Decaytimes of Leptons

I just want to ask if the Standard - Model of Leptons has a theory for the
decay times of the Myon and the Tau.
What Environment Parameters (Bonding in an Atom, Temperature etc. )
influence the decay ?

If standard Theory says nothing: Are there any empirical models or
relationships ?

I think this is a very important question and would like to collect
experience in this field.

Thanks

Josef


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