Just in case neutrinos are not the source of CP violation, I am also involved in another experiment at the ILL reactor in Grenoble, France. My British colleagues - assisted by other physicists from around the world - have been working there for many years building experiments to try to measure the static electric dipole moment of the neutron (nEDM). An nEDM would violate time-reversal invariance, but is almost unaffected by the mixing of Kobayashi and Maskawa, so just observing one would be a sign of currently unknown physics. The measurement sounds simple. You make an atomic clock out of neutrons, then you put it in a whopping electric field and look for tiny shifts in the frequency of the clock (of one part in 10¹¹, or one second in 3,000 years) as you switch the sign of the electric field. So far no shift has been seen, producing a limit that the nEDM cannot be bigger than 6.3x10^-26 e.cm. To appreciate just how tiny this number is, consider a neutron made of one unit of negative charge (that is, the charge on one electron) and one unit of positive charge. Then expand it to be the size of the Earth. To produce a nEDM equal to our limit you would only need to offset the centres of the two charges in the Earth-sized neutron by about ten microns.

We can do even better, however, and we are currently building an entirely new experiment, which should be about a factor of a hundred more sensitive than the existing one, based on storing the neutrons in liquid helium cooled to a fraction of a degree above absolute zero. We hope to see a signal this time, but whether we do or don't, we will have learned something important about the structure of the laws of physics. One of these quests to find CP violation must get an answer, because if the universe exists, those laws of physics which produced the matter are out there somewhere.