A golden probe of nonlinear Higgs dynamics

The most salient generic feature of a composite Higgs boson resides in the nonlinearity of its dynamics, which arises from degenerate vacua associated with the pseudo-Nambu–Goldstone (PNGB) nature of the Higgs boson. It has been shown that the nonlinear Higgs dynamics is universal in the IR and controlled only by a single parameter f, the decay constant of the PNGB Higgs. In this work we perform a fit, for the first time, to Wilson coefficients of O(p4) operators in the nonlinear Lagrangian using the golden H → 4L decay channel. By utilizing both the "rate" information in the signal strength and the "shape" information in the fully differential spectra, we provide limits on the Goldstone decay constant f, as well as O(p4) Wilson coefficients, using Run 2 data at the LHC. In rate measurements alone, the golden channel prefers a negative ξ=v2/f2 corresponding to a non-compact coset structure. Including the shape information, we identify regions of parameter space where current LHC constraint on f is still weak, allowing for ξ≲0.5 or ξ≳-0.5. We also comment on future sensitivity at the high-luminosity upgrade of the LHC which could allow for simultaneous fits to multiple Wilson coefficients.