Neutrinos, some of the most abundant particles in the universe, are also among the most mysterious. We know they have mass but not how much. We know they come in at least three types, or 'flavors' — but there may be more. A new study found that a mismatch between observations of galaxy clusters and measurements of the cosmic background radiation could be explained if neutrinos are more massive than is usually thought. It also offers tantalizing hints that a fourth type of hitherto unseen neutrino exists.
The tension between galaxy clusters and the cosmic microwave background (CMB) has been a brewing problem, albeit one that might be resolved simply by getting better measurements in the coming years (see 'Missing galaxy mass found'). The background radiation shows the small density variations in the early universe that would eventually cause matter to clump in some places and form voids in others. We can see the end product of this clumping in the recent universe by observing the spread of galaxy clusters across space.
Theorists have long suggested that a fourth type of neutrino might exist, but so far proof of them has been elusive. Hints at some particle accelerator experiments2 lately have begun to suggest they are out there, however. "What's really interesting is that the mass of this sterile neutrino, is consistent with what the other experiments see," says physicist Joseph Formaggio of the Massachusetts Institute of Technology in Cambridge. "I think people are starting to look at the data and say maybe there's something there." And coincidentally another study3 supporting the idea of a sterile neutrino as well as heavier neutrino masses was also published in the same issue of PRL.
For many years neutrinos were thought to be completely massless, but the discovery that they can swap flavors also proved that they have at least a little bit of mass. Each flavor’s state is thought to be a mixture of the three unknown neutrino masses — called mass 1, mass 2 and mass 3 for the time being — and this mixing is why any flavor has a chance of turning into one of the other flavors over time. The transformation is only possible if the mass states are different from one another, and such a difference is only possible if neutrinos' mass is nonzero, Formaggio explains.
Experiments aiming to catch neutrinos in the act of switching flavours could help pin down the differences between the neutrino masses and tell us which weighs more—the so-called neutrino-mass hierarchy. One such experiment, called NuMI Off-Axis νe Appearance (NOvA), measured its first neutrinos last week. The experiment creates a beam of neutrinos at the Fermi National Accelerator Laboratory (Fermilab) near Chicago and sends them to two detectors — one near Fermilab and another 800 kilometres away in Ash River, Minnesota. All of the particles start as muon neutrinos but some precious few arrive at the distant detector having turned into electron neutrinos, which create a different signature. The frequency at which this happens is related to the difference between electron and muon neutrinos' masses.
Another experiment based in Japan called the Japanese Tokai to Kamioka (T2K) project also looks for these transformations. The collaboration announced last week that it had observed a record total of 28 candidate mutations from muon into electron neutrinos, with only about five of the events predicted to be other processes masquerading as the real thing. It is the strongest evidence to date for this type of neutrino oscillation, although much more data will be needed to answer questions about neutrinos' masses. "It's sort of like a big mile marker in a long race," says Formaggio, who wrote an essay accompanying the publication of the result on 10 February in PRL. The two experiments are complementary, says NOvA deputy project leader Rick Tesarek. "There are some capabilities that NOvA has that T2K doesn’t have" and vice versa. The experiments use different detector technology that is sensitive to different effects, and the NOvA project has a longer distance between its neutrino beam and the far detectors.
As these experiments gather more data the secrets of neutrino masses may be revealed. The coming years should also clarify whether galaxy cluster measurements are truly incompatible with the cosmic background radiation data, and hence whether they point toward heavier neutrino masses and/or a sterile neutrino.