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# Decay of K mesons watch

1. Hi, in the Jan 2012 AQA PHYA1 exam question 1biii it asks to give the products of a K- meson. The accepted answers are pi0, e-, electron antineutrino OR pi0, mu- muon antineutrino.

How am I meant to know this - should I just memorise every single decay or is there a way of working it out?
2. K- mesons are made up of an anti-up quark and a strange quark. They decay via the weak interaction. At normal energies, the weak interaction will change the strange quark into an up quark, a lepton and an antineutrino.

So we're left with an up quark and an anti-up quark, which together form a neutral pi0 as well as a lepton and an antineutrino. The lepton can either be an electron or a muon.

The best way to remember it is by conservation of charge and conservation of particle minus antiparticle number. The negatively charged K- goes to a neutral pi0 so the lepton has to be negatively charge. But since you have an extra particle you need an extra antiparticle for conservation, and it must be neutral too, so the charge balance is unaffected. So you get an antineutrino.

The in depth explanation for precisely why this happens is waaaaay beyond A level, so not sure why they expect you to remember.
3. (Original post by 3nTr0pY)
K- mesons are made up of an anti-up quark and a strange quark. They decay via the weak interaction. At normal energies, the weak interaction will change the strange quark into an up quark, a lepton and an antineutrino.

So we're left with an up quark and an anti-up quark, which together form a neutral pi0 as well as a lepton and an antineutrino. The lepton can either be an electron or a muon.

The best way to remember it is by conservation of charge and conservation of particle minus antiparticle number. The negatively charged K- goes to a neutral pi0 so the lepton has to be negatively charge. But since you have an extra particle you need an extra antiparticle for conservation, and it must be neutral too, so the charge balance is unaffected. So you get an antineutrino.

The in depth explanation for precisely why this happens is waaaaay beyond A level, so not sure why they expect you to remember.
That question has baffled me. Is there any reason why the lepton is either an electron OR a muon?

I had always thought that the K-mesons decayed into Pi-mesons, and then these Pi-mesons would decay into Muons (and muon anti-neutrinos), and then these muons would decay into electrons. Is this all wrong?
4. (Original post by Jack93o)
That question has baffled me. Is there any reason why the lepton is either an electron OR a muon?

I had always thought that the K-mesons decayed into Pi-mesons, and then these Pi-mesons would decay into Muons (and muon anti-neutrinos), and then these muons would decay into electrons. Is this all wrong?
There are several decays possible, either into pions, muons or electrons+pion. Tho further decays may occur, but are not necessarily restricted to only one result only.
5. (Original post by agostino981)
There are several decays possible, either into pions, muons or electrons+pion. Tho further decays may occur, but are not necessarily restricted to only one result only.
Are you saying for the K- meson, it is possible to decay into only pi mesons?

from what OP's said of the mark scheme, there can only be two possible decay for the K- meson:

''The accepted answers are pi0, e-, electron antineutrino OR pi0, mu- muon antineutrino.''
6. (Original post by Jack93o)
Are you saying for the K- meson, it is possible to decay into only pi mesons?

from what OP's said of the mark scheme, there can only be two possible decay for the K- meson:

''The accepted answers are pi0, e-, electron antineutrino OR pi0, mu- muon antineutrino.''
It can. Take a look at this:

It's antiparticle can decay to pions only as well.

You may also want to refer to this listing:

http://pdg.lbl.gov/2008/listings/s010.pdf
7. (Original post by agostino981)
It can. Take a look at this:

It's antiparticle can decay to pions only as well.

You may also want to refer to this listing:

http://pdg.lbl.gov/2008/listings/s010.pdf
thats what I suspected, so whats up with the mark scheme then?

hey OP, can you give us the full question, did it give you a few of the decay particles and then asked you to fill in the rest?
8. I've figured out why it only lists those two possible answers - it gives three boxes to write the products of the decay. Thanks for the relatively in-depth explanation of the conservation laws, it helped me understand the decay.
9. (Original post by agostino981)
It can. Take a look at this:

It's antiparticle can decay to pions only as well.

You may also want to refer to this listing:

http://pdg.lbl.gov/2008/listings/s010.pdf
well apparently for the question, there were 3 boxes to fill in for the decay products (with none already given to you), so doesn't that mean that a k- meson, it can't decay only into pi mesons? (for example: pi-, pi0, pi0)

because the mark scheme as stated by OP only gives two possible answers, and neither of those involve only pi mesons
10. (Original post by Jack93o)
well apparently for the question, there were 3 boxes to fill in for the decay products (with none already given to you), so doesn't that mean that a k- meson, it can't decay only into pi mesons? (for example: pi-, pi0, pi0)

because the mark scheme as stated by OP only gives two possible answers, and neither of those involve only pi mesons
Yeah i don't understand why the mark scheme does not involve pi- pi0 pi0? Does anyone know please?

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