A large, electron-counting machine indirectly turned up a measurement of the slipperiest known particle in physics – and added to the records for dark matter.
of neutrinos – particles that fill our universe and determine its structure, but which we are barely able to detect at all. Neutrinos, according to the German-based Karlsruhe Tritium Neutrino experiment (KATRIN), have no more than 0.0002% of the mass of an electron. That number is so low that even if we tallied up all the neutrinos in the universe, they would explain its missing mass. And that fact adds to the pile of evidence for dark matter's existence.
KATRIN is basically a very large machine for counting super-high-energy electrons that burst out of a sample of tritium ̵
Neutrinos are more or less impossible to accurately measure on their own because they are so small in mass and tend to skip out of detectors without interacting with them. These are figures out of the mass of neutrinos, Robertson told Live Science, KATRIN counts the most energetic electrons and works backward from that number to deduce the neutrino's mass. The first results from KATRIN have been announced, and the researchers have come to an early conclusion: Neutrinos have a mass no higher than 1.1 electron volts (eV).
Electron volts are the units of mass and energy physicists use when talking about the smallest things in the universe. (At the scale of fundamental particle, energy and mass are measured using the same units, and the neutrino-electron pairs have to have combined energy levels equivalent to their neutron source.) The Higgs boson, which lends other particles to their mass, has a mass of 125 billion EV. Protons, the particles at the center of atoms, have masses of about 938 million eV. Electrons are a measure of 510,000 eV. This experiment confirms that neutrinos are incredibly tiny.
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KATRIN is a very big machine, but its methods are straightforward, Robertson said. The device's first chamber is full of gaseous tritium, whose neutrons naturally decay into electrons and neutrinos. Physicists already know how much energy is involved when a neutron decays. Some of the energy is converted into the mass of the neutrino and the mass of the electron. And the rest gets spread into those newly created particles, very roughly dictating how fast they go. Usually, that extra energy gets distributed pretty evenly between the electron and the neutrino. But sometimes most or all of the remaining energy gets dumped into one particle or another.
In that case, all of the energy left over after the neutrino and the electron are formed is dumped into the electron partner, forming a super-high-energy electron, Robertson said. That means the mass of the neutrino can be calculated: It's the energy involved in the neutron decay minus the mass of the electron and the maximum energy level of the electrons in the experiment.
measure the neutrinos; those are allowed to escape the machine untouched. Instead, the experiment funnels the electrons into a giant vacuum chamber, called the spectrometer. An electric current then creates a very strong magnetic field that only the highest-energy electrons can pass through. At the other end of the chamber is a device that counts how many electrons make it through the field. As KATRIN slowly increases the magnetic field strength, Robertson said, the number of electrons getting through shrinks – almost as if it were going to fade all the way to zero.