arXiv subjects

Cheng-Wei Chiang

Publications and source records attributed to Cheng-Wei Chiang.

At least 73 records · Page 4Linked to original sources

arXiv2021

Distinguishing $W'$ Signals at Hadron Colliders Using Neural Networks

We investigate a neural network-based hypothesis test to distinguish different $W'$ and charged scalar resonances through the $\ell+\require{cancel}\cancel{E}_T$ channel at hadron colliders. This is traditionally challenging due to a four-fold ambiguity at proton-proton colliders, such as the Large Hadron Collider. Of the neural network approaches we studied, we find a multi-class classifier based on a fully-connected neural network trained upon 2D histograms made from kinematic variables of the final state $\ell$ to be the most powerful. Furthermore, by considering the 1-jet processes, we demonstrate that one can generalize to multiple $2D$ histograms to represent different variable pairs. Finally, as a comparison to traditional approaches, we compare our method with Bayesian hypothesis testing and discuss the pros and cons of each approach. The neural network scheme presented in this paper is a powerful tool that can help probe the properties of charged resonances.

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arXiv2021

Electroweak phase transition confronted with dark matter detection constraints

We study the type-II first-order electroweak phase transition and dark matter (DM) phenomenology in both real and complex singlet extensions of SM. In the real singlet extension with a $\mathbb{Z}_2$ symmetry, we show that the parameter regions favored by the phase transition suffer from strong constraints from DM direct detection so that only a negligible fraction ($f_{X}\sim 10^{-4}-10^{-5}$) of DM composed of the real singlet scalar can survive the LUX and XENON1T constraints. In the complex singlet $S$ case, we impose a $CP$ symmetry $S\to S^{*}$ to the scalar potential. The real component of $S$ can mix with SM Higgs boson while the imaginary component becomes a DM candidate due to the protection of the $CP$ symmetry. By taking into account the current experimental constraints of invisible Higgs decays, Higgs signal strength measurements, and dark matter detections, we find that there exists a large parameter space for the type-II electroweak phase transition to occur while explaining all of the dark matter relic density. We identify a subset of parameter space that is promising for future experiments, including the di-Higgs and Higgs signal strength measurements at the HL-LHC and the dark matter direct detection in the XENONnT project.

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arXiv2021

Finite-Width Effects in Three-Body B Decays

It is customary to apply the so-called narrow width approximation $Γ(B\to RP_3\to P_1P_2P_3)=Γ(B\to RP_3)Br(R\to P_1P_2)$ to extract the branching fraction of the quasi-two-body decay $B\to RP_3$, with $R$ and $P_3$ being an intermediate resonant state and a pseudoscalar meson, respectively. However, the above factorization is valid only in the zero width limit. We consider a correction parameter $η_R$ from finite width effects. Our main results are: (i) We present a general framework for computing $η_R$ and show that it can be expressed in terms of the normalized differential rate and determined by its value at the resonance. (ii) We introduce a form factor $F(s_{12},m_R)$ for the strong coupling involved in the $R(m_{12})\to P_1P_2$ decay when $m_{12}$ is away from $m_R$. We find that off-shell effects are small in vector meson productions, but prominent in the $K_2^*(1430)$, $σ/f_0(500)$ and $K_0^*(1430)$ resonances. (iii) We evaluate $η_R$ in the theoretical framework of QCD factorization (QCDF) and in the experimental parameterization (EXPP) for three-body decay amplitudes. In general, $η_R^{\rm QCDF}$ and $η_R^{\rm EXPP}$ are similar for vector mesons, but different for tensor and scalar resonances. A study of the differential rates enables us to understand the origin of their differences. (iv) Finite-width corrections to $Br(B^-\to RP)_{\rm NWA}$ obtained in the narrow width approximation are generally small, less than 10\%, but they are prominent in $B^-\toσ/f_0(500)π^-$ and $B^-\to \overline K_0^{*0}(1430)π^-$ decays. The EXPP of the normalized differential rates should be contrasted with the theoretical predictions from QCDF calculation as the latter properly takes into account the energy dependence in weak decay amplitudes.

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arXiv2021

Collider Probes of Real Triplet Scalar Dark Matter

We study discovery prospects for a real triplet extension of the Standard Model scalar sector at the Large Hadron Collider (LHC) and a possible future 100 TeV $pp$ collider. We focus on the scenario in which the neutral triplet scalar is stable and contributes to the dark matter relic density. When produced in $pp$ collisions, the charged triplet scalar decays to the neutral component plus a soft pion or soft lepton pair, yielding a disappearing charged track in the detector. We recast current 13TeV LHC searches for disappearing tracks, and find that the LHC presently excludes a real triplet scalar lighter than 248 (275) GeV, for a mass splitting of 172 (160) MeV with $\mathcal{L}=\rm36\,$fb$^{-1}$. The reach can extend to 497 (520) GeV with the collection of $3000\,$fb$^{-1}$. We extrapolate the 13 TeV analysis to a prospective 100 TeV $pp$ collider, and find that a $\sim3$ TeV triplet scalar could be discoverable with $\mathcal{L}=30$ ab$^{-1}$, depending on the degree to which pile up effects are under control. We also investigate the dark matter candidate in our model and corresponding present and prospective constraints from dark matter direct detection. We find that currently XENON1T can exclude a real triplet dark matter lighter than $\sim3$ TeV for a Higgs portal coupling of order one or larger, and the future XENON20T will cover almost the entire dark matter viable parameter space except for vanishingly small portal coupling.

