An international team of scientists that includes researchers at the Department of Energy’s Lawrence Berkeley National Laboratory and the Karlsruhe Institute of Technology (KIT), Germany have announced a breakthrough in research which aims to measure the mass of the neutrino, one of the most abundant yet elusive elementary particles in the universe.
The Karlsruhe Tritium Neutrino (Katrin) experiment, housed at the Tritium Laboratory Karlsruhe, on the KIT Campus North site, is investigating the most important open issue in neutrino physics: what is the absolute mass scale of neutrinos?
At the 2019 Topics in Astroparticle and Underground Physics conference in Toyama, Japan, leaders from the Katrin experiment reported in September that the rest mass of the neutrino is not larger than 1 electron volt, or eV. These inaugural results, obtained earlier this year by the Katrin experiment, cut the mass range for the neutrino by more than half, by lowering the upper limit of the neutrino’s mass from 2eV to 1eV.
‘These findings by the Katrin collaboration reduce the previous mass range for the neutrino by a factor of two, place more stringent criteria on what the neutrino’s mass actually is, and provide a path forward to measure its value definitively,’ said Hamish Robertson, a Katrin scientist and professor emeritus of physics at the University of Washington.
‘Knowing the mass of the neutrino will allow scientists to answer fundamental questions in cosmology, astrophysics and particle physics, such as how the universe evolved, or what physics exists beyond the Standard Model’ of particle physics, Robertson added.
The Katrin experiment involves researchers at 20 research institutions around the globe. Berkeley Lab’s Katrin team is led by Alan Poon, deputy director of the lab’s nuclear science division.
Poon noted that Berkeley Lab’s low background facility, which measures very-low levels of natural radioactivity in materials, was used to certify the high-purity materials used in Katrin’s components and ensure minimal interference with the experiment. Berkeley Lab’s supercomputing resources also aided in Katrin simulations and data analyses.
‘Our Berkeley Lab team applied our expertise in materials testing and data analysis to help validate this important measurement,’ Poon said.
Neutrinos are one of the most common fundamental particles in our universe, second only to photons. Yet neutrinos are elusive. They are neutral particles with no charge. They interact with matter only through the aptly named ‘weak interaction,’ which means that opportunities to detect them and measure their mass are both rare and difficult.
‘If you filled the solar system with lead out to 50 times beyond the orbit of Pluto, about half of the neutrinos emitted by the sun would still leave the solar system without interacting with that lead,’ Robertson said.
Neutrinos are also mysterious particles that have shaken up our understanding of physics, cosmology and astrophysics. The Standard Model of particle physics had once predicted that neutrinos should have no mass. But in 1998, scientists published evidence from the Super-Kamiokande detector in Japan that they actually do have a nonzero mass – a breakthrough recognised in 2015 with the Nobel Prize in Physics. Since that discovery scientists have been trying to measure its precise value.
‘Solving the mass of the neutrino would lead us into a brave new world of creating a new Standard Model,’ said Peter Doe, a research professor of physics at the University of Washington, who takes part in the Katrin experiment.
The Katrin discovery stems from direct, high-precision measurements of how a rare type of electron-neutrino pair share energy. This approach is the same as neutrino-mass experiments from the early 2000s in Mainz, Germany, and Troitsk, Russia, which set the previous upper limit of the mass at 2eV.
The heart of the Katrin experiment is the source that generates electron-neutrino pairs: gaseous tritium, a highly radioactive isotope of hydrogen. As the tritium nucleus undergoes radioactive decay, it emits a pair of particles: one electron and one neutrino, both sharing 18,560eV of energy.
Katrin scientists cannot directly measure the neutrinos, but they can measure the electrons, and try to calculate neutrino properties based on electron properties.
Most of the electron-neutrino pairs emitted by the tritium share their energy load equally. But in rare cases, the electron takes nearly all the energy, leaving only a tiny amount for the neutrino.
Those rare pairs are what Katrin scientists are after because – thanks to Einstein’s famous E=mc2 equation – scientists know that the miniscule amount of energy left for the neutrino corresponds to its rest mass. If Katrin can accurately measure the electron’s energy, they can calculate the neutrino’s energy and therefore its mass.
The tritium source generates about 25 billion electron-neutrino pairs each second, only a fraction of which are pairs in which the electron takes nearly all the decay energy. The Katrin facility uses a complex series of magnets to channel these electrons away from the tritium source and toward the experiment’s 200-ton electrostatic spectrometer, which measures the energy of the electrons with high precision.
An electric potential within the spectrometer creates an “energy gradient” that electrons must ‘climb’ in order to pass through the spectrometer for detection. Adjusting the electric potential allows scientists to study these rare high-energy electrons, which carry information about the neutrino mass.
Björn Lehnert, a postdoctoral researcher at the Berkeley Lab, used the Cori supercomputer, at NERSC, to perform a comparative tritium measurement for the study using a separate analysis technique.
Lehnert’s analysis is based on a software platform developed by the Katrin team at the Technical University of Munich. The Munich team is headed by Susanne Mertens, a former Berkeley Lab postdoctoral researcher who led a study on how to use Katrin to search for hypothetical particles called sterile neutrinos. Sterile neutrinos are a possible candidate for the dark matter that, though accounting for 85 per cent of the matter in the universe, remains undetected.
Katrin researchers also used NERSC to support several studies of the electromagnetic field, used to guide beta electrons from tritium decays inside the spectrometer.
With tritium data acquisition now underway, US institutions are focused on analysing these data to further improve our understanding of neutrino mass.
Katrin co-spokespersons Guido Drexlin, of KIT, and Christian Weinheimer, of the University of Münster, said in a statement: ‘Katrin is not only a shining beacon of fundamental research and an outstandingly reliable high-tech instrument, it is a motor of international co-operation that provides first-class training of young researchers.’
Now that Katrin scientists have set a new upper limit for the mass of the neutrino, project scientists are working to narrow the range further. ‘Neutrinos are strange little particles,’ Doe said. ‘They’re so ubiquitous, and there’s so much we can learn once we determine this value.’
The Katrin project includes researchers from Europe and the US, including institutions such as; KIT, University of Münster, University of Washington, Carnegie Mellon University, Max Planck Institute for Physics (Werner Heisenberg Institute), The Technical University of Munich, Nuclear Physics Institute of the Czech Academy of Sciences and the French Alternative Energies and Atomic Energy Commission.
