Searching for New Physics in the Three-Body Decays of the Higgs-like Particle

TL;DR

The paper investigates whether new physics can imprint on Higgs three-body decays even when inclusive rates are SM-like. It adopts an effective field theory description with a limited set of higher-dimension operators that couple the Higgs to Z and leptons, considering both linear and nonlinear realizations of electroweak symmetry and mapping to effective couplings such as c_L, c_R, and c_{Zγ}. The key contribution is the analytic treatment of the doubly differential decay rate for h -> Zℓℓ and the demonstration that NP contributions can noticeably alter differential spectra, with interference shaping the observable signal strength and the largest effects arising at low dilepton invariant mass for Zγ-type operators. This provides a practical path to probe NP via differential observables at future Higgs factories or precision experiments, with semi-realistic collider studies indicating potential observability given sufficient resolution and statistics.

Abstract

We show that the three-body decays of the resonance recently discovered at the LHC are potentially sensitive to effects of new physics. Even if the fully integrated partial decay widths are consistent with the minimal Standard Model there is information that is lost upon integration, which can be uncovered in the differential decay widths. Concentrating on the decay $h \to Z \ell \bar{\ell}$, we identify the regions in the three-body phase space in which these effects become especially pronounced and could be detected in future experiments.

Figures (5)

• Figure 1: Three-body decay $h\to Z f\bar{f}$; the main focus of the present work is for the case that the final fermion pair is comprised of charged leptons, $f\bar{f} = \ell\bar{\ell}$.
• Figure 2: Contributions to $h \to Z \ell \bar{\ell}$ from $\mathcal{O}_{ZJ}$. The differential decay rate and differential signal strength as a function of $m_{23}^2$ are shown on the left and right respectively. The curves correspond to the SM (blue); $c_R = 0.99,\, c_L = 0$ (red); $c_L = -1.15,\, c_R = 0$ (yellow); and $c_R = -c_L = 1.07$ (green). $\mu = 1$ in each of these cases.
• Figure 3: Contributions to $pp \to h \to 4\ell$ at LHC8 from $O_{ZJ}$. The differential production rate and differential signal strength as a function of $m_{23}^2$ are shown on the left and right respectively. The curves correspond to the SM (blue); $c_R = 0.92,\, c_L = 0$ (red); $c_L = -1.09,\, c_R = 0$ (yellow); and $c_R = -c_L = 0.92$ (green). $\mu = 1$ within statistical uncertainty in each of these cases.
• Figure 4: Differential rates for $h \rightarrow Z \ell \bar{\ell}$ in the SM. Shown is red is the LO process, which has a tree-level $Z Z^{\star}$ intermediate state. The blue curve includes the dominant NLO corrections, which come from the loop-induced $Z \gamma^{\star}$ intermediate state.
• Figure 5: The differential rates and signal strengths in the SM, supplemented by a NP contribution to the operator $\mathcal{O}_{Z\gamma}$. The curves correspond to: SM including $c_{Z\gamma,\,SM}$ (blue); $c_{Z\gamma,\,NP} = 20$ (red); $c_{Z\gamma,\,NP} = 10$ (yellow); $c_{Z\gamma,\,NP} = -10$ (green). In order to emphasize the enhancement of NP effects for the lower values of $m_{23}$, we restrict $m_{23}$ to be less than 12 GeV in the plots on the left, while $m_{23} > 12$ GeV is considered for the plots on the right.