Identifying the Higgs Spin and Parity in Decays to Z Pairs

TL;DR

This paper develops a model‑independent approach to identifying the spin and parity of the Higgs boson from H→ZZ→4f decays by exploiting angular correlations of the Z decay products and, crucially, production mechanisms gg→H and γγ→H that carry nontrivial spin information. It derives and analyzes the angular distributions and helicity amplitudes for both above‑threshold (two real Z bosons) and below‑threshold (Z*Z) regimes, establishing criteria to exclude all odd‑J^P states and many even higher spins through threshold behavior and final‑state correlations. The work highlights how specific production channels and CP/normality constraints (including Yang’s theorem) can close remaining ambiguities, and it provides a practical framework for LHC and future linear‑collider analyses. Overall, the method offers a comprehensive, testable set of signatures to determine Higgs spin and parity in ZZ decays across mass ranges, with direct implications for experimental searches and interpretation.

Abstract

Higgs decays to Z boson pairs may be exploited to determine spin and parity of the Higgs boson, a method complementary to spin-parity measurements in Higgs-strahlung. For a Higgs mass above the on-shell ZZ decay threshold, a model-independent analysis can be performed, but only by making use of additional angular correlation effects in gluon-gluon fusion at the LHC and gamma-gamma fusion at linear colliders. In the intermediate mass range, in which the Higgs boson decays into pairs of real and virtual Z bosons, threshold effects and angular correlations, parallel to Higgs-strahlung, may be adopted to determine spin and parity, though high event rates will be required for the analysis in practice.

Figures (3)

• Figure 1: The definition of the polar angles ${\theta_i}$ ($i=1,2$) and the azimuthal angle $\varphi$ for the sequential decay $H \rightarrow Z^{(*)} Z \rightarrow (f_1\bar{f}_1) (f_2\bar{f}_2)$ in the rest frame of the Higgs particle.
• Figure 2: The azimuthal distributions, $d\Gamma/d\varphi$, for the Standard Model Higgs boson and a pseudoscalar boson, with a Higgs mass of $280$ GeV. The histogram for the Standard Model shows the expected result from $900$ signal events corresponding to an integrated luminosity of $\int {\cal L}\, dt = 300\, {\rm fb}^{-1}$ at LHC [with efficiencies and cuts included according to the experimental simulation Ref.Hohl]. The curves show the exact theoretical dependences for the scalar and pseudoscalar, appropriately normalised.
• Figure 3: The threshold behaviour of the differential distribution $d\Gamma/dM_*$ for the Standard Model and two possible examples of spin-1 [$b_1=1/M_H$, $b_2=1/M_H^3$, $b_3=1/M_H$ and $b_4=1/M_H$] and spin-2 [$c_1=0$, $c_2=1/M_H^2$, $c_3=1/M_H^2$, $c_4=1/M_H^2$ and $c_5=1/M_H^4$] even normality bosons, with a Higgs boson mass of 150 GeV. The histogram for the Standard Model shows the expected result from $203$ signal events corresponding to an integrated luminosity of $\int {\cal L}\, dt = 300\, {\rm fb}^{-1}$ at LHC [with efficiencies and cuts included according to the experimental simulation Ref.Hohl]. The curves show the exact theoretical dependences for such scenarios, appropriately normalised.