Several proposed future lepton colliders are capable of producing trillions of Z-bosons, including FCC-ee, CEPC, LEP3 and LEP-Z. Such Tera-Z factories can discover new elementary particles with couplings to the Z-boson that are orders of magnitude smaller than current bounds. For couplings near the currently excluded parameter regions they could produce sufficiently large samples to study the new particles’ properties in detail, thereby serving as both a discovery and precision machine in one. Using simple analytic estimates, we quantify the dependence of the expected event yield in long-lived particle searches on the number of produced Z-bosons and on the detector dimensions. From this, we derive estimates for both the discovery reach and the measurement precision attainable at such facilities. While the precision of such estimates of course falls short of proper simulations, the analytic approach is suitable for a quick assessment of the sensitivity for a given design. We illustrate this with two examples, heavy neutral leptons and axion-like particles. Under optimistic assumptions, these could be produced in the millions and billions, respectively, effectively turning future lepton colliders into exotics factories.

Belgium

Germany

Marco Drewes

Croatia

Juraj Klarić

Yuan-Zhen Li

New particles at the Z-pole:

Neutral triple gauge couplings (nTGCs) first arise from the dimension-8 operators of the Standard Model effective field theory (SMEFT), rather than the dimension-4 SM Lagrangian and dimension-6 SMEFT operators, opening up a unique window for probing new physics at the dimension-8 level. In this work, we formulate the nTGC form factors of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi><msup><mi>V</mi><mo>*</mo></msup></math> (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>V</mi><mo>=</mo><mi>Z</mi></mrow></math>, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>γ</mi></mrow></math>) that are compatible with the spontaneous breaking of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>SU</mi><mo stretchy="false">(</mo><mn>2</mn><mo stretchy="false">)</mo><mo stretchy="false">⊗</mo><mi mathvariant="normal">U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></mrow></math> electroweak gauge symmetry and consistently match the dimension-8 nTGC operators in the broken phase. We study the sensitivities for probing both the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi><msup><mi>V</mi><mo>*</mo></msup></math> form factors and the corresponding new physics scales through <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi></math> production (with visible/invisible fermionic <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi></math> decays) at high energy <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> colliders including CEPC, FCC-ee, LCF, ILC, and CLIC. In particular, we identify the dimension-8 operator that contributes to the pure triple <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi></math> boson coupling <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi><msup><mi>Z</mi><mo>*</mo></msup></math> alone, but not the mixed <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi><msup><mi>γ</mi><mo>*</mo></msup></math> coupling. We further study the correlations between probes of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi><msup><mi>Z</mi><mo>*</mo></msup></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi><msup><mi>γ</mi><mo>*</mo></msup></math> couplings. Using machine learning, we show that angular distributions of the final-state fermions can play key roles in suppressing the SM backgrounds. The sensitivities can be further improved by using polarized <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>∓</mo></msup></math> beams, and we find that the optimal sensitivity bounds on the nTGC correlations are given by the mixed setting including both the unpolarized operation and the follow-up polarized operation. We demonstrate that machine learning is advantageous for handling the 4-body final states from <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi></math> decays and improves significantly the sensitivity reaches of probes of nTGCs in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> collisions. We find that nTGC new physics scales can be probed up to the multi-TeV scale at the proposed <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> colliders.

John Ellis

Hong-Jian He

Rui-Qing Xiao

Shi-Ping Zeng

Probing neutral triple gauge couplings via <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mi>Z</mi></math> production at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> colliders with machine learning

We carry out an Effective Field Theory (EFT) study of the $$pp \rightarrow \bar{t} t Zh$$ process in the final state. This process can uniquely probe the $$\bar{t} t Zh$$ couplings arising from higher dimensional EFT operators and can also provide bounds on $$\bar{t} t Z$$ coupling deviations. We highlight the importance of the proposed proton-proton Future Circular Collider (FCC-$\textit{hh}$) to study this process and then perform a complete collider analysis by examining the relevant background processes. This allows us to determine the FCC-$\textit{hh}$ sensitivity to probe anomalous $$\bar{t} t Zh$$ couplings.

India

Shankha Banerjee

Rick Gupta

Shilpi Jain

Switzerland

Michelangelo Mangano

Elena Venturini

An EFT study of the $$pp \rightarrow \bar{t} t Z(ll) h(bb)$$ process at the FCC- $$\varvec{hh}$$

We achieve for the first time NNLO <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>QCD</mi><mo>+</mo><mi>NLO</mi></mrow></math> EW accuracy for doubly polarized <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>W</mi><mi>Z</mi></math> inclusive production at the LHC, in the case of fully leptonic decays. Additionally, we provide estimates for missing higher-order uncertainties in QCD associated with doubly polarized differential cross sections and joint polarization fractions, obtained both with standard scale variations and with a theory-nuisance-parameter approach. The study is carried out in the fiducial setup of a recent ATLAS analysis of Run-2 data.

