Determination of -violating HZZ interaction with polarised beams at the ILC

Cheng Li; Gudrid Moortgat-Pick

doi:10.1140/epjc/s10052-025-13828-4

2024 Impact factor 4.8

Particles and Fields

Open Access

Eur. Phys. J. C (2025) 85: 99
https://doi.org/10.1140/epjc/s10052-025-13828-4

Regular Article - Theoretical Physics

Determination of $C P$ $\mathcal{C}\mathcal{P}$ -violating HZZ interaction with polarised beams at the ILC

Cheng Li¹^a and Gudrid Moortgat-Pick²^,3

¹ School of Science, Sun Yat-sen University, Gongchang Road 66, 518107, Shenzhen, China
² II. Institut für Theoretische Physik, Universität Hamburg, Luruper Chaussee 149, 22761, Hamburg, Germany
³ Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607, Hamburg, Germany

^a cheng.li@desy.de, lich389@mail.sysu.edu.cn

Received: 7 November 2024
Accepted: 15 January 2025
Published online: 28 January 2025

Abstract

We study possible $C P$ ${\mathcal{C}\mathcal{P}}$ -violation effects of the 125 GeV Higgs to Z boson coupling at the 250 GeV ILC with transverse and longitudinal beam polarisation via the process $e^{+} e^{-} \to H Z \to H μ^{-} μ^{+}$ $e^+ e^- \rightarrow HZ \rightarrow H \mu ^-\mu ^+$ . We explore the azimuthal angular distribution of the muon pair from the Z boson decay, and constructe $C P$ ${\mathcal{C}\mathcal{P}}$ -odd observables sensitive to $C P$ ${\mathcal{C}\mathcal{P}}$ -violation effects, where we derived this observable both by analytical calculations and by Whizard simulations. Particularly, we can construct two $C P$ ${\mathcal{C}\mathcal{P}}$ -odd observables with the help of transversely-polarised initial beams and improve the statistical significance of $C P$ ${\mathcal{C}\mathcal{P}}$ -violation effects by combining two measurements. We defined the asymmetries between the signal regions with different signs of the $C P$ ${\mathcal{C}\mathcal{P}}$ -odd observables, and determine the $C P$ ${\mathcal{C}\mathcal{P}}$ -violation effect by comparing with the SM 95% CL upper bound. In this paper, we setup a scenario which assumes that the total cross-section is always fixed while $C P$ ${\mathcal{C}\mathcal{P}}$ -violation is varying, and such a scenario helps us to determine the intrinsic $C P$ ${\mathcal{C}\mathcal{P}}$ -mixing angle limit around $| ξ_{C P} | \sim 0.03 rad$ $|\xi _{\mathcal{C}\mathcal{P}}|\sim 0.03~{\operatorname {rad}}$ with (90%, 40%) polarised electron-positron beams and 5 ${ab}^{- 1}$ $\hbox {ab}^{-1}$ integrated luminosity. In addition, we determine the $C P$ ${\mathcal{C}\mathcal{P}}$ -odd coupling limit $| {\tilde{c}}_{HZZ} | \sim 0.01$ $|\widetilde{c}_{HZZ}|\sim 0.01$ as well, where we suppose that the SM tree-level cross-section is fixed and the $C P$ ${\mathcal{C}\mathcal{P}}$ -violation is the varying additional contribution. Comparing with the analysis with unpolarised beams, the sensitivity to the $C P$ ${\mathcal{C}\mathcal{P}}$ -violation effect can be improved by transverse or longitudinal polarisation.

© The Author(s) 2025

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Funded by SCOAP³.

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Determination of CP-violating HZZ interaction with polarised beams at the ILC

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Determination of $C P$ $\mathcal{C}\mathcal{P}$ -violating HZZ interaction with polarised beams at the ILC