Anomalous Higgs couplings in angular asymmetries of H --> Zl+l- and e+e- --> HZ

TL;DR

This paper uses a linear dimension-6 EFT to study how anomalous Higgs couplings modify angular distributions in $H\to Z\ell^+\ell^-$ and the crossing-symmetric process $e^+e^-\to HZ$, deriving the full angular structure in terms of six form factors and nine angular coefficients. It identifies which operators—especially $HZ\gamma$ and $HZ\ell\ell$ contact terms, as well as CP-odd couplings—produce observable CP-even and CP-odd asymmetries, with some asymmetries exhibiting photon-pole or $1/g_V$ enhancements that amplify their sensitivity beyond rate observables. The results show that the $d=6$ $HZ\gamma$ couplings can generate asymmetries at the percent level, while contact $HZ\ell\ell$ effects are generally more constrained and yield smaller signals; CP-odd couplings can also yield percent-level asymmetries and even zeros in certain observables. Crossing symmetry to $e^+e^-\to HZ$ provides complementary sensitivity, particularly at higher $q^2$, making these angular observables a valuable tool for probing BSM Higgs interactions at LHC and future $e^+e^-$ colliders, with SM loop effects estimated to be subdominant but relevant for precision fits.

Abstract

We study in detail the impact of anomalous Higgs couplings in angular asymmetries of the crossing-symmetric processes H --> Zl+l- and e+e- --> HZ. Beyond Standard Model physics is parametrized in terms of the SU(3)xSU(2)_LxU(1)_Y dimension-six effective Lagrangian. In the light of present bounds on d = 6 interactions we study how angular asymmetries can reveal non-standard CP-even and CP-odd couplings. We provide approximate expressions to all observables of interest making transparent their dominant dependence on anomalous couplings. We show that some asymmetries may reveal BSM effects that are hidden in other observables. In particular, CP-even and CP-odd d = 6 HZgamma couplings as well as (to a lesser extent) HZll contact interactions can generate asymmetries at the several percent level, while having small or no effects on the di-lepton invariant mass spectrum of H --> Zl+l-. Finally, the higher di-lepton invariant mass probed in e+e- --> HZ leads to interesting differences in the asymmetries with respect to those of H --> Zl+l- that may lead to complementary anomalous coupling searches at the LHC and e+e- colliders.

Figures (14)

• Figure 1: Feynman diagrams for the decay $H \to Z(\to \ell^+\ell^-) \ell^+\ell^-$.
• Figure 2: (a) $d\Gamma/ds$, (b) $-\mathcal{A}_\phi^{(3)}$, (c) $-\mathcal{A}_{{\rm c}\theta_1,{\rm c}\theta_2}$. Three scenarios are considered. The red solid-line is the SM case. The dotted green line corresponds to $(\widehat{\alpha}^V_{\Phi \ell}, \widehat{\alpha}^A_{\Phi \ell})=(-5,0)\times 10^{-3}$, and the dot-dashed blue line to $(\widehat{\alpha}^V_{\Phi \ell},\widehat{\alpha}^A_{\Phi \ell})=(5,0) \times 10^{-3}$. The shaded bands exclude $\sqrt{q^2} < 12$ GeV, where hadronic resonances dominate.
• Figure 3: (a) $d\Gamma/ds$, (b) $-\mathcal{A}_\phi^{(3)}$, (c) $-\mathcal{A}_{{\rm c}\theta_1,{\rm c}\theta_2}$. The red solid-line is the SM case. The dotted green line corresponds to $(\widehat{\alpha}^V_{\Phi \ell},\widehat{\alpha}^A_{\Phi \ell})=(5,5)\times 10^{-3}$, whereas the dot-dashed blue line to $(\widehat{\alpha}^V_{\Phi \ell}, \widehat{\alpha}^A_{\Phi \ell})=(5,-5)\times 10^{-3}$.
• Figure 4: (a) $d\Gamma/ds$, (b) $-\mathcal{A}_\phi^{(3)}$, (c) $-\mathcal{A}_{{\rm c}\theta_1,{\rm c}\theta_2}$. Three scenarios are considered. The red solid-line is the SM case. The dot-dashed blue line corresponds to $\widehat{\alpha}_{AZ}=-1.3\times 10^{-2}$, whereas the dotted green line corresponds to $\widehat{\alpha}_{AZ}=2.6\times 10^{-2}$.
• Figure 5: $d\Gamma/ds$ including terms of order $\mathcal{O}(1/\Lambda^4)$ in the squared amplitude. The three scenarios of Fig. 4 are considered: the red solid-line is the SM case, the dot-dashed blue line corresponds to $\widehat{\alpha}_{AZ}=-1.3\times 10^{-2}$, whereas the dotted green line corresponds to $\widehat{\alpha}_{AZ}=2.6\times 10^{-2}$.