arXiv subjects

Huayang Song

Publications and source records attributed to Huayang Song.

At least 19 recordsLinked to original sources

arXiv2026

A HEFT Perspective on the Type-II Seesaw Model and the Complete Basis of Lepton-Number-Violating Operators

We perform the complete tree-level matching of the type-II seesaw model onto the Higgs effective field theory (HEFT) through $\mathcal{O}(p^4)$ in the chiral expansion. Employing a nonlinear field representation and the primary-HEFT power-counting scheme, we retain the dependence on the independent heavy-scalar masses, the neutral-scalar mixing angle, and the triplet vacuum expectation value without introducing additional expansions in these parameters. To systematically describe lepton-number violation, we construct a complete and nonredundant basis of LNV HEFT operators through $\mathcal{O}(p^4)$, including their full flavor multiplicities, using a complex dressed spurion that encodes the $B-L$ charge and custodial orientation of the LNV insertion. The basis is independently validated by Hilbert-series counting. We then compare our matching results with those for the corresponding real-triplet extension and with a previously proposed broken-phase effective field theory. These comparisons identify the effects of the additional CP-odd and doubly charged scalar states and show that the overlapping broken-phase results are recovered after the appropriate parameter expansion, while HEFT retains the unexpanded nonlinear electroweak structure. We further discuss representative implications for Higgs and electroweak precision observables, vector-boson scattering, multi-Higgs production, anomalous gauge couplings, top-quark processes, neutrinoless double-beta decay, charged-lepton flavor violation, and same-sign dilepton production.

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arXiv2026

Establishing the Primary HEFT as a Precision Benchmark for UV-HEFT Matching

We match the real Higgs triplet model (RHTM) onto HEFT under different parameter choices and power-counting schemes, thereby obtaining several representative HEFT formulations and clarifying their relations. We establish the primary HEFT (pHEFT) as a benchmark framework, demonstrating that alternative HEFT constructions can be systematically derived from it. A key advantage of the pHEFT construction is its parameter choice, which maintains linear relations between the UV Lagrangian parameters and squared heavy masses. By strictly employing the inverse squared heavy masses as the expansion parameters without imposing additional constraints, pHEFT preserves maximal ultraviolet (UV) information and ensures higher perturbative accuracy by avoiding the additional truncations inherent in more complex, non-linear formulations or extra constraints. Through the analysis of the $Z_2$-symmetric real singlet model and the 2HDM, we illustrate the criteria for identifying viable primary HEFT constructions in UV models with scalar extensions. Furthermore, for the first time, we derive the HEFT operators of the RHTM involving fermions.

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arXiv2025

Complete Light Long-Lived Particles searches in Type-I 2HDM

Recently, the study of long-lived particles (LLPs) has attracted increasing attention. In this work, we analyze the full parameter space of the Type-I Two-Higgs-Doublet Model (2HDM) that allows for light long-lived scalar ($H$) and pseudoscalar ($A$) particles. When involving a light beyongd Standard Model (BSM) Higgs, the $\Delta S$ could be the main contribution during the global fit of the oblique parameters, which is different to $\Delta T$ being the main factor for heavy BSM Higgs cases. By imposing theoretical constraints such as vacuum stability and perturbative unitarity, together with current experimental bounds, we summarize a complete region for a potential light $H$ with $\cos(\beta - \alpha) \simeq \frac{1}{\tan \beta}$, light $A$ with $\cos(\beta - \alpha) \simeq \frac{1}{\tan\beta} \frac{2m_H^2 - m_h^2}{m_H^2 - m_h^2}$, and point out the invisible Higgs decay is the most important constraint. We further identify viable regions for LLPs and propose four benchmark regions that simultaneously accommodate a light long-lived particle and explain the $W$ boson mass anomaly. For these benchmarks, we present the reaches of FASER and FASER~2, where FASER~2 improves the sensitivity by approximately two orders of magnitude compared to FASER.

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arXiv2025

Matching the real Higgs triplet extension of Standard Model to HEFT

We make a first matching of the real Higgs triplet extension (RHTE) of the standard model to the Higgs effective field theory (HEFT), which is also known as electroweak chiral Lagrangian (EWChL). In the RHTE, the ratio $\xi$ of two VEVs separately from triplet and doublet is a tiny quantity, due to the constrained custodial symmetry violation. Based on the nonlinear representation~\cite{Song:2024kos} of RHTE and functional method, we get the HEFT in the power counting of $\xi$. By comparing its phenomenological results of $hh\to hh$, $W W \to h h$, $Z Z \to hh $ to SMEFT's, we show its convergence property is as good as SMEFT's. Besides we explicitly show that the HEFT is available in the phase transition region where SMEFT fails.

