The 2015 Nobel Prize in Physics

What's the buzz all about?

The 2015 Nobel Prize in Physics was awarded jointly to Takaaki Kajita and Arthur McDonald for their important experimental finding related to neutrinos. Neutrinos are the tiniest particles that make up matter--you know things like electrons. However unlike electrons, neutrinos are nearly massless and travel as fast as light. Also, neutrinos don't have charge which means it is not attracted to or repelled by a magnet.* The physicist Wolfgang Pauli predicted the existence of such particle in the 1930s and another great physicist Enrico Fermi coined the term neutrino to refer to "the little neutral one" because its like a neutron though a neutrino is at least a billion times less massive than a neutron, among other things. This makes neutrino very hard to detect and study. The first detection occured in 1942 which earned Wang Gangchang Nobel prize almost 40 years later.

Where do they come from? Neutrinos are produced in small quantities on Earth during decay of radioactive elements such as fission of Uranium in nuclear reactors, whereas copius amounts are produced during a supernova explosion and gamma-ray bursts as well as in the core of stars like our Sun during nuclear fusion and fission. Right this moment, countless neutrinos have passed through your body (and the entire Earth for that matter) relatively "unfelt" because they hardly interact with anything. **

What's the big deal? Neutrinos are interesting to physicists for some of the same reasons that pottery shards are interesting to archaeologists. Just as archaeologists study broken clay pieces to construct a story about the society that produced them, physicists examine neutrinos to learn more about the events and processes from which these sub-atomic particles have their origins. They’re important to our understanding of the kind of processes that go on in the Sun for example. That work could have practical application in developing nuclear fusion, not to mention understanding an important building block of the blueprint of nature.

How did they do it? Scientists used a device like the KamiokaNDE which stands for Kamioka Netrino Decay Experiment. Kamiokande uses a cylindrical tank filled with 3,000 tons of pure water that functions as both the target (source of proton) and the detector, including 1,000 photomultiplier tubes attached in the inner surface. Using water is ideal because it is inexpensive among other things. The kamiokande is buried deep underground so that it will be isolated from false signals such as muons produced by cosmic rays. Deep underground, kamiokande detects about 0.4 cosmic ray muon events per second; compare this to about 50,000 events had the detector been placed on Earth's surface. With this device, experimenters was able to directly demonstrate for the first time that the Sun was a source of neutrinos. Masatoshi Koshiba was awarded the Nobel Prize in Physics in 2002 for his work directing the Kamioka experiments, and in particular for the first-ever detection of astrophysical neutrinos--that is, neutrinos coming not only from the Sun but also from other sources like supernova explosion.*** Raymond Davis Jr. and Riccardo Giacconi were co-winners of the prize.

Super what? The Super-kamiokande (a.k.a. Super-K), a scaled up and more sensitive version of kamiokande, was designed to test a lingering problem concerning neutrinos. Since the 1960s, physicists observed that only about 1/3 of the predicted number of neutrinos from the Sun passes through Earth. What was known that time was that neutrinos have three types or flavors with fancy names like electron neutrino, muon neutrino, and tau neutrino. Four decades later, they solved this "solar neutrino problem" by proposing that a neutrino can change from one flavor into another while traveling in space, and finally changing back to its original flavor. Thus, the "missing" solar neutrinos could be electron neutrinos which changed into other flavor along the way to Earth and therefore were not seen by the detectors on Earth. The Sudbury Neutrino Observatory (SNO) in Canada was also able to make similar measurements, which together with Super-K's data, proved this back-and-forth phenomenon, now called neutrino oscillation. This resulted to the understanding that neutrinos have mass albeit very small, a modification to Standard Model of particle physics. For this reason, the Takaaki Kajita and Arthur McDonald, directors of Super-K and SNO, respectively, received the 2015 Nobel Prize in Physics.

Problem solved? Not yet! The Kamiokande is originally designed to detect the elusive neutrinos by looking for signs of proton decay. Proton decay is a hypothetical process wherein proton decays into its substituents including pions, positrons, and of course neutrinos. But even super K was not sensitive enough to observe proton decay with enough confidence level. Observing proton decay, if it really occurs, will help solve a piece of the puzzle which itself has overarching implications--things which we are just beginning to understand--such as the explanation of why there are more matter than anti-matter in our Universe. In fact, it is one of the list of unsolved problems in physics. The difficulty with this task however, is that it requires incredibly long observation since proton halflife is considered infinite which means proton decay is next to impossible as our current best theory predicts. Fortunately, there are zillions of protons in the universe and tiny probabilities might add up so that we are lucky enough to witness few spikes in the detector.

_____ *Actually, neutrinos have a tiny magnetic moment because of its tiny mass which makes them respond quitely nicely to very strong magnetic field.

**To be more precise, about 65 billion (6.5×10^10) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.

***The Kamiokande-II experiment happened to be running at a particularly fortuitous time, as a supernova took place while the detector was online and taking data. With the upgrades that had taken place the detector was sensitive enough to observe the thermal neutrinos produced by Supernova 1987A, which took place roughly 160,000 light years away in the Large Magellanic Cloud. The neutrinos arrived at Earth in February 1987, and the Kamiokande-II detector observed 11 events.

Sources: http://phys.org/news/2014-04-importance-neutrino-physics.html https://icecube.wisc.edu/info/neutrinos

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