seesaw mechanism
/ SEE-saw /
Neutrinos are real puzzles: they have mass, but a mass so tiny it is at least a million times smaller than the next-lightest particle, the electron. Why so absurdly light? The seesaw mechanism offers a clever answer using a playground image. On a seesaw, if one child is very heavy, the other is lifted very high; the heavier one side, the lighter the other floats. The mechanism links the neutrino's lightness to the existence of a partner that is fantastically heavy — the heavier that partner, the lighter the neutrino we observe.
Here is the working idea. Suppose there exists a new, very heavy neutrino partner that, unlike all known matter particles, is its own antiparticle (a 'Majorana' particle). When the ordinary neutrino mixes with this heavy partner, the mathematics produces two masses whose product is fixed by a normal energy scale; so the bigger one mass is forced to be, the smaller the other must become. The ordinary neutrino ends up with a mass roughly equal to the usual scale squared divided by the huge partner mass — naturally tiny precisely because the partner is so heavy.
The seesaw matters because it kills two birds with one stone: it explains why neutrino masses are so small, and it ties them to physics at an extremely high energy, plausibly near grand unification. As a bonus, the heavy partners decaying in the early universe could help explain why there is more matter than antimatter (an idea called leptogenesis). The honest caveat: the seesaw is unverified. It hinges on neutrinos being their own antiparticles, which would show up as a rare process called neutrinoless double beta decay — searched for intensely, but never yet seen.
If a hidden neutrino partner weighs near the grand-unification scale, the seesaw automatically squeezes the ordinary neutrino's mass down to a fraction of an electronvolt — exactly the ballpark that oscillation experiments suggest.
A super-heavy partner forces the everyday neutrino to be super-light.
The seesaw assumes neutrinos are Majorana particles — their own antiparticles. If neutrinoless double beta decay is never observed, that assumption fails and the simplest seesaw would not be how neutrinos get their mass.