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arXiv2024

Flavor Physics at the CEPC: a General Perspective

We discuss the landscape of flavor physics at the Circular Electron-Positron Collider (CEPC), based on the nominal luminosity outlined in its Technical Design Report. The CEPC is designed to operate in multiple modes to address a variety of tasks. At the $Z$ pole, the expected production of 4 Tera $Z$ bosons will provide unique and highly precise measurements of $Z$ boson couplings, while the substantial number of boosted heavy-flavored quarks and leptons produced in clean $Z$ decays will facilitate investigations into their flavor physics with unprecedented precision. We investigate the prospects of measuring various physics benchmarks and discuss their implications for particle theories and phenomenological models. Our studies indicate that, with its highlighted advantages and anticipated excellent detector performance, the CEPC can explore beauty and $\tau$ physics in ways that are superior to or complementary with the Belle II and Large-Hadron-Collider-beauty experiments, potentially enabling the detection of new physics at energy scales of 10 TeV and above. This potential also extends to the observation of yet-to-be-discovered rare and exotic processes, as well as testing fundamental principles such as lepton flavor universality, lepton and baryon number conservation, etc., making the CEPC a vibrant platform for flavor physics research. The $WW$ threshold scan, Higgs-factory operation and top-pair productions of the CEPC further enhance its merits in this regard, especially for measuring the Cabibbo-Kobayashi-Maskawa matrix elements, and Flavor-Changing-Neutral-Current physics of Higgs boson and top quarks. We outline the requirements for detector performance and considerations for future development to achieve the anticipated scientific goals.

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arXiv2024

Charged Higgs Search in 2HDM

In this paper, we present a comprehensive study of the collider search limits on the charged Higgses in the four types of Two Higgs Double Models (2HDM). In addition to constraints from flavor physics measurements, we include both the LEP charged Higgs search channels, as well as the LHC search results on the light and heavy charged Higgses. We consider both the conventional charged Higgs search channels of $H^\pm \rightarrow \tau\nu, cs, cb, tb$, and the latest search results on the exotic decay channels $H^\pm \rightarrow A W^\pm / H W^\pm$. We find that $H^\pm \rightarrow A W^\pm / H W^\pm$ are complementary to the conventional fermionic channels for $m_{H^\pm} < m_t$. For heavy $H^\pm$, $H^\pm \rightarrow A W^\pm / H W^\pm$ extend the reach of $\tan \beta$ beyond that of $H^\pm \to tb$ in the Type-L 2HDM. We also present the combined reach of all the neutral and charged Higgs searches.

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arXiv2024

A Non-linear Representation of General Scalar Extensions of the Standard Model for HEFT Matching

We introduce a non-linear representation of ultraviolet~(UV) complete model, $U$ representation, under which matching HEFT to general scalar extensions of the standard model is straightforward. The main idea is to express a scalar multiplet in its linear form rotated by $U$ matrices, where $U$ matrix is exponential form of Goldstones based on Pauli matrices and meanwhile is a special $SU(2)_L$ rotation. All together the doublet is expressed in $U$ matrix multiplying a doublet column or bidoublet matrix, which is composed of physical Higgs. We show a complete matching between HEFT and real triplet extension. Meanwhile, under tensor notation, we give $U$ representation for general scalar extensions.

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arXiv2023

Complete EFT Operator Bases for Dark Matter and Weakly-Interacting Light Particle

The standard model can be extended to include weakly-interacting light particle (WILP): real or complex singlet scalar, Majorana or Dirac neutral fermion, neutral or hidden-charged vector boson, etc. Imposing the $Z_2$ symmetry, these particles can be lifted as the weakly-interacting massive particle (WIMP), the candidate of dark matter. Instead, imposing the shift symmetry on the scalar components gives rise to the axion-like particle, dark photon, etc. Utilizing these light degree of freedom along with the standard model particles and imposing different symmetries, we construct the complete and independent sets of effective operators up to dimension eight with the Young tensor technique, consistent with counting from the Hilbert series.

