How many point-like elementary particles do we have?

Question

I think the question is self-explanatory. Besides electrons and positrons, are there any other point-like particles? Can quarks behave as dimensionless particles as well? Is there any theoretical support rather than experimental, asserting that, say, electrons must be point-like?

Answer 1

In the standard model of particle physics there are seventeen distinct species of elementary particles - twelve fermions and five bosons. Each of these elementary particles is a point particle i.e. an elementary particle has no internal structure in the standard model. Note, however, that the uncertainty principle tells us that even a point particle cannot be assigned a definite location.

However, we know that the standard model is not the final word in particle physics - it does not describe gravity, for example. And there are extensions to the standard model (such as string theory) in which the elementary particles are not point particles.

Answer 2

A possible substructure of elementary particles can be probed in high energy physics experiments, as a collision energy $E$ corresponds roughly to a spatial resolution of $$\lambda \sim \frac{h c}{E}= 1.2 \times 10^{-18} \, {\rm m} \;\frac{\rm TeV}{E}.$$ With the currently available energies at collider experiments, no substructure of the standard model particles $$e^\pm, \, \mu^\pm, \, \tau^\pm, \, \stackrel{(-)}\nu_{\!e}, \,\stackrel{(-)}\nu_{\!\mu}, \, \stackrel{(-)}\nu_{\!\tau}, \, \stackrel{(-)}u, \, \stackrel{(-)}d, \, \stackrel{(-)}c, \stackrel{(-)}s, \, \stackrel{(-)}t, \, \stackrel{(-)}b, \, W^\pm, \, Z^0, \, \gamma, \, H^0, \, 8 \, {\rm gluons} $$ could be detected. In this sense, these particles are considered as "pointlike", with their corresponding fields being the basic building blocks of the standard model. Note that quarks and gluons cannot be detected as free particles (asymptotic states) as they are confined in hadrons (like proton, neutron or pions).

There is no (known) theoretical reason why the standard model particles must be point particles. Theoretical models where leptons or quarks do have a substructure exist, but at the end only experiment can decide.