Mass of Ghostly Neutrinos Measured, Uncovering Flaws in the Standard Model

Mass of Ghostly Neutrinos Measured, Uncovering Flaws in the Standard Model

By precisely measuring the mass of neutrinos - ghostly particles that stream through your body by the billions each second - physicists could find some glaring holes in the Standard Model of particle physics. A new experiment has taken them one step closer.

A ghost on the scales

Technically, the weirdness of the quantum mechanical mixing among the three neutrino flavors means that none of them have a well-defined mass. Instead, they are combinations of three different "mass states." This means that physicists don't look for an exact reading of a neutrino's mass but for an upper limit of how big this mass could be. Nearly 99% of the mass of any object, including our own bodies, comes from the binding energy holding elementary particles together inside atoms. The remaining 1% of the mass, however, is intrinsic to those particles. To find this intrinsic mass, physicists look for something called the Q value - the difference between the sum of the masses of the initial reactants and the sum of the masses of the final products. With this value in hand, further measurements can extract the intrinsic mass from the overall mass of the atom. One neutrino-mass-measuring experiment, the Karlsruhe Tritium Neutrino experiment (KATRIN) in Germany, found a precise estimate for the neutrino's mass by measuring the energy - and, therefore, by Einstein's E = mc2, the mass difference - as superheavy hydrogen decayed into helium, emitting an electron and a neutrino in the process. The KATRIN experiment’s best result found an upper neutrino mass limit of 0.8 electronvolts, making it roughly 500,000 times smaller than the mass of an electron. This measurement can also be made in reverse by observing an electron being captured by the artificial isotope holmium-163, transforming it into dysprosium-163 and releasing a neutrino. But to do so, the isotope must be surrounded by gold atoms. "However, these gold atoms could have an influence on holmium-163," Schweiger said. "It is therefore important to measure the value of Q as precisely as possible using an alternative method" and to compare it with the mass value determined through the KATRIN method in order to detect possible sources of error. To get closer to a separate measurement of the neutrino's elusive mass, the researchers designed an experiment known as a Penantrap - a combination of five "Penning traps," which can capture atoms inside a combination of an electric field and a magnetic field, in which they swing in an intricate motion known as a "circle dance." By placing charged holmium-163 and dysprosium-163 ions inside the Penning traps and measuring the subtle differences in their swing rates, the physicists gauged the difference in their energies caused by the additional neutrino. The result was a measurement of a Q value that the researchers say is 50 times more precise than the result of any previous experiment. With this result in hand, an even better upper limit for the neutrino's mass is one tiny - but consequential - step closer.