How can one rule out the possibility of there being neutral quarks?

Question

This is a very simple question, which will most likely yield a prompt response from someone who knows more than I. The reason I ask:The quarks that we can detect (as they interact electromagnetically) are much lighter than the particles that they make up. From what I know, we say that gluons augment the energy level of the quarks when they combine to form baryons, yet if there are quarks that are made up of partial charges, how can we rule out the possibility of another type of quark made up of partial neutrals(or just a uniform neutral quark)(?). One could say that neutral doesn't exist, and it is merely a combination of partial charges that sum to zero. Yet neutrinos exist as leptons, which are independent of quarks. Is it possible that there is a relationship between the gluon, gauge bosons, and lepton pairs that all has to do with the instant something such as a beta decay occurs subsequent to an ephemeral appearance by a W boson? And that they would all turn out to be the same thing just for the instant the gauge boson acts as a mediator?

Answer 1

Neutral quarks can be excluded experimentally (up to a certain mass), because the neutrinos match the invisible decays of Z bosons perfectly. If there were neutral quarks below half the Z mass, the Z boson should also decay into these.

For higher masses, things get more involved. Finding out that you're not seeing something going on is hard, but our experimental physicists can do just that. And they don't see any "invisible" particles.

Finally, neutral quarks would also affect the running of the strong coupling which is measured to good precision. New quarks, i.e. new color charged particles, would change the rate at which the strong interaction becomes weaker with higher energies (it would become weaker even more quickly). Again, calculations using particles from the Standard Model match the observation excuisitely, just like for the invisible Z dacays.