Scientists detect most common way that Higgs bosons decay

Scientists now know the fate of most Higgs bosons produced in Large Hadron Collider

Today at CERN, the Large Hadron Collider collaborations ATLAS and CMS jointly announced the discovery of the Higgs boson transforming into bottom quarks as it decays. This is predicted to be the most common way for Higgs bosons to decay, yet was a difficult signal to isolate because background processes closely mimic the subtle signal. This new discovery is a big step forward in the quest to understand how the Higgs enables fundamental particles to acquire mass.

After several years of refining their techniques and gradually incorporating more data, both experiments finally saw evidence of the Higgs decaying to bottom quarks that exceeds the 5-sigma threshold of statistical significance typically required to claim a discovery. Both teams found their results were consistent with predictions based on the Standard Model.

“The Higgs boson is an integral component of our universe and theorized to give all fundamental particles their mass,” said Patty McBride, distinguished scientist at Fermi National Accelerator Laboratory and recently elected deputy spokesperson of the CMS experiment. “But we haven’t yet confirmed exactly how this field interacts—or even if it interacts—with all the particles we know about, or if it interacts with dark matter particles, which remain to be detected.”

Higgs bosons are only produced in roughly one out of a billion LHC collisions and live only a tiny fraction of a second before their energy is converted into a cascade of other particles. Because it’s impossible to see Higgs bosons directly, scientists use these secondary particle decay products to study the Higgs’ properties. Since its discovery in 2012, scientists have been able to identify only about thirty percent of all the predicted Higgs boson decays.

The highest priority in Higgs boson research

“We expected the Higgs would decay into bottom quarks most often, but it is crucial to see and measure it,” said Young-Kee Kim, the Louis Block Distinguished Service Professor in Physics and chair of the Department of Physics at the University of Chicago, who worked on the bottom quark decay of the Higgs with the ATLAS experiment in the past. “We now have seen all of the primary decay modes of the Higgs predicted by the current theory of particle physics, the Standard Model. This marks a very important moment in particle physics!”

According to Viviana Cavaliere, a physicist at DOE’s Brookhaven National Laboratory who works on the ATLAS experiment, finding the Higgs boson decaying into bottom quarks has been priority number one for the last several years because of its large decay rate.

“Theory predicts that sixty percent of Higgs bosons decay into bottom quarks,” said Cavaliere, who is also using this process to search for new physics. “Finding and understanding this channel is critical because it opens up the possibility for us to examine the behavior of the Higgs, such as whether it could interact with new, undiscovered particles.”

The Higgs field is theorized to interact with all massive particles in the Standard Model, the best theory scientists have to explain the behavior of subatomic particles.

“However, questions remain which cannot be answered by the Standard Model,” said Kim. “For example, is the Higgs responsible for giving mass to dark matter particles, the dominant form of matter in the universe?”

Kim's group is currently studying whether or not some fraction of Higgs bosons created in the LHC decay into dark matter particles.

A ‘delicate and labyrinthine’ challenge

Even though this decay is the most popular path, spotting it in the experimental data was no walk in the park. Every proton-proton collision at the LHC produces a splattering of subatomic byproducts, one of the most common being bottom quarks. These bottom quarks then quickly decay into other kinds of particles, leaving behind vast showers of particles in the detectors. Tracing these particle showers back to two bottom quarks (and then figuring out which ones came from a Higgs boson) is extremely delicate and labyrinthine work.

“Being able to identify and isolate bottom quarks in the experimental data is a huge challenge and required precise detector calibration and sophisticated tagging capabilities to identify b-quarks,” said Giacinto Piacquadio, a physicist at Stony Brook University who co-leads the Higgs-to-bottom-quarks analysis group. “We were only able to do these analyses thanks to years of work that came before.”

To spot this process, the ATLAS and CMS collaborations each combined data from the first and second runs of the LHC and then applied complex analysis methods to the data.

“Finding just one event that looks like two bottom quarks originating from a Higgs boson is not enough,” says Chris Palmer, a scientist at Princeton who worked on the CMS analysis. “We needed to analyze hundreds of thousands of events before we could illuminate this process, which is happening on top of a mountain of similar-looking background events.”

According to Palmer, these deceptive background events made the analyses almost impossible to perform based on isolated bottom quarks alone.

“Luckily, there are a few Higgs production mechanisms that produce identifiable particles as byproducts,” said Palmer. “We used these particles to tag potential Higgs events and separate them out from everything else. So we really got a two-for-one deal with this analysis because not only did we find the Higgs decaying to bottom quarks, but we also learned a lot about its production mechanisms.”

The next step is to increase the precision of these measurements so that scientists can study this decay mode with a much greater resolution and explore what secrets the Higgs boson might be hiding.

In addition to Kim, the UChicago ATLAS group includes 20 other scientists and staff, including faculty members Jim Pilcher, professor emeritus in physics; Melvyn Shochet, the Elaine M. and Samuel D. Kersten Jr. Distinguished Service Professor in Physics; Mark Oreglia, professor in physics; and David Miller, assistant professor in physics.

The University of Chicago co-manages Fermi National Accelerator Laboratory, which serves as the U.S. base for participation in the CMS experiment. Read more about Fermilab and the Large Hadron Collider.

U.S. Funding: Department of Energy Office of Science, National Science Foundation