An international team of scientists, including researchers at MIT has come closer to establishing the mass of the elusive neutrino. These ghostly particles penetrate the universe and are still considered almost massless. They flow through our bodies millions of times and hardly leave any physical traces.
The researchers have determined that the mass of the neutrino should not be more than 1
The new estimate was based on data collected by KATRIN, the Karlsruhe tritium neutrino experiment at the Karlsruhe Institute of Technology in Germany, last week at the 2019 Conference on Astroparticle and Underground Physics. The experiment triggers the decay of tritium gas, releasing neutrinos and electrons. As the neutrinos dissolve rapidly, the KATRIN magnetic sequence directs the tritium electrons into the heart of the experiment – a giant 200-ton spectrometer that measures the mass and energy of the electrons and from there the mass can be computed the corresponding neutrinos.
Joseph Formaggio, professor of physics at MIT, is a leading member of the KATRIN experimental group and spoke with MIT News about the new estimate and the further path of neutrino search.
Q: According to KATRIN's findings, the neutrino can not be more massive than 1 electron volt. Imagine the following context: how easy is that and how big is the fact that the maximum mass of the neutrino can be half of what people thought before?
A: Well, that's a tough question People (myself included) do not really have an intuitive sense of what the mass of a particle is, but let's try it. Consider something very small like a virus. Each virus consists of approximately 10 million protons . Each proton weighs about 2000 times more than any electron in this virus. And our results showed that the neutrino has a mass of less than 1 / 500,000 of a single electron!
Let me put it another way. There are about 300 neutrinos flying through every cubic centimeter of space around you. These are remnants of the early Universe just after the Big Bang. If you add all the neutrinos in the sun, you will get about one kilogram or less. Well, yes, it is small.
Q: How was this new mass limit set for the neutrino and what role did MIT play in the search?
A: This new mass limit comes from investigation of the radioactive decay of tritium, an isotope of hydrogen. The decomposition of tritium produces a helium-3 ion, an electron and an antineutrino. However, we never actually see the antineutrino; The electron contains information about the mass of the neutrino. By examining the energy distribution of the electrons ejected with the highest allowed energies, we can derive the mass of the neutrino from Einstein's equation E = mc 2 .
However, if we study these high-energy electrons electrons are very difficult. First, all information about the neutrino is embedded in a tiny fraction of the spectrum – less than a billionth of the decays are useful for this measurement. So we need a lot of tritium inventory. We also have to measure the energy of these electrons very, very accurately. That's why the KATRIN experiment is so difficult to build. Our very first measurement presented today is the culmination of nearly two decades of hard work and planning.
When I came to Boston in 2005, MIT participated in the KATRIN experiment. Our group helped develop the simulation tools to understand the response of our detector to high precision. More recently, we have been involved in developing tools to analyze the data collected by the experiment.
Q: Why does the mass of a neutrino matter and how will it adjust to its exact mass?  A: The fact that neutrinos have mass at all surprised many physicists. Our earlier models predicted that the neutrino should have exactly zero mass, an assumption that has been nullified by the discovery that neutrinos oscillate between different types. That is, we do not really understand the mechanism responsible for the neutrino masses, and it is likely that it is very different from how other particles reach mass. In addition, our universe is filled with original neutrinos from the Big Bang. Even a tiny mass has a significant impact on the structure and evolution of the Universe as they are so numerous.
This measurement is only the beginning of KATRIN's measurement. With only about a month of data, we were able to improve on previous experimental limits by a factor of two. Over the next few years, these limits will steadily improve, which will hopefully result in a positive signal (and not just a limit). There are also a number of other experiments with direct neutrino mass on the horizon, which are also competing to achieve greater sensitivity and therefore discovery!