Positron decay direction from muon

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• Malamala
In summary, the decay is a three body process with the two muons having opposite spins. The positron is more likely to be along the muon spin due to the correlated angular momentum.
Malamala
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!

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

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

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.

Last edited:
BvU

1. What is positron decay direction from muon?

Positron decay direction from muon is a process in which a muon, a subatomic particle with a negative charge, decays into a positron, a subatomic particle with a positive charge, and two neutrinos. This process occurs when a muon interacts with a nucleus, releasing energy and creating a positron that travels in the opposite direction of the muon's original path.

2. How is the direction of positron decay determined?

The direction of positron decay from muon is determined by the laws of conservation of energy and momentum. Since the muon and positron have opposite charges, they will travel in opposite directions. The direction of the positron can also be determined by tracking the path of the muon and observing the direction in which it decays.

3. What is the significance of studying positron decay direction from muon?

Studying positron decay direction from muon can provide valuable insights into the fundamental properties of particles and the laws of physics. It can also help scientists understand the behavior of subatomic particles and their interactions with matter.

4. Can positron decay direction from muon be controlled or manipulated?

No, positron decay direction from muon cannot be controlled or manipulated. It is a natural process that occurs spontaneously when a muon interacts with a nucleus.

5. How is positron decay direction from muon relevant to everyday life?

Positron decay direction from muon may not have a direct impact on everyday life, but the study of this process has led to advancements in particle physics and technology. For example, positron emission tomography (PET) scans use positron decay to produce images of the body, helping to diagnose and treat medical conditions.

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