# B Questions about beta decay

1. Feb 16, 2017

### Comeback City

These questions came to mind after my physics teacher told us that a neutron is actually a "proton-electron" pair. He then said in beta minus decay, when the neutron decays, the electron is released and only the proton remains. I'm pretty sure this is incorrect, though, but can any confirm it?
I get that the proton is made up of 2 up quarks and 1 down quark, and the neutron is 1 up quark and 2 down quarks. Can anyone confirm or deny if my following understanding of this process is correct?...
In beta minus decay, a down quark in a neutron decays into an up quark. This releases a W- Boson, which then decays into an electron and an electron anti-neutrino upon exiting the nucleus.
Now, the questions...
1) How does the down quark decay into an up quark? I've read several different explanations for this, including that virtual electron-positron pairs pop into and out of existence in the nucleus, and the positron can give the +1 charge to the down quark needed to change it into an up quark (explains conservation of charge).
2) Lepton number has to be conserved, which is why the electron and the electron anti-neutrino are created. Does the W- Boson have exactly the mass of the electron + neutrino?

2. Feb 16, 2017

### Q.B.

Hi,

1) you only need the W boson to turn one quark into the other. W bosons are electrically charged, and while turning a down quark into a up type quark, they leave with both the charge and the "downness"-"upness" difference (even if it's a very unclear way to put it - but these characteristics are the analog of an electric charge. The electric charge is the charge of the electromagnetic interaction, whereas the "type" would be the charge of the weak interactions - not to be confused with the flavour, which is just a repetition of the same charges: up, charm and top quarks have the same charges, which differ from those of the associated down, strange and bottom quarks). When they become an electron-neutrino pair, they release exactly these charge and "type" differences.

2) the W boson involved would be virtual, so it doesn't have to have its own mass. If it was real, its mass would feed the electron-neutrino masses, but also their kinetic energies. The eventual masses just have to add up to something smaller than the initial mass.

If you want to understand this at a deeper level, you should google "standard model of particle physics", "yang-mills theories", ... but it requires quite a background.

3. Feb 16, 2017

### Comeback City

Thanks for the response!
Another question though:
Is the "virtual w boson" created as a result of the decay, or is it residing elsewhere before the decay happens?

4. Feb 16, 2017

### Q.B.

It's "created" by the decay. This should be taken carefully since virtual particles are a way of representing an interaction within a perturbation approach in QFT, and are not to be thought of as temporary propagating particles.

5. Feb 16, 2017

### Vanadium 50

Staff Emeritus
This is not correct. Since the proton and electron both have spin-1/2, this would imply a neutron has either spin-0 or spin-1. But it also has spin-1/2.

6. Feb 16, 2017

### Comeback City

How does the W boson cause the quark decay and also get created by the quark decay?

7. Feb 16, 2017

### Comeback City

Is it even possible at all to have a proton-electron pair?

8. Feb 16, 2017

### mikeyork

Yes, a hydrogen atom. Your neutron decay description is missing one important factor. The decay also produces a neutrino and that is how the angular momentum is balanced: $n\rightarrow p + e + \nu$.

9. Feb 16, 2017

### Comeback City

I just overlooked the obvious. I was thinking of it in a different context where the electron would be bound with the quarks of the proton (which, now thinking about it, most likely cannot work anyways)
I mentioned that in the original post...

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