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arXiv2020

Width effects in resonant three-body decays: $B$ decay as an example

For three-body hadron decays mediated by intermediate resonances with large widths, we show how to properly extract the quasi-two-body decay rates for a meaningful comparison with the corresponding theoretical estimates, using several $B$ decays as explicit examples. We compute the correction factor from finite width effects in the QCD factorization approach, and make a comparison with that using the experimental parameterization in which the momentum dependence in the weak dynamics is absent. Although the difference is generally less than $10\%$ for tensor and vector resonances, it can be as large as $(25-40)\%$ for scalar resonances. Our finding can in fact be applied to general quasi-two-body decays.

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arXiv2020

Testing clockwork axion with gravitational waves

We investigate the gravitational waves (GWs) produced from the Peccei-Quinn (PQ) phase transition associated with the clockwork axion. The PQ phase transition can be first-order when the dimension-6 operator is included into the scalar potential. The GWs from the PQ phase transition at scale in the range of $10^3-10^6$ GeV are detectable for the BBO and ALIA interferometers. The LISA and Taiji interferometers can probe the GWs from the PQ scale $f\lesssim 10^4$ GeV, while the GW signals from the scale $f\gtrsim 10^5$ GeV can be detected by the ground-based GW observatories ET and CE. We find that the parameter space $κ_m\sim 0.06-0.001$, $κ_l\sim 0.04-0.001$, and $\varepsilon\sim 0.1-0.01$ at the scale $f=10^5$ GeV and most of the parameter regions at the scale $f=10^6$ GeV have been excluded by the LIGO O2 run. The LIGO O3 and design phases can further probe the remaining parameter space. We show that the GWs from the annihilation of domain walls with a PQ scale $f\simeq 2\times 10^5$ GeV can induce the stochastic signals indicated by the 12.5-year observation of NANOGrav. The LIGO O3 run has the opportunity of detecting the GW signals from the first-order PQ phase transition around this scale.

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arXiv2020

Evidence of a simple dark sector from XENON1T excess

We propose that the nearly massless dark photons produced from the annihilation of keV dark fermions in the Galaxy can induce the excess of electron recoil events recently observed in the XENON1T experiment. The minimal model for this is the extension of a $U(1)_{X}$ gauge symmetry, under which the dark photon couples to both dark and visible matter currents. We find that the best-fit parameters of the dark sector are compatible with the most stringent constraints from stellar cooling. We also show that in the freeze-out scenario, the dark fermions can explain the anomaly while contributing $\gtrsim 1\%$ of the DM relic density.

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arXiv2020

An explanation for the muon and electron $g-2$ anomalies and dark matter

We propose simple models with a flavor-dependent global $U(1)_\ell$ and a discrete $\mathbb{Z}_2$ symmetries to explain the anomalies in the measured anomalous magnetic dipole moments of muon and electron, $(g-2)_{μ,e}$, while simultaneously accommodating a dark matter candidate. These new symmetries are introduced not only to avoid the dangerous lepton flavor-violating decays of charged leptons, but also to ensure the stability of the dark matter. Our models can realize the opposite-sign contributions to the muon and electron $g-2$ via one-loop diagrams involving new vector-like leptons. Under the vacuum stability and perturbative unitarity bounds as well as the constraints from the dark matter direct searches and related LHC data, we find suitable parameter space to simultaneously explain $(g-2)_{μ,e}$ and the relic density. In this parameter space, the coupling of the Higgs boson with muons can be enhanced by up to $\sim 38\%$ from its Standard Model value, which can be tested in future collider experiments.

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arXiv2020

First-order electroweak phase transition in a complex singlet model with $\mathbb{Z}_3$ symmetry

We consider an extension of the Standard Model with a complex singlet scalar, where a global $U(1)$ symmetry is explicitly broken to $\mathbb{Z}_3$ symmetry. We study the two-step electroweak phase transition in the model and find that it can be of first-order if the heavy scalar mass falls in the range of $1-2$~TeV and the mixing angle $\left | θ\right |\gtrsim 0.2$ ($11.5^{\circ}$). The Higgs signal strength measurements at the LHC, on the other hand, restrict the mixing angle $\left | θ\right |\lesssim 0.4$ ($23^{\circ}$). Future colliders including high-luminosity LHC can probe the remaining parameter space of first-order phase transition in this scenario. After the $U(1)$ symmetry breaking, the pseudo-Goldstone boson becomes a dark matter candidate due to a hidden $\mathbb{Z}_2$ symmetry of the model. We find that the pseudo-Goldstone boson can make up a small fraction of the observed dark matter and escape from the constraints of current direct detection. We also show that the stochastic gravitational wave signals from the phase transition are potentially discoverable with future space-based interferometers.

