Borexino Collaboration receives award

European Physical Society acknowledges groundbreaking research into solar neutrinos with Giuseppe and Vanna Cocconi Prize 2021

31 May 2021

The European Physical Society (EPS) announced that it will award the Giuseppe and Vanna Cocconi Prize 2021 to the Borexino Collaboration to acknowledge their groundbreaking insights into solar neutrinos, particles which act as probes of various nuclear fusion processes in the Sun. As a result of their work, the Borexino Collaboration, which also includes scientists from the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU), was able to obtain unique insights into the two fusion processes which take place inside the Sun – the proton-proton chain and the so-called CNO cycle. The award ceremony will take place at this year's virtual European Physical Society Conference on High Energy Physics (EPS-HEP Conference) on July 26, 2021.

To generate energy, the Sun works as a gigantic fusion reactor, continuously converting hydrogen into helium – a process also referred to as 'hydrogen burning'. Essentially, this involves two types of processes within the Sun: On the one hand, there is the proton-proton chain (pp chain). It starts from the direct fusion of two hydrogen nuclei to create the intermediate hydrogen isotope deuterium from which helium is subsequently formed. On the other hand, the heavier elements carbon (C), nitrogen (N) and oxygen (O) are involved in the second type of reaction chain, known as the CNO cycle or Bethe-Weizsäcker cycle. While the pp reaction is predominant in smaller stars such as our Sun, the CNO cycle is the main process for generating energy in hotter, more massive stars.

Guides to the Sun's core

As is the case with all fusion processes that occur within the Sun, countless neutrinos are produced in addition to helium and the enormous amounts of energy which cause the Sun and its sister stars to shine. They reach Earth in their billions and normally pass through it unhindered. The Borexino experiment is able to detect and analyze these neutrinos. While the collaboration has been able to detect neutrinos originating from several reactions along the pp chain in recent years, their latest achievement has been to explicitly identify neutrinos generated during the CNO cycle, which are significantly less abundant in comparison. In doing so, they provided the first experimental evidence of the CNO cycle taking place in the Sun. What's more: The findings also pave the way for a better insight into the elements that compose the solar core, particularly with regard to how frequently heavier elements such as carbon, nitrogen and oxygen can be found in the solar plasma in addition to hydrogen and helium – or what the scientists refer to as 'metallicity'.

"Thanks to our experiment, we have now acquired an almost complete picture of the processes taking place in the Sun's interior," says Prof. Michael Wurm, a neutrino physicist at PRISMA+ and a member of the Borexino Collaboration. "This achievement is based on the common efforts of numerous colleagues from all over the world. I am delighted that this is now being acknowledged with the Guiseppe and Vanna Cocconi Prize."

About the Borexino detector

The Borexino detector has been collecting data on solar neutrinos since 2007. It is located in the largest underground laboratory in the world, the Laboratori Nazionali del Gran Sasso in Italy. At the heart of the Borexino detector is an extremely thin-walled, spherical nylon balloon that contains 280 tons of special scintillator fluid. Per day, only about a hundred neutrinos interact with the detector material. They generate tiny flashes of light which are detected by around 2,000 extremely sensitive photosensors.

In order to make sure that the detected signals actually come from neutrinos, the scientists have to switch off other potential signal sources or filter them out during data analysis – this includes natural background radioactivity and interference caused by cosmic radiation, especially that associated with muons. But even though the tank is shielded by a 1,400-meter thick layer of rock in the Gran Sasso massif near Rome, some muons are still able to reach it and to induce radioactive decays that at first glance cannot be distinguished from a neutrino event. The Mainz team in the Borexino Collaboration is specialized in developing sophisticated analysis techniques that help suppress such background events, so that the rare neutrino signals can be reliably identified.

About the Guiseppe and Vanna Cocconi Prize

The EPS has awarded the Guiseppe and Vanna Cocconi Prize every two years since 2011. The prize honors those who have made an outstanding contribution to Particle Astrophysics and Cosmology in the last fifteen years, and is awarded to one or more individuals or collaborations, for experimental, theoretical or technological work.