There is a significant exception to the independence of the cross-section on energy. Suppose that the quantity is slightly less than . As the incident energy increases, , which is given by Equation (1018), can reach the value . In this case, becomes infinite, so we can no longer assume that the right-hand side of Equation (1013) is small. In fact, at the value of the incident energy when , it follows from Equation (1013) that , or (because we are assuming that ). This implies that
Note that the cross-section now depends on the energy. Furthermore, the magnitude of the cross-section is much larger than that given in Equation (1017) for (since ).
The origin of this rather strange behaviour is quite simple. The condition
is equivalent to the condition that a spherical well of depth possesses a bound state at zero energy. Thus, for a potential well that satisfies the above equation, the energy of the scattering system is essentially the same as the energy of the bound state. In this situation, an incident particle would like to form a bound state in the potential well. However, the bound state is not stable, because the system has a small positive energy. Nevertheless, this sort of resonance scattering is best understood as the capture of an incident particle to form a metastable bound state, and the subsequent decay of the bound state and release of the particle. The cross-section for resonance scattering is generally far higher than that for non-resonance scattering.
We have seen that there is a resonant effect when the phase-shift of the -wave takes the value . There is nothing special about the partial wave, so it is reasonable to assume that there is a similar resonance when the phase-shift of the th partial wave is . Suppose that attains the value at the incident energy , so that
Let us expand in the vicinity of the resonant energy:
Recall, from Equation (984), that the contribution of the th partial wave to the scattering cross-section is
This is the famous Breit-Wigner formula. The variation of the partial cross-section with the incident energy has the form of a classical resonance curve. The quantity is the width of the resonance (in energy). We can interpret the Breit-Wigner formula as describing the absorption of an incident particle to form a metastable state, of energy , and lifetime .
- Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)