The Neutrino Mass
Meta-page: the mass that the Standard Model forgot to include.
The assumption
For fifty years, the Standard Model assumed neutrinos have zero mass. The assumption followed from the model's structure: left-handed neutrino fields exist but no right-handed ones. Without right-handed fields, there is no Dirac mass term. The model's particle content did not allow neutrino mass.
This was not a problem for most of the Standard Model's history. Neutrino oscillation had not been discovered. All experiments were consistent with massless neutrinos. The model's prediction was confirmed.
The contradiction
Oscillation requires non-zero mass. Two of the three mass eigenstates must be massive. The mass-squared differences are measured: Delta m^2_21 is approximately 7.5 times ten to the negative fifth eV squared, and |Delta m^2_31| is approximately 2.5 times ten to the negative third eV squared. If one eigenstate is massless, the heaviest is at least fifty meV. Cosmology bounds the sum of the three masses to less than approximately 0.12 eV. The masses are small but non-zero.
The Standard Model was wrong.
The possibilities
The Dirac mass
Add right-handed neutrino fields. The Higgs mechanism generates Dirac mass terms, just as for quarks and charged leptons. The right-handed neutrinos would be sterile — invisible particles with mass. Three new fermion fields, singlets under the Standard Model gauge group. Their masses would be arbitrary; achieving meV-scale masses requires Yukawa couplings a billion times smaller than the electron's. Technically possible. Aesthetically unsatisfactory.
The Majorana mass
Neutrinos are their own antiparticles. The right-handed neutrino is the CP conjugate of the left-handed neutrino. No new field is needed. The mass term violates lepton number by two units. The seesaw mechanism: a large Majorana mass scale suppresses the light neutrino mass. At the GUT scale, the light mass is approximately the electroweak scale squared divided by the GUT scale — approximately 0.1 eV, the right order of magnitude.
The seesaw explains the smallness of the mass. It connects neutrino physics to grand unification. But it predicts neutrinoless double beta decay, which has not been observed.
Radiative mass
Neutrino mass is generated by quantum loops rather than tree-level Higgs coupling. Loop suppression and new particle masses naturally produce small masses. These models often involve new scalars or fermions at the TeV scale, potentially testable at colliders.
What we know
We know the mass-squared differences. We know the mixing angles. We know the sum is less than 0.12 eV. We do not know the absolute scale. We do not know the mass ordering. We do not know if neutrinos are Dirac or Majorana. We do not know the mechanism.
What the mass means
Neutrino mass is the clearest evidence that the Standard Model is incomplete. It is not a small correction. It is a structural failure — the model's particle content is insufficient. The mass is the smallest of any fermion, small enough to remain invisible for fifty years, small enough that we are still measuring it.
The neutrino mass is the gap in the Standard Model. The smallest crack in the most successful theory of particle physics. Through that crack, something new may be entering.