arXiv subjects

Cheng-Wei Chiang

Publications and source records attributed to Cheng-Wei Chiang.

At least 37 records · Page 2Linked to original sources

arXiv2024

Tetraquark nature of the $a_0(980)$ meson in hadronic $D$ decays

The internal structure of the light scalar meson $a_0(980)$ is explored in the three-body $D$ decays of $D\to a_0(980)P\to P_1P_2P$ through the intermediate state $a_0(980)$, where $P$ denotes a pseudoscalar meson. The quasi-two-body $D\to a_0(980)^+P$ decays are governed by the external $W$-emission diagram in which $a_0(980)^+$ is emitted. The predicted branching fractions in the $q\bar q$ model of $a_0(980)$ are too small by one to two orders of magnitude compared to experiment as the amplitude is suppressed by the smallness of the $a_0(980)^+$ decay constant, while those for $D^+\to a_0(980)^0 P$ and $D^0\to a_0(980)^{-}P$ are usually too large. These discrepancies can be resolved provided that $a_0(980)$ is a tetraquark state. In this case, there exist two additional $T$-like topological amplitudes, denoted by $\overline{T}$ and $\tilde T$ which readily account for the discrepancies. An important implication of the tetraquark model is that the $D_s^+\to a_0(980)^+π^0+a_0(980)^0π^+$ decay is not a purely $W$-annihilation process as in the diquark model of $a_0(980)$; it receives dominant contributions from $\overline{T}$ newly noticed in this work. Therefore, measurements of $(D,D_s^+)\to a_0(980)P$ decays lend strong support to the tetraquark picture of $a_0(980)$.

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arXiv2024

Impact of non-thermal phase-space distributions on dark matter abundance in secluded sectors

Many new physics models include secluded sectors that interact little with the Standard Model and whose internal interactions control the dark matter abundance. If these same interactions are responsible for maintaining kinematic equilibrium within the secluded sector, it is possible that the phase-space distributions will differ considerably from their thermal values during freeze-out. This can potentially result in deviations of the dark matter abundance from that computed under the assumption of thermal distributions. In this paper, we revisit dark matter abundance computations for a benchmark secluded sector by numerically tracking the phase-space distributions. Namely, we show that the dark matter abundance can deviate considerably from standard results during the freeze-out process, but that a longer period of annihilation ultimately leaves only a slight excess.

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arXiv2024

Indirect detection constraints on semi-annihilation of inert scalar multiplets

Certain models of inert multiplets allow for semi-annihilation processes, in which two dark matter candidates annihilate to a dark matter particle and a non-dark matter particle. The existence of these processes can alleviate certain constraints and substantially modify the indirect detection signal. In this paper, we study current indirect detection constraints on the semi-annihilation of inert scalar multiplets. We show that there exist gauge numbers for which dark matter can be thermally produced and be compatible with indirect detection constraints even for very cuspy galactic dark matter density profiles.

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arXiv2024

Dark matter semi-annihilation for inert scalar multiplets

Dark matter semi-annihilation is a process through which two dark matter candidates annihilate to a single dark matter particle and a non-dark matter particle. Such processes are common when the symmetry stabilizing the dark matter differs from $\mathbb{Z}_2$ and can lead to qualitatively different phenomenology. In this work, we study the viability of semi-annihilation models including one or two inert multiplets. For one multiplet, we show that there does not exist any viable model in which semi-annihilation is efficient. For two multiplets, semi-annihilation can be efficient, but the number of viable and technically natural models is limited. We then perform a detailed study of the most promising model, showing that the correct relic abundance can be obtained for a wide range of masses.

