I would like to ask when can someone add the width in a scalar particle's propagator. In general the scalar propagator can be:
[itex] \frac{1}{k^{2}-m^{2}+i \epsilon} (\epsilon \rightarrow 0)[/itex]
However I read somewhere that if necessary one can include a width for the propagator:
[itex]\frac{1}{k^{2}-m^{2}+i 0} \rightarrow \frac{1}{k^{2}-m^{2}+i m \Gamma}[/itex]
my question is when is it necessary and why?
[itex] \frac{1}{k^{2}-m^{2}+i \epsilon} (\epsilon \rightarrow 0)[/itex]
However I read somewhere that if necessary one can include a width for the propagator:
[itex]\frac{1}{k^{2}-m^{2}+i 0} \rightarrow \frac{1}{k^{2}-m^{2}+i m \Gamma}[/itex]
my question is when is it necessary and why?
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