[stremba]

You lose me when it comes to advance theoretical physics, but isn't it possible that we know of ONLY three particle families because we haven't developed particle accelerators capable of imparting sufficient energies to particles to produce a more massive 4th family (or higher) of particles? I assume that any hypothetical fourth family of particles would be more massive than the known three. I am sure it would throw the standard model for a loop, but it is pretty much a consensus view among physicists that the standard model is not a complete theory of elementary particles, isn't it?

[Physicist]

Good question. We have evidence that there are exactly three families of fundamental fermions.

In the lepton sector, there seems to be some sort of physical principle that makes the neutral leptons--the neutrinos--extremely light. If the same rule applies to a 4th generation, we can rule it out, because for one thing, it would distort the Z lineshape in a characteristic way, and for another, we'd see it directly in the anomalous single photon cross section. (That's the probability of seeing an electron-positron collision result in a single, high-energy photon; it's caused by radiation during a neutrino pair-production event.)

So the 4th generation neutrino must be very heavy, and it must be very stable because of lepton number conservation. That in itself runs afoul of cosmological constraints.

In the quark sector, there are two more evidences. First, the Higgs mechanism causes the heaviest quark to become extremely heavy compared to the others. If there's a 4th generation, it becomes hard to explain why the top quark is so heavy.

Second, 4th-generation quarks would violate the unitarity of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix. This is a 3x3 matrix that describes how the various flavors of quarks transform into one another in weak interactions. If there are only three generations, it must be a unitary matrix, which is to say that (for example) the probability of a top quark changing to a bottom quark, plus the probability of it changing to a strange quark, plus the probability of it changing to a down quark, must all add up to 100%. (Similarly for the other quarks.) If there's a 4th generation, these won't always add up to 100%.

We can measure the various CKM matrix elements independently, and the unitarity constraints are getting pretty tight.

So while it's possible that a 4th generation exists--never say never--the cracks in which it can hide are getting awfully thin.