The relativistic Breit–Wigner distribution is a continuous probability distribution with the following probability density function, f = k 2 + M 2 Γ 2, {\displaystyle f={\frac {k}{\left^{2}+M^{2}\Gamma ^{2}}}~,} where k is a constant of proportionality, equal to k = 2 2 M Γ γ π M 2 + γ {\displaystyle k={\frac {2{\sqrt {2}}M\Gamma \gamma }{\pi {\sqrt {M^{2}+\gamma }}}}~~~~} with γ = M 2. Wikipedia