The mystery of the mass of the neutrino could soon be solved


We are one step closer to unravelling the mystery of the mass of the neutrino. It has been revealed that the particle, which was once thought to weigh nothing, probably has a mass no more than 500,000 times that of an electron. The new upper limit of the neutrino’s mass, 1.1 electronvolts, is almost half of the previous known upper limit of 2 electronvolts and brings us nearer to pinning down the exact mass of this elusive particle. “Neutrinos are a billion times more abundant in the universe than atoms, so even tiny neutrino masses would make a big contribution to the mass in the universe,” says Christian Weinheimer at the University of Münster, Germany. Identifying the mass of the neutrino will not only help us discern the structure of the early universe but may also help scientists better understand the physics of the smallest things, he says. To estimate the upper limit of the neutrino’s mass, an international team of researchers analysed the decay of a radioactive form of hydrogen called tritium – a process that emits an electron and a neutrino simultaneously. By measuring the energy of the released electrons, they were able to estimate the mass of the neutrino with greater precision than was previously possible. “We are extremely happy and proud,” says Weinheimer. He was part of the international collaboration of scientists behind the Karlsruhe Tritium Neutrino experiment. Weinheimer and his colleagues were able to measure the precise energy of the electrons by using an enormous, purpose-built electron spectrometer that is 24 metres tall and 10 metres across. “It’s very, very exciting,” says Melissa Uchida at the University of Cambridge, UK. “This is just the most precise measurement we’ve ever had,” she says. Uchida thinks the project gives us real hope of pinpointing the mass of the neutrino in the coming years. “We may finally be able to put together the puzzle of how the formation of the universe happened,” says Uchida.

New Scientist, 22 November 2019