arXiv subjects

Yi Chung

Publications and source records attributed to Yi Chung.

14 recordsLinked to original sources

arXiv2025

Two coincidences are a clue: Probing a GeV-scale dark QCD sector

The similarity between the dark matter and baryon energy densities suggests an existence of a dark sector analogous to QCD. In addition, small-scale structure anomalies can be addressed by dark matter self-interactions with cross sections comparable to those of QCD. Both observations point toward a GeV-scale dark QCD sector. Motivated by these two coincidences, we investigate the parameter space of a distinctive chiral dark QCD model featuring a MeV-scale dark photon with axial-vector couplings. We also discuss a possible third coincidence associated with the latest measurement of $N_{\rm eff}$. Current constraints leave a finite and testable region of parameter space that can be probed by future experiments such as the Gamma Factory.

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arXiv2024

Generating the Dark Matter mass from the QCD vacuum: A new approach to the Dark Matter-Baryon coincidence problem

The comparable abundances of dark matter and baryons imply a deep connection between the dark sector and the QCD sector. In models of asymmetric dark matter, the number densities of both sectors are ensured to be similar. However, a complete solution should also include a mechanism for comparable masses. In this letter, we present a solution based on a strongly coupled chiral dark sector, featuring a light composite fermion as the dark matter. Its mass is generated through the misalignment triggered by the QCD vacuum, ensuring the mass to be at the GeV scale. The model features $\mathcal{O}(1)$ GeV dark baryon dark matter together with dark pions (axion-like particles). Moreover, a composite QCD axion naturally arise in the UV model.

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arXiv2024

Comparable Dark Matter and Baryon energy densities from Dark Grand Unification

We investigate a theory of $SU(9)$ dark grand unification, where dark matter consists of asymmetric dark baryons from the $Sp(4)_D$ dark QCD sector. By unifying the dark color gauge group with the Standard Model gauge group, the asymmetry generation in both sectors originates from a common process that preserves a $U(1)_{D-(B-L)}$ symmetry, resulting in comparable number densities. Furthermore, thanks to dark grand unification, the $Sp(4)_D$ dark QCD sector shares a similar matter content with the QCD sector, leading to comparable running of the gauge couplings from the scale as high as $10^{15}$ GeV. This predicts a dark color confinement scale and thus dark baryon masses around the GeV scale, comparable to visible baryon masses. Together with the similar number densities, the model provides an explanation for the observed similarity between the energy densities of dark matter and baryons, $\rho_D \approx 5\,\rho_B$. The model also features some novel phenomenology, including a GeV-scale flavored dark QCD sector with diquark dark baryons and light dark mesons. The interaction between the dark sector and the visible sector occurs via a new $Z'$ boson with a mass of $\mathcal{O}(10)$ TeV, which could be searched for at future hadron colliders. We also briefly discuss an $SU(8)$ dark grand unified theory featuring an $SU(3)_D$ dark QCD sector.

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arXiv2023

Dynamical origin of Type-I Seesaw with large mixing

We investigate Type-I Seesaw models where the right-handed neutrino masses are dynamically generated by strong interactions. Using horizontal gauge symmetry as the source of strong dynamics, a nontrivial flavor structure can also be introduced dynamically. We find that the right-handed neutrino mass matrix with a strongly anti-diagonal structure emerges when the three right-handed neutrinos are in the triplet representation of $SU(2)_H$ horizontal gauge symmetry. With an assumption of a Dirac neutrino mass matrix with hierarchical eigenvalues and small mixing angles, analogous to the up-type quark sector, and certain substructures, the resulting light neutrino mass matrix from the type-I seesaw mechanism can accommodate the large mixing and weak hierarchy observed in low-energy neutrino data. The neutrino puzzles can, therefore, be understood as the consequence of strong horizontal gauge interactions. We also discuss the potential UV completion and the phenomenology that could be tested in the future.

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arXiv2023

Third-generation-philic Hidden Naturalness

We present a solution to the electroweak hierarchy problem, where the relevant new particles are third-generation-philic and hidden in SM processes with third-generation fermions. Due to this feature, the mass bounds from direct searches are much weaker and the required fine-tuning can be reduced drastically. A concrete model is constructed based on a $SU(6)/Sp(6)$ fundamental composite Higgs model with collective symmetry breaking and extended hypercolor mechanism. The construction allows to raise the scale $f$ to $\sim 3\,$TeV, corresponding to resonances at $M_\rho \gtrsim 10$ TeV, without much tuning - employing ingredients that are naturally inherent in the (composite) Goldstone-Higgs framework. The experimental signatures are discussed in detail. It is found that current bounds allow for a model with negligible tuning.

