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CMU researcher attempts to weigh a neutrino, the lightest known particle

Aaron Aupperlee
| Thursday, Aug. 17, 2017, 9:06 a.m.
Researchers work inside the electromagnetic spectrometer for the KATRIN experiment in Karlsruhe, Germany.
Researchers work inside the electromagnetic spectrometer for the KATRIN experiment in Karlsruhe, Germany.
The electromagnetic spectrometer for the KATRIN experiment in Karlsruhe, Germany, makes its way through houses and city streets to its lab.
Forschungszentrum Karlsruhe
The electromagnetic spectrometer for the KATRIN experiment in Karlsruhe, Germany, makes its way through houses and city streets to its lab.
A researcher stands in the electromagnetic spectrometer for the KATRIN experiment.
A researcher stands in the electromagnetic spectrometer for the KATRIN experiment.

What's the lightest thing you can think of?

A feather? Air? Maybe an atom or even better, an electron?

Carnegie Mellon University researcher Diana Parno would say a neutrino, the lightest particle known to science. It's so light, and so hard to measure, that we don't know how much it weighs, yet.

Parno is part of a team of 120 particle physicists and other scientists who will attempt to determine the mass of the neutrino over the next five years during an experiment in Karlsruhe, a city in southwest Germany.

Scientists have been able to determine that a neutrino weighs at least 250,000 times less than an electron, but Parno believes the KATRIN experiment will produce a more accurate measurement within days of starting.

"The basic idea is to try to plug a gap in our understanding of particle physics," Parno said. "We know the mass of every other particle in the standard model except for neutrinos."

The neutrino was first theorized in 1930 and later detected in 1956. A pair of scientists won the Noble Prize in Physics in 2015 for proving that neutrinos have mass.

Parno, who earned her doctorate in physics at CMU in 2011, went to the University of Washington in Seattle before returning to CMU in January. She has been working on measuring the particles for 6 1/2 years.

Neutrinos are flying around everywhere. Nuclear reactions, like those at power plants, weapons facilities or the sun, give off neutrinos. The sun alone produces enough neutrinos that 60 billion fly through a space the size of your thumbnail every second. Further study of neutrinos could help organizations that monitor nuclear power plants for possible weapons manufacturing, Parno said.

How do you measure the lightest particle in the world? With a 200-ton electromagnetic spectrometer that is 23 meters long and taller than a two-story house. A spectrometer measures energy.

The electromagnetic spectrometer used in the KATRIN experiment is so large that it wouldn't fit on the 350 kilometers of roads connecting the German city of Deggendorf to Karlsruhe so it had to be shipped by boat around Europe, an 8,600 kilometer journey down the Danube River, through the Black Sea to the Mediterranean Sea, around Spain, into the Atlantic to Antwerp, then up the Rhine River to docks in the German city of Leopoldshafen. The spectrometer wound through narrow city streets for the last seven kilometers of its journey.

Parno and the team will seek to weigh neutrinos by measuring the energy of electrons emitted by decaying tritium. Tritium is a radioactive hydrogen isotope used in hydrogen bombs and in the batteries to light exit signs. As tritium decays into a helium isotope, it will emit an electron and a neutrino. Parno and the team will use magnets to direct the electron into the electromagnetic spectrometer to measure its energy.

The scientists will measure the energy of the emitted electrons to determine how much has been lost to the neutrinos, and compare that effect to what would have happened if the neutrinos had no mass. This will allow them to calculate the mysterious particle's mass.

Parno said it will take five years for the experiment to generate a final result. She hopes to start next June. Last week, Parno and CMU hosted 40 top particle physics from around the world to plan for the experiment.

"It's a really neat, fundamental question, and we're just starting to understand how neutrinos work," Parno said. "Every time we look at neutrinos, they surprise us."

Aaron Aupperlee is a Tribune-Review staff writer. Reach him at aaupperlee@tribweb.com, 412-336-8448 or via Twitter @tinynotebook.

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