In the previous two articles, I discussed the structure of Sun in detail. Before leaving solar physics, I want to discuss an interesting problem in Astrophysics known as The Solar Neutrino Problem. The Solar Neutrino problem, when solved bragged the 2015 Nobel prize to Prof. Arthur B. McDonald and Prof. Taakaki Kajita. The aim of including this in Basics of Astrophysics series is to show how important particle physics is in Astrophysics. So in this article, I shall tell you what are neutrinos and how are they related to the sun, and eventually the solar neutrino problem. Let’s understand the famous solar neutrino problem and revisit its solution.
What Are Neutrinos?
Neutrinos are elementary particles of matter. They have no electric charge and were earlier considered to have no mass. Neutrinos only interact weakly with matter, which makes them very difficult to detect. So weakly that while you are reading this article, trillions of neutrinos are passing through your body without any significant interaction. They are of three types: Electron neutrino, Muon neutrino, and Tau neutrino. These are the three flavors of neutrinos.
Neutrinos Generated In The Sun:
The Sun mainly contains hydrogen gas. According to the standard solar model, the central temperature of the Sun is of the order of 15 million degree Kelvin. At this temperature, the most important reactions are the proton-proton chain reactions. A simple figure below illustrates these reactions.
You can easily see that when a proton collides with another proton, a neutrino is formed. The sun only produces electron neutrinos. It is theorized that nearly 1.8*10^38 (180 trillion trillion trillion) solar neutrinos are produced every second by the Sun. This means that on Earth nearly 400 trillion solar neutrinos go through our body every second. Most of these solar neutrinos have energy too low for detection. So how can we detect them?
Higher energy neutrinos are rare. They have an occurrence frequency of 2 out of 10,000 p-p reactions. To detect these solar neutrinos, we need extremely large vessels full of liquid. In these vessels, the neutrinos can be detected via Cerenkov detectors. The previous devices were only sensitive to electron neutrinos. This is why they could detect only half the number of neutrinos that were generated in the sun. Where were the other half neutrinos?
This bewildered the Physicists. Particle Physicists started blaming the long standing solar model. They said that there is something missing in the solar model itself. Maybe the total neutrino flux that is theorized by the Solar Physicists is incorrect. However, the Solar Physicists were adamant as their model had successfully explained each and every aspect of the Sun so far.
Solving The Missing Solar Neutrino Problem:
The two neutrino detectors- Sudbury Neutrino Observatory (SNO) in Canada and the Super-Kamiokande detector in Japan played a major role in solving this puzzle. At SNO, the neutrinos from the Sun, Earth, and supernovae are detected. The electrons produced in charged-current reactions emit Cerenkov radiation as they travel through water. The intensity of this Cerenkov radiation is proportional to the energy of the neutrino. Using this fact, scientists can calculate the energy distribution of the incoming neutrinos.
The scientists working at the Super-Kamiokande detector made a breakthrough discovery regarding the properties of neutrinos. They gave an experimental observation of neutrino oscillations. Neutrino oscillations occur when a neutrino produced with particular flavor later changes to a different flavor. The neutrinos have a slight mass of the order of 0.05-0.1 eV/c^2. Due to this slight mass, the neutrinos interact with matter. A particular neutrino may be born as an electron neutrino. It may later convert into a muon or tau neutrino and vice-versa.
The Sudbury team compared its value of electron-neutrino flux with a very precise measurement of the total neutrino flux measurements at Super-Kamiokande. By comparing these figures, physicists from SNO and SuperKamiokande calculated the true solar-neutrino flux. It was in excellent agreement with the “standard solar model” of energy production in the Sun. Hence, the missing neutrinos were actually changing their flavors from electron to muon neutrinos. This was the reason that they escaped from the eyes of these detectors.
The field of particle physics that studies the origin and behaviour of the elementary particles of our universe, has always played a parallel role in unlocking the mysteries of cosmos. Moving in the same direction, the chargeless and weakly interacting neutrinos prove out to be ideal astronomical messengers as they can travel through space without scattering, absorption or deflection.
A few years ago, experiments at IceCube observatory discovered neutrinos that originated beyond the Sun with energies bracketed by those of the highest energy gamma rays and cosmic rays. So, definitely the study and detection of neutrinos can reveal a lot about the origins of cosmic rays and about dark matter. This has made the field of Neutrino astrophysics a new emerging branch of research.
I have personally met Nobel Laureate Takaki Kajita at the 66th Lindau Meeting of Nobel Laureates and students. His wonderful talk was based on neutrino oscillations. I will not shy away from saying that his talk arose my interest in this wonderful phenomenon of neutrino oscillations. His calmness and dedication inspired me. After interacting with him, I realized how big a project is the Super-Kamiokande detector. Huge teams of scientists have dedicated themselves for the Super-Kamiokande’s proper working of such Nobel establishments.
In my Doctorate in Plasma Physics, among other phenomenon, I have studied the neutrino beam instabilities due to neutrino oscillations in the ultra-relativistic degenerate plasma such as that in red giant stars (Betelgeuse), with Rishabh (founder of The Secrets Of The Universe). That’s how we met!