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arXiv2024

A 95 GeV Higgs Boson in the Georgi-Machacek Model

CMS and ATLAS have reported small excesses in the search for low-mass Higgs bosons in the di-photon decay channel at exactly the same mass, $95.4~$GeV. These searches rely on improved analysis techniques, enhancing in particular the discrimination against the $Z \to e^+e^-$ background. In models beyond the Standard Model (SM) that extend the Higgs sector with triplets, doubly-charged Higgs bosons are predicted which can contribute substantially to the di-photon decay rate of a light Higgs boson. The Georgi-Machacek (GM) Model is of particular interest in this context, since despite containing Higgs triplets it preserves the electroweak $ρ$-parameter to be$~$1 at the tree level. We show that within the GM model, a Higgs boson with a mass of $\sim 95~$GeV with a di-photon decay rate as observed by CMS and ATLAS can be well described. We discuss the di-photon excess in conjunction with an excess in the $b \bar b$ final state observed at LEP and an excess observed by CMS in the di-tau final state, which have been found at comparable masses with local significances of about $2σ$ and $3σ$, respectively. The presence of a Higgs boson at about $95~$GeV within the GM model would imply good prospects of the searches for additional light Higgs bosons. In particular, the observed excess in the di-photon channel would be expected to be correlated in the GM model with a light doubly-charged Higgs boson in the mass range between $100~$GeV and $200~$GeV, which motivates dedicated searches in upcoming LHC Runs.

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arXiv2024

Clockwork axion footprint on nano-hertz stochastic gravitational wave background

The recent Pulsar Timing Arrays (PTAs) nano-Hz gravitational wave (GW) background signal can be naturally induced by the annihilation of domain walls (DWs) formed at a symmetry-breaking scale $f\simeq 200$~TeV in the clockwork axion framework. Based on our first successful and precise prediction, we for the first time suggest that the recent PTA observations strongly support the novel mechanism of the QCD instanton-induced DW annihilation in the clockwork axion framework. We also for the first time discover a novel correlation between dark matter (DM) relic abundance and nano-Hz GW background, which in turn indicates a natural connection between the axion decay constant and the symmetry-breaking scale in the clockwork framework. We find that the GW signal has a peak $h^2Ω_{\rm GW}\simeq 10^{-6.6}-10^{-6.1}$ at about 50~nHz, which is definite and testable for future PTA data at frequencies $\gtrsim 25$~nHz and CMB-S4 experiment. We also propose various phenomena that may appear in PTAs and future GW interferometers.

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arXiv2024

Pseudo-Nambu-Goldstone Dark Matter in $SU(7)$ Grand Unification

We propose a grand unified theory (GUT) pseudo-Nambu-Goldstone boson (pNGB) dark matter (DM) model based on $SU(7)$ gauge symmetry. In the GUT model, the Standard Model (SM) gauge symmetry $G_{\rm SM} := SU(3)_C\times SU(2)_L\times U(1)_Y$ and the ``dark'' gauge symmetry $SU(2)_D$ are unified, where the $SU(2)_D$ symmetry plays an important role in the stability of DM. The unification of SM fermions and dark sector fermions is partially realized. The gauge symmetry $SU(7)$ is spontaneously broken to $SU(5)\times SU(2)\times U(1)$ gauge symmetry at the GUT scale by the nonvanishing vacuum expectation values of an $SU(7)$ adjoint scalar field. The symmetry is further broken to $G_{\rm SM}\times SU(2)_D$ at an intermediate scale. Furthermore, the $SU(2)_D$ symmetry is broken by the $SU(2)_D$ doublet and triplet scalar fields at the TeV scale. In the pNGB DM model based on $G_{\rm SM}\times SU(2)_D$, the residual global $U(1)_V$ dark custodial symmetry guarantees DM stability. On the other hand, in the $SU(7)$ pNGB DM model, this global symmetry is explicitly broken by the Yukawa interaction and the effective Majorana mass terms. To maintain $U(1)_V$ symmetry and thus the DM stability, we need to tune Yukawa coupling constants and cubic scalar couplings at high accuracy. We find that the allowed DM mass region is quite restricted as the gauge coupling constant of $SU(2)_D$ is determined by the condition of the gauge coupling unification. To satisfy gauge coupling unification and the current experimental constraint on proton lifetime, we find that three generations of $SU(3)_C$ adjoint fermions and another three generations of $SU(2)_L$ adjoint fermions with the intermediate mass scale are required. We also find that there is no other solution to satisfy simultaneously the gauge coupling unification and the proton decay constraint if one assumes the other symmetry breaking schemes.

