Adriana Bercebal Ruiz

The Higgs boson was proposed by Robert Brout, François Englert and Peter Higgs in 1964. This particle gives mass to W and Z bosons when they interact with the Higgs field. In 2012, CERN announced the discovery of a new particle at the Large Hadron Collider (LHC), by the CMS and ATLAS experiments. [1][2], which agreed with the standard model Higgs boson of mass 125 GeV, as described in Ref. [3].

There exists direct evidence of the Higgs boson coupling to W and Z bosons directly and to photons indirectly [4][5]. Standard model predicts that the Higgs boson couples to fermions through the Yukawa interaction, where coupling strength is proportional to the mass acquired by those fermions. Indirect evidence of the Higgs boson coupling to a top quark already existed in 2014. However, direct evidence of it coupling to down-type fermions was necessary to confirm its nature. According to Yukawa interaction, the most abundant decays will happen between the Higgs boson and third generation quarks and leptons. Hence, the most appropriate experiments investigate Higgs boson decay to a bottom quark-antiquark pair (H → bb) and to a tau lepton-antilepton pair (H → ττ).

The Higgs boson lifetime is 10-22 seconds. Hence, LHC can only detect its decay products. Background from other processes make detection of H → bb and H → ττ decays challenging, especially as they do not produce sharp peaks. Quantum chromodynamics (QCD) predicts vast direct production of bb quark pairs, treated as background. An excess over this background is produced for a Higgs boson with mass, mH, in the range 120-135 GeV [6]. Additionally, τ lepton’s lifetime is 2.9 x 10-13 s [7], which cannot be detected by the CMS detector systems. τ leptons decay to a lighter, detectable lepton and neutrinos, which are inferred by the overall imbalance in transverse momentum. Difference in mass and kinematic properties help to distinguish non-fermionic contributions from H → bb and H → ττ decays. CMS tracking system and mass reconstruction techniques [7] reduce significantly the effect of QCD background by tracking individual interactions and identifying the desired decays.

Results from H → bb and H → ττ decays are consistent with each other and the standard model Higgs boson. They were analysed together using statistical methods developed by the LHC Higgs Combination Group. The test used compares the profile likelihood ratio of both the background-only to the signal-plus-background hypotheses [8][9]. For all mH values, evidence against the background-only hypothesis is found to be at least 3σ, with a maximum of 3.8σ for mH = 125GeV. The Higgs boson was determined to have a mass of 125 GeV from non-fermionic decays too. Therefore, conclusion obtained from fermionic-decay contributions agree with non-fermionic ones and reveal a strong evidence for direct coupling of Higgs bosons with down-type fermions.

The Higgs boson could be essential for dark-matter discovery. Theory proposes that dark matter interacts with itself through “dark photons”, which would interact weakly with standard-model particles. A Higgs boson would decay into a photon and a dark photon, which could be detected at the LHC [10].


[1] CMS Collaboration, “Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC”, Phys. Lett. B 716 (2012) 30, doi:10.1016/j.physletb.2012.08.021, arXiv:1207.7235. 

[2] ATLAS Collaboration, “Observation of a new particle in the search for the standard model Higgs boson with the ATLAS detector at the LHC”, Phys. Lett. B 716 (2012) 1, doi:10.1016/j.physletb.2012.08.020, arXiv:1207.7214.

[3] CMS Collaboration, “Evidence for the direct decay of the 125 GeV Higgs boson to fermions”, Nature Phys. 10, 557-560 (2014). doi:10.1038/nphys3005, arXiv:1401.6527v3.

[4] ATLAS Collaboration, “Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC”, Phys. Lett. B 726 (2013) 88, doi:10.1016/j.physletb.2013.08.010, arXiv:1307.1427.

[5] CMS Collaboration, “Measurement of Higgs boson production and properties in the WW decay channel with leptonic final states”, JHEP 01 (2014) 096, doi:10.1007/JHEP01(2014)096, arXiv:1312.1129.

[6] CMS Collaboration, “Search for the standard model Higgs boson produced in association with a W or a Z boson and decaying to bottom quarks”, Phys. Rev. D 89 (2014) 012003, doi:10.1103/PhysRevD.89.012003, arXiv:1310.3687.

[7] CMS Collaboration, “Evidence for the 125 GeV Higgs boson decaying to a pair of τ leptons”, (2014). arXiv:1401.5041. Submitted for publication in JHEP.

[8] CMS Collaboration, “Combined results of searches for the standard model Higgs boson in pp collisions at √ s = 7 TeV”, Phys. Lett. B 710 (2012) 26, doi:10.1016/j.physletb.2012.02.064, arXiv:1202.1488.

[9] G. Cowan, K. Cranmer, E. Gross, and O. Vitells, “Asymptotic formulae for likelihood-based tests of new physics”, Eur. Phys. J. C 71 (2011) 1554, doi:10.1140/epjc/s10052-011-1554-0, arXiv:1007.1727.

[10] A. Lopes, “CMS hunts for dark photons coming from the Higgs boson”, CERN, May 24, 2019. [Online]. Available at: (Accessed: 3 March 2020.)

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