Meetings: Neutrino Woodstock

The early neutrino studies of Kajita, McDonald, and others relied on neutrinos from the Sun and from cosmic-ray interactions in the atmosphere. Many of the newer experiments use “artificially made” neutrinos from nuclear reactors and particle accelerators. One of the most sophisticated examples is the Daya Bay experiment, which is made up of eight identical detectors arranged around a nuclear power plant in China. At the meeting, Zeyuan Yu from China’s Institute of High Energy Physics reported the latest Daya Bay results, confirming earlier evidence of relatively strong neutrino oscillations at distances of several hundred meters.

The physical explanation for neutrino oscillations is that the flavor neutrinos are superpositions of separate neutrino mass states (deemed $m_1$, $m_2$, and $m_3$). This so-called mixing causes the probability of detecting a certain flavor to vary with distance, like a wave. This wave’s amplitude is characterized by three angles ( ${𝜃}_{12}$, ${𝜃}_{23}$, ${𝜃}_{13}$), and its frequency depends on the mass squared differences (say, $m_1^2-m_2^2$). Earlier work on solar and atmospheric neutrinos constrained the first two angles ( ${𝜃}_{12}\approx 34^{\circ }$, ${𝜃}_{23}\approx 45^{\circ }$) and the mass squared differences (roughly $10^{-4}{\text{eV}}^2$), but the third mixing angle, ${𝜃}_{13}$, remained largely unknown. Many researchers assumed it would be close to zero—implying little mixing between the first and third mass states.

In 2012, the Daya Bay experiment caused a stir when it reported conclusive evidence that ${𝜃}_{13}$ is not zero (see 23 April 2012 Viewpoint). The latest results from Daya Bay are consistent with the previous findings: they show that ${𝜃}_{13}$ is around $8^{\circ }$, with the highest precision of any experiment.

The value of ${𝜃}_{13}$ has important implications for other experiments that are looking for a potential difference between neutrinos and antineutrinos (see 10 February 2014 Viewpoint). If such a difference existed, neutrinos could oscillate more (or less) than antineutrinos, and a nonzero ${𝜃}_{13}$ means that this disparity will be easier to spot. Taking advantage of the relatively large ${𝜃}_{13}$, two accelerator-based experiments in Japan and the U.S. are finding possible hints of this neutrino/antineutrino asymmetry, which may help explain why the Universe contains far more matter than antimatter.

Outside of their electric charge, most particles are identical to their antiparticle partner: an electron, for example, has the same interaction with photons as does a positron. This equivalence is referred to as charge-parity (or $CP$) symmetry. In terms of neutrinos, a breaking of $CP$ symmetry—characterized by the $CP$ violating phase, ${𝛿}_{CP}$—would leave a detectable imprint on oscillation results.

One experiment hunting for $CP$ violation is the T2K experiment in Japan, which generates a beam of muon neutrinos at the Japan Proton Accelerator Research Complex and detects them 295 kilometers away at the Super-Kamiokande detector. Recently, the T2K project switched from a neutrino beam to an antineutrino beam. Hirohisa Tanaka of the University of Toronto in Canada said that T2K has so far detected four antineutrino events, which is less than would be expected under $CP$ symmetry. A preliminary analysis by T2K suggests that ${𝛿}_{CP}$ is near one of its maximum values of $3𝜋∕2$. A related experiment in the U.S., called NOvA, presented a similar ${𝛿}_{CP}$ value.

The preliminary agreement between the T2K and NOvA results may seem promising, but Enrique Fernández Martínez, from the Autonomous University of Madrid, argued in his talk that current experiments will not be able to reach a $5𝜎$ result—the gold standard of a discovery. That level of statistics will require a next-generation machine, such as the Deep Underground Neutrino Experiment (DUNE). Considered the next global flagship neutrino project, DUNE will have the world’s most intense neutrino beam (1.2 megawatts) and largest liquid-argon detector (40 kilotons). In his DUNE status report, Jon Urheim of Indiana University said the experiment, which already has 800 international collaborators, aims to be operational by 2026.