Unraveling the Mysteries of Pion Decays

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In summary, the conversation discusses the decays of pions and the questions that arise from them. One question is why the decay of a neutral pion is not simply an annihilation qqbar -> gamma -> e+e-, to which it is explained that this is not allowed due to the pion being spin 0 and the photon being spin 1. Another question is about the high branching fraction of the charged pion decaying to muon+neutrino, which is explained by the allowed vertex pion -> vector boson, but suppressed in some way. The conversation also touches on the confusion about the neutral pion's C number and its role in explaining its decay.
  • #1
alsey42147
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hi,

yesterday i had a discussion with some friends about the decays of pions which produced a few questions that we couldn't resolve properly.

if anyone can shed some light on this matter, or point out flaws in the reasoning below, that would be great.

so starting with the decay of a neutral pion, and the question of why it is not simply an annihilation qqbar -> gamma -> e+e-; my understanding is that this diagram would not be allowed since the pion is spin 0 and the photon is spin 1. is this correct?

there is a non-zero (~10^-8) branching fraction for pi0 -> e+e- (with no final state photons); I'm assuming this proceeds by some higher order diagram and is hence supressed.

so anyway, if the tree diagram qqbar(pion) -> gamma -> e+e- is not allowed because of spin, how is it that the charged pion decays to muon+neutrino with very high branching fraction? i have seen in textbooks and on wikipedia ( http://en.wikipedia.org/wiki/Pion ) this decay being drawn as ud -> W -> mu nu, but as in the case for the neutral pion, the pion is spin zero and the W is spin 1, so how can this happen?

or is it that the vertex pion -> vector boson is allowed, but just supressed in some way? if so what is the nature of that supression?

i have looked at the matrix element calculation for pi+ -> mu+ nu and it seems that the current on the quark side of the diagram cannot be written with gamma^mu(1 - gamma^5) ; does this mean that the propagator is not a W? if so then what is it?

so, if anyone has any thoughts on this, or you think I'm just being retarded, please let me know.

thanks :)
 
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  • #2
ok, i was being retarded, as i expected. the pi0 is C = +1 and the photon is C = -1, and C must be conserved in EM interactions. I'm fairly sure someone taught me that at some point but i'd completely forgotten.
 
  • #3
well, i am still confused. looking at some textbooks, they all state that C(gamma) = -1 since the EM field is produced by charges that change sign under C transformation. fine.

they then say that because the neutral pion decays to two photons, it has C = +1. if the C number of the neutral pion is defined by its decay, then to use this number to explain how it decays would be circular. without already knowing how it decays, i can only say that it has C = +/- 1.

:confused: :confused: :confused:

i would really appreciate some thoughts on this...
 

1. What are pion decays?

Pion decays are a type of subatomic particle decay that involves the transformation of a pion particle into other types of particles, such as electrons, neutrinos, and photons.

2. Why is it important to study pion decays?

Studying pion decays can provide insights into the fundamental forces and interactions of the universe, as well as help us understand the properties of subatomic particles and their role in the structure of matter.

3. How do scientists unravel the mysteries of pion decays?

Scientists use a variety of experimental techniques, such as particle accelerators and detectors, to study pion decays and gather data on the particles involved. They also use theoretical models and calculations to interpret the data and make predictions about pion decay processes.

4. What have scientists learned from studying pion decays?

Through studying pion decays, scientists have been able to confirm the existence of certain particles, such as the W and Z bosons, and have made important discoveries related to the Standard Model of particle physics. They have also gained a deeper understanding of the fundamental forces and interactions of the universe.

5. How do pion decays contribute to our understanding of the universe?

Pion decays provide valuable information about the structure of matter and the behavior of particles at the subatomic level. This knowledge can help us better understand the origin and evolution of the universe, as well as potentially lead to new technologies and advancements in various fields.

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