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The 2015 Nobel Prize in Physics was given on October 6 to Takaaki Kajita of the Superkamiokande experiment and to Arthur McDonald of the Sudbury Neutrino Observatory.  The work at both those sites is thoroughly discussed in The Cosmos (see Section 12.7, p. 322-325) about the solar-neutrino experiment.


By showing definitively that a mix of the three types of neutrinos reaches the Earth, combining the knowledge that only electron-neutrinos leave the Sun shows that neutrinos change in type en route.  Only if neutrinos have mass can such changes take place, so the discovery is a major challenge to the Standard Model of particle physics.

This is the third Nobel Prize for neutrino research.  Half the 1995 Nobel Prize went Fred Reines for the discovery of neutrinos in an atomic-reactor beam (his co-discoverer, Clyde Cowan, having died before the prize was given, making him ineligible).  Half the 2002 Nobel Prize went to Ray Davis, who ran the chlorine version of the neutrino experiment at the Homestake Mine, and Masatoshi Koshiba, who was in charge of Kamiokande (the Neutrino Detection Experiment [NDE] in the Kamioka mine in Japan). John Bahcall from the Institute for Advanced Study, who had done the bulk of the theoretical work involved, was omitted from the prize, unfortunately (again, as the prize is not awarded posthumously).

Links: 2015 Nobel Prize in Physics at the Royal Swedish Academy of Sciences; Dennis Overbye’s analysis for the NY Times (including a discussion of the next investigations via The Deep Underground Neutrino Experiment, DUNE).

From an article in Physics World by Ken Croswell, August 29, 2014:

Physicists working on the Borexino experiment in Italy have successfully detected neutrinos from the main nuclear reaction that powers the Sun. The number of neutrinos observed by the international team agrees with theoretical predictions, suggesting that scientists do understand what is going on inside our star. (See Section 12.7, p. 322.)

Credit: Borexino Collaboration

Credit: Borexino Collaboration

Each second, the Sun converts 600 million tons of hydrogen into helium, and 99% of the energy generated arises from the so-called proton–proton chain. And 99.76% of the time, this chain starts when two protons form deuterium (hydrogen-2) by coming close enough together that one becomes a neutron, emitting a positron and a low-energy neutrino. It is this low-energy neutrino that physicists have now detected. Once this reaction occurs, two more quickly follow: a proton converts the newly minted deuterium into helium-3, which in most cases joins another helium-3 nucleus to yield helium-4 and two protons.

Neutrinos normally pass through matter unimpeded and are therefore very difficult to detect. However, the neutrinos from this reaction in the Sun are especially elusive because of their low energy. The measurement therefore took scientists by surprise.

The Borexino detector is a large sphere containing a benzene-like liquid that is located deep beneath a mountain at the Gran Sasso National Laboratory to shield the experiment from cosmic rays. Occasionally, a neutrino will collide with an electron in the liquid and the recoiling electron will create a flash of ultraviolet light that can then be detected.

Links: the full Physics World article; Borexino website.

As described in The Cosmos, 4ed, Section 12.7, the solar-neutrino and other experiments have been detecting subsidiary nuclear interactions, but could not reach the energy range of the most fundamental process that fuels the sun and stars like it–the interaction of two protons. Scientists of the Borexino project in Italy has announced that they have finally detected this fundamental process. Here is an abridged version of their press release of August 27, 2014:

Scientists working on the neutrino experiment in the Italian National Institute for Nuclear Physics (INFN) Gran Sasso Laboratories have managed to measure the energy of our star in real time: the energy released today at the center of the Sun is exactly the same as that produced 100,000 years ago. For the first time in the history of scientific investigation of our star, solar energy has been measured at the very moment of its generation. The study was published on August 28, 2014, in the journal Nature.

INFN Borexino infographic

Credit: INFN/Borexino experiment

Borexino has managed to measure the Sun’s energy in real-time, detecting the neutrinos produced by nuclear reactions inside the solar mass: these particles take only a few seconds to escape from it and eight minutes to reach us. Previous measurements of solar energy, on the other hand, have always taken place on radiation (photons) which currently illuminate and heat the Earth and which refer to the same nuclear reactions, but which took place over a hundred thousand years ago: this, in fact, is the time it takes, on average, for the energy to travel through the dense solar matter and reach its surface. The comparison between the neutrino measurement now published by Borexino and the previous measurements concerning the emission of radiant energy from the Sun shows that solar activity has not changed in the last one hundred thousand years.

The Borexino detector, installed in the INFN underground Laboratories of Gran Sasso, has managed to measure the flux of neutrinos produced inside the Sun in the fusion reaction of two hydrogen nuclei to form a deuterium nucleus: this is the seed reaction of the nuclear fusion cycle which produces about 99% of the solar energy. Up until now, Borexino had managed to measure the neutrinos from nuclear reactions that were part of the chain originated by this reaction or belonging to secondary chains, which contribute significantly less to the generation of solar energy, but which were central to the discovery of certain crucial physical properties of this “ephemeral” elementary particle, the neutrino.

Links: Full INFN press release on; Borexino homepage.