Letter
Open Access

Constraining $A \to Z H$ with $H \to t \bar{t}$ in the low-mass region

Saiyad Ashanujjaman ^1,2,*, Guglielmo Coloretti ^3,†, Andreas Crivellin^3,‡, Siddharth P. Maharathy^4,5,§, and Bruce Mellado ^6,4,∥

¹Institut für Theoretische Teilchenphysik, Karlsruhe Institute of Technology, Engesserstraße 7, D-76128 Karlsruhe, Germany
²Institut für Astroteilchenphysik, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
³Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH–8057 Zürich, Switzerland
⁴School of Physics and Institute for Collider Particle Physics, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa
⁵Indian Institute of Science Education and Research Pune, Dr. Homi Bhabha Road, Pune 411008, India
⁶Institute of High Energy Physics, 19B, Yuquan Road, Shijing District, 100049, Beijing, China

^*Contact author: saiyad.ashanujjaman@kit.edu
^†Contact author: guglielmo.coloretti@protonmail.com
^‡Contact author: andreas.crivellin@cern.ch
^§Contact author: siddharth.prasad.maharathy@cern.ch
^∥Contact author: bmellado@mail.cern.ch

PDF

Phys. Rev. D 113, L051702 – Published 10 March, 2026

DOI: https://doi.org/10.1103/9jdj-hg8y

Abstract

The decay $A \to Z H$ is a characteristic signal of two-Higgs-doublet models (2HDMs), where $A$ and $H$ lie primarily within the same $S U (2)_{L}$ multiplet, leading to a coupling of order $g_{2}$ to the $Z$ boson. The subsequent decay $H \to t t^{(*)}$ is particularly promising, as it gives rise to distinct final states involving multiple leptons and $b$ -jets. The required splitting between $m_{A}$ and $m_{H}$ can naturally occur near the electroweak scale while being consistent with perturbative unitarity. Whereas dedicated ATLAS and CMS searches focused on the region with both top-quarks on-shell, we cover lower masses where one top quark is off-shell by recasting Standard Model $t \bar{t} Z$ measurements of ATLAS and CMS. The obtained limits on $σ (A \to Z H) \times Br (H \to t \bar{t})$ are between 0.12 pb and 0.62 pb. Interestingly, we observe these stringent limits despite a preference (up to $2.5 σ$ ) for a nonzero new physics signal, most pronounced around for $m_{A} \approx 450 - 460 GeV$ and $m_{H} \approx 290 GeV$ , with a best-fit value of $σ (A \to Z H) \times Br (H \to t \bar{t}) \approx 0.3 pb$ . This cross section can be accommodated within a top-philic 2HDM for a top-Yukawa coupling of the second Higgs doublet of $0.16 ≲ μ_{t} ≲ 0.33$ .

Physics Subject Headings (PhySH)

Article Text

Supplemental Material

References (65)

