Neutrinos are the most common particles in the universe. Not having an electric charge and being very light, they can move to almost everything, with trillions flowing through any of us any second. The small mass of the neutrino has been a crucial quest in physics and we still do not know exactly how light they are – but brand new research has shrunk our region of ignorance.
The KATRIN collaboration improved on its previous result by a factor of two. According to the experiment, the effective electron neutrino mass cannot be more than 0.45 electron volts, which is equivalent to 8×10-37 kilograms (1.76×10-36 pounds). That’s the proportion between 1 gram (0.04 ounces) and the mass of 40,000 Suns. Yes, neutrinos are that tiny.
“The neutrino mass measuring campaign of the KATRIN experiment will end in 2025 after reaching 1,000 days of data acquisition,” writes Loredana Gastaldo in a related Perspective in the journal Science. “Analysis of the full data set gained from this grand project will allow for estimating the effective electron neutrino mass close to the projected value of 0.3 eV at 90% confidence level.”
The current measurement was based on “only” 259 measurement days where the team analyzed a whopping 36 million electrons. As we have said, neutrinos are not easy to study directly so the KATRIN experiment looks at them indirectly. They use a heavier version of hydrogen called tritium, which instead of being made of one electron and one proton is made of one electron, one proton, and two neutrons.
Tritium is radioactive and it turns into helium-3. In doing so, it turns one neutron into a proton, releasing one electron and one antineutrino – the antimatter version of neutrino. The antineutrino has the same properties as the regular neutrino including the mass but an opposite spin state. The transformation from tritium to helium-3 releases energy that is carried away by the electron and the antineutrino.
KATRIN has particles going around a 70-meter-long (230-foot) structure measuring the energy of the electrons with a 200-tonne spectrometer. The missing energy from the transformation is on the antineutrino, so the calculation to get the mass is quite simple. It’s the measuring that’s a challenge! To get high precision you need millions of measurements, and that’s why this one was based on 36 million electrons. When the experiment finishes taking measurements later this year it will have measured 250 million electrons.
The original Standard Model of Particle Physics – the theory that underpins most physics – did not originally expect neutrinos to have mass. Special tweaks have to be made to fix that. Understanding the mass of the neutrino might lead us to the next theory beyond the standard model.
A paper announcing the results is published in the journal Science.
Source Link: The Possible Mass Of The Neutrino Is Now Half What It Used To Be
