What do neutrinos tell us about the sun?

The low number of observed neutrinos coming from the sun can be explained by the oscillation between the three species of neutrinos, which transform into each other.

While it takes thousands of years for the fusion-produced Gamma rays to reach the Sun's surface, neutrinos travel at the speed of light and reach us instantly. Therefore, while light provides information about fusion that occurred millions of years ago, neutrinos provide information about the current state of affairs.

Since lepton number is not conserved unless another lepton is formed, beta decay is the beginning of it all. An electron is released during the decay of a nucleus; since electric charge is conserved, the particle needed to be neutral and was designated as a neutrino. There is one neutrino associated with each lepton: one for the electron, one for the muon, and one for the tau.

Because they are neutral, their mean free path is larger than that of charged particles. Neutrinos are produced during Proton-Proton reactions, which are fusion reactions that produce nuclei of increasingly large atomic number, starting with hydrogen and moving up the Mendeleev ladder. Neutrinos interact via gravity, weak interaction, but not electromagnetic interaction.

In the sun, hydrogen is converted into deuterium through the chain PP I. P P III produces the most energetic neutrinos and contains the reaction that causes the sun to heat up and keep from collapsing under its own weight.

John Bacall designed his experiment specifically for these reactions; when he observed about 1/3 of the expected number of neutrinos, he had to revise the standard model and demonstrate that a tiny mass can cause neutrino oscillations. Since all species of neutrinos can turn into each other, the neutrino count was off by 2/3 when a detector was only aimed at one species. Now that this issue has been resolved, we can and must use neutrinos to explore the interior of the sun.

Assuming the path of a photon to be a random walk, the average size of each step was estimated to be 9/100 centimeters. Taking into account that the photon has almost 700000 km to travel to reach the surface and that it does so through a diffusion process, one finds that it takes 1.7 10^5 years for photons to emerge out of the sun, which is why we need neutrinos to understand what is going on inside the sun.

Neutrinos travel near the speed of light and reach detectors much faster than photons, making them an inferior tool for exploring the sun's interior.

Since neutrinos are generated in the Sun's core by nuclear reactions, they offer important insights into the Sun's composition, temperature, and density. Moreover, by analyzing the neutrinos released by the Sun, scientists can investigate the basic characteristics of neutrinos themselves and gain an indirect understanding of the nuclear fusion processes that drive the Sun.