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arXiv2024

Phenomenological study of a gauged ${L_μ-L_τ}$ model with a scalar leptoquark

A $Z'$ gauge boson with sub-GeV mass has acquired a significant interest in phenomenology, particularly in view of the muon $g-2$ anomaly and coherent elastic neutrino-nucleon scattering. The latter is challenged by the nuclear recoil energy of a few tens of keV but has been observed by the COHERENT experiment. To further reconcile the observed excesses in $R(D^{(*)})$ from semileptonic charmful $B$ decays and in the $W$ boson mass, we investigate a model with a gauged $U(1)_{L_μ-L_τ}$ symmetry and a scalar leptoquark. In contrast to the mechanism that involves kinetic mixing between the gauge bosons of $U(1)_{\rm em}$ and $U(1)_{L_μ-L_τ}$, we adopt a dynamical symmetry breaking of $U(1)_{L_μ-L_τ}$ by incorporating an additional Higgs doublet. Through mixing with the $U(1)_{L_μ-L_τ}$-charged Higgs doublet, new Higgs decay channels $h\to Z_1 Z_1/Z_1 Z_2$ occur at percent-level branching ratios, which could be accessible at the LHC. The $W$-mass anomaly observed by CDF II can be potentially resolved through the enhancement in the oblique parameter $T$. Due to the flavored gauge symmetry, the introduced scalar leptoquark $S^{\frac{1}{3}}=(\bar{3},1,2/3)$ exhibits a unique coupling to the $τ$-lepton, offering an explanation for the excesses observed in $R(D^{(*)})$. Moreover, $τ\to μ(Z_1\to ) e^- e^+$ via the resonant light gauge boson decay can reach the sensitivity of Belle II at an integrated luminosity of 50 ab$^{-1}$.

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arXiv2024

Rare $B$ and $K$ decays in a scotogenic model

A scotogenic model can radiatively generate the observed neutrino mass, provide a dark matter candidate, and lead to rare lepton flavor-violating processes. We aim to extend the model to establish a potential connection to the quark flavor-related processes within the framework of scotogenesis, enhancing the unexpectedly large branching ratio (BR) of $B^+\to K^+ ν\barν$, observed by Belle II Collaboration. Meanwhile, the model can address tensions between some experimental measurements and standard model (SM) predictions in flavor physics, such as the muon $g-2$ excess and the higher BR of $B_s \to μ^- μ^+$. We introduce in the model the following dark particles: a neutral singlet Dirac-type lepton ($N$); two inert Higgs doublets ($η_{1,2}$), with one of which carrying a lepton number; a charged singlet dark scalar $(χ^+)$, and a singlet vector-like up-type dark quark ($T$). The first two entities are responsible for the radiative neutrino mass, and $χ^+$ couples to right-handed quarks and leptons and can resolve the tensions existing in muon $g-2$ and $B_s\to μ^- μ^+$. Furthermore, the BR of $B^+ \to K^+ ν\barν$ can be enhanced up to a factor of 2 compared to the SM prediction through the mediations of the dark $T$ and the charged scalars. In addition, we also study the impacts on the $K\to πν\barν$ decays.

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arXiv2024

Updated analysis of $D\to PP, V\!P$ and $VV$ decays: Implications for $K_S^0-K_L^0$ asymmetries and $D^0$-$\overline {D}^0$ mixing

An updated analysis of the two-body $D\to PP, V\!P$ and $VV$ decays within the framework of the topological diagram approach is performed. A global fit to the Cabibbo-favored (CF) modes in the $V\!P$ sector gives many solutions with similarly small local minima in $χ^2$. The solution degeneracy is lifted once we use them to predict for the singly Cabibbo-suppressed (SCS) modes. Topological amplitudes are extracted for the $η-η'$ mixing angles $ϕ=40.4^\circ$ and $43.5^\circ$. The $K_S^0-K_L^0$ asymmetries in $D\to K_{S,L}^0M$ decays denoted by $R(D,M)$ are studied. While the predicted $R(D^0,P)$ for $P=π^0, η$ and $η'$ agree with experiment, the calculated $R(D^+,π^+)$, $R(D_s^+, K^+)$, $R(D^0,ω)$ and $R(D^0,ϕ)$ deviate from the data. We conjecture that the relative phase between the topological amplitudes $(C+A)$ and $(T+C)$ should be slightly smaller than $90^\circ$ in order to explain the first two discrepancies and that additional singlet contributions due to the SU(3)-singlet nature of $ω$ and $ϕ$ are needed to account for the last two. For doubly Cabibbo-suppressed (DCS) $D\to V\!P$ decays, their topological amplitudes (double-primed) cannot be all the same as the corresponding ones in the CF modes. The assumption of $E_{V,P}''=E_{V,P}$ for the $W$-exchange amplitude leads to some inconsistencies with the experiment. Through the measured relative phases between CF and DCS channels, the relations of $E_{V,P}''$ with $E_{V,P}$ are determined. Long-distance contributions to the $D^0$-$\overline {D}^0$ mixing parameter $y$ are evaluated in the exclusive approach. In particular, we focus on $D\to PP$ and $V\!P$ decays where $y$ can be reliably estimated. We conclude that $y_{_{P\!P}}\sim (0.110\pm 0.011)\%$ and the lower bound on $y_{_{V\!P}}$ is $(0.220\pm 0.071)\%$.

