We revisit the radiative decays of the Higgs boson to a fermion pair h→ff¯γ where f denotes a fermion in the Standard Model (SM). We include the chirality-flipping diagrams via the Yukawa couplings at the order O(y2fα), the chirality-conserving contributions via the top-quark loops of the order O(y2tα3), and the electroweak loops at the order O(α4). The QED correction is about Q2f×O(1%) and contributes to the running of fermion masses at a similar level, which should be taken into account for future precision Higgs physics. The chirality-conserving electroweak-loop processes are interesting from the observational point of view. First, the branching fraction of the radiative decay h→μ+μ−γ is about a half of that of h→μ+μ−, and that of h→e+e−γ is more than four orders of magnitude larger than that of h→e+e−, both of which reach about 10−4. The branching fraction of h→τ+τ−γ is of the order 10−3. All the leptonic radiative decays are potentially observable at the LHC Run 2 or the HL-LHC. The kinematic distributions for the photon energy or the fermion pair invariant mass provide non-ambiguous discrimination for the underlying mechanisms of the Higgs radiative decay. We also study the process h→cc¯γ and evaluate the observability at the LHC. We find it comparable to the other related studies and better than the h→J/ψ γ channel in constraining the charm-Yukawa coupling.
Monday, April 3, 2017