When it comes to the most exciting problems that have ever been witnessed in the field of Astrophysics, how can one ignore the famous “solar neutrino problem.” The solar neutrino problem puzzled the scientists for decades, and when solved, it bagged four researchers, the Nobel Prize in Physics in 2002 and 2015. But what this problem exactly was and what was its ultimate solution? Before addressing these questions, let’s start with an introduction to the neutrino.

Note: This article is a part of our Basics of Astrophysics series. You can read the entire series free of cost here.

What are neutrinos?

In the 1970s, the theory of the Standard Model in particle physics was developed that integrated all different types of fundamental particles in nature and the particles that govern their interaction with each other. At present, the standard model comprises 17 building blocks of nature: 6 quarks, 6 leptons, 4 force-carrying particles, and the Higgs boson, as shown below. The 6 leptons and 6 quarks are the building blocks of matter called Fermions, while the remaining 5 are called bosons. So, where are the neutrinos in this model?

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The Standard Model of Particle Physics

Coming to neutrinos, they are an important subgroup within the leptons. They come in three flavors named for their partner leptons. The electron, muon, and tau match the electron neutrino, muon neutrino, and tau neutrino. Neutrinos have very little mass and hence, interact extremely feebly with the rest of the particles.  This makes them exceptionally difficult to detect. You would be surprised to know that even while reading this article, trillions of neutrinos pass through your body without any significant interaction. But here we are talking about solar neutrinos, so are these particles related to the host star of our solar system in any way? The answer is “Yes.”

Solar neutrinos and the solar neutrino problem

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 degrees Kelvin. At this temperature, the most important reactions are the proton-proton chain reactions. When a proton collides with another proton, a neutrino is formed. The sun only produces electron neutrinos. It is theorized that nearly 1.8 x 1038 (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 bodies every second. Most of these solar neutrinos have energy too low for detection. So how can we detect them?

Formation of a neutrino (known as solar neutrino when reaction takes place in sun) during proton proton reaction
Formation of a neutrino during proton proton reaction (Image: Academic kids)

The 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 huge 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. So where were the other half neutrinos?

This question bewildered the physicists. Particle physicists started blaming the long-standing solar model. They said that something is missing in the solar model itself. Maybe the total neutrino flux that the solar physicists have theorized is incorrect. However, the solar physicists were adamant as their model had successfully explained 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.

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Sudbury Neutrino Detector played a huge role in solving the solar neutrino problem (Image courtesy: SNO)

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Neutrino oscillations: The ultimate answer

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 a particular flavor later changes to a different flavor. The neutrinos have a slight mass of the order of 0.05-0.1 eV/c2. 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.

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A model of Kamiokande

The Sudbury team compared its electron-neutrino flux value with a 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.

Neutrino Astrophysics

The field of particle physics that studies the origin and behavior of the elementary particles of our universe has always played a parallel role in unlocking the mysteries of the 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, the study and detection of neutrinos can definitely reveal a lot about the origins of cosmic rays and dark matter. This has made the field of neutrino astrophysics a newly emerging branch of research.

This article on solar neutrino problem has been written by Dr. Yashika Ghai, who is currently working as a Theory and Computation Plasma Physicist at Oak Ridge National Laboratory, Tennessee, USA.

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