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arXiv2024

Improving the performance of weak supervision searches using transfer and meta-learning

Weak supervision searches have in principle the advantages of both being able to train on experimental data and being able to learn distinctive signal properties. However, the practical applicability of such searches is limited by the fact that successfully training a neural network via weak supervision can require a large amount of signal. In this work, we seek to create neural networks that can learn from less experimental signal by using transfer and meta-learning. The general idea is to first train a neural network on simulations, thereby learning concepts that can be reused or becoming a more efficient learner. The neural network would then be trained on experimental data and should require less signal because of its previous training. We find that transfer and meta-learning can substantially improve the performance of weak supervision searches.

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arXiv2024

Deep Learning to Improve the Sensitivity of Di-Higgs Searches in the $4b$ Channel

The study of di-Higgs events, both resonant and non-resonant, plays a crucial role in understanding the fundamental interactions of the Higgs boson. In this work we consider di-Higgs events decaying into four $b$-quarks and propose to improve the experimental sensitivity by utilizing a novel machine learning algorithm known as Symmetry Preserving Attention Network (\textsc{Spa-Net}) -- a neural network structure whose architecture is designed to incorporate the inherent symmetries in particle reconstruction tasks. We demonstrate that the \textsc{Spa-Net} can enhance the experimental reach over baseline methods such as the cut-based and the Deep Neural Networks (DNN)-based analyses. At the Large Hadron Collider, with a 14-TeV centre-of-mass energy and an integrated luminosity of 300 fb$^{-1}$, the \textsc{Spa-Net} allows us to establish 95\% C.L. upper limits in resonant production cross-sections that are 10\% to 45\% stronger than baseline methods. For non-resonant di-Higgs production, \textsc{Spa-Net} enables us to constrain the self-coupling that is 9\% more stringent than the baseline method.

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arXiv2024

Contributions of inert electroweak multiplets to Higgs properties

New physics could manifest itself in the form of electroweak multiplets that interact at tree level with the Higgs boson but do not mix with Standard Model fields or acquire expectation values. In this paper, we study the potential contributions of such inert multiplets to several crucial Higgs properties, namely, the branching ratio of the Higgs to a $Z$ boson and a photon (or massless dark photon) and the triple Higgs coupling. Constraints from the Higgs signal strengths, oblique parameters and unitarity are taken into account.

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arXiv2023

Flavor anomalies in leptoquark model with gauged $U(1)_{L_μ-L_τ}$

Leptoquarks (LQs) have been extensively studied in the context of $B$ anomalies. When $U(1)_{L_μ-L_τ}$ is introduced to a scalar LQ model with the LQ $S_1$ charged under the new symmetry, $S_1$ primarily couples to the third-generation leptons while its couplings to first and second-generation leptons are naturally suppressed. Furthermore, only $S_1$ in the scalar LQ models has the feature that down-type quarks merely couple to neutrinos but not the charged leptons, avoiding strict restrictions from $b\to s μ^+ μ^-$. With this distinctive characteristic of $S_1$, we investigate its impact on rare processes involving the $d_i \to d_j ν\barν$ transitions. Under the dominant constraints from $ΔF=2$ processes, we find that the $S_1$ contributions to the branching ratios (BRs) of $B\to K(K^*) ν\barν$ and $K_L \to π^0 ν\barν$ can be factorized into the same multiplicative factor multiplying the standard model predictions. Enhancement in the BRs can possibly exceed a factor of 2. In particular, ${\cal B}(K^+\to π^+ ν\barν)$ can reach the upper $1σ$ error of the experimental value, i.e., $\simeq 15.4 \times 10^{-11}$. We also show that the model can fit the new world averages of $R(D)$ and $R(D^*)$.