P. W. Higgs, Broken symmetries, massless particles and gauge fields, Phys. Lett. 12, 132 (1964).
F. Englert and R. Brout, Broken symmetry and the mass of gauge vector mesons, Phys. Rev. Lett. 13, 321 (1964).
P. W. Higgs, Broken symmetries and the masses of gauge bosons, Phys. Rev. Lett. 13, 508 (1964).
G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble, Global conservation laws and massless particles, Phys. Rev. Lett. 13, 585 (1964).
G. Aad et al. (ATLAS Collaboration), Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC, Phys. Lett. B 716, 1 (2012).
S. Chatrchyan et al. (CMS Collaboration), Observation of a new boson at a mass of 125 GeV with the CMS Experiment at the LHC, Phys. Lett. B 716, 30 (2012).
J. M. Langford (ATLAS Collaboration and CMS Collaboration), Combination of Higgs measurements from ATLAS and CMS: Couplings and $κ - framework$ , Proc. Sci., LHCP2020 (2021) 136.
Combined measurements of Higgs boson production and decay using up to $139 {fb}^{- 1}$ of proton-proton collision data at $\sqrt{s} = 13 TeV$ collected with the ATLAS experiment, (2021).
V. Silveira and A. Zee, Scalar phantoms, Phys. Lett. 161B, 136 (1985).
M. Pietroni, The Electroweak phase transition in a nonminimal supersymmetric model, Nucl. Phys. B402, 27 (1993).
J. McDonald, Gauge singlet scalars as cold dark matter, Phys. Rev. D 50, 3637 (1994).
T. D. Lee, A theory of spontaneous T violation, Phys. Rev. D 8, 1226 (1973).
P. Fayet, Supergauge invariant extension of the Higgs mechanism and a model for the electron and its neutrino, Nucl. Phys. B90, 104 (1975).
P. Fayet, Spontaneously broken supersymmetric theories of weak, electromagnetic and strong interactions, Phys. Lett. 69B, 489 (1977).
H. E. Haber and G. L. Kane, The search for supersymmetry: Probing physics beyond the Standard Model, Phys. Rep. 117, 75 (1985).
J. E. Kim, Light pseudoscalars, particle physics and cosmology, Phys. Rep. 150, 1 (1987).
R. D. Peccei and H. R. Quinn, $C P$ conservation in the presence of instantons, Phys. Rev. Lett. 38, 1440 (1977).
N. Turok and J. Zadrozny, Electroweak baryogenesis in the two doublet model, Nucl. Phys. B358, 471 (1991).
D. A. Ross and M. J. G. Veltman, Neutral currents and the Higgs mechanism, Nucl. Phys. B95, 135 (1975).
W. Konetschny and W. Kummer, Nonconservation of total lepton number with scalar bosons, Phys. Lett. 70B, 433 (1977).
T. P. Cheng and L.-F. Li, Neutrino masses, mixings and oscillations in $SU (2) \times U (1)$ models of electroweak interactions, Phys. Rev. D 22, 2860 (1980).
G. Lazarides, Q. Shafi, and C. Wetterich, Proton lifetime and fermion masses in an SO(10) model, Nucl. Phys. B181, 287 (1981).
J. Schechter and J. W. F. Valle, Neutrino masses in $SU (2) \times U (1)$ theories, Phys. Rev. D 22, 2227 (1980).
M. Magg and C. Wetterich, Neutrino mass problem and gauge hierarchy, Phys. Lett. 94B, 61 (1980).
R. N. Mohapatra and G. Senjanovic, Neutrino masses and mixings in gauge models with spontaneous parity violation, Phys. Rev. D 23, 165 (1981).
J. F. Gunion, R. Vega, and J. Wudka, Higgs triplets in the standard model, Phys. Rev. D 42, 1673 (1990).
T. G. Rizzo, Updated bounds on Higgs triplet vacuum expectation values and the tree level rho parameter from radiative corrections, Mod. Phys. Lett. A 6, 1961 (1991).
P. Chardonnet, P. Fayet, and P. Salati, Heavy triplet neutrinos as a new dark matter option, Nucl. Phys. B394, 35 (1993).
T. Blank and W. Hollik, Precision observables in $SU (2) \times U (1)$ models with an additional Higgs triplet, Nucl. Phys. B514, 113 (1998).
G. Aad et al. (ATLAS Collaboration), ATLAS searches for additional scalars and exotic Higgs boson decays with the LHC Run 2 dataset, Phys. Rep. 1116, 184 (2025).
