"Catch me if you can" - the story of Neutrino hunting

Part I

Neutrino interaction in a bubble chamber. (Credit: Fermilab)

The heading of the article (borrowed from a famous Hollywood movie) is best fitted to describe the story. I am going to tell a story about a particle that is the second most abundant particle in this Universe. Every second, a hundred billion neutrinos, passing through the tip of our fingers but it's very hard to catch them. It is so shy that it doesn't want to interact with others and that makes this particle so elusive. So the hero of today's story is "Neutrino". If scientists were Sherlock Holmes, then neutrino hunting would be one of the most critical and fascinating cases for them. So the journey begins...

Let us first jump into the atomic world like the "Ant-man". Imagine you have entered a radioactive atom (such as Uranium). You are watching the revolution of the electrons around something at the center. Now you are going more closer to the center to figure out what it is. You find that the protons and neutrons are making a colony at the center. The name of the colony is "Nucleus". At that time magic happens...

Suddenly you discover an amazing incident. Neutrons are changing their identity and they are becoming protons by emitting an electron. Now, let us come back from the magic show. Scientists carefully measured when neutrons are changing into protons by releasing an electron, the energy was not conserved in this process. This measurement puzzled the whole scientific community. Where is that missing energy? 

                                                  

                          Enrico Fermi

                        Wolfgang Pauli

In 1930, Wolfgang Pauli put a solution for that. He realized that the extra energy must be carried off by a very tiny neutral particle which is very hard to detect. He named that particle as neutron but this name already exists. After giving the solution Pauli wrote a fascinating letter to the Federal Institute of Technology, Zurich, which is kept in the Pauli archive at CERN, where he expressed his own confusion and dilemma - "I admit that my remedy may seem almost improbable because one probably would have seen those neutrons, if they exist, for a long time. But nothing ventured nothing gained...". To overcome this confusion another famous physicist Enrico Fermi named this elusive particle "Neutrino" which is an Italian name meaning "the little neutral one". The phenomena of decaying neutrons are famous as "beta decay" because at that time electrons were known as beta particles.

Now the actual suspense begins...

There are four forces that exist in nature. Two that are familiar to us, they are - gravitational force and electromagnetic force. The rest of the two are a little bit unfamiliar to us - strong nuclear force and weak force. Neutrinos only give responses to gravity and weak force. Their nature is so bizarre that makes neutrinos famous as the ghost of the atomic world. Let me give one example of its strangeness. Neutrinos originate from nuclear reactions and the biggest nuclear reactor around us is the Sun. The Sun produces an enormous amount of neutrino each and every second. More precisely around 650 trillion neutrinos hit the earth each and every second. But as you know neutrinos are so shy to interact even if you make one shield of a lead bar of a size of 5 light-years (that means the distance of Alpha Centauri to earth), that can only stop 50% of the solar neutrinos. 

Be with me to enjoy the suspense of neutrino hunting...