Neutrino Decay Reconciles Two Worlds express
In the thirties, Pauli realized: without tiny ghost-like particles, the energy of stars doesn't add up. Thus neutrinos were born — intangible wanderers that permeate everything. They must have mass, otherwise they can't transform into one another, but observations of the expansion of the Universe showed their total weight was nearly zero. Meanwhile, laboratory experiments insisted on a larger number.
A new study compares neutrinos to potatoes in a leaky sack. If half the tubers go missing along the way, weighing the remainder will leave you short. It's the same with cosmology: if neutrinos decay into completely invisible particles over a period of ten million to a billion years, today we measure only their surviving fraction. Fresh data from the DESI survey and the cosmic microwave background from the Big Bang allow a mass up to four times higher than previous estimates — enough to settle the dispute.
An unexpected twist: decay into familiar neutrino types yields almost no benefit. The sack still seems full. So nature is hiding something beyond the Standard Model inside it — particles we don't yet know about. That's where the search should go.
Original abstract
as written by the authors arXiv:2601.04312v2
A new tension is emerging between the tight cosmological upper bounds on the total neutrino mass ($$\sum m_ u \lesssim 0.06 \, \rm{eV}$$) and the lower limits from oscillation experiments, with potentially far-reaching implications for cosmology and particle physics. Neutrinos decaying into massless BSM particles with lifetimes $$\tau_ u \sim 0.01-1\, \rm{Gyr}$$ represent a theoretically well-motivated mechanism to reconcile such measurements. Using DESI DR2 and CMB datasets, we show that such decays relax the bound on the total neutrino mass up to $$\sum m_ u < 0.23 \, \rm{eV}$$ (95%), restoring full agreement with oscillation data. We also present the first late-time cosmological analysis of neutrino decays into lighter neutrinos in a manner consistent with the measured mass splittings. In contrast to the decays into massless BSM particles, we find that this scenario only marginally alleviates - or even tightens - the cosmological neutrino mass bounds, depending on the mass ordering.
There are almost as many neutrinos in the Universe as photons of the cosmic microwave background — about 330 per cubic centimeter.