Constraining anomalous HVV interactions at proton and lepton colliders

TL;DR

The paper develops a unified, matrix-element–based framework to constrain anomalous HVV couplings and CP properties of the Higgs across multiple production and decay channels at the LHC and future e+e- colliders. It combines a dedicated Monte Carlo generator with likelihood discriminants to extract the CP-even/odd admixtures, quantified by fractions such as f_a2 and f_a3, from observables in H→VV decays, VBF, and VH production. Across channels and collider scenarios, it shows that CP-odd fractions as small as 10^-4 can be probed, with higher-energy e+e- machines offering enhanced sensitivity in certain regimes, and demonstrates how discriminants and multi-dimensional fits optimize information extraction. The study provides practical guidance on which channels and observables yield the strongest constraints and how to treat potential form-factor effects and backgrounds in a robust, model-independent way.

Abstract

In this paper, we study the extent to which CP parity of a Higgs boson, and more generally its anomalous couplings to gauge bosons, can be measured at the LHC and a future electron-positron collider. We consider several processes, including Higgs boson production in gluon and weak boson fusion and production of a Higgs boson in association with an electroweak gauge boson. We consider decays of a Higgs boson including $ZZ, WW, γγ$, and $Z γ$. Matrix element approach to three production and decay topologies is developed and applied in the analysis. A complete Monte Carlo simulation of the above processes at proton and $e^+e^-$ colliders is performed and verified by comparing it to an analytic calculation. Prospects for measuring various tensor couplings at existing and proposed facilities are compared.

Figures (24)

• Figure 1: Illustrations of $H$ particle production and decay in $pp$ or $e^+e^-$ collision $gg/q\bar{q}\to H\to ZZ\to 4\ell^\pm$ (left), $e^+e^-(q\bar{q})\to Z^*\to ZH\to\ell^+\ell^-b\bar{b}$ (middle), or $e^+e^-(q{q^\prime})\to e^+e^-(q{q^\prime})H \to e^+e^-(q{q^\prime}) b\bar{b}$ (right). The $H\to b\bar{b}$ decay and $HZZ$ coupling are shown as examples, so that $Z$ can be substituted by other vector bosons. Five angles fully characterize the orientation of the production and decay chain and are defined in the suitable rest frames.
• Figure 3: Distributions of the observables in the $e^+e^-\to ZH\to (\ell^+\ell^-)H$ analysis at $\sqrt{s}=250$ GeV, from left to right: $\cos\theta_1$, $\cos\theta_2$, and $\Phi$. Points show simulated events and lines show projections of analytical distributions. Four scenarios are shown: SM scalar ($0^+$, red open circles), pseudoscalar ($0^-$, blue diamonds), and two mixed states corresponding to $f_{a3}=0.5$ with $\phi_{a3}=0$ (green squares) and $\pi/2$ (magenta points). In all cases we choose $f_{a2} = 0$.
• Figure 4: Comparison of transverse momentum $p_T$ distribution of a SM Higgs boson with $m_H=125$ GeV in MC simulation of 14 TeV $pp$ collisions at the LHC. Higgs production in the gluon fusion is generated by JHU generator combined with Pythia parton shower (solid red) and by POWHEG (dashed red) where NLO QCD approximation is matched to parton shower. The decay $H\to ZZ\to 4\ell$ is simulated using the JHU generator in both cases. Also shown in the order of decreasing peak position: $VH$ production (solid green), WBF production (solid blue), and gluon fusion $H+2$ jets production (solid magenta) with the JHU generator. For $VH$ and WBF production, parton shower is included and comparison with NLO QCD POWHEG simulation (dashed distributions) is shown. All distributions are normalized to unit area except for $H+2$ jets, which is normalized with respect to inclusive gluon fusion production according to its relative cross section with selection requirements on jets $p_T>15$ GeV and $\Delta R_{jj}>0.5$ as discussed in text.
• Figure 5: Cross sections for $e^+e^-\to Z^*\to ZX$ process as a function of $\sqrt{s}$ for three models: SM Higgs boson ($0^+$, solid), scalar with higher-dimension operators ($0^+_h$, short-dashed), and pseudoscalar ($0^-$, long-dashed). All cross sections are normalized to SM value at $\sqrt{s}=250$ GeV. Different high-energy behavior of cross sections related to point-like interactions (solid) and higher-dimensional non-renormalizable operators (dashed) is apparent from the right panel.
• Figure 6: Distribution of fitted values of $f_{a3}$, $\phi_{a3}$, and $f_{a2}$ in a large number of generated experiments in $e^+e^-\to ZH$ process at $\sqrt{s}=250$ GeV. Left plot: $f_{a3}$ results from simultaneous fit of $f_{a3}$ and $\phi_{a3}$. Middle and right plots: simultaneous fit of $f_{a3}$ and $\phi_{a3}$ or $f_{a3}$ and $f_{a2}$, with 68% and 95% confidence level contours shown.