The probability of producing the resonance at a given energy E is proportional to f (E), so that a plot of the production rate of the unstable particle as a function of energy traces out the shape of the relativistic Breit-Wigner distribution. Note that for values of E off the maximum at M such that |E2 - M2| = M?, (hence |E - M| = ?/2 for M >> ?), the distribution f has attenuated to half its maximum value, which justifies the name for ?, width at half-maximum.
In the limit of vanishing width, ? -> 0, the particle becomes stable as the Lorentzian distribution f sharpens infinitely to 2M?(E2 - M2).
In general, ? can also be a function of E; this dependence is typically only important when ? 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 M2 that multiplies ?2 should also be replaced with E2
(or E4/M2, etc.) when the resonance is wide.
The form of the relativistic Breit-Wigner distribution arises from the propagator of an unstable particle, which has a denominator of the form p2 - M2 + iM?. (Here, p2 is the square of the four-momentum carried by that particle in the tree Feynman diagram involved.) The propagator in its rest frame then is proportional to the quantum-mechanical amplitude for the decay utilized to reconstruct that resonance,
The resulting probability distribution is proportional to the absolute square of the amplitude, so then the above relativistic Breit-Wigner distribution for the probability density function.
The form of this distribution is similar to the amplitude of the solution to the classical equation of motion for a driven harmonic oscillator damped and driven by a sinusoidal external force. It has the standard resonance form of the Lorentz, or Cauchy distribution, but involves relativistic variables s = p2, here = E2. The distribution is the solution of the differential equation for the amplitude squared w.r.t. the energy energy (frequency), in such a classical forced oscillator,
In experiment, the incident beam that produces resonance always has some spread of energy around a central value. Usually, that is a Gaussian/normal distribution. The resulting resonance shape in this case is given by the convolution of the Breit-Wigner and the Gaussian distribution,
This function can be simplified  by introducing new variables,
where the relativistic line broadening function  has the following definition,
is the relativistic counterpart of the similar line-broadening function  for the Voigt profile used in spectroscopy (see also Section 7.19 of ).
See Pythia 6.4 Physics and Manual (page 98 onwards) for a discussion of the widths of particles in the PYTHIA manual. Note that this distribution is usually represented as a function of the squared energy.