In general, Γ can also be a function of E ; this dependence is typically only important when Γ is not small compared to M and the phase space -dependence of the width needs to be taken into account. (For example, in the decay of the rho meson into a pair of pions.) The factor of M that multiplies Γ should also be replaced with E (or E / M, etc.) when the resonance is wide.