Neutrinos, some of nature’s weirdest fundamental particles, are nearly massless—emphasis on nearly. They were predicted to be completely massless, but experiments roughly 20 years ago found they surprisingly do have some mass. Just how much has remained a mystery. Now a new calculation based on cosmological observations places an upper limit on how heavy the lightest kind of neutrino can be.
There are many strange things about neutrinos: their unexpected heft, for one thing, and that they rarely interact with other matter and are passing through our bodies by the billions each moment. Perhaps the oddest aspect of these particles is their tendency to switch identities, cycling between the three possible “flavors,” or types. In fact, it was the observation of this shape-shifting ability in the first place that told scientists the three neutrino flavors must have different masses—which means, of course, that all of their masses cannot be zero.
Scientists would desperately like to know what they actually weigh, which would be a vital clue about why they have mass, given that they do not seem to acquire it the way other particles do: through the Higgs field (associated with the Higgs boson, which was discovered in 2012). “Understanding why particles have mass is something very fundamental in how we understand physics,” says physicist Joseph Formaggio of the Massachusetts Institute of Technology. “What neutrinos pose is the possibility that the mechanism we think gives rise to masses for all the particles may not apply, for some strange reason, to neutrinos. I find that exciting.”
The new mass limit comes from a supercomputer calculation that combined data on the distribution of galaxies throughout the universe, the remnants of the first light released after the big bang and supernova measurements that reflect the expansion rate of the cosmos. The analysis also used laboratory data on neutrinos, such as measurements of the rates that they switch between flavors, to arrive at an estimate of the maximum weight of the smallest one: 0.086 electron volt, or 0.00000000000000000000000000000000000015 kilogram—making it at least six million times lighter than an electron.
“What they have done is really nice work,” says Olga Mena of the Institute of Corpuscular Physics in Spain, who has worked on similar calculations. André de Gouvêa, a theoretical physicist at Northwestern University, says, “It’s a slightly more detailed analysis of cosmological data than people had done before. It’s quite a nice paper.” The results, initially posted to the preprint server arXiv.org, were published on August 22 in Physical Review Letters (PRL). Another limit, calculated through similar methods by physicists Shouvik Roy Choudhury and Steen Hannestad, was also recently posted on arXiv.org and is now under peer review at the Journal of Cosmology and Astroparticle Physics.
Why should far-flung measurements of galaxies and supernovae have any bearing on the mass of the lightest matter particle in the universe? Because neutrinos, though small, do have a gravitational effect on everything else through their tiny mass. As they travel through space at nearly the speed of light, they tend to pull other particles with them ever so slightly, leading to an overall blurriness in the spread of galaxies throughout the cosmos. “It’s like if you're shortsighted, and you take your glasses off,” says Arthur Loureiro of University College London, first author of the PRL study. “They make things fuzzier.” And the extent of this fuzziness can tell us how much mass neutrinos have.
The estimate complements other efforts to weigh neutrinos that focus on laboratory experiments. For instance, a project called the Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany aims to measure neutrino mass by observing beta decays in which a neutron transforms into a proton by releasing a neutrino and an electron. By carefully measuring the energy of the electron, scientists can infer the mass of the neutrino. In contrast to cosmology-based estimates, which include uncertainties from assumptions about unknowns such as dark matter and dark energy, this kind of experiment is more direct. “It sort of makes the least assumptions, but unfortunately, it’s the least sensitive right now,” says Formaggio, who works on KATRIN and similar experiments.
A third class of studies search for a fabled decay process known as “neutrinoless double beta decay,” in which two neutrons transform into two protons, releasing the expected electrons but not the corresponding neutrinos. This phenomenon could happen if neutrinos turn out to be their own antimatter partner particles—a theoretical possibility but far from a certainty. If so, the two neutrinos emitted would annihilate each other, as all matter and antimatter partners do when they meet. If neutrinoless double beta decay can be measured, the strength of the decay would be proportional to the lightest neutrino mass. So far, though, no experiment has seen it.
The Theory’s Missing Piece
Ultimately, scientists must compare the results from all these different methods. “Only by combing all the possible ways of measuring the neutrino mass will we have a finite and robust answer,” Mena says. But if the estimates differ, some scientists say, all the better. “One thing that’s exciting is: What if we make a measurement from cosmology, and we get an answer that doesn’t agree with particle physics measurements?” de Gouvêa says. “That would be indicative of the fact that there’s something in this picture that’s just wrong. Maybe there’s something wrong with our understanding of the early universe. Or maybe there’s something unusual about the mechanism for neutrino masses, like the mass depends on where you are or when you make the measurement. It sounds crazy, but it’s possible.”
Even without evidence for such outlandish scenarios, finding a reliable estimate of neutrino mass would push physics in a new direction. The Standard Model of particle physics, the best theory researchers have to describe the particles and forces in the universe, predicted neutrinos were weightless. The fact that they are not presents the possibility of expanding the theory. “The Standard Model is one of the most precise theories that humanity has ever built,” Loureiro says, “but it’s missing a bit. Finding the missing piece about neutrinos could definitely be the key to understanding what dark energy and dark matter are, because they are also not in the Standard Model.”
The cosmological piece of the answer stands to get more precise in the next decade, as some eagerly awaited new telescopes come online. The European Euclid telescope, for instance, will drastically improve the precision of 3-D cosmic maps after it launches in 2022. And the Dark Energy Spectroscopic Instrument in Arizona will soon begin surveying the distances of 30 million galaxies. Finally, the Large Synoptic Survey Telescope, under construction in Chile, will image the whole sky every few nights, starting in 2022. “Everybody is very excited,” de Gouvêa says, “because in the next five-ish years, they should get to a sensitivity that they should actually see something—they will be in a position to make an observation, as opposed to just setting a bound.”