospin is also known as isobaric spin or isotopic spin.
Isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions of baryons and mesons. The name of the concept contains the term spin because its quantum mechanical description is mathematically similar to that of angular momentum. But unlike angular momentum, it is a dimensionless quantity and is not actually any type of spin. Before the concept of quarks was introduced, particles that are affected equally by the strong force but had different charges were considered different states of the same particle, but having isospin values related to the number of charge states. A close examination of isospin symmetry ultimately led directly to the discovery and understanding of quarks and to the development of Yang–Mills theory. Isospin symmetry remains an important concept in particle physics.