Borexino is a particle physics experiment located in Hall C of the Laboratori Nazionali del Gran Sasso, approximately 100 miles north-east of Rome - Italy. It is a large liquid scintillator detector whose main goal is the study of the properties of low energy solar neutrinos and has started data-taking in 2007. The exceptional levels of radiopurity Borexino has reached through the years, have made it possible to accomplish not only its primary goal but also to produce many other interesting results both within and beyond the Standard Model of particle physics.
The properties of the subatomic elementary particle called neutrino have been studied by several experiments in the past decades. Borexino is one of the most advanced neutrino real time detectors and it works by exploiting an extremely intense source of neutrinos: our Solar System star, the Sun. The Sun is essentially a self-confining highly efficient nuclear fusion reactor whose production rate is regulated by the Weak Nuclear interaction. The result of a typical chain of reactions initiated by a proton-proton fusion in the core of the Sun can be written as:
in which four protons are used to form an alpha particle and two positrons. Two neutrinos (ν) are emitted and energy is released (actually 26 MeV, mega-electron volts per one of these reactions). Indeed the Sun is a very powerful source of neutrinos: about ~ 6 x 1010 neutrinos of solar origin hit a centimeter square of the Earth per second!
The fusion processes that generate the Sun’s energy take place in the core, its innermost part. Lots of photons and neutrinos are produced there. While photons are stopped in the Sun core and the relative energy takes typically a million years to reach the Sun’s surface (and us), neutrinos emitted in the fusion reactions are able to traverse the material of the Sun (mostly ionized hydrogen and helium) and arrive in 8 minutes (travelling at the speed of light) to the Earth’s surface.
This is possible because of a feature that makes the neutrino peculiar in yet another sense: its interaction capability with matter (called cross section) is unbelievably small (~ 10-46 cm2) . Most of the neutrinos traverse the Sun and the Earth with almost no interactions at all.
It is precisely inside a huge detector like Borexino that an impressive compensation takes place between an unbelievably small number (the neutrino cross section, 10-46 cm2) and an incredibly large number (the solar neutrino flux, ~ 1010 cm-2 s-1) to produce a number which is still rather small (~ 10-36 interactions per second per unit target) but can be made reasonable by having a big (several tons) detector (tons of material, so that one has about 1030 targets). The detector features about 1300 tons of scintillator and 2400 tons of water.
A scintillator is a material that emits light when it is traversed by a subatomic charged particle, for instance an electron. The detection of neutrinos in Borexino is made by using electrons of the liquid scintillator as the target, according to the following the reaction:
in which the neutrinos hit the electrons of the material. While the electron on the left-hand side is practically at rest, the one on the right hand side has received energy from the impinging neutrino and is therefore able to generate a scintillation signal (composed by light) within a special liquid material, called scintillator. This light burst is then detected by the 2200 photomultipliers of the detector which essentially are light sensors: the detection of light pulses by the photomultiplier system is the signature of a neutrino interaction.
However, the investigation of the solar neutrino spectrum is far from being an easy game. The background is a very important experimental problem that makes neutrino detection very difficult. The background has several components but the main ones are the radioactivity of the detector materials, of the Gran Sasso facility and cosmic ray particles.
Especially in the energy region below 1 MeV, experiments can be severely affected by background: in particular, the cosmic ray background practically imposes the use of an underground laboratory to shield the detector. In the Gran Sasso underground laboratory, the average rock cover of about 1,400 m results in a reduction (with respect to the surface) in the muon flux of a factor 106. To give an idea, the expected rate of 7Be solar neutrinos in 100 ton of Borexino scintillator is about 10-9 Bq/Kg while the common natural water is about 10 Bq/Kg in 238U, 232Th and 40K.
Borexino has been specifically designed to study the low energy solar neutrinos. The success of Borexino comes as a result of a 15 year long R&D study carried out by the collaboration to develop the best techniques of scintillator purification, allowing to reach and exceed the required levels of radiopurity. At present, Borexino is the less radioactive place in the world!
The Borexino experiment started taking data in 2007. Since then, it has produced a considerable amount of interesting results which include the first direct spectroscopy of proton-proton solar neutrinos, the precision measurement of the 7Be solar neutrino rate (with a total error of less than 5%), the first direct measurement of the so-called pep solar neutrinos and the measurement of the 8B solar neutrino rate with an unprecedented low energy threshold. Borexino has also published significant results on non-solar neutrino physics, such as the first observation of anti-neutrinos from the Earth (the geoneutrinos) and several limits on rare or forbidden processes.
