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

Part IV- The Conclusion

Everything was created from a bang- "The Big Bang". During the creation, the amount of the matter and the antimatter should be equal, but then an asymmetry can be seen everywhere. There is matter everywhere, but antimatters are hard to find. This is a long cherishing puzzle for the physicists, that why there is more matter than antimatter. During the creation, there must be some small amount of imbalance in between them. We know, that matter and antimatter annihilate each other and create some energy. But by some mysterious cause matter dominates and the annihilation process stops. And that is why we exist. Our existence is just an accident of matter-antimatter asymmetry. This asymmetry is also known as CP (charge, parity) violation. 

Super Kamiokande experiment in Japan for Neutrino hunting

To uncover this challenging puzzle, physicists are trying to find out some clues. Can neutrino shade some light in this? In 2016 Japanese scientists found a bizarre thing that neutrinos behave differently to their antimatter counterparts. This observation leads particle physicists to think to have a solution for the matter-antimatter asymmetry problem. In previous episodes, we discussed the three flavors of neutrino- electron type, muon type, and taw type. These three types of neutrinos also have their antiparticle counterpart. But you know neutrinos are super weird. For any other particle, let's take an electron it is always an electron irrespective of its position or speed, or interaction with other particles. But neutrinos don't have this kind of deterministic identity. They change their identity from time to time. For example, an electron neutrino might be changed into a muon neutrino and vice versa. When they travel through space and interact with other objects, they change their identity just like fiction, "a handkerchief gets converted into a cat". Is it a kind of magic or its a joke? The answer is - its quantum mechanics. The three flavors of the neutrinos exist as a superposition state just like a chocolate cake with vanilla cream and strawberry topping on it. When neutrinos travel through space one of the flavor emerge. The three flavors of the neutrinos having different mass and they interact differently. But magic doesn't stop here because this particle world is like Hogwart. An electron neutrino changes into muon neutrino and after some time it becomes an electron neutrino again. This phenomenon is known as neutrino oscillations.

The way these three flavors of neutrino exchange their identity in between them, in the same way, their antiparticle counterpart do not change themselves. This difference between the neutrinos and antineutrinos brings a smile in the physics community just like when Sherlock Holmes gets some clue about the investigation. The difference in the oscillation rates for the neutrinos and antineutrinos is a CP violation, a symmetry that says, more or less, that physics should look the same if you swap particles for antiparticles and reverse all their spins. Some CP violation seems necessary because the infant universe apparently generated more matter than antimatter; otherwise, the two would have annihilated each other completely. Since the 1960s, physicists have observed a hint of CP violation involving subatomic particles called quarks, but too little to explain the cosmic imbalance. After finding the asymmetry between the neutrino and antineutrino world, Physicists got more fuel to solve this long cherishing unsolved problem.

Standing within one of the most abandoned particles, we are trying to catch them so hard. What a fallacy! Definitely, the day will come when the wizards (read "physicists") will understand the sorcery, and one of the powerful wands will be the neutrino.

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

 Part III

In the first episode of the neutrino story, we discussed beta decay. That time it was predicted that the new particle produced in this process as the neutrino. But the story is not so simple. Later on, it was discovered that it is not the neutrino, rather it is the antineutrino, antiparticle of neutrino. Wait a minute... What is an antiparticle then? Well, in general antiparticles possesses opposite charge and they interact through electromagnetic force. Just like electron having its antiparticle named positron, which is having the same mass but opposite charge. The concept of antiparticle was predicted by famous Physicist Paul Dirac. 

                           Paul Dirac

Later it was proved by Carl Anderson by finding the positron in his cloud chamber experiment. Again a confusion...When neutrino is chargeless, then how it can have an antiparticle? To understand this concept we need to know about the fundamental forces of nature. There are four fundamental forces of nature. They are - 1. Gravitational Force, 2. Electromagnetic Force, 3. Strong Force, 4. Weak Force. Neutrino does not interact through electromagnetic force. It only responds to the weak force. The concept of the antiparticle in weak force is different from the concept of antiparticle in electromagnetic force. Here, the charge is denoted by the lepton number. Neutrino having lepton number +1, whereas, antineutrino is -1. To understand the physical interpretation, we need to visualize the nature of neutrino and antineutrino. 

