Neutrino Physics of the 21st Century and Future

Dear Colleagues,

Neutrinos are as mysterious as they are ubiquitous, and they are one of the most abundant particles in the universe. Physicists have spent a lot of time exploring the properties of these invisible particles.

Neutrinos are neutral particles produced abundantly during the Big Bang, by the Sun, and by cosmic rays striking the Earth’s atmosphere. However, they are extremely hard to detect, because they have very little mass with no electric charge and only weak interactions with other fundamental particles. After photons, the second most abundant known particle in the universe is the neutrino. 

Despite being the most successful theory of particle physics to date, the Standard Model is not perfect. The Standard Model of particle physics agrees very well with the research, but many important questions remain unanswered. There are fundamental physical phenomena in nature that the Standard Model (SM) does not adequately explain, a few experimental results are not explained by the SM, and certain theoretical predictions are not observed as predicted by the SM—the most puzzling one being neutrino mass. According to the Standard Model, neutrinos are massless particles. However, neutrino oscillation experiments have shown that neutrinos do have mass. The mass terms for neutrinos can be added to the Standard Model by hand, but this leads to new theoretical problems. For example, the mass terms need to be extraordinarily small and it is not clear if the neutrino masses arise in the same way that the masses of other fundamental particles do in the Standard Model. 

The 2015 Nobel Prize in Physics was shared by Arthur B. McDonald, the leader of the Sudbury Neutrino Observatory (SNO), and Takaaki Kajita, a leader of the Super-Kamiokande collaboration, “for the discovery of neutrino oscillations, which shows that neutrinos have mass”. In a single stroke, it both solved a long-standing puzzle about these most elusive of fundamental particles and exposed an incompleteness in the current bedrock theory of physics called the Standard Model.

The discovery of neutrino oscillations in the atmospheric Super-Kamiokande experiment in the SNO, in other solar neutrino experiments, and in the long-baseline reactor KamLAND experiment is one of the most important recent discoveries in particle physics. The phenomenon of neutrino oscillations was further investigated in the long-baseline accelerator K2K, MINOS, and T2K experiments, in the reactor experiments Daya Bay, RENO, and Double Chooz, in the solar BOREXINO experiment, and in many other experiments. 

Neutrinos are standard-model members, but the theoretical predictions are wrong. The prevailing theory says that neutrinos are massless; the Nobel-Prize-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. Until now, neutrinos have been deemed the “only palpable evidence of physics beyond the Standard Model”. Everything we learn about neutrinos in the coming years is new physics. Now the physics community is gearing up for new neutrino studies that could lead to answers to some big questions: 

How much do neutrinos weigh and do they have the magnetic moment?

Are neutrinos their own antiparticles?

Are there more than three kinds of neutrinos?

Do neutrinos get their mass the same way that other elementary particles do?

Why is there more matter than antimatter in the universe?

Prof. Dr. Chitta Ranjan Das
Dr. Timo J. Kärkkäinen
Dr. Sampsa Vihonen
Guest Editors

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