Evidence for the direct decay of the 125 GeV Higgs boson to fermions

Abstract

The discovery of a new boson with a mass of approximately 125 GeV in 2012 at the Large Hadron Collider(1-3) has heralded a new era in understanding the nature of electroweak symmetry breaking and possibly completing the standard model of particle physics(4-9). Since the first observation in decays to gamma gamma, WW and ZZ boson pairs, an extensive set of measurements of the mass(10,11) and couplings to W and Z bosons(11-13), as well as multiple tests of the spin-parity quantum numbers(10,11,13,14), have revealed that the properties of the new boson are consistent with those of the long-sought agent responsible for electroweak symmetry breaking. An important open question is whether the new particle also couples to fermions, and in particular to down-type fermions, as the current measurements mainly constrain the couplings to the up-type top quark. Determination of the couplings to down-type fermions requires direct measurement of the corresponding Higgs boson decays, as recently reported by the Compact Muon Solenoid (CMS) experiment in the study of Higgs decays to bottom quarks(15) and tau leptons(16). Here, we report the combination of these two channels, which results in strong evidence for the direct coupling of the 125 GeV Higgs boson to down-type fermions, with an observed significance of 3.8 standard deviations, when 4.4 are expected.