# Why can only the weak interaction change quark flavour?

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1. May 19, 2015

### AlanKirby

Hi there, so my question is as follows.

I understand that only the weak interaction can change the flavour of a quark, but why?

Idea 1: It's due to the change in flavour also meaning a change in mass, thus a massive exchange particle is needed (gravity is negligible so forget the massive graviton). But then since the exchange particles are themselves virtual, could a photon for example, not simply be 'off mass shell' and produce the same result?

Idea 2: It's to do with a change in electric charge (assuming we always deal with U,C,T to S,B,D quarks flavours or vice versa, which i don't know if that is true), thus an electrically charged exchange particle is needed, i.e. W boson (and hence Z boson doesn't cause a change in quark flavour since it's electrically neutral)?

Idea 3: The different quark flavours have a different value of weak charge (which is a different quantum number), thus to change this from one value to another requires an exchange particle with weak charge, thus the W bosons are the only gauge bosons that can cause this flavour change (and again, thus the Z boson cannot since it has no weak charge)?

I'm sure that someone is about to confuse me with some guage field theory stuff, but I would appreciate an answer, despite how confusing it may seem to an undergrad such as myself.

Thank you kindly for any response.

2. May 19, 2015

### Orodruin

Staff Emeritus
Weak interactions are the only interactions whichd deal with the SU(2) doublets containing quarks with different charges. All other interactions are blind to this difference. In essence, it is akin to asking why only strong interactions can change the colour of a quark with the addition that electroweak symmetry breaking makes the W charged under the residual U(1) symmetry, i.e., electromagnetism.

3. May 19, 2015

### AlanKirby

Thank you for your response. Can I please ask for a recommended text for this material, preferably an introductory text and then separately a more advanced text.

Thanks again.

4. May 19, 2015

### Staff: Mentor

This has nothing to do with masses.
A hypothetical graviton would be massless.
The W boson is the only boson with electric charge, but you could have changes like charm->up or muon->tau with uncharged bosons. It is easy to allow a boson to mediate those interactions, but we never observed those interactions.

5. May 19, 2015

### AlanKirby

Thanks for your response. So are you suggesting that 'idea 2' is the correct way to be thinking about it on a simple level?

6. May 19, 2015

### Staff: Mentor

Well, charge conservation tells you "up -> down + photon" is impossible. It does not tell you "charm -> up + photon" is impossible.