Scientists can’t take pictures of the Higgs boson. But they can find proof of its existence by watching “E=mc2” play out in hundreds of millions of particle collisions per second and detecting how it decays into other particles they do know how to spot. Now, six years after officially discovering the Higgs boson, particle physicists are announcing that they’ve spotted the Higgs in another way.
This announcement isn’t a surprise. It matches the predictions of the Standard Model of particle physics, the rock-solid but probably incomplete blueprint of the Universe on the smallest scales. But the news is certainly important; you might say it closes the first chapter of the Higgs boson’s story, and offers a potential window to explore some of most confounding questions in the Universe.
“It’s the first time we’ve seen the Higgs coupling to bottom quarks, which had been predicted,” John Huth, a Harvard University particle physicist who works on the ATLAS collaboration at CERN in Switzerland, told Gizmodo. “We thought it would happen, but until we saw it, we wouldn’t know for sure that it coupled to quarks in this way.”
Fifty years ago, a team of scientists, including Peter Higgs, first theorized the Higgs boson’s existence. The theoretical particle would explain why certain particles that should be massless actually have mass, and potentially why all fundamental particles have mass. A billion-dollar search ensured, resulting in the discovery of a particle that looked a whole lot like the Higgs boson, which scientists at the Large Hadron Collider’s ATLAS and CMS experiments announced on July 4, 2012.
Those original experiments spotted the Higgs candidate by slamming together protons and observing what came out. They compared those outcomes to simulations of how the resulting collisions would look like if the new particle didn’t exist. Around 30 percent of the Higgs bosons produced in these collisions should produce either a set of photons or a set of W and Z bosons, the particles that carry the weak nuclear force (one of the four fundamental forces). But almost 60 percent of the Higgs bosons should decay into a pair of bottom quarks—the second-heaviest of the six quarks.
All available evidence has made it pretty clear that the new particle was, in fact, the Higgs boson, but spotting the Higgs through the bottom quark decay was much more difficult than finding it via photons or the W and Z bosons. Photons are obvious in the detectors, and Ws and Zs themselves decay into pairs of muons or electrons, also easy-to-understand-and-detect particles, Sarah Charley writes for Symmetry Magazine. But the bottom quarks look much messier in the detector, and it’s easy to confuse bottom quark pairs that come from Higgs bosons with those produced in other ways.
The ATLAS collaboration has finally said that it has seen enough proton collisions and resulting bottom quarks to warrant an announcement of the discovery at the 2018 International Conference on High Energy Physics (ICHEP) in Seoul, South Korea. You might remember the phrase “standard deviations,” or “sigmas,” a threshold that physicists use to determine how unlikely it is that their measurement would have happened by chance if their hypothesis were false. Five sigma is the benchmark particle physicists use to say that, after taking lots of data, something is so unlikely to have happened by chance that they’ve made a discovery. The physicists here got their five-sigma observation, a finding consistent with the Standard Model’s predictions.
Physicists on the other Higgs-hunting Large Hadron Collider experiment, CMS, noted the discovery’s difficulty. “ATLAS had to combine all data ever collected by the LHC since 2011... and even then, fancy tricks like deep artificial neural nets and other machine learning was necessary,” Freya Blekman, physicist at the Vrije Universiteit Brussel, told Gizmodo. CMS will release similar results on the discovery soon.
Huth was excited about the result for some existential reasons. On one hand, the Higgs boson offers a way for particles to get their mass through a kind of physical field that permeates the Universe. On the other hand, the force that acts between masses, gravity, doesn’t seem to fit into the same quantum theory that describes the Higgs boson and the paradigm right after the Big Bang, in which the weak nuclear force and electromagnetic force were united. One way to better solve that problem could be to understand the strange field that accompanies the Higgs. That requires learning about how the Higgs interacts with itself, a behavior that could be probed by looking at the four bottom quarks resulting from such an interaction.
“We’re at this interesting juncture where we need more information to help fill out this puzzle,” said Huth.
Huth thought the discovery was rock-solid, but he pointed out that it did take a lot of math and variables in order to get the five-sigma discovery—there’s always the concern that the computer did something stupid, but they’ve done many cross checks. Another physicist from the CMS collaboration, André David, tempered the excitement, telling Gizmodo that Higgs bosons decaying to bottom quarks is the Standard Model prediction—it’s the null result. Nothing weird has been learned yet. But he was enthusiastic about how the results can better explain how quarks get their mass, too, since it’s a way to measure how quarks and the Higgs boson interact.
This discovery is yet another tool to probe the very deepest underpinnings of the Universe. But there’s so much more to learn, and therefore, more observations to be done. Said Huth: “You just gotta keep chipping away at it.”