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arXiv2023

Effective Field Theories of Axion, ALP and Dark Photon

With the help of Young tensor technique, we enumerate the complete and independent set of effective operators up to $dim$-8 for the extension of the standard model with a Goldsonte boson by further imposing the Adler's zero condition in the soft momentum limit. Such basis can be reduced to describe the axion or majoron effective Lagrangian if further (symmetry) constraints are imposed. Then reformulating dark photon as combination of Goldstone boson and transverse gauge boson, the effective operators of the Goldstone boson can be extended to effective chiral Lagrangian description of the dark photon. For the first time we obtain 0 (0), 6 (44), 1 (1), 44 (356), 32 (520) operators in Goldstone effective field theory, and 9 (49), 0 (0), 108 (676), 10 (426), 1904 (40783) operators in dark photon effective field theory at the dimension 4, 5, 6, 7, 8 for one (three) generation of fermions.

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arXiv2022

Light Scalars at FASER

FASER, the ForwArd Search ExpeRiment, is a currently operating experiment at the Large Hadron Collider (LHC) that can detect light long-lived particles produced in the forward region of the LHC interacting point. In this paper, we study the prospect of detecting light CP-even and CP-odd scalars at FASER and FASER 2. Considering a model-independent framework describing the most general interactions between a CP-even or CP-odd scalar and SM particles using the notation of coupling modifiers in the effective Lagrangian, we develop the general formalism for the scalar production and decay. We then analyze the FASER and FASER 2 reaches of light scalars in the large $\tanβ$ region of the Type-I two Higgs double model as a case study, in which light scalars with relatively long lifetime could be accommodated. In the two benchmark scenarios we considered, the light (pseudo)scalar decay length varies in $(10^{-8}, 10^5)$ meters. Both FASER and FASER 2 can probe a large part of the parameter space in the large $\tanβ$ region up to $10^5$, extending beyond the constraints of the other existing experiments.

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arXiv2022

Custodial Symmetry Violation in Scalar Extensions of the Standard Model

The new measurement of the W boson mass from the CDF collaboration shows a significant tension with the Standard Model prediction, which evidences violation of custodial symmetry in the scalar sector. We study the scalar extensions of the Standard Model, which can be categorized into two classes, scalar sector with custodial symmetry (Georgi-Machacek model and its generalizations) and scalar sector without custodial symmetry, and explore how these extensions fit to the electroweak precision data and the CDF new $m_W$ . The favored oblique parameters are coming from either the large mass splitting in the multiplet via the loop contribution or the large vacuum expectation value which breaks custodial symmetry at the tree level. In particular, we find that $\mathcal{O}(100)$ GeV new particles are allowed in the scalar extension scenarios.

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arXiv2022

MSSM Under Higgs Factories

The high precision measurements of the Higgs mass and couplings at the future Higgs factories are sensitive to the parameter space of the Minimal Supersymmetric Standard Model (MSSM). Focused on the dominant stop sector contributions, we study the implication of the Higgs precision measurements on MSSM using multi-variable $χ^2$ fit. The results show nice complementarity between the indirect searches at Higgs factories and the direct searches at the current LHC program.

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arXiv2022

Exotic Higgs Decays in the Type-II 2HDMs at Current and Future pp Colliders

The exotic decay modes of non-Standard Model Higgses can serve as powerful search channels to explore the parameter space of extended Higgs sectors. In this Snowmass contribution, we illustrate this using the Two-Higgs Doublet Model (2HDM) as an example. We first review the current experimental constraints on the parameter space of a Type-II 2HDM arising from existing searches for the exotic Higgs decay mode $A/H\rightarrow HZ/AZ$. We then present the sensitivity of future colliders to discover addition Higgs bosons using the exotic decay channels $A\rightarrow HZ$, $A\rightarrow H^\pm W^\mp$ and $H^\pm\rightarrow H W^\pm$. We find that a 100 TeV collider can probe almost the entire region of the Type-II 2HDM parameter space that survives current theoretical and experimental constraints and would therefore be an ideal machine to search for heavier Higgses in hierarchical scalar sectors.