A. Hayrapetyan et al. (CMS Collaboration), Searches for Higgs boson production through decays of heavy resonances, Phys. Rep. 1115, 368 (2025).
G. C. Dorsch, S. J. Huber, K. Mimasu, and J. M. No, Echoes of the electroweak phase transition: Discovering a second Higgs doublet through $A_{0} \to Z H_{0}$ , Phys. Rev. Lett. 113, 211802 (2014).
D. Gonçalves, A. Kaladharan, and Y. Wu, Resonant top pair searches at the LHC: A window to the electroweak phase transition, Phys. Rev. D 107, 075040 (2023).
T. Biekötter, S. Heinemeyer, J. M. No, K. Radchenko, M. O. O. Romacho, and G. Weiglein, First shot of the smoking gun: Probing the electroweak phase transition in the 2HDM with novel searches for $A \to ZH$ in $ℓ^{+} ℓ^{-} t \bar{t}$ and $ν ν b \bar{b}$ final states, J. High Energy Phys. 01 (2023) 107.
P. Basler, M. Krause, M. Muhlleitner, J. Wittbrodt, and A. Wlotzka, Strong first order electroweak phase transition in the $C P$ -conserving 2HDM revisited, J. High Energy Phys. 02 (2016) 121.
G. C. Dorsch, S. J. Huber, K. Mimasu, and J. M. No, The Higgs vacuum uplifted: Revisiting the electroweak phase transition with a second Higgs doublet, J. High Energy Phys. 12 (2017) 086.
F. Arco, T. Biekötter, P. Stylianou, and G. Weiglein, Top-quark spin correlations as a tool to distinguish pseudoscalar $A \to ZH$ and scalar $H \to ZA$ signatures in $Z t \bar{t}$ final states at the LHC, J. High Energy Phys. 06 (2025) 170.
G. Aad et al. (ATLAS Collaboration), Search for a $C P$ -odd Higgs boson decaying into a heavy $C P$ -even Higgs boson and a Z boson in the $ℓ^{+} ℓ^{-} t \bar{t}$ and $ν \bar{ν} b \bar{b}$ final states using $140 {fb}^{- 1}$ of data collected with the ATLAS detector, J. High Energy Phys. 02 (2023) 197.
A. Hayrapetyan et al. (CMS Collaboration), Search for heavy neutral Higgs bosons A and H in the $t \bar{t} Z$ channel in proton-proton collisions at 13 TeV, Phys. Lett. B 866, 139568 (2025).
G. Aad et al. (ATLAS Collaboration), Inclusive and differential cross-section measurements of $t \bar{t} Z$ production in pp collisions at $\sqrt{s} = 13 TeV$ with the ATLAS detector, including EFT and spin-correlation interpretations, J. High Energy Phys. 07 (2023) 163.
A. Hayrapetyan et al. (CMS Collaboration), Measurements of inclusive and differential cross sections for top quark production in association with a Z boson in proton-proton collisions at $\sqrt{s} = 13 TeV$ , J. High Energy Phys. 02 (2024) 177.
S. Ashanujjaman, A. Crivellin, S. P. Maharathy, and B. Mellado, Searching for a charged Higgs boson in top-quark decays via the $W Z$ Mode, arXiv:2509.07094.
J. Alwall, R. Frederix, S. Frixione, V. Hirschi, F. Maltoni, O. Mattelaer, H. S. Shao, T. Stelzer, P. Torrielli, and M. Zaro, The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations, J. High Energy Phys. 07 (2014) 079.
R. Frederix, S. Frixione, V. Hirschi, D. Pagani, H. S. Shao, and M. Zaro, The automation of next-to-leading order electroweak calculations, J. High Energy Phys. 07 (2018) 185; 11 (2021) 85.
R. D. Ball et al. (NNPDF Collaboration), Parton distributions from high-precision collider data, Eur. Phys. J. C 77, 663 (2017).
S. Frixione, B. Fuks, V. Hirschi, K. Mawatari, H.-S. Shao, P. A. Sunder, and M. Zaro, Automated simulations beyond the Standard Model: supersymmetry, J. High Energy Phys. 12 (2019) 008.
T. Sjöstrand, S. Ask, J. R. Christiansen, R. Corke, N. Desai, P. Ilten, S. Mrenna, S. Prestel, C. O. Rasmussen, and P. Z. Skands, An introduction to PYTHIA 8.2, Comput. Phys. Commun. 191, 159 (2015).