Let us imagine, one neutrino is moving along a horizontal axis. During its linear motion, it also possesses a spin, which is left-handed or anticlockwise, whereas, an antineutrino possesses right-handed spin or clockwise while moving through the same horizontal axis. So there is a difference in their helicity. This property is known as chirality in physics. To visualize this factor, let us design a game. Take a card in your left hand and hold it diagonally; draw an arrow vertically upward; flip the card diagonally and draw another arrow symmetrical to the other side. Now hold the card in your right hand and start flipping it in the opposite direction. You will notice that an arrow is facing upward and another facing downward. Using this activity we can easily understand the notion of left-handedness and right-handedness.

Now for the ice cream lovers (mostly everyone), I have a good analogy to the next feature of neutrino particle. Suppose in an ice-cream parlor there are three flavors of ice cream- chocolate, vanilla, and strawberry. Similarly, neutrinos also have some flavors. They are electron type neutrino, muon type neutrino, and tau type neutrino, and also they have their antiparticle counterpart. And here the suspense begins...   

Let's have a trip to 13.7 billion years ago just after the big bang. From this huge energy of the big bang, an equal amount of matter and antimatter were created at that time. But the mystery is our observable Universe is only made up of matters. We cannot find any trace of antimatters in our observable Universe. The question is where are they! Why there are matters everywhere? It is one of the biggest unsolved mystery of physics. 

Can neutrino put a light on this mystery? To know about it keep an eye on my next episode.

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

 Part II

The reason behind picking up the neutrino story at first, just because "they have an attitude". They just give a damn to others. But the question is when neutrinos are so reluctant about others why should we care about them, why neutrino matters? So the answer is - if we can catch them, they will reveal to us so many unknown secrets of the Universe. These tiniest particles are fundamental, elementary in nature. That means it cannot be chopped further. The existence of the neutrino was experimentally proved by Fredrick Reines and Clyde Cowan. Before starting the hunting story, let's figure out the motivation of neutrino hunters.

The neutrinos hardly interact with anything and can pass through any matter very easily. This fact leads those hunters to a thought that neutrinos must travel in a straight line for millions and billions of years. So they could be the best storyteller of the universe, as they can carry the information of their source. Neutrinos take birth from various sources such as, from any nuclear reactor, particle accelerator (large hadron collider, at CERN), or maybe from Earth surface due to the presence of radioactive elements and also from our body because of radioactive .decay of potassium. One of the largest sources of neutrinos is our Sun. They are known as solar neutrinos but the highest energetic neutrinos are born in deep space, as the deep space of the Universe is full of violent and mysterious events such as supermassive black holes, supernova explosion, pulsars. So these high energy neutrinos are the messenger of those far reached unknown things of the Universe which bears tantalizing physics mysteries. Scientists immediately understood that there is a sea of possibilities that can be obtained if they can catch them. All the secrets of this Universe lie in the past if we want to uncover them, the neutrino could be the best clue. 

These facts not only excites the particle physicists but also draws the attention of cosmologists and astrophysicists. This is a new way perhaps the best way to look into the secrets of the universe. Scientists thought about a new kind of telescope, known as the neutrino telescope. This ambition lead scientists to the making of the world's largest neutrino observatory in south-pole, named as Icecube neutrino observatory.  Though the project is ambitious, the challenges are enormous. The biggest challenge is to catch the neutrino as they don't interact and can pass through any object.

Ice Cube Neutrino Observatory, South Pole (Antarctica)

A beam of light produced by neutrino

But why the South Pole is chosen as the place for this neutrino observatory? That is also an interesting story. The ice of the South Pole itself acts as a detector in this project. The specialty of the South Pole's ice is- it's the clearest thing available on Earth. The detector is one cubic kilometer in size which is buried under the ice between 1.5 km to 2.5 km. Within this one cubic kilometer volume, there are 5160 light detector sensors have been installed to detect any light source within it. We know when a jet plane travels faster than the speed of sound, it produces a sonic boom behind it. Similarly, when high energetic neutrinos came in contact with ice, it produces a zoo of particles. These particles leave a beam of light behind them which are to be detected by those optical sensors eventually. Does anyone have any idea what is the frequency of detection of those neutrino particles? You may think that it is a joke. But believe me, the possibility is only 10 neutrinos are being detected in a whole year in this project. Don't you think it is a far more thrilling and tiring job than what detectives do? Sometimes I think nothing can be more thrilling and poetic than the neutrino story... Just think these dark particles are carrying messages from the most energetic cosmic engines of our Universe when they finally reach, they left a little bit of light behind them to show their presence. What an insignificant way of presenting the most significant mysteries of the Universe.

Keep an eye for more stories on neutrino hunting and stay amazed...

 

"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...