KATRIN halves the estimation of the elusive neutrino mass

An international team of scientists has announced a breakthrough in its quest to measure the mass of neutrino, one of the most abundant and elusive particles in our universe.

At the 2019 astroparticle and underground physics conference in Toyama, Japan, KATRIN experiment leaders reported on September 13 that the estimated distance for the neutrino resting mass is not greater than 1 electronvolt, or eV. These inaugural results obtained earlier this year by the Karlsruhe Tritium Neutrino experiment – or KATRIN – have reduced the neutrino mass range by more than half by lowering the upper bound of the neutrino mass by 2 eV to 1. eV. The lower limit of the neutrino mass, 0.02 eV, was fixed by previous experiments conducted by other groups.

"Knowing the neutrino mass will allow scientists to answer fundamental questions in cosmology, astrophysics and particle physics, such as the evolution of the universe or physics going beyond the standard model," said Hamish Robertson , KATRIN scientist and professor emeritus of physics. at the University of Washington. "These discoveries from the KATRIN collaboration reduce the neutrino mass range by a factor of two, set stricter criteria for the actual mass of the neutrino, and provide a way to measure its value permanently."

The KATRIN experiment is based at the Karlsruhe Institute of Technology in Germany and involves researchers from 20 research institutes around the world. In addition to the University of Washington, KATRIN member institutions in the United States are:

• The University of North Carolina at Chapel Hill, led by Professor of Physics and Astronomy John Wilkerson, former faculty member of the University of Washington.
• The Massachusetts Institute of Technology, led by physics professor Joseph Formaggio
• Lawrence Berkeley National Laboratory, led by Deputy Director of the Nuclear Science Division, Alan Poon
• Carnegie Mellon University, led by Diana Parno, Assistant Professor of Physics
• Case Western Reserve University, led by Associate Professor of Physics Benjamin Monreal

Under Robertson and Wilkerson, the University of Washington became one of KATRIN's founding institutions in 2001. Mr. Wilkerson then joined the University of North Carolina at Chapel Hill. Formaggio and Parno began their collaboration with KATRIN as UW researchers, then moved on to their current institutions. Besides Robertson, other UW scientists currently working on the KATRIN experiment are physics research professor Peter Doe, associate professor of physics research Sanshiro Enomoto and Menglei Sun, postdoctoral researcher at the Center. of experimental physics and astrophysics UW.

Neutrinos are abundant. They are one of the most common fundamental particles of our universe, just after photons. Yet neutrinos are also elusive. These are neutral, non-charged particles that interact with other materials only through "weak interaction", which means that the possibilities of detecting neutrinos and measuring their mass are both rare and difficult.

"If you filled the solar lead system fifty times beyond Pluto's orbit, about half of the neutrinos emitted by the sun would still leave the solar system without interacting with that lead," Robertson said.

Neutrinos are also mysterious particles that have already disrupted physics, cosmology and astrophysics. The standard model of particle physics predicted that neutrinos should not have mass. But in 2001, scientists had demonstrated with two detectors, Super-Kamiokande and the Sudbury Neutrino Observatory, that they had a non-zero mass, a breakthrough recognized in 2015 with the Nobel Prize in Physics. Neutrinos have a mass, but how much?

"The resolution of the neutrino mass would take us into a new world of creating a new standard model," Doe said.

KATRIN's discovery stems from direct and accurate measurements of how a rare type of electron-neutrino pair shares energy. This approach is the same as that of the mass neutrino experiments of the 1990s and early 2000s in Mainz (Germany) and Troitsk (Russia), both of which set the previous upper limit of mass at 2 eV. The heart of the KATRIN experiment is the source generating electron-neutrino pairs: gaseous tritium, a highly radioactive isotope of hydrogen. When the tritium nucleus undergoes radioactive decay, it emits a pair of particles: an electron and a neutrino, both sharing 18,560 eV of energy.

KATRIN scientists can not measure neutrinos directly, but they can measure electrons and try to calculate the properties of neutrinos based on the properties of electrons.

Most electron-neutrino pairs emitted by tritium share their energy charge equally. But in rare cases, the electron consumes almost all the energy, leaving only a tiny amount to the neutrino. KATRIN scientists are looking for these rare pairs because, thanks to E = mc2, scientists know that the minute amount of energy left to the neutrino has to include its mass at rest. If KATRIN can accurately measure the energy of the electron, he can calculate the energy of the neutrino and thus its mass.

The tritium source generates about 25 billion electron-neutrino pairs every second, of which only a fraction is made up of pairs where the electron absorbs almost all the energy of disintegration. The KATRIN installation from Karlsruhe uses a complex series of magnets to channel the electron from the tritium source to an electrostatic spectrometer, which measures the energy of electrons with high accuracy. An electrical potential in the spectrometer creates an "energy gradient" that electrons must "climb" to pass through the spectrometer for detection. By adjusting the electrical potential, scientists can study the rare high-energy electrons, which contain information about the mass of neutrinos.

US institutions have made many contributions to KATRIN, including providing the electron detection system – "the eye" of KATRIN – which examines the heart of the spectrometer, an instrument built at UW . The University of North Carolina at Chapel Hill led the development of the data acquisition system of the detector, KATRIN's "brain." The contribution of MIT was the design and development of the simulation software used to model KATRIN's response. Lawrence Berkeley National Laboratory contributed to the creation of the physical analysis program and provided access to national computer facilities. The rapid analysis was made possible through a series of applications created at the UW. Case Western Reserve University was responsible for the design of the electron gun, the central element of the KATRIN calibrator. Carnegie Mellon University was instrumental in the analysis, paying particular attention to background and fit, and contributed to the coordination of the analyzes for the experiment.

With the acquisition of tritium data underway, US institutions are focusing on analyzing this data to improve our understanding of the mass of neutrinos. These efforts may also reveal the existence of sterile neutrinos, a possible candidate for dark matter that, although it accounts for 85% of the matter in the universe, remains undetected.

"KATRIN is not only a beacon of fundamental research and a highly reliable high-tech instrument, but also a driving force for international cooperation, providing first-rate training for young researchers," said Guido Drexlin, co-leader of KARRIN's speech, from the Karlsruhe Institute. of technology and Christian Weinheimer of the University of Münster in a statement.

Now that scientists at KATRIN have set a new upper limit for neutrino mass, scientists at the project are working to further reduce the range.

"Neutrinos are strange small particles," said Doe. "They are so ubiquitous and we can learn a lot once we have determined that value."

The Office of Nuclear Physics of the US Department of Energy has been funding the country's participation in the KATRIN experiment since 2007.

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