Searches for exotic Higgs decays are a particularly rich and fruitful way to seek evidence of new physics.
  • The SM-like Higgs boson has an extremely narrow width, Γh ≅ 4.07 MeV, so that Γh/mh ≅ 3.3 ×10−5. Even a small coupling to another light state can easily open up additional sizable decay modes.
  • There are very good reasons to suspect that new physics couples preferentially to the Higgs boson, since it provides one of only a few "portals" that allow SM matter to interact with hidden-sector matter that is SM-neutral via potentially (super-)renormalizable interactions (see e.g.  [11,12,13,14]). This is the famous Higgs portal, which includes terms like


    for a singlet scalar s or singlet fermion ψ. Exotic branching fractions Br(h→ ss, ψψ)  ∼ 10% only require a coupling ζ ∼ 0.01, or Λ as large as several TeV (for μ ∼ mψ). Thus exotic Higgs decays can indirectly probe new physics scales beyond the kinematic reach of the LHC, and may even provide the only evidence of a new sector at the LHC.
  • Higgs "coupling fits" constrain Br(h→BSM) <~ 20 − 60% at 95% CL, depending on assumptions, see [1,2,3,4,5,6,7] for some recent fits. Future projections suggest an ultimate precision at the LHC on this indirect measurement of Br(h→BSM) of O(5−10%) [8,9,10]. Branching fractions of O(10%) into exotic decay modes are therefore not only still allowed by existing data but will remain reasonable targets for the duration of the physics program of the LHC.
  • The data collected at the LHC7 and LHC8 may easily contain O(50,000) exotic Higgs decays per experiment, assuming  ∼ 10% exotic branching fraction. For those decay modes which pass trigger thresholds with high enough efficiency, dedicated searches represent a tremendous potential for discoveries of new physics.
  • Branching fractions as small as O(10−6) could be detected at the LHC14 with 300  fb−1, if the decay signature is both visible and clean.


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