An international team of scientists has announced a breakthrough in measuring the mass of the neutrino, one of the most abundant, elusive elementary particles in our universe.
At the Topics Conference in Astroparticle and Underground Physics 201
"Knowing the mass of the neutrino will enable scientists to answer fundamental questions of cosmology, astrophysics, and particle physics, such as how the universe evolves or which physics exists beyond the standard model," said Hamish Robertson, a KATRIN. Scientist and professor emeritus of physics at the University of Washington. "These results of the KATRIN collaboration reduce the previous mass range for the neutrino by a factor of two, set stricter criteria for the actual mass of the neutrino, and provide a way to finally measure its value."
The KATRIN The experiment is based at the Karlsruhe Institute of Technology in Germany and involves researchers at 20 research institutions around the world. In addition to the University of Washington, KATRIN member institutions in the US include:
- The University of North Carolina at Chapel Hill, headed by Professor of Physics and Astronomy John Wilkerson, a former UW faculty member
- The Massachusetts Institute of Technology under the direction of physics professor Joseph Formaggio
- The Lawrence Berkeley National Laboratory led by Deputy Director of Nuclear Science Division Alan Poon
- The Carnegie Mellon University led by Deputy Professor of Physics Diana Parno
- Case Western Reserve University Under the leadership of Professor Benjamin Monreal
the University of Washington under Robertson and Wilkerson in 2001 became one of the founding institutions of KATRIN. Wilkerson later moved to the University of North Carolina at Chapel Hill. Formaggio and Parno began their employment with KATRIN as UW researchers and later switched to their current facilities. Other current UW scientists working on the KATRIN experiment include Robertson, PhD physics researcher Peter Doe, Sanshiro Enomoto physicist and Menglei Sun, a postdoctoral fellow at the UW Center for Experimental Nuclear Physics and Astrophysics.
Neutrinos are abundant. They are one of the most common fundamental particles in our universe, after photons. But also neutrinos are elusive. They are neutral particles with no charge and interact with other substances only by the aptly named "weak interaction," which means that the ability to detect neutrinos and measure their mass is rare and difficult.
"If you fill the solar system with lead up to fifty times beyond the orbit of Pluto, about half of the neutrinos emitted by the Sun would still leave the solar system without interacting with this lead," Robertson said.
Neutrinos are also mysterious particles that have already disrupted physics, cosmology and astrophysics. The standard model of particle physics had once predicted that neutrinos should have no mass. By 2001, scientists with two detectors, Super-Kamiokande and Sudbury Neutrino Observatory, had shown that they actually have a non-zero mass – a breakthrough recognized in 2015 with the Nobel Prize in Physics. Neutrinos have mass, but how much?
"Solving the mass of the neutrino would lead us into a brave new world of creating a new standard model," said Doe.
The discovery of KATRIN relies on direct, high-precision precision measurements of how a rare type of electron-neutrino pair shares energy. This approach is the same as in the 1990s and early 2000s neutrino mass experiments in Mainz and Troitsk, Russia, where both set the previous upper limit of mass to 2 eV. At the heart of the KATRIN experiment is the source that produces electron-neutrino pairs: gaseous tritium, a highly radioactive isotope of hydrogen. As the tritium nucleus decays radioactively, it emits a pair of particles: an electron and a neutrino, both sharing 18,560 eV of energy.
KATRIN scientists can not measure neutrinos directly, but they can measure electrons and try to calculate neutrinos. Properties are based on electron properties.
Most tritium-emitted electron-neutrino pairs share their energy charge in equal proportions. In rare cases, however, the electron absorbs almost all the energy leaving only a small amount of energy for the neutrino. These rare couples seek out the KATRIN scientists, because thanks to E = mc2, the scientists know that the tiny amount of energy remaining for the neutrino must encompass its rest mass. If KATRIN can accurately measure the energy of the electron, they can calculate the energy of the neutrino and thus its mass.
The tritium source generates about 25 billion electron-neutrino pairs per second, of which only a fraction are pairs, in which the electron almost all absorbs the decay energy. The KATRIN facility in Karlsruhe uses a complex array of magnets to direct the electron away from the tritium source towards an electrostatic spectrometer that measures the energy of the electrons with high precision. An electrical potential within the spectrometer produces an "energy gradient" that electrons must "scale up" to reach for detection by the spectrometer. By adjusting the electrical potential, scientists can study the rare high-energy electrons that contain information about the neutrino mass.
US. The institutes have made extensive contributions to KATRIN, including providing the electron detector system – the "eye" of KATRIN – that looks into the heart of the spectrometer, a UW-built instrument. The University of North Carolina at Chapel Hill led the development of the data acquisition system of the detector, the "brain" of KATRIN. MIT's contribution was the design and development of the simulation software that modeled KATRIN's response. The Lawrence Berkeley National Laboratory contributed to the development of the Physics Analysis Program and provided access to national computing facilities. The rapid analysis was made possible by a number of applications that originated at the UW. Case Western Reserve University was responsible for the design of the electron gun, which was central to the calibration of the KATRIN instrument. Carnegie Mellon University contributed primarily to the analysis, with background and adaptation in the foreground, and supported analysis coordination for the experiment.
With tritium data acquisition in progress, US institutions are focusing on analyzing this data to further improve our understanding of neutrino mass. These efforts may also reveal the existence of sterile neutrinos, a potential candidate for dark matter, which, while accounting for 85% of matter in the universe, remains undetected.
"KATRIN is not only a beacon of basic research and" An outstandingly reliable high-tech instrument, but also an engine of international cooperation that enables first-class training of young scientists, "said KATRIN spokesman Guido Drexlin from the Karlsruhe Institute of Technology and Christian Weinheimer of the University of Münster in a statement.
After the KATRIN scientists set a new upper limit for the mass of the neutrino, the project scientists are working to narrow the scope even further.
"Neutrinos are strange small particles "Doe says," They are so omnipresent, and there is so much we can learn from that value. "
The US Department of Energy has funded US participation in the KATRIN experiment since 2007.
Important advances in the understanding of neutrino properties
KATRIN halves the mass estimate for the elusive neutrino (2019, September 16)
September 16, 2019 retrieved
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