Topic: Solar Neutrinos: History and Observations
Speaker: Aldo Ianni
How does the sun shine? This is a long-standing question that scientists have answered by considering nuclear fusion of hydrogen into helium. In the late 1930s two reaction mechanisms were proposed to turn hydrogen into helium in the core of stars, namely the pp-chain and the CNO-cycle. These nuclear processes produce electron neutrinos besides energy. Neutrinos can propagate through the high density solar core easily. Therefore, they are a unique probe for understanding the energy production mechanism.
In 1960s it was proposed to search for electron neutrinos from the sun (solar neutrinos) in deep underground laboratories. The first observation was carried out in Homestake in 1968. This has been a breakthrough in solar neutrino physics and was awarded a Nobel prize in physics in 2002. With the Homestake experiment the so-called solar neutrino problem was born: 2/3 of the expected signal was missing. A new experimental program started to solve this problem. In the following 40 years several experiments were deployed in underground laboratories in different continents. This intense activity from an experimental and theoretical point of view eventually solved the problem consolidating the model of neutrino mixing and the origin of solar energy. In 2020 the Borexino experiment observed neutrinos from the CNO-cycle which accounts for only 1% of the solar energy. With this observation both energy production mechanisms claimed 80 years before were proved.
In the talk a review of solar neutrino observations and a discussion on the physics implications are presented. Details on the experimental challenges to detect low energy neutrinos are discussed.
Aldo Ianni is a staff researcher at the Laboratori Nazionali del Gran Sasso. His research interests lie in the field of astroparticle physics (at the intersection between particle physics, astrophysics and cosmology). This is a new field in which researchers work to solve the problem of dark matter, to observe neutrinos from the sun and supernovae, to search and understand the origin of high energy cosmic rays, to determine whether a neutrino is a so-called Majorana or Dirac particle. The Gran Sasso Laboratory is a unique place where one can carry out this research.
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