Subatomic Physics: Super Kamiokande

Neutrinos being detected at the Super Kamiokande (“Super-K”) detector

For my subatomic physics class, taught by Francisco Yumiceva, we had to choose a final project to do a report and presentation on. Mine was on neutrinos being detected at the Super Kamiokande (“Super-K”) detector in Gifu, Japan.

Super Kamiokande is a giant water tank 1000 meters under the ground, 42 meters tall and 39 meters in diameter, filled with 50,000 tons of pure water. It was designed to discover properties of neutrinos via Cherenkov radiation, which is light emitted when a particle enters a new medium (such as water, in this case) and is traveling faster than the speed of light measured in that medium (ie vacuum: 2.998×108 m/s; water: 2.254×108 m/s).

In the picture above, you can see the 13,000 beach-ball-sized photomultiplier tubes covering the walls, which detect, amplify and record any interactions the neutrinos cause. Some major types of neutrinos Super Kamiokande has studied are solar, atmospheric and supernova neutrinos.

Solar neutrinos are electron neutrinos (νe) formed during the fusion process in the core of the sun. Based on models of the sun, the Standard Model was able to predict the number of νe hitting the Earth. But in 1968, the Homestake Experiment conducted by Ray Davis and John Bahcall (using a chlorine-rich solution to detect solar neutrinos) came up short on the number of νe predicted. So either the solar model was incorrect or the Standard Model was incorrect. Later experiments by various detectors came up with the same problem, including Super-K, which provided the most precise solar neutrino measurements at the time. This became known as “the solar neutrino problem” and was not resolved for over 30 years.

Super Kamiokande
Neutrinos allow us to verify our theories about the inner workings of stars like our sun. Also, because they rarely interact with matter, they only take about eight minutes to reach Earth’s surface after they are created, unlike a photon, which takes thousands of years to make it to the sun’s surface due to the number of collisions inside that interrupt the photon’s path. Therefore, neutrinos allow us to see the current state of our sun’s core.

Atmospheric neutrinos come from cosmic rays constantly bombarding the earth from all directions. They interact with our atmosphere and form all kinds of elementary particles, including muon and electron neutrinos (νμ and νe). Super-K and other detectors were not long in discovering a discrepancy between νμ and νe, the number of downward-going νμ twice that of the upward ones when theory predicted them to be uniform all over Earth.

Oscillation between different flavors of neutrinos (ie electron, muon, or tau) was suspected and the KEK to Kamioka, or “K2K,” experiment was set up to investigate (KEK is the Ko Enerugi Kusokuki high energy laboratory in Japan and Kamioka is the name of the observatory inside of Super Kamiokande). K2K created a stream of νμ at one end and then the number of neutrinos was measured again after a distance, a change in number indicating some of them probably changed to another type of neutrino.

Super Kamiokande
Cosmic rays come in from every direction, but the number of a particular flavor of neutrinos was not the same everywhere as predicted, leading to the resolution of the “solar neutrino problem” by the theory of neutrino oscillations. A lot of evidence suggests this is occurring, which means neutrinos are not massless as originally thought and calls for a modification of the Standard Model.

The supernova in 1987 (SN1987a) provided Earth with its first observations of supernova neutrinos. Super-K observed eleven events, all practically at the same time. SN1987a happened in the Large Magellanic Cloud, near enough for us to record data; now we are waiting for a supernova to occur within our own galaxy. Super-K is estimated to observe nearly 8,000 events if this does happen. Supernovae usually occur every ten to fifty years in a galaxy, so we are hopefully due for one!

An on-going neutrino search is for what are referred to as “relic” neutrinos, also as a result of supernovae. The only difference: these neutrinos were formed too far away and too long ago for us to get the kind of data as from SN1987a. Instead, these relic neutrinos are thought to be remnants of past supernovae, spread out in space after being ejected from the explosion. They should be measurable because every supernova to ever happen in our Universe released approximately 1046 Joules of energy, 99 percent of that in the form of neutrinos. Where did they all go? Our current hypothesis — nowhere. We just haven’t been able to detect them yet.

Super Kamiokande
The supernova SN1987a produced a burst in neutrino observations by various detectors world-wide, including Super-K. More supernova in our neighborhood of the galaxy are needed so we can verify the mechanisms predicted for supernova (note: detectors like Ice Cube in Antarctica are capable of detecting astrophysical neutrinos not in the neighborhood!).

Neutrinos and their oscillations between the electron, muon and tau flavors are still not understood and detectors like Super-K will continue their research to discover the truth. They are essential to making a complete Standard Model of subatomic particles and have already helped us verify other theories we rely on in astrophysics, such as fusion of stars and supernova. I found the class very interesting and I’m glad I got the opportunity to study our world at the smallest possible levels!

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