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arXiv2022

Electroweak Phase Transition in 2HDM under Higgs, Z-pole, and W precision measurements

In this work we revisit the existence of a strong first order electroweak phase transition (SFOEWPT) and recent $m_W$ precision measurement in the Type-I and Type-II 2HDMs. The O(100) GeV new scalars in 2HDMs are favored by SFOEWPT, which is necessary for electroweak baryogenesis, and observed $m_W$ shift as well. % We find that under current constraints, both Type-I and Type-II 2HDM can explain the SFOEWPT, Z-pole, Higgs precision measurements and $m_W$ precision measurement of CDF-II at same time, and all these precision measurements are sensitive to heavy Higgs mass splitting in 2HDM. The allowed regions are $Δm_{A/C} \in (-400, 400) GeV, \tanb\in(1,50)$, and $Δm_{A/C} \in (-200, 300) GeV, \tanb\in(1,12)$ for Type-I and Type-II 2HDM respectively. Furthermore future lepton collider measurements on Higgs and $Z$ boson properties can explore this scenario in more detail or even rule out it.

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arXiv2020

MSSM at future Higgs factories

In this work, we study the implication of Higgs precision measurements at future Higgs factories on the MSSM parameter space, focusing on the dominant stop sector contributions. We perform a multi-variable fit to both the signal strength for various Higgs decay channels at Higgs factories and the Higgs mass. The chi-square fit results show sensitivity to mA, tan beta, stop mass parameter mSUSY as well as the stop left-right mixing parameter Xt. We also study the impact of the Higgs mass prediction on the MSSM and compare the sensitivities of different Higgs factories.

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arXiv2020

Precision Higgs Couplings in Neutral Naturalness Models: an Effective Field Theory Approach

The Higgs sector in neutral naturalness models provides a portal to the hidden sectors, and thus measurements of Higgs couplings at current and future colliders play a central role in constraining the parameter space of the model. We investigate a class of neutral naturalness models, in which the Higgs boson is a pseudo-Goldstone boson from the universal SO(N)/SO(N-1) coset structure. Integrating out the radial mode from the spontaneous global symmetry breaking, we obtain various dimension-six operators in the Standard Model effective field theory, and calculate the low energy Higgs effective potential with radiative corrections included. We perform a chi-square fit to the Higgs coupling precision measurements at current and future colliders and show that the new physics scale could be explored up to 2.7 (2.8) TeV without (with) the Higgs invisible decay channels at future Higgs factories.

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arXiv2020

Probing Exotic Charged Higgs Decays in the Type-II 2HDM through Top Rich Signal at a Future 100 TeV pp Collider

The exotic decay modes of non-Standard Model Higgs bosons are efficient in probing the hierarchical Two Higgs Doublet Models (2HDM). In particular, the decay mode $H^\pm\to HW^\pm$ serves as a powerful channel in searching for charged Higgses. In this paper, we analyzed the reach for $H^\pm\to HW^\pm \to t\bar{t}W$ at a 100 TeV $pp$ collider, and showed that it extends the reach of the previously studied $ττW$ final states once above the top threshold. Top tagging technique is used, in combination with the boosted decision tree classifier. Almost the entire hierarchical Type-II 2HDM parameter space can be probed via the combination of all channels at low $\tanβ$ region.

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arXiv2020

Non-Minimal Dark Sectors: Mediator-Induced Decay Chains and Multi-Jet Collider Signatures

A preponderance of astrophysical and cosmological evidence indicates that the universe contains not only visible matter but also dark matter. In order to suppress the couplings between the dark and visible sectors, a standard assumption is that these two sectors communicate only through a mediator. In this paper we make a simple but important observation: if the dark sector contains multiple components with similar quantum numbers, then this mediator also generically gives rise to dark-sector decays, with heavier dark components decaying to lighter components. This in turn can even give rise to relatively long dark decay chains, with each step of the decay chain also producing visible matter. The visible byproducts of such mediator-induced decay chains can therefore serve as a unique signature of such scenarios. In order to examine this possibility more concretely, we examine a scenario in which a multi-component dark sector is connected through a mediator to Standard-Model quarks. We then demonstrate that such a scenario gives rise to multi-jet collider signatures, and we examine the properties of such jets at both the parton and detector levels. Within relatively large regions of parameter space, we find that such multi-jet signatures are not excluded by existing monojet and multi-jet searches. Such decay cascades therefore represent a potential discovery route for multi-component dark sectors at current and future colliders.

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