UC Davis Physicists Help Detect Higgs Boson Decay

Six years after its discovery, the Higgs boson has at last been observed decaying to fundamental particles known as bottom quarks. The finding, presented Aug. 28 at CERN 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 quarks. 

A number of UC Davis physicists were involved in the observation. Professors Maxwell Chertok, John Conway, Robin Erbacher, Michael Mulhearn and Mani Tripathi are members of the CMS (Compact Muon Solenoid) collaboration. UC Davis has a long history with the Large Hadron Collider and CMS. Professors emeritus Winston Ko, Richard Lander and David Pellett have also been part of the CMS team, together with research scientists Richard Breedon, Timothy Cox, and John Smith. Ko, Lander, Tripathi and Pellett helped develop technology for the CMS detector in the 1990s, and UC Davis was one of the first U.S. universities to sign on to help build the Large Hadron Collider. Professor emeritus John (Jack) Gunion is one of the theorists behind work on the Higgs boson, as co-author of “The Higgs Hunter’s Guide.” Gunion and his co-authors were awarded the 2017 Sakurai Prize for their contributions.

The Standard Model and new physics

By discovering the Higgs, the Large Hadron Collider has helped confirm the Standard Model of particle physics. Now physicists hope that the machine will provide evidence of “new physics” which must lie beyond the Standard Model.

CMS detector illustration
The CMS detector catches a Higgs boson decaying to two bottom quarks (b) in association with a Z boson decaying to an electron (e-) and an antielectron (e+). (Image: CMS/CERN)

The Standard Model predicts that about 60 percent of the time a Higgs boson will decay to a pair of bottom quarks, the second-heaviest of the six flavors of quarks. If this prediction turned out to be incorrect, it would rock the foundations of the Standard Model 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 Higgs 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.

The ATLAS and CMS teams each combined their data from the first and second runs of the LHC and applied complex analysis methods. Both ATLAS and CMS teams were able to detect decay of the Higgs boson to a pair of bottom quarks with a significance exceeding five standard deviations. Their results are consistent with the Standard Model prediction.

With more data, the collaborations will improve the precision of these and other measurements and probe the decay of the Higgs boson into a pair of much-less-massive fermions called muons, always watching for deviations in the data that could point to physics beyond the Standard Model.

Initial evidence for the decay of the Higgs boson to pairs of bottom quarks actually came in 2012, shortly before the discovery of the Higgs itself, based on data collected by the CDF and DZero experiments at the Tevatron proton-antiproton collider at Fermilab in Illinois by teams including UC Davis physicists. The Tevatron was for 25 years the world’s highest-energy accelerator. It delivered its last collisions in late 2011, shortly after the LHC began operating at high intensity.

— Andy Fell, UC Davis News and Media Relations