Investigating neutrinos

Other hints of physics beyond the present Standard Model concern the neutrinos. In the Standard Model these particles have zero mass, so any measurement of a nonzero mass, however small, would indicate the existence of processes that are outside the Standard Model. Experiments to measure directly the masses of the three neutrinos have yielded only limits; that is, they give no sign of a mass for the particular neutrino type but do rule out any values above the smallest mass the experiments can measure. Other experiments have measured neutrino mass indirectly by investigating whether neutrinos can change from one type to another. Such neutrino “oscillations”—a quantum phenomenon due to the wavelike nature of the particles—can occur only if there is a difference in mass between the basic neutrino types.

The first indications that neutrinos might oscillate came from experiments to detect solar neutrinos. By the mid-1980s several different types of experiments, such as those conducted by the American physical chemist Raymond Davis, Jr., in a gold mine in South Dakota, had consistently observed only one-third to two-thirds the number of electron-neutrinos arriving at Earth from the Sun, where they are emitted by the nuclear reactions that convert hydrogen to helium in the solar core. A popular explanation was that the electron-neutrinos had changed to another type on their way through the Sun—for example, to muon-neutrinos. Muon-neutrinos would not have been detected by the original experiments, which were designed to capture electron-neutrinos. Then in 2002 the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, announced the first direct evidence for neutrino oscillations in solar neutrinos. The experiment, which is based on 1,000 tons of heavy water, detects electron-neutrinos through one reaction, but it can also detect all types of neutrinos through another reaction. SNO finds that, while the number of neutrinos detected of any type is consistent with calculations based on the physics of the Sun’s interior, the number of electron-neutrinos observed is about one-third the number expected. This implies that the “missing” electron-neutrinos have changed to one of the other types. According to theory, the amount of oscillation as neutrinos pass through matter (as in the Sun) depends on the difference between the squares of the masses of the basic neutrino types (which are in fact different from the observed electron-, muon-, and tau-neutrino “flavours”). Taking all available solar neutrino data together (as of 2016) and fitting them to a theoretical model based on oscillations between the electron- and muon-neutrinos indicate a difference in the mass-squared of 7.5 × 10⁻⁵ eV².

Earlier evidence for neutrino oscillations came in 1998 from the Super-Kamiokande detector in the Kamioka Mine, Gifu prefecture, Japan, which was studying neutrinos created in cosmic ray interactions on the opposite side of Earth. The detector found fewer muon-neutrinos relative to electron-neutrinos coming up through Earth than coming down through the atmosphere. This suggested the possibility that, as they travel through Earth, muon-neutrinos change to tau-neutrinos, which could not be detected in Super-Kamiokande. These efforts won a Nobel Prize for Physics in 2002 for Super-Kamiokande’s director, Koshiba Masatoshi, and a Nobel Prize in 2015 for Japanese physicist Kajita Takaaki. Davis was awarded a share of the 2002 prize for his earlier efforts in South Dakota. SNO director Arthur B. McDonald shared the 2015 prize with Kajita.

Experiments at particle accelerators and nuclear reactors have found no conclusive evidence for oscillations over much-shorter distance scales, from tens to hundreds of metres. “Long-baseline” experiments have found oscillations of muon-neutrinos created at accelerators over distances of a few hundred kilometres. The aim is to build up a self-consistent picture that indicates clearly the values of neutrino masses.

Linking to the cosmos

Massive neutrinos and supersymmetric particles both provide possible explanations for the nonluminous, or “dark,” matter that is believed to constitute 26.5 percent of the mass of the universe. This dark matter must exist if the motions of stars and galaxies are to be understood, but it has not been observed through radiation of any kind. It is possible that some, if not all, of the dark matter may be due to normal matter that has failed to ignite as stars, but most theories favour more-exotic explanations, in particular those involving new kinds of particles. Such particles would have to be both massive and very weakly interacting; otherwise, they would already be known. A variety of experiments, set up underground to shield them from other effects, are seeking to detect such “weakly interacting massive particles,” or WIMPs, as Earth moves through the dark matter that may exist in the Milky Way Galaxy.

Other current research involves the search for a new state of matter called the quark-gluon plasma. This should have existed for only 10 microseconds or so after the birth of the universe in the big bang, when the universe was too hot and energetic for quarks to coalesce into particles such as neutrons and protons. The quarks, and the gluons through which they interact, should have existed freely as a plasma, akin to the more-familiar plasma of ions and electrons that forms when conditions are too energetic for electrons to remain attached to atomic nuclei, as, for example, in the Sun. In experiments at CERN and at the Brookhaven National Laboratory in Upton, New York, physicists collide heavy nuclei at high energies in order to achieve temperatures and densities that may be high enough for the matter in the nuclei to change phase from the normal state, with quarks confined within protons and neutrons, to a plasma of free quarks and gluons. One way that this new state of matter should reveal itself is through the creation of more strange quarks, and hence more strange particles, than in normal collisions. CERN has claimed to have observed hints of quark-gluon plasma, but clear evidence will come only from experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the Large Hadron Collider at CERN. These experiments, together with those that search for particles of dark matter and those that investigate the differences between matter and antimatter, illustrate the growing interdependence between particle physics and cosmology—the sciences of the very small and the very large.