Nobel Prize winners prove neutrinos have mass

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Takaaki Kajita of the Super-Kamiokande Collaboration, University of Tokyo, and Arthur B. McDonald of the Sudbury Neutrino Observatory Collaboration, Queen’s University, were jointly awarded the Nobel Prize in Physics “for the discovery of neutrino oscillations, which shows that neutrinos have mass,” according to the Nobel Prize’s website.

Beyond the common particles of protons, neutrons and electrons, which many a high schooler has had to memorize, is a whole set of particles known as elementary particles.

Among these are quarks, muons, taus and neutrinos.

Neutrinos have puzzled physicists for many years despite their bountiful abundance.

According to the Nobel Prize press release, “thousands of billions of neutrinos are flowing through your body every second,” released mainly by the sun. However, the press release notes that these particles “hardly ever interact with matter.”

Quantifying the neutrino has proven very difficult because of its lack of reactivity.

However, two separate research facilities have successfully used Cherenkov radiation to explore the neutrino.

One of these facilities is the Super-Kamiokande Neutrino Detector in Kamioka, Japan. Located about one kilometer underground, the Super-Kamiokande is basically a large cylinder full of ultrapure water.

The purpose of the cylinder is to observe neutrinos passing through the earth.

Since neutrinos travel at the speed of light, it is virtually impossible to tell them apart from photons of light – at least, in air it is impossible.

However, when light passes through a denser medium such as water, it is slowed slightly by about 25 percent.

Neutrinos, on the other hand, are unaffected by water, and continue to travel at the speed of light.

Although the neutrinos’ speed is not altered by the water, the water is affected by the neutrinos.

When a charged neutrino passes through water, it disrupts the electromagnetic field of the water, sending out a shockwave of cone-shaped light, called Cherenkov radiation.

Determining the angle of the cone of radiation can give both the direction and the velocity of the disrupting particle, and in this case neutrinos.

To detect these shockwaves of light, 11,000 photo-multiplier tube detectors surround the cylinder, which contains 50,000 tons of water.

These detectors take an incoming signal of light and multiply it, increasing the signal’s intensity.

Scientists working on a platform above the cylinder are then able to transform the electrical signal from the detectors into useful information about the neutrino.

The Super-Kamiokande is capable of detecting two of the three types of neutrinos: electron and muon neutrinos, but not tau neutrinos.

The number of cones of radiation that these two neutrinos spark in water can be used to tell them apart.

Muon neutrinos spark a single cone, while electron neutrinos spark multiple cones.

A second neutrino detector located 2,100 meters underground in Ontario, Canada, uses a similar design to the Super-Kamiokande in Japan but changes some key features.

The Sudbury Neutrino Observatory (SNO) uses 1,000 tons of heavy water instead of pure water.

Heavy water is so named because the nuclei of its hydrogen atoms contain an extra neutron, making them “heavy.” A “heavy” hydrogen is called deuterium.

The heavy water is housed in a large, acrylic, spherical tank, which is itself surrounded by an 18-meter high geodesic dome.

The dome contains 9,600 photomultiplier tube detectors, and surrounding the dome is excess pure water, used to shield the detectors.

The effect of all these differences is that SNO can detect the third type of neutrinos: tau neutrinos.

Previous research at the Super-Kamiokande suggested that neutrinos coming from one side of the earth differed in number compared to those coming from the other side of the earth.

Considering that neutrinos do not interact with any matter, this was puzzling to scientists. They hypothesized that somehow, the neutrinos must have changed forms.

Indeed, as research at SNO indicates, the missing muon neutrinos observed by the Super-Kamiokande were transformed into tau neutrinos.

Armed with this knowledge and the applications of quantum physics, McDonald and Kajita were able to conclude that the different neutrinos have different masses.

 

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