Positron decay direction from muon

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SUMMARY

The discussion centers on the preferential emission direction of positrons in the decay of positive muons, specifically the decay process ##\mu^+\to e^+\nu_e\bar{\nu_\mu}##. It is established that positrons are emitted in the direction of the muon spin due to angular momentum conservation, where the left-handed electron neutrino and right-handed muon antineutrino momenta cancel each other. The correlation between the positron's emission direction and the muon's spin is further illustrated in slide 6 of the referenced TRIUMF document. The discussion emphasizes that any deviation from this configuration would violate angular momentum conservation principles.

PREREQUISITES
  • Understanding of particle decay processes, specifically muon decay.
  • Familiarity with concepts of helicity and chirality in particle physics.
  • Knowledge of angular momentum conservation in quantum mechanics.
  • Ability to interpret scientific illustrations and diagrams related to particle interactions.
NEXT STEPS
  • Study the principles of angular momentum conservation in particle physics.
  • Review the TRIUMF document, particularly slide 6, for visual understanding of muon decay.
  • Examine the differences between helicity and chirality in particle interactions.
  • Explore the implications of spin configurations in three-body decays.
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Physicists, students of particle physics, and researchers interested in muon decay processes and angular momentum conservation in quantum mechanics.

Malamala
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Hello! I read that in the rest frame of a positive muon, decay positrons are preferentially emitted in the direction of the muon spin. Why is that the case? The decay is ##\mu^+\to e^+\nu_e\bar{\nu_\mu}##. Assuming that the positron is emitted at almost the speed of light, it will be a left handed particle. The 2 muons are right handed and left handed. So we have a 3 body decay, with the 3 particles having their spin aligned or anti-aligned with their momentum. But I am not sure how can I get from here that the positron is more likely to be along the muon spin. Can someone help me? Thank you!
 
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Malamala said:
The 2 muons are right handed and left handed.
I believe one is referring to two neutrinos, or rather neutrino and antineutrino.

A muon decay positron is also correlated with the angular momentum of the muon in a more complicated way. Here, the positron with maximum energy takes the angular momentum of the decaying muon, since the left-handed electron neutrino and the right-handed muon antineutrino angular momenta cancel.
See slide 6 in https://www.triumf.ca/sites/default/files/sslec_201310.pdf

See also - https://www.hep.lu.se/staff/c.jarlskog/nffl_7184.pdf
 
Astronuc said:
I believe one is referring to two neutrinos, or rather neutrino and antineutrino.

See slide 6 in https://www.triumf.ca/sites/default/files/sslec_201310.pdf

See also - https://www.hep.lu.se/staff/c.jarlskog/nffl_7184.pdf
Thank you so much for this! The slide 6 illustration is useful, however I am not totally clear I understand. In the upper illustration, it is clear why the spin and momenta are the way they are, as there is no other way. But why does the configuration in the bottom plot has to be like that? Assuming helicity and chirality eigenstates are the same, can't we switch the position of ##e^+## and ##\bar{\nu}_\mu##, and still conserve the overall spin? In that case the positron would be emitted in the opposite direction from the muon spin.
 
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