Physics

MAJORANA

Hubble's law

Neutrino oscillation

Particle physics

Sterile neutrino

Engineering

Coupling (piping)

Coupling constant

Neutrino

Dark energy

Weak interaction

Cosmology

Sigma

Astrophysics

Quantum mechanics

Mechanical engineering

Physical Review Letters

Large, nonstandard neutrino self-interactions have been shown to resolve the $\ensuremath{\sim}4\ensuremath{\sigma}$ tension in Hubble constant measurements and a milder tension in the amplitude of matter fluctuations. We demonstrate that interactions of the necessary size imply the existence of a force carrier with a large neutrino coupling ($>{10}^{\ensuremath{-}4}$) and mass in the keV--100 MeV range. This mediator is subject to stringent cosmological and laboratory bounds, and we find that nearly all realizations of such a particle are excluded by existing data unless it carries spin 0 and couples almost exclusively to $\ensuremath{\tau}$-flavored neutrinos. Furthermore, we find that the light neutrinos must be Majorana particles, and that a UV-complete model requires a nonminimal mechanism to simultaneously generate neutrino masses and appreciable self-interactions.

Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension