Left and right-handed neutrinos
Neutrinos like electrons and quarks have two possible states of rotation or spin. Only one of the two states are produced : the neutrinos having a "left-handed helicity" , as indicated on the figure with respect to the direction of motion. Riht-handed neutrinos are not observed: they are not produced and do not interact. In the world of antimatter is the opposite: only rights antineutrinos are produced and observed. This remarkable property is a feature of the weak interactions.