July 4, 2012. Besides being the United States’ 236th birthday, it was the day that physicists announced that they had found strong evidence of the Higgs boson, an elusive particle that imparts mass to other elementary particles in the universe. It was one of the most important achievements in physics in the last century, and it took the construction of the Large Hadron Collider, the giant particle accelerator based outside Geneva, Switzerland, to test for it. Following that triumph, the physics community was confident that more discoveries would follow from CERN. But literally quadrillions of proton collisions in the collider later, nothing new has emerged. Now, however, after sifting through years of data, researchers working on the LHC’s ATLAS experiment announced that they can confirm something new: the decay of the Higgs boson produces bottom quarks, lending support to a theoretical framework of physics known as the Standard Model of particle physics.
According to a press release, the 2012 Higgs sighting was incomplete. While actually observing a Higgs boson is not currently possible, detecting the bits the particle decays into is something the particle accelerator can do. At that time, two predicted particles called W and Z bosons were observed, which are expected in about 30 percent of decaying Higgs bosons. But researchers did not see the particles expected 60 percent of the time—bottom quarks.
Or at least, so they thought. The problem, explains the Wire, is that they did see bottom quarks, just too many of them; the collider produces lots of bottoms quarks through various interactions besides the streams of protons it's been designed to slam into one another. So figuring out whether a bottom quark detected in the LHC came from a decaying Higgs boson or from somewhere else proved extremely difficult. That’s why it took so long for scientists to reach the point of reasonable certainty that some of the bottom quarks they were observing were coming from Higgs decay. Looking at all the data since 2011 and using new analytical techniques like deep artificial neural nets and machine learning, they finally found statistically significant evidence of the Higgs-generated bottom quarks.
“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 experiment, tells Ryan F. Mandelbaum at 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.”
While the finding is a triumph of analysis and another confirmation that we’ve likely found the Higgs boson, it’s also mildly disappointing. That’s because it fits nicely in the Standard Model, which physicists have been working with since the early 1970s. While the model explains a lot about particle physics, it has some gaping holes. For instance, it doesn’t address gravity or explain dark matter. Since the LHC was switched on, researchers had hoped for evidence of “weird” particles that would break or expand the Standard Model or confirm supersymmetry, an addendum to the model that helps explain mass. That has simply not happened, at least not yet.
For now, physicists may have to wait a few more years before the laws of nature are rewritten. The LHC is undergoing a series of power upgrades that will be complete in 2026 and when it's switched back on the juiced-up particle accelerator may lead to the type of weird science the field needs to move beyond even John Hughes' wildest imagination.