The Triple Helix @ UChicago

Fall 2015

"Why Do Those Pesky Neutrinos Have Mass? - The 2015 Physics Nobel Prize" by Tom Klosterman

 

On October 6, the Royal Swedish Academy of Sciences awarded the 2015 Nobel Prize in Physics to Takaaki Kajita from Japan and Arthur B. McDonald from Canada for the discovery that neutrinos have mass. Independently, the two scientists led teams which discovered that neutrinos traveling through the earth and from the sun, respectively, undergo metamorphosis: frequent oscillation between three flavors, a condition that can only hold if they have mass. 

In your typical high school chemistry class, one of the first things you learned is that matter is composed of tiny particles called ‘atoms.’ These are in turn composed of protons and neutrons in the nucleus, with orbiting electrons. Later on, you learned that some nuclei are unstable, and split up, or decay, by emitting radiation. One way this can happen is Beta (β) decay. As any old Wiki article can tell you, one electron is emitted in β decay as the nucleus splits. Scientists assumed the energy released in β decay would always be constant, but in 1914, James Chadwick showed that it can take a spectrum of values.[1] An Austrian physicist, Wolfgang Pauli, soon proposed that a new electrically neutral particle was released in β decay alongside the electron and differences in its energy accounted for the variance in the energy. 

Consider that at this time, all matter was thought to consist solely of protons and electrons. Needless to say, Pauli’s imaginary particle, which he dubbed the “neutron,” electrified the scientific community. When Chadwick proved the existence of the neutron in the nucleus in 1932[2], Pauli realized it was far too massive to be the neutral particle in β decay. The theoretical particle was renamed the “neutrino.” Then, more than forty years after the first glimpse of neutrinos, Cowan and Reines conclusively proved their existence in 1956.[3] Because of their neutrality and minuscule size, the particles are almost impossibly hard to detect, but by setting up a detector near a nuclear reactor, the team was able to detect the byproducts of several β decays that could have only been caused by the elusive neutrino. Finally the miniature neutrinos had been found. Fast forward to the 1990s. The Standard Model of Physics was now almost fully developed. In particular, the Muon (μ−) and the Tau (τ−) particles, had been confirmed as more massive analogues to the electron (e−). When any of the three are produced, corresponding neutrinos are typically produced as well: electron neutrinos (ve), muon neutrinos (νμ), or tau neutrinos (ντ). For example, ve particles are produced with the electron in β decay. Importantly, The Standard Model calculates the mass of neutrinos to be zero. Lastly, astrophysicists had shown that only ve particles are emitted from the sun, in staggeringly huge numbers. However, even the most sensitive detectors, built hundreds of feet under the ground, only produced a handful of neutrino collisions a day. But scientists began to notice a discrepancy: they were only capturing about a third of the neutrinos that they should capture. Was neutrino theory wrong or did scientists need to rethink all of solar science? 

This is where Takaaki Kajita and the Super Kamiokande detector came in. During the 1990s, the Japanese detector showed a similar result with a νμ source across the earth, hinting that perhaps neutrinos ‘disappear.’[4] However, the disappearance was proportional to the distance from the source, so a theory was postulated in which neutrinos can oscillate between the three types. The farther away from the source, the more the neutrinos would settle into a one to one to one ratio between the three types. The exact mechanism for this was unknown, but experimentally it held up. 

Then, in Canada, the Sudbury Neutrino Observatory (SNO), led by Arthur McDonald, showed more accurate results that the number of ve particles detected on Earth is only a third of the number produced in the sun. After showing νμ and ντ particles in the detector as well, the SNO was able to conclude in 2002 with 99.999% accuracy that neutrinos from the sun oscillated between the three flavors during the trip to Earth.[5] 

How then does this oscillation prove the existence of neutrino mass? Unfortunately the exact reason is very mathematical and complicated, and involves words like eigenstates and phase factors. But qualitatively, it is a consequence of Quantum Mechanics, where matter simultaneously takes the form of waves and particles. The three flavors of neutrinos (ve, νμ and ντ) each describe a certain wave, with energy related to the mass of the particle. When a neutrino is created, it is a certain combination of these three waves with one wave (the particular flavor of the neutrino) dominating. If the three flavors of waves all had the same energy, they would never shift around and the particle would never oscillate. Therefore, while one of the neutrinos could still have zero mass, the other two may not. 

This has large scale implications on physics as a whole. As mentioned, the Standard Model only allows for massless neutrinos. So the Standard Model, which the entire field of particle physics is based on, will have to be updated. In addition, even though the neutrino masses are incredibly small, there are so many generated in stars, supernovae, and anywhere with radioactive decay that the total mass of neutrinos could outweigh all of the stars in the universe combined. It is clear that much will come out of Kajita and McDonald’s discovery of neutrino mass, hence the two were awarded the 2015 Nobel Prize in Physics.

References


[1] Chadwick J 1914 Verh. der Deutschen Physikalischen Ges. 16 383. 
[2] Chadwick J 1932 Nature 129 312. 
[3] Reines F and Cowan C L Jr 1956 Science 124 103. 
[4] Fukuda, Y., et al. 1998 Physical Review Letters 81 1562. 
[5] Q.R. Ahmad et al 2002 Physical Review Letters 89 011301.

 
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