Scientists now know the fate of the majority of Higgs bosons particles produced in the LHC

Aug. 29, 2018

Lawrence Livermore physicist Finn O’Neill Rebassoo in front of the CMS detector. The detector is 21.6 meters long, 15 meters in diameter and weighs about 14,000 metric tons. (Download Image)

Scientists now know the fate of the majority of Higgs bosons particles produced in the LHC

Anne M Stark, stark8@llnl.gov, 925-422-9799

At CERN, the Large Hadron Collider experiments ATLAS and CMS have taken a big step forward in the quest to understand how the Higgs boson enables fundamental particles to acquire mass.

The team announced the discovery of the Higgs boson particle 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 it closely mimics ordinary background processes.

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. The finding confirms the existence of the Higgs boson as it decayed down to a known particle. 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, a scientist at the Department of Energy’s (DOE) Fermi National Accelerator Laboratory (Fermilab). “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.”

Higgs bosons are only produced in roughly one out of a billion LHC collisions and live only one-septillionth 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 particles to study the Higgs’ properties. Since its discovery in 2012, scientists have been able to identify only about 30 percent of all the predicted Higgs boson decays. According to Viviana Cavaliere, a physicist at Brookhaven National Laboratory, finding the Higgs boson decaying into bottom quarks has been priority No. 1 for the last several years because of its huge favorability.
 
Lawrence Livermore National Laboratory (LLNL) has been part of the CMS experiment since 2005, seven years before the Higgs boson was discovered. The Livermore team of Doug Wright and Finn O’Neill Rebassoo contributed to the trigger, an elaborate system made up of both hardware and software, that works to filter out events where scientists think there could be interesting particles. In addition, they contributed to the development of software algorithms that reconstruct the data into the physics objects that are the basis of all analysis.

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 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 complex work.
 
“Theory predicts that 60 percent of Higgs bosons decay into bottom quarks,” said Cavaliere, who is 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. But many scientists suspect that the Higgs also could interact with massive particles outside the Standard Model, such as dark matter. By finding and mapping the Higgs bosons’ interactions with known particles, scientists can simultaneously probe for new phenomena.

“A fraction of Higgs bosons could be producing dark matter particles as part of their decay,” said Giacinto Piacquadio, a physicist at Stony Brook University who co-leads the Higgs-to-bottom-quarks analysis group. “Because the decay of the Higgs boson to bottom quarks is so common, we can use it to put constraints on potentially invisible decays as well as use it to probe for new physics directly.”

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.” Palmer said. “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.