By Robin Yadav
Neutrinos(not to be confused with neutrons) are one of the most abundant particles in the universe. Yet we still have to go through great lengths to detect them. Why is this particle so difficult to detect even with our current technology and understanding of physics?
What are Neutrinos?
Neutrinos are fundamental particles, meaning that they can’t be broken down any further. An example of a fundamental particle that you might be more familiar with is an electron. This is because there is no other particle or smaller part of matter that makes up an electron. Since electrons and neutrinos are fundamental particles, they are present on the “Standard Model” which is a collection of all the known fundamental particles in the universe.
Neutrinos have some very interesting properties that help explain why they are so hard to detect. Firstly, they have no electric charge i.e. they are neutral just like a neutron. They also have very little mass, in fact, the mass of all three types of neutrinos(electron, muon, and tau) combined is still less than one-millionth of the mass of an electron.
The most important fact to remember is that a neutrino is a Lepton. Leptons are particles that interact via the weak force. Notably, neutrinos only interact through the weak force and no other fundamental forces. The weak force operates at extremely small distances about 0.1% the length of a proton(or 10^-18 meters). An instance of the weak force would be when a neutrino comes very close to a proton. Due to the weak interaction between the neutrino and proton, the neutrino will change into an electron and the proton will change into a neutron. Also, beta decay is a process where a neutron decays into a proton producing an electron and a neutrino.
Those are essentially the only ways a neutrino interacts with the environment. Now with the properties of neutrinos in mind, it is easier to understand why they are so difficult to detect.
Just to give you an idea about how abundant neutrinos are; there are around a trillion neutrinos passing through your body every second! Supernovas, our Sun and the Big Bang are responsible for the neutrinos that travel millions of miles to reach our location.
Even all these neutrinos are hard to detect. Neutrinos don’t have any charge so you can’t use any form of electromagnetic detection. They also only interact with matter through the weak force which acts only at very small distances. Therefore, they just end up passing right through most things and never interacting with them. Most neutrinos can travel right through the Earth without a single interaction.
However, it is still possible that out of the trillions of neutrinos that some make interactions. Scientists build large scale detectors with incredibly sensitive instruments to pick up neutrinos.
The Super-Kamiokande Detector
The Super-Kamiokande(or just Super-K) detector in Japan is the world’s largest neutrino detector. It is 1000 meters deep in Mount Ikeno to avoid any external interference. The 15 story detector has an enormous water tank that fits 50,000 tons of super pure water. Since neutrinos travel faster than light when in water (remember that light slows down in water since it is bumping into a lot more atoms along the way) they end up producing their own light. The scientific name for this phenomenon is Cherenkov Radiation. This is analogous to a plane producing a sound shockwave when traveling faster than the speed of sound.
Over 13,000 extremely sensitive light sensors called PMTs(Photo Multiplier Tubes) capture the light produced from the neutrinos.
The water is extremely purified so there are no obstacles blocking the path of the light from the neutrinos. In fact, the water is so pure that metal can actually dissolve in it.
Despite all of this technology and the trillions of neutrinos passing through Earth every day, the detections are few. The Sudbury Neutrino Observatory which is around 50 times smaller than Super K detects only 30 neutrinos per day out of 10^12.
Why are Neutrinos Important?
It’s so amazing to realize that trillions and trillions of neutrinos are going undetected. Also note that just because neutrinos are small and hard to detect, doesn’t make them insignificant. Firstly, since they’re fundamental particles, they’re embedded into the nature of our universe. Furthermore, understanding neutrinos can improve our technology and help our civilization.
As mentioned before, neutrinos play a critical role in beta decay. Beta-decay and consequently neutrinos are by-products of a reaction which is known as nuclear fission. Nuclear reactors use this process to produce a huge amount of energy as unstable nuclei split into neutrons and smaller nuclei which then undergo beta decay. Understanding the roles of different particles in a nuclear reactor can help scientists create better and more efficient reactors which can produce energy to power our homes.
Another process called nuclear fusion which happens in the Sun to produce energy also creates neutrinos. Scientists and engineers are still trying to figure out how to create fusion reactors. Studying neutrinos and their effects can advance research in that area.
Not only are neutrinos a useful particle for humans to research, but they are also critical in understanding the formation and evolution of our universe. The Big Bang created many neutrinos and physicists can use them to determine the state and properties of the primordial universe. Also, since supernovas eject a huge amount of neutrinos, physicists use neutrinos to understand the properties of stars.
The neutrino still remains a mysterious and elusive particle to study. They fly through our detectors without leaving a trace of their existence. That is what really fascinates me about neutrinos. Their existence is not common knowledge and I feel that it’s important to share information on other particles that are just as important as, for example, electrons. Neutrinos are a challenge to detect even with the high tech equipment that scientists employ. Even the trillions of neutrinos that pass through our home planet go undetected. This makes me wonder what else is out there in the universe to discover, and what other strange phenomena are right under our nose. The search for more neutrino data continues as physicists are determined to understand the key components of the universe