🚀 go-pugleaf

s5169180@deimos.ucc.umass.edu (S5169180) asks:
> What is the mass ?

Why, the mass is the pole in the propagator!  In a Feynman diagram, a
particle passing from one interaction to the next is represented
diagrammatically by a line, and algebraically by 1/(p^2-m^2).  If it is
kinematically allowed (by energy-momentum conservation), this factor is
(usually) dominated by p^2=m^2, and we call the particle "on-shell", a
"resonance", or an "external particle."  [Recall here p^2 = (energy)^2 -
(momentum)^2.]  The location of that pole can by shifted from the "bare
mass" (the thing in the Lagrangian) by loop effects (renormalization).

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