Re: resonances in particle physics

[Jack Uretsky <jlu@HEP.ANL.GOV>]

        A "resonance" in particle and atomic physics is a state (in the
sense of quantum mechanics) that can be characterized by a mass, a
lifetime, and an angular momentum.  The lifetime makes the mass uncertain,
so the characteristic mass is associated with a width.  The state decays
in accordance with its lifetime, emitting "decay products".
        The resonant state may be observed as an enhancement in the
scattering of the decay products in the appropriate angular momentum
channel.
        Example: the delta resonance is a marked enhancement in the
p-wave, J=3/2, scattering of a positive pion from a proton.
        Example: the L=1, n=1 state of the hydrogen atom is a resonance
in the scattering of (approximately) 10 eV photons from hydrogen atoms
in their ground state.
        At resonance, the phase of the part of the scattering amplitude
having the resonant angular momentum passes upward through 90 degrees.
This is similar to the change of sign of the amplitude of a driven
classical harmonic oscillator as the oscillation frequency passes through
resonance.

On Thu, 7 Dec 2000, John Denker wrote:

> At 09:33 PM 12/7/00 -0500, Chuck Britton wrote:
>  > And will someone who IS familiar with the Particle Physics use of the
> > term 'resonance' please comment on this 'distinction'.
>
> I'm not sure what "distinction" we are talking about.  Particle physicists
> use this word in an intelligent way.
                                        Regards,
                                                Jack