The discovery of the Higgs boson in 2012 explained something no less foundational than how subatomic particles gain mass. The particle’s discovery marked the beginning of a comprehensive effort to measure its properties. In June 2018, the CMS collaboration reported observing the Higgs boson produced in association with a pair of top quarks. This represented the first direct measurement of the Higgs boson coupling to quarks, a building block of matter. Now the CMS collaboration announces another major observation: The Higgs boson decays into bottom quarks, also known as the beauty quarks.
The CMS measurement of the Higgs boson coupling to the bottom quark is consistent with expectations detailed by the Standard Model, the scientific theory that describes how the particles that make up all known matter in the universe behave. The study strengthens the case that the Higgs boson couples to a class of particles called fermions, including the up and down quarks, which make up the familiar proton and neutron. The current precision of the measurement still leaves room for contributions to Higgs decays from new physics beyond the Standard Model.
The 2012 discovery of the Higgs boson marked the beginning of an experimental program aimed at determining the properties of the newly discovered particle. Milestones since the discovery include the observation of Higgs boson decays into a pair of photons (carriers of the electromagnetic force), a pair of Z bosons (carriers of the weak force), a pair of W bosons (also carriers of the weak force), a pair of tau leptons (matter particles, or fermions), as well as precise measurements of the Higgs boson’s mass, spin, and couplings.
The recent observation of Higgs boson decays into bottom quarks is a major achievement and an important step toward establishing the Higgs mechanism as the source of mass generation in the fermion sector of the Standard Model—the sector contains the particles that make up matter.
The bottom-quark decay mode is the most abundant of all Higgs boson decays, but despite its frequency, it has been very difficult to observe. This is because the huge background contribution from other processes can mimic the experimental signature of the Higgs-to-bottom-quark decay, one that is characterized by the appearance of a bottom quark and its antiparticle.
CMS overcame this challenge by deploying modern sophisticated analysis tools, including machine learning, and by focusing on the particular signatures of a Higgs boson produced in association with a W or Z boson, a weak interaction process known as VH(bb), leading to a significant reduction in the background.
Meenakshi Narain of Brown University is a scientist on the CMS experiment and chair of the U.S. CMS Collaboration Board.