Oak Ridge National Laboratory is managed by UT-Battelle LLC for the US Department of Energy
The story of the neutrino
The Science
The Story of the Neutrino
A neutrino is one of the elementary particles that makes up the matter in our Universe. As one of the most abundant particles found in nature, neutrinos are continuously produced in large numbers from nuclear reactions in our Sun, through cosmic interactions in our atmosphere, man-made nuclear reactors and particle accelerators, within the Earth, and even inside our own bodies. The neutrino have many remarkable properties that makes them rich for scientific study. After its existence was first proposed in 1930 and detected in 1956, the decades since have yielded a wealth of new information, and new questions. Originally expected to be massless, neutrinos have a surprisingly small mass and the lightest of all subatomic particles. They are created in one of several flavors, yet can transform into a different flavor before detection – the principle behind the now-solved Solar Neutrino Problem. Given these properties, the neutrino may be hiding a few more surprises that will explain their role in the Universe.
Ordinary beta decay of many heavy even-even nuclei is energetically forbidden or strongly spin inhibited. However, a process in which a nucleus changes its atomic number by two while simultaneously emitting two beta (β) particles, which are ordinary electrons, and two anti-neutrinos (ν) is energetically possible for some even-even nuclei (this process is abbreviated as 2νββ defining the emitted particles in the final state). It has been hypothesized that zero-neutrino double-beta decay 0νββ is also possible, where only the two β particles are emitted with no neutrinos. This decay violates lepton number conservation, which is a symmetry of nature that so far has been empirically conserved. Further, the observation of this 0νββ decay would definitively show that neutrinos are indistinguishable from their anti-matter partners, and therefore would be so-called Majorana particles. Establishing this case would provide a neutrino mass mechanism that is understood to be lepton-generating; a process referred to as leptogenesis. An exciting prospect for establishing a leptogenetic process would be its ability to drive baryogenesis: a net accumulation of baryons over anti-baryons that is visible in our universe by the small a amount of matter over antimatter that survived the initially equal conditions of the Big Bang.
Should it exist, an observation of neutrinoless double-beta decay will be a technical triumph, as well as a scientific breakthrough. The rate of decay is astonishingly small. Its half-life – a common measure to define the time it takes for half of a collection of nuclei to decay to a new state – exceeds 1026 years (the age of the Universe is just over 1010 years old!). In order to see such a decay in reasonable amount of time, one must start with a large enough number of atoms where the probability of one, or a few, atomic nuclei undergoing the decay is likely – the next-generation of experiments aim for a target mass on the order of 1000 kg and operate on the order of 10 years. Once a neutrinoless double-beta decay occurs, its only detectable signature is in the form of the two energetic elections (once dubbed beta particles). A detector optimized to search for this rare decay must first distinguish it from the competing two-neutrino double-beta decay that occurs at a greater rate but through the emission of two electrons of reduced energy. Further, there are many sources of natural nuclear decay that would be observed in the detector at a very high rate that can obstruct the faint signal of interest. For these reasons, the experimental design has a large influence on the ultimate discovery potential of an experiment searching for neutrinoless double-beta decay. Key considerations include excellent energy resolution, ability to identify or separately detect the interfering background, and the use of ultra-pure materials to shield the target of the decay.
The importance of reducing interfering backgrounds and excellent energy resolution is demonstrated in the graphic for the case of the isotope 76Ge. The various curves denote the total number of background events that would be observed per the detector’s standard unit of energy resolution (FWHM: full-width at half-maximum, which defines the region to search for the signal) per ton of material (1000 kg) per year of operating. Only the ideal background free scenario and the realistic red curve allow an experiment to reach the shaded region beyond a half-life of 1028 years when running for a reasonable amount of time: 10 years with a target with a mass of 1 ton. This means only an extremely small fraction of background events (0.03!) are allowed in the signal region in the detector per year, which sets a tight goal for the experimental design.
Oak Ridge National Laboratory is managed by UT-Battelle LLC for the US Department of Energy
