Why Can't a Top Quark Decay Into a Charm or Up Quark?

[Paul159]

Hello,

I have a very basic question : why a top quark for example cannot decay into a charm or up quark ?

The fact is that I don't really understand where the concept of up- and down-type quark come from (except that they have the same charge). Why a up-type quark cannot transform into another up-type quark ? Is this only an experimental fact or is there a theoretical result prohibiting it ?

Thanks.

 

[mfb]

The Standard Model has no vertex with a top quark and charm or up. It can still happen via loop processes, but it's so rare we have never seen it. We do see b->s and b->d transitions, however. This is called flavor-changing neutral current.
Why is there no such vertex? Well, ultimately it's a result of observations. You can write down theories that have such an interaction, but they don't seem to exist, or they must be so rare that we haven't seen them. The strong interaction doesn't allow any quark transitions, and the weak interaction only allows up-type to down-type and vice versa via coupling to the W boson (which has an electric charge).

 

[Paul159]

Thank you very much.