An experiment that has been going on for nearly two decades has finally revealed its measurements to the mass of the most abundant matter particle in the universe: the neutrino.
The neutrino could be the strangest subatomic particle; Although abundant, some of the most sensitive detectors are required for observation. Scientists have worked for decades to find out if neutrinos have mass, and if so, what mass they have. The Karlsruhe Tritium Neutrino Experiment (KATRIN) in Germany has now shown its first result, which limits the maximum limit of this mass. The work has an impact on our understanding of the entire cosmos, as these particles formed shortly after the Big Bang and shaped structural formation in the early Universe.
"There are not many ways to measure a cosmos parameters that shape the evolution of the universe in the laboratory," said Diana Parno, a research associate at Carnegie Mellon University, who works on the experiment, opposite Gizmodo.
Neutrinos are available in three variants: electron, muon and tau over how they interact with the corresponding electron, muon and tau particles. Already in 1957, the physicist Bruno Pontecorvo predicted that neutrinos would oscillate between these three different flavors, but this oscillation would require that the particle has mass. Experiments have since shown that oscillation exists, a discovery that brought Arthur B. McDonald and Takaaki Kajita the Nobel Prize in 2015.
But finding their mass is difficult for a number of reasons – first of all, neutrinos only interact with matter via the weak core to gain access to a difficult fundamental force for man-made experiments. Then there is the madness of quantum mechanics; Each neutrino flavor is composed of a probabilistic combination of three "mass states". Due to the madness of quantum mechanics, you can measure either the mass state or the flavor of a neutrino, but not both.
Detecting a particle that does not work does not interact with typical sensors that scientists need to get creative. The KATRIN experiment starts with 25 grams of a type of radioactive hydrogen called tritium, stored in a 30-foot container that is stored at cryogenic temperatures – cold enough that even neon gas is a liquid. These atoms undergo a kind of radioactive decay, the so-called beta decay, in which one of their neutrons is transformed into a proton, spewing out an electron and an electron antineutrino (which has the same mass as the electron neutrino). These decay products pass into a house size detector called a spectrometer, which measures the energy of the electrons. The electron and the neutrino each carry away a portion of the reaction energy, but how much they take away can vary. The scientists need to study the spectrum of all the different electron energies, focusing in particular on the electrons that have been deprived of the maximum energy whose neutrinos would have deprived the minimum energy. Analysis of the shape of the resulting graphs shows the maximum energy of any neutrino mass state.
The mere fact that there is an oscillation sets the lowest possible average mass of the three mass states, less than 0.1 electron volts (eV). After one month of operation and 18 years of planning and construction, KATRIN has now predicted an upper limit for one of the three mass states of 1.1 eV, with one electron weighing around 500,000 eV and one proton nearly one billion.
KATRIN scientists announced the results at the Topics conference in Astroparticle and Underground Physics 2019 in Toyama, Japan, last Friday.
The collaboration with KATRIN began in 2001, but "it was a long time because it's a really complicated experiment," said Hamish Robertson, a KATRIN scientist and professor of physics at the University of Washington, to Gizmodo.  The pressure and temperature of the gas source require precise control, and there are many moving parts.It took years to develop and build the giant spectrometer, which rejects unwanted electrons and precisely measures the resulting electron energies.
"It is somewhat fractal, "Parno said." Increasing part of the experiment and starting to ask questions gives you the same level of complexity again. "
KATRIN is just one of several different mass-calculation strategies Just last month, researchers used cosmological data to argue that the sum of the three neutrinos at most 0.26 electron volts. Other experiments hope to calculate the neutrino mass using rare atomic decays. However, KATRIN's findings are valuable because they are not based on great theories of how the universe works, said Phillip Barbeau, Associate Physics Professor at Duke University, who was not involved in the study.
This latest limit on mass halves the maximum mass found in other experimental setups and comes from just one month of data. Much work remains to be done, including five-year data collection, which will further restrict the masses. Scientists want to know more than just the maximum mass of states; They want to know the absolute mass of all three states and how they compare with each other. The solution to this problem has an impact on understanding the behavior of the early universe, whether the neutrino is its own antiparticle and why there is more matter in the universe than antimatter. Many physicists are interested in the result.
"It's a fundamental parameter," said Kate Scholberg, a professor of physics at Duke University who was not involved in the study, to Gizmodo. "If you're trying to develop overarching models of fundamental physics, unified theories, and the like, you want all the information you can – like the masses of all the particles."