Let’s get back to the question we started this series with. How do you see something that weighs nothing, looks like nothing and interacts with nothing? The answer is that it does weigh a little. So little is their mass, that for a long time neutrinos were thought to be mass less. Now we know they have a finite mass, but we haven’t been able to put a number on it. The interaction too is very little. It is this precious little that we exploit in large neutrino detectors. Various clever methods have been used to detect neutrinos. But one of them stands out as it is used in most of the present detectors. Here is how it works.
Light Amidst Darkness
A large tank is filled with a clear substance like water or ice and instruments are placed inside the tank to sense the tiniest amounts of light. Most neutrinos that arrive here simply move out of the other side. However, some do interact with the nucleus of hydrogen or oxygen atoms from the water molecule. The collision creates a small charged particle that emits light when it moves with a speed that is greater than the speed of light in water. The instruments will then detect this light. So, we can detect a neutrino by filling a large tank with some transparent substance in a dark environment surrounded by light detectors.
If we do get a signal from our light sensors, we know that’s a high energy neutrino passing through our detector. However, two problems plagued physicists as they tried to detect neutrinos. The first one is that the nucleus takes up a very small space in an atom, the rest being empty. The chances of neutrinos colliding with a nucleus are, therefore, pretty low. The second one is that a charged particle so created needs to travel at more than 225 thousand kilometers per second to emit the light we can see. For a charged particle to get to this speed, it needs a lot of energy which has to be provided by the neutrino. Therefore, we detect a very small fraction of all the neutrinos passing from the detector.
The way to get around this problem is less interesting though. We simply make a detector of large volume, so that there are enough interactions for studying neutrinos. Hence, larger and larger neutrino observatories are being built. The largest one proposed as of now is the Hyper-Kamiokande neutrino detector in Japan. It will hold 260,000 tonnes of water in a huge tank that has hyper-sensitive photo-sensors on the inside. These detectors will detect the glow of charged particles when they move with speeds higher than that of light in the medium. This glow has a special name: Cherenkov Radiation.
Physicists use experimental setups that cost billion of dollars to learn about these subatomic particles. These experiments not only satisfy human curiosity but increase our knowledge base as a human civilization. Every once in a while, the experiments churn up unexpected surprises as we venture into the unknown. Their influence is not always limited to physics. Recently, a basic discovery in mathematics was made while studying neutrinos. Many a times, these experiments will point towards gaps in our understanding of the universe. Scientists will then acknowledge their mistake and create a model that better emulates our reality.
One such event happened in the Kamiokande neutrino detector in Japan. As we have discussed earlier, we can analyse the light emitted after collision. This is done by using huge computers to find out the direction and flavour of the neutrino. Neutrinos, like ice-creams, come in flavours viz. Electron neutrino, muon neutrino and tau neutrino. By analysing the light signals from a detector, physicists found out that neutrinos oscillate between flavours.
Imagine, you ordered an vanilla ice-cream from the icy moons of Jupiter but a chocolate one arrived at your door. You might be pissed-off but if you were Takaaki Kajita you would try to find out what did it (He analysed solar neutrinos). Then you, like Takaaki Kajita, would get a Nobel Prize for finding out that neutrinos change flavours as they travel. By exploring this phenomenon further, scientists found that neutrinos have a finite mass. And it became evident that the standard model of physics had a hole that needs to be filled. This hole, called CP-Symmetry violation, is expected to lead the way towards a new understanding of the existence of our universe.
Larger and more specialised projects are underway to make their own discoveries that might change the way we think about the universe and ourselves. America’s Fermilab is surely one of them as they are building the strongest neutrino beam ever. We, as a civilization are still novices when it comes to understanding our reality. Projects like Kamiokande, CERN and Fermilab will take us further, one step at a time, in this journey of collective learning.