First they found the Higgs boson using the world’s largest atom smasher. Now, thanks to observations of an ultra-rare particle interaction, scientists have more evidence that the Higgs does what it’s supposed to do.

For forty years physicists have been using the standard model of particle physics to explain how forces of nature operate. And an essential feature is the Higgs boson, a particle that’s thought to provide mass to all matter. As New Scientist explains it, the particles that make us up have mass, and without the Higgs, these particles would be massless, like photons. Its discovery in 2012 might be considered the crowning achievement of the Large Hadron Collider (LHC), and it greatly bolstered physicists confidence in the model they'd been working with.

But finding the Higgs isn't the end of the story. For one thing, some physicists are chasing even greater levels of confidence in the standard model; for another, the standard model isn't a complete description of the way the subatomic world works. "The Standard Model has so far survived all tests, but we know that it is incomplete because there are observations of dark matter, dark energy, and the antimatter/matter asymmetry in the universe that can't be explained by the Standard Model," says Marc-André Pleier of Brookhaven National Laboratory in a news release.

It took years of collisions to confirm the Higgs discovery, and the mountain of data LHC has created hides more secrets for physicists to uncover. Take, for example, collisions of two particles called W bosons. When they collide, they scatter in a way that can tell physicists whether the Higgs does its job of imparting mass to matter in the way they expect -- and possibly eliminate some of the competing additional theories.

The problem? These interactions are harder to find than even the Higgs itself. “Only about one in 100 trillion proton-proton collisions would produce one of these events,” Pleier explains. “We looked through billions of proton-proton collisions produced at the LHC for a signature of these events -- decay products that allow us to infer like Sherlock Holmes what happened in the event.” He and the ATLAS collaboration observed 34 of these events.

To test the Higgs mechanism, the scientists compared distributions of decay products of the W scattering process -- how often particular products are observed at a particular energy and geometrical configuration.

“It’s like a fingerprint,” Pleier says. “We have a predicted fingerprint and we have the fingerprint we measure. If the fingerprints match, we know that the Higgs does its job of mass generation the way it should.” Sure enough, the data indicate that the Higgs is working as expected. The work will be published in Physical Review Letters.