Six years after its discovery, the Higgs boson has at last been observed decaying to fundamental particles known as bottom quarks. The finding, presented today at CERN1 by the ATLAS and CMS collaborations at the Large Hadron Collider (LHC), is consistent with the hypothesis that the all-pervading quantum field behind the Higgs boson also gives mass to the bottom quark. Both teams have submitted their results for publication today.
The Standard Model of particle physics predicts that about 60% of the time a Higgs boson will decay to a pair of bottom quarks, the second-heaviest of the six flavors of quarks.
Testing this prediction is crucial because the result would either lend support to the Standard Model – which is built upon the idea that the Higgs field endows quarks and other fundamental particles with mass – or rock its foundations and point to new physics.
Spotting this common Higgs-boson decay channel is anything but easy, as the six-year period since the discovery of the boson has shown. The reason for the difficulty is that there are many other ways of producing bottom quarks in proton-proton collisions. This makes it hard to isolate the Higgs-boson decay signal from the background “noise” associated with such processes. By contrast, the less-common Higgs-boson decay channels that were observed at the time of discovery of the particle, such as the decay to a pair of photons, are much easier to extract from the background.
To extract the signal, the ATLAS and CMS collaborations each combined data from the first and second runs of the LHC, which involved collisions at energies of 7, 8 and 13 TeV. They then applied complex analysis methods to the data. The upshot, for both ATLAS and CMS, was the detection of the decay of the Higgs boson to a pair of bottom quarks with a significance that exceeds 5 standard deviations. Furthermore, both teams measured a rate for the decay that is consistent with the Standard Model prediction, within the current precision of the measurement.
The findings are another big step along the journey to better understand the Higgs boson and our universe. And each new discovery or observation, like the discovery of the Higgs boson, has the potential to give way to new questions and experiments. “First [you] discover the thing,” Beacham said about the Higgs boson. “Then, you want to measure everything about it.”
Additionally, this work represents “a significant landmark in our tests of the Standard Model,” Shelton said. “The Higgs’ main job in the Standard Model is to give masses to the matter fermions and the weak force carriers,” Shelton continued, “observing this decay, then, is our first direct piece of evidence that the Higgs boson gives masses to quarks as the Standard Model predicts. Observing this decay mode also leaves less room for potential undiscovered particles to contribute to fermion masses.”
In confirming that this particle does, in fact, decay into b quarks, these physicists have shown that the Higgs field, the field behind Higgs boson particles described by Beacham as the “invisible jelly that permeates all of space,” gives b quarks mass. The Higgs field uses the Higgs boson to interact with other particles, like the b quark, and give them mass.
Experiments like these allow physicists to not only validate what the Standard Model predicts about the Higgs boson and b quarks but also challenge what the Standard Model predicts. “So far, the Standard Model is continually winning,” Beacham said. “[But] if we find evidence that it’s not so standard,” Beacham continued — if we find things like “extra quarks, weirdo things like vector-like quarks, leptoquarks, dark matter” — then we might also find a totally new understanding of how our universe works.