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  • Andrew Grant

Six reasons to get excited about neutrinos

Extra Dimensions: New results and upcoming experiments offer hope that neutrinos hold the key to expanding the standard model.

The headlines from the recent International Conference on High Energy Physics (ICHEP) in Chicago trended sad, focused on the dearth of discoveries from the Large Hadron Collider. (See, for example, “Prospective particle disappears in new LHC data.”) Yet there was some optimism to be found in the Windy City, particularly among neutrino physicists. Here are six reasons to believe that neutrinos might provide the window into new physics that the LHC has not:

Neutrinos are proof that the standard model is wrong. Sure, we know that dark matter and dark energy are missing from the standard model. But neutrinos are standard-model members, and the theoretical predictions are wrong. Prevailing theory says that neutrinos are massless; the Nobel-winning experiments at the Sudbury Neutrino Observatory and Super-Kamiokande demonstrated definitively that neutrinos oscillate between three flavors (electron, muon, and tau) and thus have mass. André de Gouvêa, a theoretical physicist at Northwestern University, deems neutrinos the “only palpable evidence of physics beyond the standard model.” Everything we learn about neutrinos in the coming years is new physics.

This signal from May 2014 denotes the detection of an electron neutrino by Fermilab’s NOvA experiment. Credit: NOvA Neutrino Experiment. (Click to enlarge.)

Neutrinos’ ability to morph from one flavor to another is only now starting to be understood. Each of neutrinos’ three flavors is actually a quantum superposition of three different mass states. By understanding the interplay of the three mass states, characterized by parameters called mixing angles, physicists can pin down how neutrinos transform between flavors. Fresh data from the NOvA experiment at Fermilab near Chicago suggest that neutrino mixing may not be as simple as most theories predict.

Neutrinos may exhibit charge conjugation–parity (CP) violation. All known examples of CP violation, in which particle decays proceed differently with matter than with antimatter, take place in processes involving quark-containing particles like kaons and B mesons. But at the Neutrino 2016 meeting in London and at ICHEP, the T2K experiment offered fresh data hinting at matter–antimatter asymmetry for neutrinos. After firing beams of muon neutrinos and antineutrinos at the Super-Kamiokande detector in Japan, scientists expected to detect 23 electron neutrinos and 7 electron antineutrinos; instead they have spotted 32 and 4, respectively. T2K isn’t anywhere close to achieving a 5 σ result, but the evidence for CP violation seems to be growing as the experiment acquires more data.

Neutrinos may be the first fundamental particles that are Majorana fermions. Because the neutrino is the only fermion that is electrically neutral, it is also the only one that could be a Majorana fermion, a particle that is identical to its antiparticle. Learning whether neutrinos are Majorana particles or typical Dirac fermions would provide invaluable insight as to how neutrinos acquired mass at the dawn of the universe, de Gouvêa says. To determine the nature of neutrinos, physicists are hunting for a process called neutrinoless double beta decay. In typical double beta decay, two neutrons transform into protons and emit a pair of antineutrinos. If those antineutrinos are Majorana particles, they could annihilate each other. A 16 August paper from the KamLAND-Zen experiment in Japan reports the most stringent limits for the rate of neutrinoless double beta decay, further constraining the possibility that neutrinos are Majorana particles.

Another neutrino flavor may be waiting to be discovered. The discovery of a fourth neutrino flavor, the sterile neutrino, would make every particle physicist forget about the LHC’s particle drought. Such a neutrino could enable physicists to explain dark matter or the absence of antimatter in the universe. The Antarctic detector IceCube just reported a negative result in the hunt for a sterile neutrino, but results from prior experiments still leave some wiggle room for the particle’s existence.

Multiple powerful neutrino experiments are on the horizon. The NOvA experiment is up and running and delivering data that, at least so far, seem to complement T2K’s hints of CP violation. Fermilab scientists are already excited about the Deep Underground Neutrino Experiment, which should come on line around 2025. Hyper-Kamiokande, a megadetector in Japan with a million-ton tank of water for neutrino detection, should start operations around the same time.

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