High-Performance Computing 2019-20

HPC Yearbook 19/20

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 soſtware

Members of the Katrin experiment pose in front of the spectrometer, aſter it was installed at the Karlsruhe Institute of Technology

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 exist beyond the Standard Model

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. Te Katrin discovery stems from direct,

high-precision measurements of how a rare type of electron-neutrino pair share energy. Tis 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. Te 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. Tose rare pairs are what Katrin scientists

are aſter because – thanks to Einstein’s famous E=mc2 equation – scientists know that the miniscule amount of energy leſt 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. Te 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. Te Katrin facility uses a complex series of magnets to channel these electrons

platform developed by the Katrin team at the Technical University of Munich. Te 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. ‘Tey’re so ubiquitous, and there’s so much we can learn once we determine this value.’ Te 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), Te Technical University of Munich, Nuclear Physics Institute of the Czech Academy of Sciences and the French Alternative Energies and Atomic Energy Commission. n


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