Researchers close in on rare neutrinos

Researchers close in on rare neutrinos

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Dissection of a cryogenic CRES cell, where electrons are produced by radioactive decay and are magnetically trapped. The cell waveguide has a cold inner diameter of 10.03 mm and a height of 132 mm (distance between the rf windows). Cyclotron radiation travels axially up the waveguide (on the left in the rotated view), to the amplifiers and readout electronics. Credit: Physical review letters (2023). DOI: 10.1103/PhysRevLett.131.102502

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Dissection of a cryogenic CRES cell, where electrons are produced by radioactive decay and are magnetically trapped. The cell waveguide has a cold inner diameter of 10.03 mm and a height of 132 mm (distance between the rf windows). Cyclotron radiation travels axially up the waveguide (on the left in the rotated view), to the amplifiers and readout electronics. Credit: Physical review letters (2023). DOI: 10.1103/PhysRevLett.131.102502

The humble neutrino, a tiny subatomic particle that passes through ordinary matter, plays a major role among the particles that comprise our universe. In order to properly explain how the universe came to be, we need to know its complexity. But, like most of us, it avoids limitations.

Now, a group of international researchers from the United States and Germany leading an ambitious project called Project 8 reports that their unique strategy is a real contender to be the first to measure the mass of the neutrino. Once fully scaled up, Project 8 could help reveal how neutrinos influenced the evolution of the universe as we know it.

In 2022, the KATRIN research team set an upper limit for how hard a neutrino can be. That milestone was a tourism achievement that had been decades in the making. But these results simply narrow the search window. KATRIN will reach faster and may one day even exceed the limits of the target, but the featherweight neutrino may be easier, begging the question: “What’s next?”

In their latest study, the Project 8 team reports Physical review letters if they can use a brand new method to reliably track and record a natural phenomenon called beta decay. Each event releases a small amount of energy when a rare radioactive form of hydrogen—called tritium—splits into three subatomic particles: a helium ion, an electron, and a neutrino.

The ultimate success of Project 8 depends on an ambitious plan. Rather than trying to detect neutrinos—which quickly pass through detector technology—the research team went after a simple measurement strategy that can be summarized as follows:

We know that the total mass of a tritium atom is equal to the energy of its atoms, thanks to Einstein. When we measure the free electron produced by beta decay, and we know the total mass, the “lost” energy is the neutrino’s mass and momentum.

“In principle, with technological development and scale-up, we have a definite shot at getting into the range necessary to suppress neutrino mass,” said Brent VanDevender, one of the principal investigators of Project 8 at the Department of Energy’s Northwest Territories. Lab.

Why Project 8?

These researchers chose to pursue an ambitious plan because they had worked through the pros and cons and concluded that it would work.

Talia Weiss is a graduate student in nuclear physics at Yale University. He and his colleagues at Project 8 spent years trying to find a way to accurately extract electron signals from electronic background noise. Christine Claessens is a postdoctoral fellow at the University of Washington who received her Ph.D. in Project 8 at the University of Mainz, Germany. Weiss and Claessens performed a final analysis that placed limits on the neutrino mass derived from the new technique for the first time.

Credit: Pacific Northwest National Laboratory

“The neutrinos are incredibly bright,” Weiss said. “It is more than 500,000 times lighter than an electron. So, when neutrinos and electrons are created at the same time, the mass of the neutrino has a small effect on the motion of the electron. We want to see that small effect. Therefore, we need a precise method of measuring how fast electrons travel.”

Project 8 relies on such a technique, one that was conceived over a decade ago by physicists Joe Formaggio and Ben Monreal, then working at the Massachusetts Institute of Technology. An international team came together on this idea and established Project 8 to turn the idea into a practical tool. The resulting method is called Cyclotron Radiation Emission Spectroscopy (CRES).

It captures microwave radiation emitted from newly born electrons as they orbit the magnetic field. These electrons carry most—but not all—of the energy released during beta decay. It is this missing energy that can reveal the neutrino’s mass. This is the first time that tritium beta decay has been measured, and an upper limit has been placed on the neutrino mass, with the CRES technique.

How can scientists measure neutrinos? Credit: Sara Levine / Pacific Northwest National Laboratory

The team is only interested in tracking these electrons because their energy is the key to producing neutrino mass. While this strategy has been used before, the CRES detector measures significant electron energy with a power boost beyond any existing technology. And that scalability is what sets Project 8 apart. Elise Novitski is an assistant professor at the University of Washington and led many aspects of the newly published work.

“Nobody else is doing this,” Novitski said. “We don’t take existing techniques and try to tweak it a little bit. We’re kind of the Wild West.”

In their latest experiment, built at the University of Washington in Seattle, the team tracked 3,770 beta decay events over an 82-day window in a sample of cells the size of one pea. The cell sample is cooled carefully and placed in a magnetic field that traps the emerging electrons long enough for the recording antenna to register the microwave signal.

Most importantly, the team registered false zero signals or background events that could be confused with the real thing. That is important because too little background can obscure the neutrino mass signal making the interpretation of a useful signal more difficult.

From chips to signs

A subset of Project 8 researchers, led by PNNL experimental physicist Noah Oblath, but including a dozen others at multiple institutions, have developed specialized software—each named after a different insect—to take raw data and convert it into signals. that can be analyzed. And the project engineers put their hats on to create the various parts that make Project 8 come together.

Credit: Pacific Northwest National Laboratory

“We have engineers that are critical to the effort,” Novitski said. “It’s kind of out there from an engineer’s point of view. Experimental physics is kind of the frontier of physics and engineering. You’ve got to get really passionate engineers and physicists who have the brains to work together, make these things happen because this stuff isn’t in the books.

Getting to the finish line

Now that the team has demonstrated the design and test system works using tritium molecules, they have another pressing task ahead. A subset of the entire team is now working on the next step: the process that will produce, cool and capture each atom of tritium. This step is tricky because tritium, like its more abundant cousin hydrogen, prefers to form molecules. Those molecules would make the ultimate goals of the Project 8 team unachievable. The researchers, led by physicists at the University of Mainz, are developing a testbed to create and capture atomic tritium with complex magnetic devices that will keep it from touching the walls of the sample sample – where it almost turns back into a molecule. form.

This advanced technology, as well as increasing the entire apparatus, will be important steps to reach and eventually exceed the sensitivity achieved by the KATRIN group.

Currently, the research team, with contributing members from ten research institutions, is working on testing a design to scale up the experiment from a pea-sized sample room to one a thousand times larger. The idea is to capture many beta decay events using a large listening device – from the size of a pea to a beach ball.

Credit: Pacific Northwest National Laboratory

“Project 8 is not only the biggest and best CRES experiment, it’s the first CRES experiment and the first to use this detection method,” Oblath said. “It’s never been done before. Most experiments have a 50- or 100-year history, at least for the detection method they’re using, while this is brand new.”

More information:
A. Ashtari Esfahani et al, Tritium Beta Spectrum Measurement and Neutrino Mass Limit from Cyclotron Radiation Emission Spectroscopy, Physical review letters (2023). DOI: 10.1103/PhysRevLett.131.102502

Journal information:
Physical review letters

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