Search at SLAC Finds No Millicharged Particles
The quantization of electric charge is very well tested: The proton and electron charges ±e are equal and opposite to better than a part in 1021, and the neutrality of the neutron, photon and neutrino have been experimentally affirmed with comparable precision. And all attempts to find naked fractionally charged quarks have failed thus far, as the theory of quark confinement tells us they must.