Fermions are particles with an intrinsic angular momentum (i.e. spin) equal to a "half integer" number of fundamental units: $\frac{2n+1}{2} \hbar$ for integer $n$. Fermions are required to be in a quantum state that is globally anti-symmetric, which leads to the Pauli Exclusion Principle barring identical fermions from occupying the same quantum state.

In supersymmetric theories, spin-3/2 particles exist.

Further Classification

The known fermions divide further those that interact via the strong interaction (hadrons and quarks) and those that do not (leptons).

Hadrons include the proton and neutron as well as mesons such as pions and kaons.

Lepton come in three (known) generations each including a charged species (electrons, muons and taus) and an uncharged species (the corresponding neutrinos).

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