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Question Radiative muon decay 

Forum: Particles
Date: 25 Nov, 2013
From: Tom Roberts <Tom Roberts>

The PDG lists the radiative muon decay channel with a branching ratio of (1.4+-.4)%, with the note: for gamma energies > 10MeV (presumably in the muon rest frame). This decay is mu+ => e+ anti_nu_mu nu_e gamma.

This channel is not implemented in the normal Decay process, but is included in DecayWithSpin. Unfortunately, its branching ratio is much too small -- it is 1.4% for ALL gammas, which makes it about 0.08% for energy > 10MeV. So it looks to me like the note was not noticed.

If I assume the energy distribution is correct, then the overall branching ratio is about 24%. That's quite large and cannot reasonably be ignored. Why is the conventional wisdom "muons decay into an electron plus two neutrinos", when a quarter of the time they don't?

This can be a serious error for experiments involving muon decay backgrounds, because for high-energy muons, those gammas from ~24% of the decays get boosted to significant energies. Even for stopped muons, some 5-10% of the decays have gammas above the pair-production threshold. Surely many experiments ought to see this....

Does anybody know the real story here? What should we do about it? IMHO the Decay process is in error by omitting a channel with a branching ratio of ~24%, as is DecayWithSpin.

(Note there is no obvious forum for discussing Decay processes; this one seems like the best.)

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1 Feedback: Re: Radiative muon decay   (Gumplinger Peter - 25 Nov, 2013)
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