V. Khachatryan et al. (CMS Collaboration), Event generator tunes obtained from underlying event and multiparton scattering measurements, Eur. Phys. J. C 76, 155 (2016).
M. Cacciari, G. P. Salam, and G. Soyez, The anti- $k_{t}$ jet clustering algorithm, J. High Energy Phys. 04 (2008) 063.
M. Cacciari, G. P. Salam, and G. Soyez, FastJet user manual, Eur. Phys. J. C 72, 1896 (2012).
See Supplemental Material at http://link.aps.org/supplemental/10.1103/9jdj-hg8y for additional details.
S. Banik, G. Coloretti, A. Crivellin, and B. Mellado, Uncovering new Higgses in the LHC analyses of differential $t \bar{t}$ cross sections, J. High Energy Phys. 01 (2023) 155.
M. Czakon and A. Mitov, Top++: A program for the calculation of the top-pair cross-section at Hadron Colliders, Comput. Phys. Commun. 185, 2930 (2014).
G. C. Branco, P. M. Ferreira, L. Lavoura, M. N. Rebelo, M. Sher, and J. P. Silva, Theory and phenomenology of two-Higgs-doublet models, Phys. Rep. 516, 1 (2012).
S. Davidson and H. E. Haber, Basis-independent methods for the two-Higgs-doublet model, Phys. Rev. D 72, 035004 (2005); 72, 099902(E) (2005).
D. Atwood, L. Reina, and A. Soni, Phenomenology of two Higgs doublet models with flavor changing neutral currents, Phys. Rev. D 55, 3156 (1997).
A. Crivellin, A. Kokulu, and C. Greub, Flavor-phenomenology of two-Higgs-doublet models with generic Yukawa structure, Phys. Rev. D 87, 094031 (2013).
Y. Heo, D.-W. Jung, and J. S. Lee, Higgs boson precision analysis of the full LHC run 1 and run 2 data, Phys. Rev. D 110, 013003 (2024).
S. Navas et al. (Particle Data Group), Review of particle physics, Phys. Rev. D 110, 030001 (2024).
M. Misiak and M. Steinhauser, Weak radiative decays of the B meson and bounds on $M_{H^{\pm}}$ in the Two-Higgs-Doublet model, Eur. Phys. J. C 77, 201 (2017).
G. Aad et al. (ATLAS Collaboration), Search for heavy neutral Higgs bosons decaying into a top quark pair in $140 {fb}^{- - 1}$ of proton-proton collision data at $\sqrt{s} = 13 TeV$ with the ATLAS detector, J. High Energy Phys. 08 (2024) 013.
A. Hayrapetyan et al. (CMS Collaboration), Search for heavy pseudoscalar and scalar bosons decaying to a top quark pair in proton-proton collisions at $\sqrt{s} = 13 TeV$ , Rep. Prog. Phys. 88, 127801 (2025).
A. M. Sirunyan et al. (CMS Collaboration), Search for a charged Higgs boson decaying into top and bottom quarks in events with electrons or muons in proton-proton collisions at $\sqrt{s} = 13 TeV$ , J. High Energy Phys. 01 (2019) 096.
G. Aad et al. (ATLAS Collaboration), Search for charged Higgs bosons decaying into a top quark and a bottom quark at $\sqrt{s} = 13 TeV$ with the ATLAS detector, J. High Energy Phys. 06 (2021) 145.
G. Aad et al. (ATLAS Collaboration), Search for a heavy Higgs boson decaying into a Z boson and another heavy Higgs boson in the $ℓ ℓ b b$ and $ℓ ℓ W W$ final states in $p p$ collisions at $\sqrt{s} = 13 TeV$ with the ATLAS detector, Eur. Phys. J. C 81, 396 (2021).

Phys. Rev. D 113, L051702– Published 10 March, 2026

Vol. 113, Iss. 5 — 1 March 2026

Received 30 November 2025
Accepted 20 February 2026

DOI: https://doi.org/10.1103/9jdj-hg8y

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society

Physical Review D

Reuse & Permissions

Export citation

Export citation

Constraining $A \to Z H$ with $H \to t \bar{t}$ in the low-mass region

Abstract

Physics Subject Headings (PhySH)

Article Text

Supplemental Material

References (65)

Outline

Information

Filter

Filter

Article Lookup

Enter a citation

Constraining A→ZH with H→tt¯ in the low-mass region

Abstract

Physics Subject Headings (PhySH)

Article Text

Supplemental Material

References (65)

Constraining $A \to Z H$ with $H \to t \bar{t}$ in the low-mass region