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arXiv2023

Naturalness-motivated composite Higgs model for generating the top Yukawa coupling

The large top Yukawa coupling results in the top quark contributing significantly to the quantum correction of the Higgs mass term. Traditionally, this effect is canceled by the presence of top partners in symmetry-based models. However, the absence of light top partners poses a challenge to the Naturalness of these models. In this paper, we study a model based on composite Higgs with the top Yukawa coupling originating from dimension-six four-fermion operators. The low cutoff scale of the top quark loop required by the Naturalness principle can be realized with a light gauge boson $E_\mu$ which connects the hyperfermions and top quarks. A scalar-less dynamical model with weakly coupled extended $SU(4)_{EC}$ gauge group is presented. The model features an $E_\mu$ boson and a $Z'_E$ boson both at the sub-TeV scale, which lead to a rich phenomenology, especially in the top physics.

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arXiv2023

The Hierarchy Problem and the Top Yukawa

In this talk, an alternative to top partner solutions and its consequences on phenomenology are discussed. The hierarchy problem from the top loop contribution is solved by mitigating the top Yukawa coupling at high scales. In this scenario, the new degrees of freedom appearing at the cut-off scale of the top loop should then be some new top-philic particles instead of traditional top partners. The idea can be directly tested through measurements in top physics, including $t\bar{t}h$, $t\bar{t}$ differential cross section, and $t\bar{t}t\bar{t}$ cross section.

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arXiv2023

Explaining the $R_{K^{(*)}}$ anomalies and the CDF $M_W$ in Flavorful Top Seesaw Models with Gauged $U(1)_{L(-R)}$

A new heavy $Z'$ vector boson provides a possible explanation for the neutral current B anomalies and points to the scale where we expect the solution to the hierarchy problem to appear. In this paper, we explore a modified Top Seesaw model based on the global symmetry breaking of $U(3)_L$ to $U(2)_L$. The symmetry is partially gauged such that the breaking introduces not only a composite Higgs doublet but also a TeV-scale $Z'$ boson. The $U(3)_L/U(2)_L$ Top Seesaw model predicts a light Higgs boson as well as a large top Yukawa coupling. Additional $U(1)'$ symmetry is gauged such that a flavorful TeV-scale $Z'$ boson is introduced to address the B anomalies. The existence of a heavy vector-like top quark and $Z-Z'$ mixing breaks the custodial symmetry, which predicts a heavier $W$ mass as observed by the CDF collaboration. Two possible candidates for the $U(1)'$ symmetry corresponding to the $Z'$ boson, $U(1)_L$ and $U(1)_{L-R}$, are discussed in detail. The parameter space for the models to address both anomalies is presented. The direct searches of the $Z'$ boson from dilepton channels are also studied. Based on the assumptions, a TeV-scale $Z'$ boson is still viable and can be probed in the near future.

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arXiv2022

The Hierarchy Problem and the Top Yukawa: An Alternative to Top Partner Solutions

We discuss the role of the top-quark Yukawa coupling $y_t$ concerning the hierarchy problem and construct an alternative scheme to the conventional solutions with top partners. In traditional models, like SUSY or composite Higgs, top partners cancel the top loop contribution to the Higgs quadratic term. The lack of evidence for such colored partners however drives these models into more and more fine-tuned regions. Here, an alternative means to mitigate the top loop, allowing for natural electroweak symmetry breaking, is presented. Emphasizing that we have not measured the top-Higgs interactions at high scales yet, we envisage scenarios where this interaction is only approaching its sizable strength in the infra-red, but gets strongly suppressed at high scales. We first discuss possible effects via a modification of the running of the top Yukawa coupling. Then, we turn to models where the top Yukawa is generated at one-loop level. Originated from a dimension-six operator, it drops when crossing the mass threshold of new degrees of freedom. In either case, the top partners are replaced by some new top-philic particles with strong interaction. Thus, a very different phenomenology, such as large top mass running and signals in four top final states, is introduced, which will be discussed in detail. With the assistance of this mechanism, the solution to the hierarchy problem can be pushed to a (well-defined) higher scale, and a final test of naturalness might be deferred to a 100 TeV Collider, like the FCC.

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arXiv2022

Explaining the $R_{K^{(*)}}$ anomalies in a Fundamental Composite Higgs Model with Gauged $U(1)_{SM_3-HB}$

A new heavy $Z'$ vector boson provides a possible explanation for the neutral current B anomalies. Various $U(1)'$ gauge groups have been proposed and studied. In this paper, we explore a new type of $U(1)'$ gauge symmetry inspired by fundamental composite Higgs models with hyperfermions. It is also the first attempt to connect such a $Z'$ boson with a solution to the hierarchy problem. The $U(1)'$ symmetry is identified as the quantum number of the difference between the Standard Model fermion number and hyperbaryon number, written as $SM-HB$. This type of $U(1)'$ gauge symmetry is naturally broken in some fundamental composite Higgs models, which leads to a TeV-scale $Z'$ boson. We present a concrete example based on the minimal fundamental composite Higgs model with $SU(4)/Sp(4)$ coset, where the $Z'$ boson is the only state below the compositeness scale beside the composite Higgs. We also show that if the $U(1)'$ symmetry is third-generation-philic, written as $SM_3-HB$, the corresponding $Z'$ boson can explain the neutral current B anomalies. The model introduces the composite Higgs and the $Z'$ boson from the same symmetry breaking scale, so Higgs physics and flavor physics are now connected. After considering all the experimental constraints and theoretical preferences, we found that there is still a natural parameter space for $SU(4)/Sp(4)$ fundamental composite Higgs model with $N_{HC}=2$, which can be probed in the near future.

