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Weighing: Physicists halve the upper limit of neutrino mass



  The spectrometer for the KATRIN experiment, which is making its way through the German city of Eggenstein-Leopoldshafen in 2006 on the way to the nearby Karlsruhe Institute of Technology.
Enlarge / The spectrometer for the KATRIN experiment, which sets off in 2006 on the way to the nearby Karlsruhe Institute of Technology through the German town of Eggenstein-Leopoldshafen.

Isaac Asimov called the neutrinos "ghost particles". John Updike immortalized them in verse. They have already been awarded the Nobel Prize several times, because these strange tiny particles are always surprising physicists. And now, thanks to the first results of the Karlsruhe Tritium Neutrino Experiment (KATRIN) in Germany, we have a much better idea of ​​the upper limit of their rest mass. The leaders of the experiment announced their findings at a scientific conference in Japan last week and sent a preprint for physics arXiv.

"If the mass of the neutrino is known, scientists can answer fundamental questions of cosmology, astrophysics, and particle physics, how the universe has evolved, or what physics exists beyond the standard model," said Hamish Robertson, a KATRIN scientist and professor emeritus for physics at the University of Washington. "These results of the KATRIN collaboration reduce the previous mass range for the neutrino by a factor of two, set stricter criteria for the actual mass of the neutrino, and provide a way to finally measure its value."

Particles are devilishly hard to detect because they so rarely interact with other particles, and when they do, they only interact via the weak nuclear force. Most neutrino fighters bury their experiments deep underground to better compensate for interference from other sources, particularly the cosmic radiation that constantly bombards the Earth's atmosphere. The experiments usually require enormous fluid tanks – chemical cleaning fluid, water, heavy water, mineral oil, chlorine, or gallium, depending on the setup of the experiment. This increases the likelihood that a neutrino will hit one of the atoms in the medium of choice and trigger the decay process. The atom transforms into another element, emitting an electron that can be detected.

  The Sudbury Neutrino Observatory. "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 / The Sudbury Neutrino Observatory.

Lawrence Berkeley National Laboratory.

Neutrinos were first proposed by Wolfgang Pauli in 1930 in a letter to colleagues. He tried to explain some startling experimental results on radioactive beta decay in atomic nuclei, which seemed to lack energy, something he considered (properly) impossible. He thought a new kind of subatomic particle with no charge and no mass could have carried off the missing energy; it was Enrico Fermi who later called it Neutrino.

Clyde Cowan and Frederick Reines were the first to observe these ghostly particles in 1956, thanks to fusion reactions in nuclear power plants that proliferated after World War II. Ten years later, physicists discovered the sun's first solar neutrinos. As a result, Ray Davis Jr. and Masatoshi Koshiba received a Nobel Prize in 2002, shared with Riccardo Giacconi ("honored for pioneering contributions to astrophysics that led to the discovery of cosmic X-ray sources").

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

At that time, physicists already suspected that neutrinos may be able to switch from one flavor to another, largely in the wake of Japan's Super 1998 was due to -Kamiokande cooperation (Super-K). In 2002, scientists from the Sudbury Neutrino Observatory (or SNO) announced that they had solved the solar neutrino problem. The missing solar (electron) neutrinos were only obscured, as they had taken on the long journey between the sun and earth a different taste. SNO and Super-K shared the 2015 Nobel Prize in Physics for their respective breakthroughs.

When neutrinos vibrate, they must at least have a tiny piece of mass. As Adrian Cho explained in an article for Science of 2016, "If neutrinos were massless, they would have to move at the speed of light, at least in vacuum, according to Einstein's Theory of Relativity, if that were the case, time would stand still for them, and change would be impossible. "

But determining exactly what that mass is is another nodal neutrino problem. There are three neutrino tastes, but none of them has a well-defined mass. Rather, different types of "bulk states" mix in different ways to produce electron, muon, and tau neutrinos. That's quantum craze for you.

  The layout and essential features of the KATRIN test facility at the Karlsruhe Institute of Technology. "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
Enlarge / The layout and essential features of the KATRIN test facility at the Karlsruhe Institute of Technology.

Karlsruhe Institute of Technology.

The new results of KATRIN establishing an upper limit for the neutrino mass actually apply to the average of all three masses. The lower limit is 0.02 eV (electron volts); Neutrinos can not have a lower mass. And the data from KATRIN suggests that they can not weigh more than 1 eV or 1/500 000 of the mass of the electron.

The experiment uses tritium (a highly radioactive hydrogen isotope with one proton and two neutrons) to create electron-neutrino pairs: an electron and a neutrino sharing 18,650 eV of energy. Normally, this energy is split evenly, but there are rare pairs – only a fraction of the approximately 25 billion electron-neutrino pairs that are produced per second – where the electron accumulates almost completely, leaving only a small amount for the Neutrino is left over. These couples are the focus of the KATRIN scientists. They can not directly measure the neutrinos, but subtract the energy of the electron to deduce that of the neutrino and thus its mass (because E = mc 2 ).

These preliminary results are based on these results Since the data is only available for 28 days, it is not yet a definitive measurement. More data is needed. But it's already half the previous estimate of what the physicists considered the upper limit of mass, and the actual value could be even lower. Or neutrinos could give the physicists another curve, and the extra data would give a higher upper limit.

The experiment could also shed light on the possible existence of an exotic fourth neutrino called a "sterile" neutrino, interacting with regular matter at all, except perhaps the other neutrinos. This would have a major impact on the nature of dark matter, although sterile neutrinos have proved elusive in 2018, despite a tantalizing suggestion.

"André de Gouvêa, a theoretical physicist at Northwestern University who was not involved in the measurement, told Scientific American." However, the indirect evidence does not replace what KATRIN can do, so the result itself is of great magnitude Perhaps more important is that KATRIN has shown that things are working and that they seem to be on the right path to get much further. "


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