Fermi–Dirac statistics : The Fermi–Dirac probability distribution function, Fig. 4, is used to find the probability that a fermion occupies a specific quantum state in a system at thermal equilibrium. Fermions are particles which obey the Pauli exclusion principle (e.g. electrons, protons, neutrons). The distribution function can be written as f F D ( E ) = 1 exp ⁡ ⁡ ( E − − μ μ k B T ) + 1 .