From Forbes Magazine : “Researchers Find That Ghostly Subatomic Particles Are Even Lighter Than Previously Thought”

From Forbes Magazine

May 18, 2021

LEOPOLDSHAFEN, Germany: The main spectrometer of the Karlsruhe Tritium Neutrino Experiment (KATRIN) is manoevred through a road of Leopoldshafen, southern Germany, 25 November 2006. The experiment is designed to measure the mass of the electron neutrino and is a joint effort of several European and US institutions. Credit: MICHAEL LATZ/DDP/AFP via Getty Images.

Of all the subatomic particles that have any mass at all, the neutrino is the lightest by far. In comparison, the electron, itself a quantum featherweight, has a mass that is at least 500,000 times bigger. In fact, the mass of the neutrino is so low that we don’t have a measurement that tells us what its mass is. Instead, all we know that it is crazy small. And an experiment in Germany has improved our understanding of the mass of this insubstantial denizen of the microcosm.

This experiment is called the Karlsruhe Tritium Neutrino Experiment, or the KATRIN experiment for short. It uses a neutrino-emitting form of radiation of the isotope tritium to try to measure the mass of the neutrino. This form of radiation is called beta decay and it occurs when a neutron decays into a proton. Tritium is a form of hydrogen, but with a different number of neutrons. Hydrogen contains a single proton and no neutrons, while tritium contains not only a proton, but also two neutrons.

In the beta decay of tritium, the atomic nucleus converts a neutron into a proton, thereby converting the nucleus from a form of hydrogen, to a form of helium, called helium-3. In the process, an electron and a neutrino are emitted. (Technically, an antimatter neutrino, but this is an unimportant distinction here, and the term “neutrino” will be used in this article.)

If the KATRIN researchers were trying to measure the mass of the electron, they would try to capture the particle and use a series of techniques to ascertain the particle’s mass, however this technique is not useful for investigating the properties of neutrinos. Neutrinos interact only via the weak nuclear force, which means that they interact extremely rarely. For instance, to ensure that a neutrino emitted from beta decay would interact in a detector made of, for example, solid lead, the detector would need to be many light years thick. Obviously, researchers need a different approach.

Instead researchers use the laws of energy and momentum conservation introduced in a first-year physics class. Here’s how it works. The basic structure of beta decay is a neutron decays into a proton, electron, and neutrino. The neutron and proton are both very heavy – about two thousand times heavier than the electron – while the electron and neutrino are very light. A tritium nucleus is heavier than a helium-3 nucleus, which is why this form of radioactive decay can occur.

Einstein’s famous equation E = mc2 is relevant. It says that mass can be converted into energy. And, because tritium is heavier than helium-3, that extra mass gets converted into motion energy of the daughter particles (e.g. the helium-3 nucleus, the electron, and neutrino).

Einstein’s equation doesn’t determine the energies of the daughter particles, indeed in any particular decay, any of the particles can have any amount of energy, as long as the three particles’ energy and momentum add up to the same as the tritium nucleus before the decay. What researchers measure is the energy spectrum of the electron. That’s because the helium-3 nucleus moves so little that its motion is essentially unobservable, and the neutrino can’t be detected. The electron is the only thing that can be detected.

What researchers then do is ask what is the absolutely maximum energy an electron can carry? It happens when the helium-3 nucleus isn’t moving, and the electron and neutrino are emitted in opposite directions. If the neutrino had zero mass, the absolutely maximum motion energy that the electron could have is 18,577 electron volts. (To give context, the energy of a stationary electron is about 511,000 electron volts and the energy of a stationary proton is about 938,000,000 electron volts. The mass energy of a tritium or helium-3 nucleus is nearly three billion electron volts.)

However, that’s for the scenario where the mass of the neutrino is zero. If the neutrino has mass, some of that motion energy of the daughter particles must be used up to make the mass of the neutrino. So, researchers look at decaying tritium nuclei, focusing only on electrons with the highest energy. If they found ones with an energy of 18,577 electron volts, it would mean that neutrinos have no mass. But they don’t find electrons with that high energy, just very near it. By taking the observed maximum energy of the very small number of highest energy electrons, they can determine a maximum mass that the neutrino can carry.

In 2019, the Katrin experiment reported its first result, determining that the mass of neutrinos emitted in tritium decay could be no larger than 1.1 electron volts. This tiny mass is so small that, were that the actual mass of neutrinos, it would take about half a million of them to make up the same mass as the lightweight electron.

However, at the 2021 meeting of the American Physical Society, the Katrin Experiment reported an updated measurement. They now can say with confidence that the mass of neutrinos must be below 0.8 electron volts.

The KATRIN experiment is not done. It is expected that with further effort, the experimental apparatus will be capable of either ruling out all neutrino masses higher than 0.2 electron volts or to measure the mass of the neutrino if it exceeds 0.35 electron volts.

So, the story is not over, which is a good thing. Understanding neutrinos is a key goal of the particle physics community and researchers look forward to future announcements from KATRIN.

See the full article here .

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