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arXiv2023

Observability of the Higgs boson decay to a photon and a dark photon

Many collider searches have attempted to detect the Higgs boson decaying to a photon and an invisible massless dark photon. For the branching ratio to this channel to be realistically observable at the LHC, there must exist new mediators that interact with both the standard model and the dark photon. In this paper, we study experimental and theoretical constraints on an extensive set of mediator models. We show that these constraints limit the Higgs branching ratio to a photon and a dark photon to be far smaller than the current sensitivity of collider searches.

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arXiv2023

CP Violation in a Model with Higgs Triplets

We discuss CP-violation in a model with a real and a complex isospin triplet Higgs fields without introducing any symmetries except for the electroweak gauge symmetry. This corresponds to the minimal extension of the Higgs sector with the following properties: (i) providing new source of CP violation, (ii) absence of quark flavor changing neutral currents at tree level, and (iii) enabling the electroweak rho parameter to be unity at tree level in the scenario without imposing any new symmetries. Our model can be regarded as the generalized version of the Georgi-Machacek model, in which the global $SU(2)_L\times SU(2)_R$ symmetry is explicitly broken due to CP-violating terms in the potential. We present analytic formulas for theoretical constraints from perturbative unitarity and vacuum stability as well as contributions to the electron electric dipole moment (EDM) and the neutron EDM from all the Barr-Zee type diagrams. We then examine the parameter space allowed by the constraints mentioned above and also those from the uniqueness of the vacuum, measurements at Tevatron and LHC by using \texttt{HEPfit} to perform a global parameter fit. We find that the decays of the two lightest extra neutral (singly-charged) scalars, $H_1$ and $H_2$ ($H_1^\pm$), into $hZ$ ($WZ$) can be significant at the same time under the constraints, which can serve as direct evidence of CP violation in our model, but not from models with multi-doublet extensions.

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arXiv2023

Is the decay of the Higgs boson to a photon and a dark photon currently observable at the LHC?

Many attempts have been made to observe the decay of the Higgs boson to a photon and an invisible massless dark photon. For this decay to be potentially observable at the LHC, new mediators that communicate between the Standard Model and the dark photon must exist. In this Letter, we study bounds on such mediators coming from the Higgs signal strengths, oblique parameters, electric dipole moment of the electron and unitarity. We find that the branching ratio of the Higgs boson to a photon and a dark photon is constrained to be far smaller than the sensitivity of current collider searches, thus calling for a reconsideration of current experimental efforts.

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arXiv2023

Top-quark FCNC decays, LFVs, lepton $g-2$, and $W$ mass anomaly with inert charged Higgses

The observed flavor-changing neutral-current (FCNC) processes in the standard model (SM) arise from the loop diagrams involving the weak charged currents mediated by the $W$-gauge boson. Nevertheless, the top-quark FCNCs and lepton-flavor violating processes resulting from the same mechanism are highly suppressed. We investigate possible new physics effects that can enhance the suppressed FCNC processes, such as $t\to q(h,V)$ with $V=γ,Z,g$, $h\to \ell \ell'$, and $\ell\to \ell' γ$. To achieve the assumption that the induced-FCNCs are all from quantum loops, we consider the scotogenic mechanism, where a $Z_2$ symmetry is introduced and only new particles carry an odd $Z_2$ parity. With the extension of the SM to include an inert Higgs doublet, an inert charged Higgs singlet, a vector-like singlet quark, and two neutral leptons, it is found that, with relevant constraints taken into account, the $t\to c (h, Z)$, $h\to μτ$, and $τ\to \ell γ$ decays can be enhanced up to the expected sensitivities in experiments. The branching ratios of $h\to μ^+ μ^-/τ^+ τ^-$ from only new physics effects can reach up to ${\cal O}(10^{-3})$. Intriguingly, the resulting muon $g-2$ can fit the combined data within $2σ$ errors, whereas the electron $g-2$ can have either sign with a magnitude of ${\cal O}(10^{-13}-10^{-12})$. In addition, we examine the oblique parameters in the model and find that the resulting $W$-mass anomaly observed by CDF II can be accommodated.

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