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Weighing: Physicists cut the upper limit of neutrino mass in half



  The spectrometer for the KATRIN experiment as it operates on the German city of Eggenstein-Leopoldschafen in 2006 en route to the nearby Karlsruhe Technical Institute.
Click to enlarge / The KATRIN spectrometer is an experiment as it makes its way through the German city of Eggenstein-Leopoldshafen in 2006 to the nearby Karlsruhe Technical Institute.

Isaac Azimov called neutrino "ghost particles". John Updike immortalized them in verse. They have been the subject of several Nobel Prize quotes because these weird little particles just keep surprising physicists. And now we have a much better idea of ​​the upper limit of what their rest table might be, thanks to the first results of the Kartsruhe Tritium Neutrino (KATRIN) experiment in Germany. The experiment's leaders announced their results last week at a scientific conference in Japan and published a preface to arXiv physics.

"Knowing the mass of neutrinos will allow scientists to answer basic questions in cosmology, astrophysics and particle physics, such as how the universe evolves or what physics exists beyond the standard model," says Hamish Robertson, a KATRIN scientist and lecturer "These findings from the collaboration with KATRIN reduce the previous neutrino mass range by a factor of two, set stricter criteria for what the neutrino mass actually is, and provide a path forward for and the final measurement of its value. "

Ghost particles are devilishly difficult to recognize because they so rarely interact with other particles, and when they do, they only interact through weak nuclear force. Most neutrino hunters bury their experiments deep underground, the better to eliminate noisy disturbances from other sources, in particular the cosmic rays, which are constantly bombarding the earth's atmosphere. Experiments usually require huge tanks of liquid – liquid dry cleaning, water, heavy water, mineral oil, chlorine or gallium, depending on the experimental setup. This increases the chances of neutrinos hitting one of the atoms in a selected environment, triggering the decay process. An atom changes into a different element, emitting an electron in the process that can be detected.

  The neutrino observatory in Sudbury. "Src =" https://cdn.arstechnica.net/wp-content/uploads/2019/ 09 / SNOpic-640x422.jpg "width =" 640 "height =" 422 "srcset =" https: //cdn.arstechnica .net / wp-content / uploads / 2019/09 / SNOpic.jpg 2x
Enlarge / Neutrino Observatory in Sudbury.

Lawrence Berkeley National Laboratory

Neutrino was first proposed by Wolfgang Pauli in a 1930 letter to colleagues. He was trying to explain some astounding experimental results for radioactive beta decay in atomic nuclei where energy seemed to be missing – something he thought (right) was impossible. He thought that a new kind of charge-free and mass-free subatomic particle might have carried away the missing energy; Enrico Fermi later called it neutrino.

Clyde Cowan and Frederick Raines were the first to observe these ghostly particles in 1956, thanks to the synthesis reactions in nuclear power plants that had spread since World War II. Ten years later, physicists discovered the first solar neutrinos from the sun. This one-shot Ray Davis Jr. and Masatoshi Koshiba – Nobel Prize in 2002, shared with Ricardo Jaconi (who was distinguished "for his pioneering contribution to astrophysics that led to the discovery of cosmic X-ray sources").

The only problem was that far fewer solar neutrinos were detected than predicted by the theory, a puzzle that became known as the solar neutrino problem. In 1962, physicists discovered a second type ("fragrance") of neutrinos, the muon neutrinos. This was followed by the discovery of a third aroma, Tau Neutrino, in 2000.

By then, physicists had already suspected that a neutrino might be able to switch from one fragrance to another, thanks in large part to the 1998 observations of Japanese Super -Camiokande Cooperation (Super-K). In 2002, scientists at the Sudbury Netrino Observatory (or SNO) announced that they had solved the problem of solar neutrinos. The missing solar (electronic) neutrinos were simply obscured, changing to a different aroma during the long journey between the Sun and Earth. SNO and Super-K shared the 2015 Nobel Prize in Physics for their breakthroughs.

If they neutrally fluctuate, then they must have a small tiny mass after all. As Adrian Cho explained in a 2016 article in Science, "If neutrinos were massless, they had to move at light speed, at least in a vacuum, according to Einstein's theory of relativity. If that were the case, time would be stationary for them and change would be impossible. "

But determining exactly what this mass is is another problem related to neutrinos. There are three neutrino flavors, but none of them has a well-defined mass. Rather, different types of" mass states "are mixed in different ways to obtain electron, muon and tau neutrinos. This is a quantum weirdness for you.

  The layout and main features of the KATRIN experimental facility at the Karlsruhe Technical Institute. "Src =" https://cdn.arstechnica.net/wp-content/uploads / 2019/09 / neutrino2-640x425.jpg "width =" 640 "height =" 425 "srcset =" https://cdn.arstechnica.net/wp-content/uploads/2019/09/neutrino2.jpg 2x [1 9659002] Click to enlarge <span class= / The design and main features of the KATRIN Experimental Facility at the Karlsruhe Institute of Technology

Karlsruhe Institute of Technology

for the average of all three masses, the lower limit is 0.02 eV (electronic volts); neutrinos cannot have a lower mass than this. The KATRIN data suggest that they cannot weigh more than 1 eV – or 1 / 500,000th of the mass of an electron.

The experiment uses tritium (a highly radioactive isotope of hydrogen, with one proton and two neutrons) to generate electron-neutrino pairs: electron and neutrino, which share 18 650 eV of energy between them. Usually this energy is divided equally, but there are rare pairs – only a fraction of the approximately 25 billion electron-neutrino pairs produced every second – where the electron shrinks almost all of it, leaving only a small amount for neutrino. These couples are the focus of KATRIN scientists. They cannot measure neutrino directly, so instead they subtract the energy of the electron to derive the energy of the neutrino and, therefore, its mass (because E = mc 2 ).

These preliminary results are only based on 28 days of data, so it is not yet a definitive measurement; more data is needed. But this is already half of the previous estimate of what physicists thought would be the upper limit of the mass and the real value could be even lower. Or neutrinos can throw another curve ball at the physicists and the extra data will give an upper limit.

The experiment may also shed light on the possible existence of an exotic fourth type of neutrino, called "sterile" neutrino, which does not interact with regular matter at all, except, perhaps, on its neutrino counterparts. This would have major implications for the nature of dark matter, although despite the painstaking hint in 2018, sterile neutrinos are still inevitable.

"There is circumstantial evidence that neutrinos are smaller than what KATRIN taught us last week," Andre de Gouvka, a theoretical physicist at Northwestern University who did not participate in the measurement, told Scientific American. however, the evidence does not replace what KATRIN can do, so the result itself is very important. Perhaps more importantly, KATRIN demonstrates that things work and that they seem to be on the way to much more. "


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