Have We Already Detected a Dark Matter Particle?

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Dark matter supposedly makes up 85% of the matter in the universe, but so far, efforts to catch hypothesized dark matter particles have all ended in failure. Weakly interacting massive particles (WIMPs) are no-shows at grand experiments housed in Italy, Canada, and the United States. Even tinier axions have not been detected either. Neutralinos, born out of supersymmetry, may look nice on paper but so far have no bearing on reality.

By definition, dark matter is hard to find, but its glaring absence is growing harder and harder to ignore. As Korey Haynes pointed out at Discover:

Decades ago, scientists were confident about the existence of the “luminiferous aether” as a medium to carry light. Now, that’s looked back on as a clumsy belief that should have been dropped far earlier than it was. Scientists persisted because they were sure that light, like sound, required a medium to move through in spite of the evidence piling up against that concept. Having been fooled once, scientists have to ask: Is dark matter the new ether?

Astrophysicist and blogger Ethan Siegel thinks that claim is overblown.

"We are after the most fundamental stuff in the Universe, and we’ve only recently begun to understand it. It shouldn’t be a surprise if the search takes a little — or even a lot — longer," he wrote.

But what if the search has already been successful? What if dark matter is made up of particles that scientists discovered more more than 48 years ago?

Those particles are neutrinos. Trillions of these subatomic particles pass through you each second, a mind-bending feat that owes to the neutrino's infinitesimally tiny mass and size. For decades, physicists didn't even think neutrinos had mass, but that notion was abruptly tossed in the garbage when, in 1998, an experiment showed that neutrinos oscillate between different "flavors". These changes require energy, which means that neutrinos must have mass. 

Exactly what that mass is has yet to be conclusively determined – it's not easy to weigh something that's at least 500,000 times lighter than an electron! But when it is determined, it could greatly impact our theories of dark matter.

A researcher looks inside the main spectrometer of a machine designed to determine the mass of neutrinos at the Karlsruhe Institute of Technology, Monday, June 11, 2018. (Uli Deck/dpa via AP)

Neutrinos neither emit nor absorb light, making them ideal candidates for dark matter particles. Most predictions of their mass, however, pin them as being too light to make any dent at all in the "dark" universe, though a recent experiment suggests they could account for as much as 5% of dark matter.

If this is true, the brand new KArlsruhe TRItium Neutrino (KATRIN) experiment could provide an answer. The gigantic apparatus, featuring a 200-ton spectrometer, could measure the mass of a neutrino provided the mass is above 0.35 electron volts (a mind-bogglingly small unit of measurement). KATRIN began operations in June and will continue running for five years. It's currently our best hope for pinpointing the precise mass of the neutrino.

If KATRIN finds that the mass of the neutrino is much, much higher than estimated, then things get really interesting.

"There are so many neutrinos that even if they only had a mass one five thousandth that of the electron, the mass of all the neutrinos in the universe could make up for the missing matter," CalTech research scientist Sabrina Stierwalt wrote.

Such a finding would effectively end the mystery of dark matter! Most astrophysicists, however, aren't holding their breaths. High mass neutrinos have been effectively ruled out by prior observations.

And so, while many hope that KATRIN reveals neutrinos as true dark matter particles, scientists must continue fumbling around in the dark for other hypothesized particles that may or may not exist.

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