I would like to ask when can someone add the width in a scalar particle's propagator. In general the scalar propagator can be:(adsbygoogle = window.adsbygoogle || []).push({});

[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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# Scalar Propagator form with width

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