http://en.wikipedia.org/wiki/Standard_Model
Look at the 'periodic table' on the right. Everything has a place, and makes sense. The quarks and leptons have their pattens with increasing mass ect, and there is one Gauge boson for each basic force of nature. If there was a second Z boson... where would you put it?
The model ain't the reality.
The standard model has six quarks, up from the three initially proposed.
In fairness, discarding data which does not fit the model has not been without it's practitioners. However, the method's history is somewhat less than distinguished.
> In fairness, discarding data which does not fit the model has not been without it's practitioners.
I'm not trying to say we should discard data, I am pointing out that we would need to discard our model. (or at least a part of it) And discarding a model that has been around for so long is always surprising. (or at least to some people)