neutrino weak interaction

Why is a neutrino so hard to catch? The answer is that it can only talk to the rest of the world through one channel — the weak force — and that channel is extraordinarily narrow. Electrically charged particles can bump into each other electromagnetically, and quarks are bound tightly by the strong force, but a neutrino feels neither. The weak force is its only handle on reality, and the weak force acts only at incredibly short range and with feeble strength.

Concretely, when a neutrino does interact it does so by exchanging a W or a Z boson — the heavy carriers of the weak force. In a charged-current event it trades a W boson and turns into its partner charged lepton: an electron neutrino can strike a neutron and come out as an electron, for instance. In a neutral-current event it exchanges a Z boson and simply gives a passing nudge, recoiling off without changing identity. Either way the exchange is rare because the W and Z are so heavy that they can exist only for the merest instant, making the effective reach of the force about a thousandth the size of a proton.

This near-invisibility is both a curse and a gift. It is a curse because detecting neutrinos demands enormous targets — tanks holding thousands of tonnes of water or liquid — and patience for the few that interact. It is a gift because a particle that ignores almost everything can carry pristine information out of the densest, most violent places in the cosmos. The same weakness that hides neutrinos is exactly what makes them unique messengers from the cores of stars and supernovae.