What is the mass of a neutrino?

It’s a simple question to which we can provide an answer for every other particle: What is the weight of it?

The neutrino is extremely small. For many years, scientists thought they had no mass. Experiments demonstrating that neutrinos shift type disproved this, although the neutrino’s absolute mass remains unknown. What we do know is that the three types of neutrinos have distinct masses, and that the total mass of all three is still less than one millionth that of an electron.

Electronvolts are commonly used to measure (and weigh) particles. According to current best estimates, the sum of the three neutrinos’ masses should be less than one electronvolt. One electron, for example, has a mass of 511,000 electronvolts. A neutrino is 10 billion, billion times smaller than a grain of sand, to put it another way. This is already stunning; the Standard Description, physicists’ best model of the cosmos, says that neutrinos should have no mass.

It’s not easy to design an experiment that will provide the answer to this riddle. It’s a challenging puzzle to solve since neutrinos are so tiny and interact so infrequently. Why are neutrinos so tiny, how can they have mass, and how much do they weigh? These are just a few of the issues that keep neutrino scientists awake at night.

Neutrinos are the lightest of the Standard Model’s large basic particles. We know neutrinos have mass because we’ve seen them change from one flavor to another, which is only possible if the neutrinos have mass. Surprisingly, that technique also necessitates different masses for the various flavors. However, the flavor change process is only dependent on the masses of the various flavors being different; it does not reveal anything about the neutrino’s real mass.

The simplest answer to the question “Why don’t we know the neutrino’s absolute mass?” is that it’s extremely difficult to measure and scientists haven’t been able to do so yet. Experiments exploring the cosmological evolution of the universe, in which neutrinos played a major role, as well as experiments looking at the decay of radioactive isotopes, have all attempted to make this measurement.

Neutrinos have been proposed as a probable dark matter candidate in the past: we know the cosmos is filled of neutrinos, and we know dark matter accounts for the majority of the universe’s mass. However, because neutrinos’ mass and energy do not fit within the range required by current dark matter models, we now know that they cannot make up a major fraction of dark matter.

Charged leptons and quarks get their masses via interaction with the Higgs boson, but neutrinos don’t always get theirs that way. It’s also not fully ruled out. According to one idea, the neutrino has an extremely heavy partner that only exists for a short period and interacts with the Higgs boson to generate the light neutrinos we see. The mass of the light neutrino would thus be in a “seesaw” connection with the mass of the heavy partner through the Higgs mechanism—as the mass of the partner increases, the mass of the light neutrino decreases. However, this seesaw association has yet to be tested in the lab.