Italy

Giovanni Pelliccioli

Poland

Rene Poncelet

Precise predictions for joint polarization fractions in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>W</mi><mi>Z</mi></math> production at the LHC

We compute two-loop corrections to the associated production of a Higgs boson with a $\textit{Z}$ boson and to off-shell $\textit{Z}$ boson pair production in the gluon fusion channel, mediated by a heavy quark. We perform deep expansions in the high-energy region and around the forward limit and show that their combination covers the whole phase space. Our results constitute the next-to-leading order virtual corrections to these processes. Their numerical evaluation is fast and the dependence on all parameters is maintained, thus a change of parameter values or renormalization scheme is straightforward. As a by-product of our calculation, we also obtain the two-loop heavy quark mediated virtual corrections to the processes of off-shell di-photon and photon-$\textit{Z}$ production.

United Kingdom

Joshua Davies

Dominik Grau

Kay Schönwald

Matthias Steinhauser

Daniel Stremmer

Marco Vitti

Two-loop QCD corrections to $\textit{ZH}$ and off-shell $\textit{Z}$ boson pair production in gluon fusion

We consider the transverse momentum ($\textit{q}$ ) distribution of neutral charged bosons at hadron colliders. We perform the resummation of the logarithmically-enhanced effects due to simultaneous QCD and QED initial-state radiation, up to mixed next-to-next-to-leading logarithmic (NNLL) accuracy. We study the impact of such mixed QCD⊗QED resummed contributions on top of pure QCD corrections, finding percent-level effects.

Spain

Andrea Autieri

Stefano Camarda

Leandro Cieri

Giancarlo Ferrera

German Sborlini

Transverse-momentum resummation at mixed QCD⊗QED NNLL accuracy for $\textit{Z}$ boson production at hadron colliders

The Three-Higgs-Doublet Model (3HDM) extends the Standard Model by introducing two additional scalar doublets, leading to a rich spectrum of new particles: three neutral CP-even Higgs bosons $$(h_1,$$ $$H_2,$$ $$H_3),$$ two neutral CP-odd Higgs bosons $$(A_2,$$ $$A_3),$$ and two charged Higgs bosons $$(H_2^+,$$ $$H_3^+).$$ In this work, we present a phenomenological study of the 3HDM at the future International Linear Collider (ILC) with a center-of-mass energy of $$\sqrt{s} = 1000\,\text {GeV}.$$ Applying a comprehensive set of theoretical and experimental constraints, we identify promising new physics signals with sufficiently large production cross-sections. Our analysis shows that $$e^+e^- \rightarrow H_2 A_2,$$ $$e^+e^- \rightarrow H_2 H_2 Z,$$ $$A_2 A_2 Z,$$ $$H_2 H_2^{\pm } W^{\mp },$$ $$A_2 H_2^{\pm } W^{\mp }$$ and $$H_1 H_2 A_2$$ are among the most sensitive channels to probe this extended Higgs sector. We demonstrate that a future ILC would offer a powerful platform to test these interactions and discover these heavier Higgs bosons thus providing evidence of physics beyond the Standard Model.

Baradhwaj Coleppa

Akshat Khanna

Gokul Krishna

ILC phenomenology of the $$Z_3$$ symmetric type-Z three Higgs doublet model

We identify two distinct repulsive mechanisms in the Cho-Maison monopole-antimonopole pair (MAP) configuration. Our results show that the Higgs-mediated repulsion exhibits a nonmonotonic dependence on both topological charge and Higgs self-coupling, confirming its topological origin while revealing a mass-controlled range transition that deviates from the exponential form of a Yukawa potential. Simultaneously, the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi></math>-boson field generates localized repulsive cores of radius <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>R</mi><mi>c</mi></msub><mo>≈</mo><mn>0.8</mn><msubsup><mi>m</mi><mi>W</mi><mrow><mo>−</mo><mn>1</mn></mrow></msubsup></math>, consistent with the weak interaction scale. The collaborative effect of these mechanisms—operating in different physics regimes—counteracts the magnetic attraction, establishing a stabilization paradigm for the Cho-Maison MAP that extends naturally to other topological solitons in the Standard Model.

