The discovery of neutrinos led to a rather startling realization concerning the omnipresence of these particles. Scientists have known since the early 20th century that stars such as the Sun generate energy through nuclear fusion, especially of hydrogen into helium. In addition to producing radiation that eventually leads to what we see as sunlight, every one of these reactions releases neutrinos. As a result, the Earth is continually bathed in a stream of neutrinos: every second, billions of neutrinos pass through every square centimeter of the Earth's surface. A vast, vast majority of these pass through the planet unimpeded and resume their course through space, just as discussed in the previous post. As we will see, studying the properties of these solar neutrinos later led to an revolutionary discovery.
In 1967, an experiment began that had much in common with many of the neutrino experiments to come. Known as the Homestake experiment after its location, the Homestake Gold Mine in South Dakota, the main apparatus of the experiment was an 100,000 gallon tank of perchloroethylene (a common cleaning fluid) located deep underground, nearly a mile below the Earth's surface. The purpose of holding the experiment underground was to minimize the influence of cosmic rays, which would react with the perchloroethylene and produce experimental noise. Cosmic rays do not penetrate deep underground, however, while neutrinos do. The immense volume of liquid was necessary to obtain statistically significant data from the small rate of neutrino interactions. The number of argon atoms that were produced through the reaction was measured to determine how many reactions were occurring.
Simultaneously, physicists made theoretical calculations using knowledge of the Sun's composition, the process of nucleosynthesis, the Earth's distance from the Sun, and the size of the detector to estimate what the rate of interactions should have been. However, the results were not consistent with the data collected from the experiment. Generally, theoretical estimates were around three times as large as the actual results. Two-thirds of the expected reactions were missing! This disagreement became known as the "solar neutrino problem."
The models of the Sun were not at fault. In fact, the cause of the problem was an incorrect feature of the otherwise quite powerful Standard Model of Particle Physics, namely that neutrinos have mass. As far back as 1957, Italian physicist Bruno Pontecorvo considered the implications of neutrinos having mass.
He and others realized that neutrinos with mass would undergo what is known as neutrino oscillation when traveling through space. For example, an electron neutrino emitted from nuclear fusion would become a "mix" of all three flavors of neutrinos: electron, muon, and tau. When a solar neutrino reaches Earth and interacts with matter, it only has roughly a 1 in 3 chance of "deciding" to be an electron neutrino. This would explain the observed missing neutrinos, since the Homestake detector only accounts for electron neutrinos.
For the remainder of the 20th century, several more experiments were performed to investigate whether neutrino oscillation was in fact the solution to the solar neutrino problem. One experiment that was crucial in conclusively settling the matter was Super-Kamiokande, a neutrino observatory located in Japan. Like the Homestake experiment, it was located deep underground in a mine and consisted of a large volume of liquid (in this case, water).
When neutrinos interact with the water molecules in the detector, charged particles are produced that propagate through the chamber. These release radiation which is amplified and recorded by the photomultipliers that surround the water tank on every side. The number of photomultipliers allows a more detailed analysis of this radiation, yielding the energy and direction of origin for each neutrino interaction. It was this added precision that helped to resolve the solar neutrino problem: neutrinos indeed have mass and undergo oscillation. This discovery led to Japanese physicist Takaaki Kajita (who worked on the Super-Kamiokande detector as well as its predecessor the Kamiokande detector) sharing the 2015 Nobel Prize in Physics.
The exact masses of the different flavors of neutrinos are not yet known, nor do we completely understand why they have mass. However, despite the mysteries of particle physics that remain, further applications of neutrino detection continue in a different field: astronomy. The use of neutrinos to observe extraterrestrial objects is known as neutrino astronomy. In theory, if one could accurately measure the direction from which every neutrino arrives at Earth, the result would be an "image" of the sky highlighting neutrino sources. In reality, the scattering that occurs in detectors such as Super-Kamiokande when incoming particles hit and change direction limits angular resolution and so few interactions occur that there are insufficient samples to construct such an image. Only two extraterrestrial objects have ever been detected through neutrino emissions, in fact: the Sun, and a nearby supernova event, known as SN1987a after the year in which it took place. Theoretical calculations indicate that sufficiently bright supernovae may be located with reasonable accuracy using neutrino detectors in the future.
There is one major advantage to using neutrinos as opposed to light in making observations: neutrinos pass through nearly all matter unimpeded. The above discussion indicated that the Sun is a neutrino source. This is true, but not fully precise; the solar core is the source of the neutrinos, as it is where fusion occurs, and its radius is only about a quarter of the Sun's. There is no way to see the light emanating from the core because it interacts with other solar particles. However, we can see the core directly through neutrino imaging. In fact, the data from the Super Kamiokande experiment should be enough to approximate the radius in which certain fusion reactions take place. Future detectors could tell us even more about the Sun's interior.
Neutrino astronomy is still a nascent field and we do not yet know its full potential. Further understanding and detection of neutrinos will tell us more about the fundamental building blocks of matter, allow us to peer inside our own Sun, and measure distant supernovae.
Sources: http://www.sns.ias.edu/~jnb/SNviewgraphs/snviewgraphs.html, https://arxiv.org/pdf/hep-ph/0410090v1.pdf, http://slideplayer.com/slide/776551/, https://www.bnl.gov/bnlweb/raydavis/research.htm, https://arxiv.org/pdf/hep-ph/0202058v3.pdf, https://j-parc.jp/Neutrino/en/intro-t2kexp.html, https://arxiv.org/pdf/1010.0118v3.pdf, https://www.scientificamerican.com/article/through-neutrino-eyes/, https://arxiv.org/pdf/astro-ph/9811350v1.pdf, https://arxiv.org/pdf/1606.02558.pdf
