A spin-half particle refers to a particle with a specific type of intrinsic angular momentum called "spin." In quantum mechanics, particles are not thought of as tiny spinning objects in the classical sense but possess an intrinsic angular momentum, often referred to as spin.
The term "spin-half" indicates that the particle has a spin quantum number of ½. Spin is quantized, meaning it can only have certain discrete values determined by the laws of quantum mechanics. The spin quantum number can take on integer or half-integer values: 0, 1/2, 1, 3/2, and so on. A spin-half particle has the lowest non-zero value of spin.
The concept of spin is not easily visualized in classical terms. It is an intrinsic property of particles that is unrelated to their physical rotation. Spin does, however, exhibit some similarities to classical angular momentum. Like classical angular momentum, spin can have different orientations, and its projection along a particular axis can be measured.
Intrinsic angular momentum, including spin, is a fundamental property of elementary particles. It is a form of angular momentum that exists even when a particle is at rest, unlike orbital angular momentum, which is associated with the motion of a particle in an orbit.
In summary, a spin-half particle refers to a particle with a spin quantum number of ½, representing a type of intrinsic angular momentum. It is a fundamental property of particles in quantum mechanics and is unrelated to their physical rotation.