|An Asymmetric interaction ...|
Neutrinos are particles that travel at the speed of light and that are only sensitive to the weak force. In principle they exist under two states of spin around the axis defined by their direction of propagation. These two states are called left and right neutrino and they behave very differently. While the first becomes temporarily a W boson and an electron under the effect of the weak interaction, this transition is forbidden for the second to the point that it can not interact or be produced. Similar asymmetries are observed with electrons, quarks and nucleons, but they are however less important. Physicists are used to say that the weak interaction "violates parity".