Study of the spin and parity of the Higgs boson in diboson decays with the ATLAS detector

TL;DR

This ATLAS study tests the Higgs boson's spin, parity, and tensor couplings in diboson decays using 7 and 8 TeV LHC data across $H\rightarrow ZZ^*\to 4\ell$, $H\rightarrow WW^*\to e\nu\mu\nu$, and $H\rightarrow \gamma\gamma$. Utilizing a Higgs-boson characterisation EFT framework, the analysis examines fixed-spin hypotheses (0^+, 0^-, 0^+_h, 2^+) and CP-mixing, employing matrix-element-based (MELA) discriminants, BDTs, and ME reweighting, with a rigorous likelihood treatment and pseudo-experiments. The results show all tested non-SM spin hypotheses excluded at >99.9% CL when channels are combined, and the tensor structure analyses place tight constraints on CP-odd/even HVV couplings, consistent with the SM 0^+ Higgs. This work strengthens the SM interpretation of the Higgs boson and constrains possible Beyond-Standard-Model tensor interactions in HVV couplings. It also demonstrates the power of multi-channel diboson analyses and ME-based techniques in precision Higgs property measurements.

Abstract

Studies of the spin, parity and tensor couplings of the Higgs boson in the $H\rightarrow ZZ^*\rightarrow 4\ell$, $H\rightarrow WW^* \rightarrow eνμν$ and $H\rightarrow γγ$ decay processes at the LHC are presented. The investigations are based on $25$ fb$^{-1}$ of $pp$ collision data collected by the ATLAS experiment at $\sqrt{ s} = 7$ TeV and $\sqrt{ s }= 8$ TeV. The Standard Model (SM) Higgs boson hypothesis, corresponding to the quantum numbers $J ^P=0^+$, is tested against several alternative spin scenarios, including non-SM spin-0 and spin-2 models with universal and non-universal couplings to fermions and vector bosons. All tested alternative models are excluded in favour of the SM Higgs boson hypothesis at more than 99.9% confidence level. Using the $H\rightarrow ZZ^*\rightarrow 4\ell$ and $H\rightarrow WW^* \rightarrow eνμν$ decays, the tensor structure of the $HVV$ interaction in the spin-0 hypothesis is also investigated. The observed distributions of variables sensitive to the non-SM tensor couplings are compatible with the SM predictions and constraints on the non-SM couplings are derived.

Figures (11)

• Figure 1: Expected distributions of kinematic variables sensitive to the spin of the resonance considered in the analysis, (a) transverse momentum of the $\gamma \gamma$ system and (b) the production angle of the two photons in the Collins-Soper frame , for a SM Higgs boson and for spin-2 particles with three different choices of the QCD couplings.
• Figure 2: Observed signal fraction per category for the analysis, and comparison to expected values for a SM Higgs boson and for a spin-2 particle with different choices of QCD couplings. (a) the 11 categories described in the text are displayed, corresponding to the $\pTgg<300~\GeV$ selection; (b) the high- category is discarded and the signal fractions are renormalised over the 10 remaining categories, corresponding to the $\pTgg<125~\GeV$ selection.
• Figure 3: Definitions of the angular observables sensitive to the spin and parity of the resonance in the $X \to ZZ^{*}\to 4\ell$ decay.
• Figure 4: Distributions of some of the final-state observables sensitive to the spin and parity of the resonance in the $H\to ZZ^{*}\to 4\ell$ signal region $115~\GeV<m_{4\ell}< 130~\GeV$ for data (points with errors), backgrounds (filled histograms) and predictions for two spin hypotheses (SM solid line and alternatives dashed lines). (a), (b) and (c): invariant masses $m_{12}$ , $m_{34}$ and decay $\cos \theta _1$, respectively; (d), (e) and (f): $\Phi$, $\cos \theta ^*$ and $\Phi _1$, respectively.
• Figure 5: The distributions of the discriminant BDT$_{ZZ}$ versus the $J^P$--MELA discriminant for the SM $J^P=0^+$ Higgs boson and for the backgrounds in the signal region $115~\GeV\ <m_{4\ell} <130~\GeV$.