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A spin triplet is a set of three quantum states of a system, each with total spin S = 1 (in units of ). The system could consist of a single elementary massive spin 1 particle such as a W or Z boson, or be some multiparticle state with total spin angular momentum of one.
In physics, spin is the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. In quantum mechanics, spin is particularly important for systems at atomic length scales, such as individual atoms, protons, or electrons. Such particles and the spins of quantum mechanical systems ("particle spin") possess several unusual or non-classical features, and for such systems, spin angular momentum cannot be associated with rotation but instead refers only to the presence of angular momentum.
Almost all molecules encountered in daily life exist in a singlet state, but molecular oxygen is an exception. At room temperature, O2 exists in a triplet state, which would require the forbidden transition into a singlet state before a chemical reaction could commence, which makes it kinetically nonreactive despite being thermodynamically a strong oxidant. Photochemical or thermal activation can bring it into singlet state, which is strongly oxidizing also kinetically.
Two spin-1/2 particles 
In a system with two spin-1/2 particles - for example the proton and electron in the ground state of hydrogen, measured on a given axis, each particle can be either spin up or spin down so the system has four basis states in all
using the single particle spins to label the basis states, where the first and second arrow in each combination indicate the spin direction of the first and second particle respectively.
and since for spin-1/2 particles, the basis states span a 2-dimensional space, the basis states span a 4-dimensional space.
Now the total spin and its projection onto the previously defined axis can be computed using the rules for adding angular momentum in quantum mechanics using the Clebsch–Gordan coefficients. In general
substituting in the four basis states
returns the possible values for total spin given along with their representation in the basis. There are three states with total spin angular momentum 1
and a fourth with total spin angular momentum 0.
The result is that a combination of two spin-1/2 particles can carry a total spin of 1 or 0, depending on whether they occupy a triplet or singlet state.
See also 
- Singlet state
- Doublet state
- Angular momentum
- Pauli matrices
- Spin multiplicity
- Spin quantum number
- Spin tensor