Exclusive radiative Higgs decays as probes of light-quark Yukawa couplings

A bstract We present a detailed analysis of the rare exclusive Higgs boson decays into a single vector meson and a photon and investigate the possibility of using these processes to probe the light-quark Yukawa couplings. We work with an effective Lagrangian with modified Higgs couplings to account for possible new-physics effects in a model-independent way. The h → Vγ decay rate is governed by the destructive interference of two amplitudes, one of which involves the Higgs coupling to the quark anti-quark pair inside the vector meson. We derive this amplitude at next-to-leading order in α s using QCD factorization, including the resummation of large logarithmic corrections and accounting for the effects of flavor mixing. The high factorization scale μ ∼ m h ensures that our results are rather insensitive to the hadronic parameters characterizing the light-cone distribution amplitude of the vector meson. The second amplitude arises from the loop-induced effective hγγ * and hγZ * couplings, where the off-shell gauge boson converts into the vector meson. We devise a strategy to eliminate theoretical uncertainties related to this amplitude to almost arbitrary precision. This opens up the possibility to probe for O(1) O 1 modifications of the c - and b -quark Yukawa couplings and O(30) O 30 modifications of the s -quark Yukawa coupling in the high-luminosity LHC run. In particular, we show that measurements of the ratios Br( h → Υ( nS ) γ )/Br( h → γγ ) and Br(h→ bb̄)/Br(h→ γ γ ) B r h → b b ¯ / B r h → γ γ can provide complementary information on the real and imaginary parts of the b -quark Yukawa coupling. More accurate measurements would be possible at a future 100 TeV proton-proton collider.