Why neutrinos are the windows to the dark universe

2015 Nobel Laureates uncover the secrets of the most ephemeral particles in the universe

Last week, Takaaki Kajita and Arthur B. McDonald were awarded the Nobel Prize in Physics “for the discovery of neutrino oscillations, which shows that neutrinos have mass.” Here, Elsevier Publisher Dr. Eleonora Presani, who was a particle physicist at CERN, writes about why their discovery is so important.

Construction of the Super-Kamiokande Neutrino Observatory in Japan. Here, collaborators are working on the detector photomultipliers. (Photo by Amber Case, <a href="https://creativecommons.org/licenses/by-nc/2.0/">CC BY-NC 2.0</a>)

Neutrinos are what I consider the most interesting particles known to man right now – so interesting, in fact, that they have been the reason for four Nobel Prize in the last three decades.

Nobel Prizes for neutrino research

  • In 1988, Leon M. Lederman, Melvin Schwartz and Jack Steinberger shared the Nobel Prize "for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino."
  • The Nobel Prize for detecting the first neutrino in 1956 was awarded four decades later, in 1995, to Frederick Reines.
  • In 2002, Raymond Davis Jr. and Masatoshi Koshiba received the Nobel Prize for detecting cosmic neutrinos, meaning neutrinos produced in stars far away and reaching our planet after having traveled in space.
  • Last week, the 2015 Nobel Prize in physics went to Takaaki Kajita and Arthur B. McDonald for the discovery of neutrino oscillations.

Physicists have spent a great part of the last century identifying and cataloging elementary particles and their interactions. Nowadays, we have the luxury of describing this structure in a neat and rather simple fashion via what is called the “Standard Model of particle physics.” Yet there are aspects of our universe that cannot be described by this model – gravity, for example. This makes us aware that there is something more, something “beyond the Standard Model.”

That neutrinos had something to do with this unknown part of physics became clear immediately after their discovery. In the 1960s, Karl Schwarzchild and John Bahcall described in details the neutrino yield we should expect from the sun, as a product of the nuclear fusion that powers our star. During nuclear fusion, there are a number of nuclear reactions that produce neutrinos: in fact, there is always a neutrino every time that a neutron decays into a proton, to compensate for the missing energy. This neutrino has a particular “flavor.” Neutrinos, in fact, come in three different types, called flavors: “e” (for electron), “mu” and “tau.” In the great majority of cases, especially in interactions in the Sun, are of type “e.” Yet, when Raymond Davis Jr. started to measure these neutrinos in the mine of Homestake, South Dakota, in the 1960s, he found only a third of what he was expecting. What happened to the missing neutrinos?

We have to wait 1989 to know more about this mystery. At that time, the Kamioka Observatory in Japan reported their first measurements of the solar neutrino flux. Kamiokande – this was the name of the experiment – used a different reaction than the Homestake experiment to be able to detect neutrinos. In their experiments, researchers were able to see all neutrino “flavors,” not only electrons. So the results started to arrive: if you measure also “muon” neutrinos and “tau” neutrinos, all of a sudden everything falls into place and the predictions made by Bahcall are respected.

Construction of the Super-Kamiokande Neutrino Observatory in Japan. (Photo by Amber Case, <a href="https://creativecommons.org/licenses/by-nc/2.0/">CC BY-NC 2.0</a>)The follow up of Kamiokande – Super Kamiokande – has improved their results considerably, especially measuring atmospheric neutrinos, which are neutrinos produced by interactions within the Earth’s atmosphere. At the same time, another similar experiment measured compatible results of SuperKamiokande. Under the rocks of the Gran Sasso mountain in Italy, the MACRO experiment measured similar results, and the results were published in Physics Letters B.

An experiment that proved to be even more conclusive in measuring all neutrinos flavors at the same time was the SNO (Sudbury Neutrino Observatory) solar neutrino experiment, which started collecting data in 1999. In fact, the phycists who went on to win the 2015 Nobel Prize wrote an article explaining these results better than I can do in this article.

These discoveries raised an important question: If the Sun produces a number of electron neutrinos, why do we see neutrinos of all flavors? The combined results of these experiments demonstrated that neutrinos can actually “oscillate” – change their flavor while they’re traveling in space.

The discovery of oscillating neutrinos has extremely important implications for the way we describe our universe, because it doesn’t naturally fit into the so called “Standard Model.” In order to be able to change flavor while traveling, neutrinos must have mass, and this was not predicted in the “Standard Model.” The consequences of this discovery extend to many aspects of physics. For example, with a mass – even if minuscule – neutrinos could contribute to the missing mass of the universe: Dark Matter. And they may help to explain the asymmetry between matter and anti-matter that we see in the universe today (see this article in Progress of Particle and Nuclear Physics).

In fact, this discovery is the keyhole through which we can look forward to a new physics – one that will enable us to understand gravity and the most fundamental nature of our universe.

Elsevier Connect Contributor

Eleonora Presani, PhDBefore Dr. Eleonora Presani (@HEPPublisher) joined Elsevier in 2012, she was a particle physicist at CERN. As a publisher, she is responsible for 14 academic journals in Nuclear and High Energy Physics, including Physics Letters B, Nuclear Physics A, Nuclear Physics B, Nuclear Instruments and Methods and Physics of the Dark Universe. Her role is to organize and improve the editorial processes, appoint editors and attend international conferences.

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