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arXiv2022

A Flavorful Composite Higgs Model : Connecting the B anomalies with the hierarchy problem

We present a model which connects the neutral current B anomalies with composite Higgs models. The model is based on the minimal fundamental composite Higgs model with $SU(4)/Sp(4)$ coset. The strong dynamics spontaneously break the symmetry and introduce five Nambu-Goldstone bosons. Four of them become the Standard Model Higgs doublet and the last one, corresponding to the broken local $U(1)'$ symmetry, is eaten by the gauge boson. This leads to an additional TeV-scale $Z'$ boson, which can explain the recent B anomalies. The experimental constraints and allowed parameter space are discussed in detail.

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arXiv2021

Composite Flavon-Higgs Models

We consider a composite Higgs model based on the $SU(6)/Sp(6)$ coset, where an $U(1)$ subgroup of $Sp(6)$ is identified as the flavor symmetry. A complex scalar field $s$, which is a pseudo-Nambu-Goldstone boson of the broken symmetry, carries a flavor charge and plays the role of a flavon field. The $U(1)_F$ flavor symmetry is then broken by a VEV of the flavon field, which leads to a small parameter and generates the mass hierarchy between the top and bottom quarks. A light flavon below the TeV scale can be naturally introduced, which provides a fully testable model for the origin of flavor hierarchy. A light flavon also leads to substantial flavor changing neutral currents, which are strongly constrained by the flavor precision tests. The direct search of additional scalar bosons can also be conducted in HL-LHC and future hadron colliders.

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arXiv2020

A More Natural Composite Higgs Model

Composite Higgs models provide an attractive solution to the hierarchy problem. However, many realistic models suffer from tuning problems in the Higgs potential. There are often large contributions from the UV dynamics of the composite resonances to the Higgs potential, and tuning between the quadratic term and the quartic term is required to separate the electroweak breaking scale and the compositeness scale. We consider a composite Higgs model based on the $SU(6)/Sp(6)$ coset, where an enhanced symmetry on the fermion resonances can minimize the Higgs quadratic term. Moreover, a Higgs quartic term from the collective symmetry breaking of the little Higgs mechanism can be realized by the partial compositeness couplings between elementary Standard Model fermions and the composite operators, without introducing new elementary fields beyond the Standard Model and the composite sector. The model contains two Higgs doublets, as well as several additional pseudo-Nambu-Goldstone bosons. To avoid tuning, the extra Higgs bosons are expected to be relatively light and may be probed in the future LHC runs. The deviations of the Higgs couplings and the weak gauge boson couplings also provide important tests as they are expected to be close to the current limits in this model.

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arXiv2019

Dynamical origin of flavor hierarchies in a warped extra dimension

Extensions of the Standard Model featuring a warped extra dimension compactified on an $S^1/\mathbb{Z}_2$ orbifold, in which the fermions and gauge bosons live in the bulk of the fifth dimension, offer one of the most compelling mechanisms for addressing both the hierarchy problem and the flavor puzzle of the Standard Model. However, the five-dimensional mass terms of bulk fermions must be odd functions on the orbifold, and as such they should be described by a field depending on the coordinate of the extra dimension. We demonstrate the feasibility of dynamically generating these fermion bulk masses with a bulk scalar field in warped extra dimensions. The bulk scalar acquires a vacuum expectation value, which is odd under the orbifold symmetry and gives rise to the fermion bulk masses through non-universal Yukawa-like interactions. Like in the conventional Randall-Sundrum setup, the localization of the different fermion zero modes along the extra dimension naturally explains the observed flavor structure and four-dimensional mass hierarchy of the SM fermions. We study the phenomenological implications of the backreaction on the metric and the modified fermion profiles due to the bulk scalar field on electroweak precision and flavor observables. Using up-to-date data, we show that the contributions to the $S$, $T$, and $ε_K$ parameters require the mass of the first Kaluza-Klein gluon resonance to be of order 14 and 10 TeV in the minimal and the custodial model, respectively, regardless of the effect of the backreaction. Furthermore, effective flavor-changing interactions among the SM fermions induced by the bulk scalar are discussed. We also comment on the potential impact of the Higgs portal interaction of the bulk scalar on the couplings of the Higgs boson.

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