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

The discovery of a new boson with a mass of approximately 125GeV in 2012 at the Large Hadron Collider1–3 has heralded a new era in understanding the nature of electroweak symmetry breaking and possibly completing the standard model of particle physics4–9. Since the first observation in decays to γγ, WW and ZZ boson pairs, an extensive set of measurements of the mass10,11 and couplings to W and Z bosons11–13, as well as multiple tests of the spin-parity quantum numbers10,11,13,14, 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 quarks15 and τ leptons16. Here, we report the combination of these two channels, which results in strong evidence for the direct coupling of the 125GeV Higgs boson to down-type fermions, with an observed significance of 3.8 standard deviations, when 4.4 are expected. The CMS and ATLAS experiments at the Large Hadron Collider (LHC) have reported the discovery of a new boson1–3 with a mass near 125GeV and with production rates, decay rates and spinparity quantum numbers10–14 compatible with those expected for the standard model Higgs boson4–9. In the standard model, the Higgs boson is a spin-zero particle predicted to arise from the Higgs field which is responsible for electroweak symmetry breaking17,18. As such, the standard model Higgs boson couples directly to theW and Z bosons, and indirectly to photons. To date, significant signals have been reported in channels where the boson decays to either γ γ ,WW, or ZZ boson pairs11–13, as predicted by the theory. Overall, these results directly demonstrate that the new particle is intimately related to the mechanism of spontaneous electroweak symmetry breaking, whereby the W and Z bosons become massive, and thus it has been identified as a Higgs boson. The standard model also predicts that the Higgs field couples to fermions through a Yukawa interaction, giving rise to the masses of quarks and leptons. The structure of the Yukawa interaction is such that the coupling strength between the standard model Higgs boson and a fermion is proportional to the mass of that fermion. As the masses of many quarks and leptons are sufficiently well known from experiment, it is possible within the standard model to accurately predict the Higgs boson decay rates to these fermions. The existence of such decays and the corresponding rates remain to be established and measured by experiment. Indirect evidence for the Higgs coupling to the top quark, an up-type quark and the heaviest elementary particle known to date, is implied by an overall agreement of the gluon–gluon fusion production channel cross-section with the standard model prediction. However, the masses of down-type fermions may come about through different mechanisms in theories beyond the standard model19. Therefore, it is imperative to observe the direct decay of this new particle to down-type fermions to firmly establish its nature. As a consequence of the Yukawa interaction discussed above, the most abundant fermionic Higgs boson decays will be to third-generation quarks and leptons, namely the bottom quark and the τ lepton, as the decay of a Higgs boson with a mass around 125GeV to top quarks is kinematically not allowed. Therefore, the most promising experimental avenue to explore the direct coupling of the standard modelHiggs boson to fermions is in the study of the decay to bottom quark–antiquark pairs (denoted as bb) as well as to tau lepton– antilepton pairs (denoted as ττ). Recently, the CMS Collaboration reported on a search for the decays of the new boson to bb quark pairs15 as well as to ττ lepton pairs16 based on data collected in 2011 and 2012. In this Letter, we report on the combination of the results from the study of these two decays to down-type fermion–antifermion pairs, performed for the first time at the LHC. The CMS apparatus comprises several detectors specialized in identifying different types of particles. These detectors are arranged inside and outside a superconducting solenoid of 6m internal diameter that provides a magnetic field of 3.8 T. The detector electronics process collision information at a rate of up to 40MHz, and decide whether or not the crossing of proton beam bunches that took place at the centre of the detector produced proton–proton collisions of sufficient interest. Through a layered decision system, from the 20MHz of proton bunch crossings, fewer than 1 kHz are saved for further analysis. A detailed description of the full CMS apparatus can be found in ref. 20. Data were collected at the LHC which delivered proton–proton collisions at centre-of-mass energies of 7 TeV (2011) and 8 TeV (2012). A total integrated luminosity of 5.1 (4.9) fb−1 and 18.9 (19.7) fb−1, for the H→ bb (H→ ττ) decay, has been analysed at 7 and 8 TeV, respectively. Each proton bunch crossing gave rise to a large number of simultaneous proton– proton collisions, on average 9 in 2011 and 21 in 2012. Such a large number of overlapping collisions presented exceptional challenges in reconstructing the individual particles produced in the collision where an interesting interaction took place. Those challenges were successfullymet thanks to the CMS tracking system, able to separate collision vertices as close as 0.5mm along the beam direction. The Higgs boson lifetime is ∼10−22 s. As a consequence, the detectors at the LHC only record the interactions of its decay products. For aHiggs bosonmass of 125GeV, the channels expected to be experimentally accessible include the decays to two photons, twoW or Z bosons, a bb quark pair and a ττ lepton pair. The last two