Borexino sheds light on solar neutrinos

The Borexino experiment is running at the Gran Sasso subterranean laboratory, which is maintained by the Italian National Institute for Nuclear Physics (INFN). Deep below the Earth's surface, the experimental facilities are well-shielded from cosmic rays. Thus, the experiment is able to detect the weak signal produced by solar neutrinos. Borexino was originally conceived 30 years ago and began recording data in 2007. This extensive collaborative project involves institutes from Italy, Germany, France, Poland, the USA, and Russia.

Our understanding of the Sun's interior once again confirmed

The project's scientific success is primarily due to the outstanding radiopurity of the experiment. At the innermost core of the facility only infinitesimal amounts of natural radioactivity are present, i.e., only one trillion times fewer atoms per gram of the elements uranium and thorium than in the surrounding rock. This extraordinary purity is crucial for precisely measuring the energy spectrum of solar neutrinos. It makes it possible to determine the rates of the fusion processes taking place inside the Sun, which are highly dependent on the temperature and elemental composition of the Sun's core. Alongside this glimpse into the conditions deep within our star, the results also provide detailed insights into the oscillation process the neutrinos undergo inside the Sun. Oscillations describe the transformation of the three different types of neutrinos into each other, a process that was only conclusively confirmed for solar neutrinos in 2001. "Borexino's findings far exceed the most optimistic predictions we made when we first started," said Gianpaolo Bellini, one of the pioneers of the INFN experiment.

Professor Michael Wurm, a physicist at JGU and a Borexino partner, affirmed this: "The new results generated by Borexino impressively confirm our current understanding of fusion processes inside the Sun. Our measurements of the complete neutrino spectrum clearly demonstrate the effect solar matter has on the oscillations of the neutrinos produced at the Sun's center." While, at the lower end of the spectrum, neutrinos leave the Sun unaltered, at the upper end of the spectrum, the effect of the oscillations is intensified.

The Mainz Borexino team is mainly concerned with studying the background conditions caused by cosmic muons in the detector. "These muons are the only cosmic ray particles that make it through the 1.5-kilometer-thick mountain shield above the underground laboratory," said Wurm. Reducing this background signal is crucial to detecting the solar neutrinos. The work of the group from Mainz is supported by JGU's Precision Physics, Fundamental Interactions and Structure of Matter (PRISMA) Cluster of Excellence.