Since 2007, the Borexino detector - situated in the world's largest underground laboratory, Laboratori Nazionali del Gran Sasso in Italy - has been used to obtain data on solar neutrinos. These data reveal important findings on the processes responsible for producing energy in the sun. The properties of the neutrinos themselves can also be investigated using the Borexino data. Livia Ludhova, one of the two current scientific coordinators of the Borexino Collaboration - and head of the neutrino group at the Nuclear Physics Institute at Forschungszentrum Jülich - stated: "For the first time, we subjected all Borexino data to a uniform, homogeneous analysis in all energy ranges. Our data thus provide the best direct insight to date into what is happening inside the sun. And young scientists from the Nuclear Physics Institute have played a major role in these findings: they fitted thousands of data sets using Jülich supercomputers."
Neutrinos are likely the most common elementary particles in the universe. They are formed during a whole range of processes, such as radioactive decays, nuclear fusion in stars, or supernova explosions. Billions of solar neutrinos penetrate each of our fingertips every second - completely unnoticed and undisturbed. However, this ability to pass through matter like a ghost makes them extremely difficult to detect. Measuring them requires large detectors, in which a few of the hundred-thousand billions of neutrinos per day interact with matter and can therefore be detected.
One experiment using this kind of detector is Borexino. At the heart of the Borexino detector is an extremely thin-walled, spherical nylon balloon containing 300 tonnes of a special scintillator fluid. The fluid lights up during the rare reactions with neutrinos. This is observed by the roughly 2,000 highly sensitive light detectors mounted on the walls of the stainless steel sphere enclosing the scintillator. The energy deposited during the neutrino interactions can be determined from the amount of detected light. In order to perform this sensitive measurement in the first place, the natural radioactivity of the Borexino detector had to be reduced by several orders of magnitude, thus setting a global standard for this kind of measurements. To protect against cosmic radiation, the apparatus is located below a 1,400-metre-deep layer of rock at the Gran Sasso massif near Rome, Italy.
The sun is a constant source of neutrinos. In its core, hydrogen nuclei continuously fuse with each other and form the element helium via a chain of different reactions. This releases the vital energy which we know and use as heat and light from the sun. Borexino has been specifically designed to detect the neutrinos formed during nuclear fusion inside the sun. Livia Ludhova explained: "We cannot see into the sun's core using conventional astronomical methods - we only see the surface of the sun in all wavelength ranges of electromagnetic radiation. Only neutrinos allow us a direct glimpse of nuclear fusion."
Various fusion reactions occur inside the sun. Exploiting the unique data, the Borexino team has now provided more precise and more significant measurements of all components of the so-called "pp fusion cycle" in one shot. Borexino demonstrates, for the first time, that the less frequent "pep interaction" also contributes to parts of the neutrinos hitting the Earth. Their data also provide indications that solar models predicting higher abundances of elements heavier than helium could be closer to reality than those predicting lower abundances.
Furthermore, the energy production rate inside the sun was calculated and - within the scope of neutrino measurement accuracy - shown to be in very good agreement with the solar photon luminosity. This means that the sun has been in thermodynamic equilibrium for at least around 100,000 years - the amount of time it takes for energy from inside the sun to reach the surface through photons, while the neutrinos escape immediately.
From the new Borexino data, the researchers were able to obtain a significant result on the properties of the neutrinos themselves. Neutrinos have the characteristic of changing back and forth between three different "families" during their flight - a process otherwise known as neutrino oscillations. This transformation is dependent on whether the neutrinos fly through empty space or dense matter, such as inside the sun. The new data permit a much better description of how the transformation is influenced by matter. This helps to improve our understanding of another aspect of these small particles.
Borexino will continue to record neutrino data until at least 2020. "We are working intensively to understand whether our data can unlock an additional discovery: the detection of neutrinos from the CNO fusion process", stated Livia Ludhova with an eye to the future. The CNO process is another nuclear fusion process which is thought to be responsible for energy production primarily in stars that are heavier than the sun. It is expected to be a secondary process in the sun and therefore difficult to detect. The notion that it occurs in nature is so far only a theoretical prediction.
Borexino is an international collaboration of more than 100 scientists. In Germany, the collaboration partners are the Nuclear Physics Institute of Forschungszentrum Jülich, the Excellence Cluster Universe of the Technical University of Munich, the Institute of Experimental Physics of Hamburg University, RWTH Aachen University, the PRISMA Cluster of Excellence and the Institute of Physics at Mainz University, and the Faculty of Physics at Technische Universität Dresden. The Borexino programme is funded by INFN, Italy; NSF, USA; BMBF, DFG, HGF, and MPG, Germany; RFBR and RSF, Russia; and NCN, Poland.
The title of the paper is " Comprehensive measurement of pp-chain solar neutrinos ".