China

Dan Zhu

Xurong Chen

Malaysia

Khai-Ming Wong

Topological stabilization via Higgs and Z-boson mediated repulsions in electroweak monopole-antimonopole pairs

The decay <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>A</mi><mo stretchy="false">→</mo><mi>Z</mi><mi>H</mi></math> is a characteristic signal of two-Higgs-doublet models (2HDMs), where <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>A</mi></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>H</mi></math> lie primarily within the same <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>2</mn><msub><mo stretchy="false">)</mo><mi>L</mi></msub></math> multiplet, leading to a coupling of order <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>g</mi><mn>2</mn></msub></math> to the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi></math> boson. The subsequent decay <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>H</mi><mo stretchy="false">→</mo><mi>t</mi><msup><mi>t</mi><mrow><mo stretchy="false">(</mo><mo>*</mo><mo stretchy="false">)</mo></mrow></msup></math> is particularly promising, as it gives rise to distinct final states involving multiple leptons and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi></math>-jets. The required splitting between <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>A</mi></msub></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>H</mi></msub></math> 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 <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>t</mi><mover accent="true"><mi>t</mi><mo stretchy="false">¯</mo></mover><mi>Z</mi></math> measurements of ATLAS and CMS. The obtained limits on <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>σ</mi><mo stretchy="false">(</mo><mi>A</mi><mo stretchy="false">→</mo><mi>Z</mi><mi>H</mi><mo stretchy="false">)</mo><mo>×</mo><mi>Br</mi><mo stretchy="false">(</mo><mi>H</mi><mo stretchy="false">→</mo><mi>t</mi><mover accent="true"><mi>t</mi><mo stretchy="false">¯</mo></mover><mo stretchy="false">)</mo></math> are between 0.12 pb and 0.62 pb. Interestingly, we observe these stringent limits despite a preference (up to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>2.5</mn><mi>σ</mi></math>) for a nonzero new physics signal, most pronounced around for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>A</mi></mrow></msub><mo>≈</mo><mn>450</mn><mi>–</mi><mn>460</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>H</mi></msub><mo>≈</mo><mn>290</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>, with a best-fit value of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>σ</mi><mo stretchy="false">(</mo><mi>A</mi><mo stretchy="false">→</mo><mi>Z</mi><mi>H</mi><mo stretchy="false">)</mo><mo>×</mo><mi>Br</mi><mo stretchy="false">(</mo><mi>H</mi><mo stretchy="false">→</mo><mi>t</mi><mover accent="true"><mi>t</mi><mo stretchy="false">¯</mo></mover><mo stretchy="false">)</mo><mo>≈</mo><mn>0.3</mn><mtext> </mtext><mtext> </mtext><mi>pb</mi></math>. This cross section can be accommodated within a top-philic 2HDM for a top-Yukawa coupling of the second Higgs doublet of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>0.16</mn><mo>≲</mo><msub><mi>μ</mi><mi>t</mi></msub><mo>≲</mo><mn>0.33</mn></math>.

Saiyad Ashanujjaman

Guglielmo Coloretti

Andreas Crivellin

South Africa

Siddharth P. Maharathy

Bruce Mellado

Constraining <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>A</mi><mo stretchy="false">→</mo><mi>Z</mi><mi>H</mi></math> with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>H</mi><mo stretchy="false">→</mo><mi>t</mi><mover accent="true"><mi>t</mi><mo stretchy="false">¯</mo></mover></math> in the low-mass region

We present the phenomenology of the charged Higgs boson $\textit{H}$ appearing in a fermionic dark matter model mediated by an additional scalar doublet. In order to couple the dark matter fermion to the scalar doublet, we introduce a U(1) gauge symmetry, which is spontaneously broken at electroweak symmetry breaking, resulting in a massive $\textit{Z′}$ gauge boson. Since $\textit{Z′}$ is generically light, the model is subject to strong constraints from electroweak precision observables. As a result, the charged Higgs boson mass allowed by current experimental bounds is typically light in this model, 110 GeV $\textit{<}$ $$ {m}_{H^{\pm }} $$ $\textit{<}$ 170 GeV. Such a light charged Higgs boson will be produced mainly through top-quark decays at the LHC. Additionally, depending on the mass of the additional neutral Higgs boson $\textit{h}$ and the dark gauge boson $\textit{Z′}$, the direct production channels $\textit{pp}$ → $\textit{H}$ $\textit{Z′}$ and $\textit{pp}$ → $\textit{H}$ $\textit{h}$ can become sizable. We investigate the corresponding signal processes at the LHC to assess the discovery potential for $\textit{H}$ . Current ATLAS and CMS searches for light charged Higgs bosons already impose further constraints on the model. We also discuss the implications of dark matter in relation to the charged Higgs boson phenomenology.

South Korea

Kyu Bae

Jinn-Ouk Gong

Dong-Won Jung

Kang Lee

Chaehyun Yu

Chan Park

Charged Higgs boson phenomenology in the dark $\textit{Z}$ mediated fermionic dark matter model