Exclusive Decays of Z and Higgs Bosons in QCD Factorisation
Stefan Alte (Universität Mainz)

We present an analysis of exclusive radiative decays of Z and Higgs bosons into final states containing a highly energetic meson. We employ the Quantum Chromodynamics (QCD) factorisation approach. In this approach, perturbatively calculable hard functions account for effects at energy scales corresponding to the masses of the decaying bosons, whereas light-cone distribution amplitudes (LCDAs) describe the hadronic bound-state effects inside the meson.

We explore the decays of a Z boson into an eta(') meson and a photon within the Standard Model (SM). The decay amplitude receives two contributions, one involving the leading order (LO) quark LCDA and one involving the LO gluon LCDA of the meson. We find branching ratios which exhibit a very strong sensitivity to the hadronic input parameters. Measuring these decays in the future at an electron-positron collider running at the Z pole could provide valuable insights into the hadronic parameters in general and the gluon LCDA in particular.

In our analysis of the decays of a Higgs boson into a meson and a Z or W boson, our focus lies on the effect of flavour-changing Yukawa couplings present in models beyond the Standard Model. We find two different contributions to the decay amplitude. The first contribution arises from diagrams where the Higgs boson couples to the quark-antiquark pair forming the meson. For this contribution, we apply the QCD factorisation framework. The second contribution involves diagrams where the Higgs boson couples to two gauge bosons. One gauge boson is virtual and converts into the meson. In the SM, the first contribution is strongly suppressed. This changes when we allow for flavour-changing Yukawa couplings in a model-independent way by introducing an effective Lagrangian. In particular in the decays into B^+_(c) mesons, the first contribution is very sensitive to the off-diagonal Yukawa couplings Y_ut or Y_ct. In the most extreme scenario allowed by current constraints on these parameters, the branching ratio for the decay into B^+ can be enhanced by a factor of 1000.