New Prospects for Higgs Compositeness in h -> Z gamma

TL;DR

The paper investigates how a composite Higgs, treated as a Nambu-Goldstone boson in SO(5)/SO(4) theories, can produce large new-physics effects in the h -> Z gamma decay without conflicting with precision electroweak data. It develops an effective field theory framework, identifies the hZ gamma contact interaction as arising from LR-odd operators and shows that both tree-level vector resonance exchange and one-loop fermionic-resonance effects can enhance or suppress the decay relative to the SM. The authors derive the contributions to the S parameter from fermionic loops, explore two explicit LR-breaking models, and provide numerical analyses showing potentially sizable shifts in h -> Z gamma while maintaining EWPT compatibility. This work highlights h -> Z gamma as a particularly sensitive channel to test the dynamics of electroweak symmetry breaking in composite-Higgs scenarios.

Abstract

We discuss novel effects in the phenomenology of a light Higgs boson within the context of composite models. We show that large modifications may arise in the decay of a composite Nambu-Goldstone boson Higgs to a photon and a Z boson, h -> Z gamma. These can be generated by the exchange of massive composite states of a strong sector that breaks a left-right symmetry, which we show to be the sole symmetry structure responsible for governing the size of these new effects in the absence of Goldstone-breaking interactions. In this paper we consider corrections to the decay h -> Z gamma obtained either by integrating out vectors at tree level, or by integrating out vector-like fermions at loop level. In each case, the pertinent operators that are generated are parametrically enhanced relative to other interactions that arise at loop level in the Standard Model such as h -> gg and h -> gamma gamma. Thus we emphasize that the effects of interest here provide a unique possibility to probe the dynamics underlying electroweak symmetry breaking, and do not depend on any contrivance stemming from carefully chosen spectra. The effects we discuss naturally lead to concerns of compatibility with precision electroweak measurements, and we show with relevant computations that these corrections can be kept well under control in our general parameter space.

Figures (7)

• Figure 1: One-loop contribution to the Green function $\langle E^a_\alpha d^{\hat{a}}_\mu d^{\hat{b}}_\nu \rangle$ from composite fermions in the representations $r$ and $r^\prime$ of $SO(4)$.
• Figure 2: Mixed rho-fermion contribution to the Green function $\langle E^a_\alpha d^{\hat{a}}_\mu d^{\hat{b}}_\nu \rangle$ which arises at the one-loop level.
• Figure 3: One loop diagrams contributing to the $\langle d_\mu d_\nu \rangle$ (left) and $\langle E_\mu E_\nu \rangle$ (right) self-energies.
• Figure 4: Two-particle contribution to the spectral function of unbroken (upper row) and broken (lower row) currents from the composite fermions. The dashed line denotes the propagator of a NG boson.
• Figure 5: Left plot: shift of the $h\to Z\gamma$ decay amplitude in units of the SM top contribution, $\delta A/A^{top}_{SM}$, in the second model of Section \ref{['sec:models']} as a function of the $LR$ mass splitting. Right plot: total decay rate of $h\to Z\gamma$ normalized to its SM value, $\Gamma/\Gamma_{SM}$, in the same model. The left plot assumes one family of colored fermions ($N_\chi =3$), while the right plot assumes three degenerate families of composites ($N_\chi =9$). The horizontal lines indicate the value obtained by including only the effect of the modified tree-level Higgs couplings.