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New experiment halves weight limit of elusive neutrinos

Scientists trying to discover the elusive mass of neutrinos, tiny "ghost particles" that could solve some of the universe's biggest mysteries, announced a new limit on Thursday for how much they could weigh, halving the previous estimate.

Since the existence of neutrinos was proposed nearly a century ago, scientists around the world have struggled to learn much about them—particularly their mass.

This is important because the neutrino, as the most abundant particle in the universe, "weaves a thread that connects the infinitely small and the infinitely large," Thierry Lasserre, a physicist at France's Alternative Energies and Atomic Energy Commission, told AFP.

Its mass "influences the structures that make up the cosmos," he added.

These invisible particles have been washing across the universe since the Big Bang 13.8 billion years ago.

The number of neutrinos out there is difficult to comprehend—there are roughly a billion for every atom in the cosmos.

However, because they have so little mass and lack an electric charge, neutrinos very rarely interact with matter.

For example, trillions of these so-called ghost particles are thought to be streaming through human bodies every second, with us none the wiser.

This makes them exceedingly difficult to study. But not impossible.

Chasing ghosts

More than a hundred scientists from six countries have been hunting down the neutrino since 2019 as part of the KATRIN collaboration at Germany's Karlsruhe Institute of Technology.

In a study in the journal Science on Thursday, the collaboration announced that a neutrino's mass cannot exceed 0.45 electron volts.

That is less than a billionth of the mass of a proton, which is inside the nucleus of every atom.

The new upper limit for a neutrino's mass is around half the figure that KATRIN announced in 2022 after its first measurements.

KATRIN uses a massive spectrometer to record the decay of tritium, a radioactive form of hydrogen which releases both electrons and neutrinos.

These particles spin around a 70-meter-long (230-foot-long) structure dominated by the 200-tonne spectrometer, which operates in a vacuum.

The electron and neutrino share the energy produced by the decaying tritium. So the trick is to measure the electron's energy to infer information about the neutrino.

This requires measuring a lot of electrons.

Six million had to be measured to achieve KATRIN's first results in 2022.

And it took 36 million to reach the more accurate figure announced on Thursday.

"When we have collected all our data by the end of the year," the team will have measured around 250 million electrons, Lasserre said.

That will be the moment of truth.

Either the experiment will have finally uncovered a "trace" of the neutrino—or have determined that its mass is less than 0.3 electron volts, Lasserre explained.

Dark energy

Scientists hope that pinning down the neutrino's mass will help unravel several stubborn secrets of the cosmos.

Despite their incredible lightness, neutrinos have been included in some models seeking to explain dark energy—the unknown force thought to be driving the ever-faster expansion of the universe.

Roughly 95% of the universe is thought to be composed of dark energy and the equally unknown dark matter, leaving just 5% for everything else.

The KATRIN collaboration is planning to set up a new detection system, called TRISTAN, to hunt for a new breed of neutrinos called sterile neutrinos.

These hypothetical particles do not interact with matter, but have much more mass than normal neutrinos.

Some scientists have proposed that these strangely heavy neutrinos could actually be what we know as dark matter.