# I W Boson and different generations of quarks

1. Aug 8, 2017

### Richard McCarthy

Can a W Boson convert an up quark into a down quark of the same generation? Where can I find a resource to understand what all the possibilities are for an up quark converting into other quarks of same or other generations?

2. Aug 8, 2017

### Staff: Mentor

The first generation of quarks is "up-quarks and down-quarks" by definition, up and down are always in the same generation. The second generation is charm and strange and the third one is top and bottom.
Coupling to the W is the only option, and while the coupling with an up-quark to a down is the strongest one, coupling to strange and bottom is also possible.
If the W is not real (e.g. in a beta decay), conservation of energy can make some processes impossible.

3. Aug 8, 2017

### ChrisVer

By looking at the CKM matrix you can immediately see that the transitions between up and down-type quarks are all possible in principle (yet not equally favored)...So for example you could see the transition $t \rightarrow b$, but you would rarely see $t \rightarrow s$... yet again you could see a $c \rightarrow s$ but it's also possible (not extremely rare) to see $c \rightarrow d$. In general I think the CKM matrix tells you that transitions are more likely to occur within the same generation rather than the other...
The transitions happen via the W bosons (or charged currents), since there are no flavor changing neutral currents observed.

4. Aug 9, 2017

### ofirg

I understand that by up and down quark you mean up type quark (t,c,u) and down type quark ( b,s,d). The answer is yes.

The couplings of the W boson to any up type down type quark pair are described by the CKM matrix ( see http://pdg.lbl.gov/2014/reviews/rpp2014-rev-ckm-matrix.pdf for their measured values).

As mfb mentioned, for a process to be possible other conditions have to be met as well, such as Energy and Momentum conservation.

So, for example, the decay $W\rightarrow tb$ is impossible.