The Borexino experiment is performed by an international collaboration featuring about 140 scientists from Italy (INFN and Universities of Milano, Ferrara, Genova, Perugia, Napoli, Laboratori Nazionali del Gran Sasso and GSSI), France (CEA Saclay and APC Paris), Germany (TUM Munich, MPI-K Heildelberg, TU Dresden, Universities of Tuebingen, Hamburg and Mainz), Russia (JINR Dubna, Lomosonov State University, Kurchatov Institute Moscow and NPI St. Petersburg), Poland (Jagellonian University Krakow), Ukraine (Kiev INR) and United States (Universities of Princeton, Hawaii, Massachusetts Amherst, Houston, UCLA and Virginia Polytechnic Institute).
INFN and University of Milano: D. Basilico, G. Bellini, A. Brigatti, B. Caccianiga, D. D'Angelo, M.G. Giammarchi, P. Lombardi, L. Ludhova, E. Meroni, L. Miramonti, S. Parmeggiano, G. Ranucci, A. Re, P. Saggese
INFN and University of Ferrara: G. Fiorentini, F. Mantovani, B. Ricci
INFN and University of Genova: A. Caminata, M. Cariello, L. Di Noto, S. Farinon, C. Ghiano, R. Musenich, L. Pagani, M. Pallavicini, L. Perasso, G. Testera, S. Zavatarelli
INFN and University of Perugia: F. Ortica, N. Pelliccia, A. Romani
INFN Laboratori Nazionali del Gran Sasso: G. Bonfini, G. Di Pietro, F. Gabriele, A. Ianni, M. Laubenstein, F. Lombardi, M. Orsini, R. Roncin, N. Rossi, R. Tartaglia
Gran Sasso Science Institute: S. Davini, I. Drachnev, S. Marcocci
CEA Saclay: X. Avery, N. Berton, M. Cribier, T. Cuvillier, G. Durand, M. Durero, V. Fischer, J. Gaffiot, W. Gamache, T. Houdy, N. Jonqueres, T. Lassere, D. Leterme, D. Loiseau, P. Lotrus, G. Mention, H. Przybilsky, G. Rampal, Y. Reinert, L. Scola, C. Veyssiere, M. Vivier, P. Yala
APC Paris: D. Franco, D. Kryn, M. Obolensky, D. Vignaud
TUM Munich: M. Agostini, K. Altenmuller, S. Appel, M. Goeger-Neff, B. Neumair, L. Oberauer, L. Papp, S. Schoenert, F. Von Feilitzsch
MPI-K Heidelberg: W. Maneschg, H. Simgen
TU Dresden: B. Lehnert, J. Thurn, K. Zuber
University of Tuebingen: T. Lachenmaier
University of Hamburg: D. Bick, C. Hagner, M. Kaiser, M. Meyer
University of Mainz: S. Weinz, J. Winter, M. Wurm
JINR Dubna: K. Fomenko, A. Formozov, D. Korablev, O. Smirnov, A. Sotnikov, A. Vishneva, O. Zaimidoroga
Lomonosov State University Moscow: A. Chepurnov, M. Gromov
Kurchatov Institute Moscow: V. Atroshchensko, L. Borodikhina, A. Etenko, E. Litvinovich, G. Lukyanchenko, I. Machulin, V. Orekhov, D. Pugachev, M. Skorokhvatov, S. Sukhotin, M. Toropova
NPI St. Petersburg: A. Derbin, V. Muratova, D. Semenov, E. Unzhakov
Jagellonian University Krakow: M. Misiaszek, M. Wojcik, G. Zuzel
Kiev INR: V. Kobychev
University of Princeton: J. Benziger, F.P. Calaprice, C. Galbiati, A. Goretti, A. M. Ianni
University of Hawaii: K. Choi, J. Maricic
University of Amherst Massachusetts: A. Pocar, J. Sainz
University of Houston: E. Hungerford, G. Korga
UCLA: Y. Suvorov, H. Wang
Virginia Polytechnic Institute: D. Bravo, P. Cavalcante, B. Vogelaar, Z. Yokley
G. Bellini et al., Nature, vol. 512, 383-386 (2014).
G. Bellini et al., Physical Review D, vol. 89, 112007 (2014).
G. Bellini et al., Physics Letters B, vol. 722, 295 (2013).
G. Bellini et al., Journal of High Energy Physics, vol. 8, 038 (2013).