# 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?

## 1 Answer

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.

We have another 14 orders of magnitude of energy to explore between the LHC scale and the Planck scale, so who knows what we might find if we could do physics at significantly higher energies.

As far as I know, there is no theoretical reason why the weak bosons can’t be composite. For example, here is a 2009 paper that discusses such a model: https://arxiv.org/abs/0907.0906

• One should add that there are theories where H, and I think W and Z,are not fundamental , the technicolor ones . With the discovery of the higgs they are not so alive now, arxiv.org/abs/hep-ph/9401324. – anna v Jun 13 '19 at 6:06