Complex mass and decay rate

The mass of an unstable particle is formally a complex number, with the real part being its mass in the usual sense, and the imaginary part being its decay rate in natural units.

When the imaginary part is large compared to the real part, the particle is usually thought of as a resonance more than a particle.

For a particle of mass , the particle can travel for time 1/M, but decays after time of order of . If > M then the particle usually decays before it completes its travel.

— Wikipedia on Particle decay

2012.09.11 Tuesday ACHK