# How do we know that the $W$ and $Z$ bosons aren't composite?

Background to my reasoning: When the strong force was first discovered, it was considered to be the force that interacts between nucleons to keep the nucleus together and that the mediator bosons of this force were the massive pions ($$\pi^0, \pi^+, \pi^-$$). After this, we figured out that the pions were composite particles and the strong force we were observing was a residual effect. The fundamental strong force has eight massless mediator bosons (eight gluons).

Keeping this in mind, the weak force is mediated by three massive bosons ($$Z^0, W^+, W^-$$). Is it possible that these particles are composite and there is a more fundamental weak force mediated by massless bosons?

Question: Ignoring the success of electroweak theory...

Could the W and Z bosons be composite?

And could there be a more fundamental weak force mediated by massless bosons?

We don’t know that the $$W$$ and $$Z$$ are fundamental, but we have no experimental evidence that they aren’t. So the Standard Model makes the simplest assumption compatible with the evidence, and that is that they are fundamental.