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arXiv2020

Thermal Loop Effects on Large-Scale Curvature Perturbation in the Higgs Inflation

It is known that the Higgs potential in the Standard Model can drive a successful inflation as long as the Higgs field couples non-minimally to gravity. It is then inevitable to take into account the loop corrections of the Standard Model particles to the Higgs potential in the Higgs inflation. In this paper, we discuss the one-loop corrections at finite temperature to the curvature perturbation generated during the Higgs inflation. We find that the thermal loop effects can suppress the power of the curvature perturbation at large scales, thus resulting in a low quadrupole of the temperature anisotropy in the cosmic microwave background.

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arXiv2020

Boosted $W/Z$ Tagging with Jet Charge and Deep Learning

We demonstrate that the classification of boosted, hadronically-decaying weak gauge bosons can be significantly improved over traditional cut-based and BDT-based methods using deep learning and the jet charge variable. We construct binary taggers for $W^+$ vs. $W^-$ and $Z$ vs. $W$ discrimination, as well as an overall ternary classifier for $W^+$/$W^-$/$Z$ discrimination. Besides a simple convolutional neural network (CNN), we also explore a composite of two CNNs, with different numbers of layers in the jet $p_{T}$ and jet charge channels. We find that this novel structure boosts the performance particularly when considering the $Z$ boson as signal. The methods presented here can enhance the physics potential in SM measurements and searches for new physics that are sensitive to the electric charge of weak gauge bosons.

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arXiv2019

Displaced heavy neutrinos from $Z'$ decays at the LHC

We study the LHC sensitivity to probe a long-lived heavy neutrino $N$ in the context of $Z'$ models. We focus on displaced vertex signatures of $N$ when pair produced via a $Z'$, decaying to leptons and jets inside the inner trackers of the LHC experiments. We explore the LHC reach with current long-lived particle search strategies for either one or two displaced vertices in association with hadronic tracks or jets. We focus on two well-motivated models, namely, the minimal $U(1)_{B-L}$ scenario and its $U(1)_{X}$ extension. We find that searches for at least one displaced vertex can cover a significant portion of the parameter space, with light-heavy neutrino mixings as low as $|V_{lN}|^2\approx 10^{-17}$, and $l=e,μ$ accessible across GeV scale heavy neutrino masses.

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arXiv2019

Revisiting CP violation in $D\to P\!P$ and $V\!P$ decays

Direct CP violation in the hadronic charm decays provides a good testing ground for the Kobayashi-Maskawa mechanism in the Standard Model. Any significant deviations from the expectation would be indirect evidence of physics beyond the Standard Model. In view of improved measurements from LHCb and BESIII experiments, we re-analyze the Cabibbo-favored $D \to P\!P$ and $V\!P$ decays in the topological diagram approach. By assuming certain SU(3)-breaking effects in the tree-type amplitudes, we make predictions for both branching fractions and CP asymmetries of the singly Cabibbo-suppressed decay modes. While the color-allowed and -suppressed amplitudes are preferred to scale by the factor dictated by factorization in the $P\!P$ modes, no such scaling is required in the $V\!P$ modes. The $W$-exchange amplitudes are found to change by 10\% to 50\% and depend on whether $d\overline{d}$ or $s\overline{s}$ pair directly emerges from $W$-exchange. The predictions of branching fractions are generally improved after these SU(3) symmetry breaking effects are taken into account. We show in detail how the tree-type, QCD-penguin, and weak penguin-annihilation diagrams contribute and modify CP asymmetry predictions. Future measurements of sufficiently many direct CP asymmetries will be very useful in removing a discrete ambiguity in the strong phases as well as discriminating among different theory approaches. In particular, we predict $a_{CP}(K^+K^-)-a_{CP}(π^+π^-) = (-1.14 \pm 0.26) \times 10^{-3}$ or $(-1.25 \pm 0.25) \times 10^{-3}$, consistent with the latest data, and $a_{CP}(K^+K^{*-})-a_{CP}(π^+ρ^-) = (-1.52 \pm 0.43) \times 10^{-3}$, an attractive and measurable observable in the near future. Moreover, we observe that such CP asymmetry differences are dominated by long-distance penguin-exchange through final-state rescattering.

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arXiv2019

A simple model for explaining muon-related anomalies and dark matter

We propose a model to explain several muon-related experimental anomalies and the abundance of dark matter. We introduce an vector-like exotic lepton that form an iso-doublet and three right-handed Majorana fermions as an iso-singlet. A real/complex scalar field is added as a dark matter candidate. We impose a global $U(1)_ μ$ symmetry under which fields associated with the SM muon are charged. To stabilize the dark matter, we impose a $Z_2$ (or $Z_3$) symmetry under which the exotic lepton doublet and the real (or complex) scalar field are charged. We find that the model can simultaneously explain the muon anomalous magnetic dipole moment and the dark matter relic density in no conflict with any lepton flavor-violating/conserving observables, with some details depending upon whether the scalar field is real or complex. Besides, we extend the framework to the quark sector in a way similar to the lepton sector, and find that the recent anomalies associated with the $b \to s μ^+ μ^-$ transition can also be accommodated while satisfying constraints such as the $B_{s(d)} \to μ^+ μ^-$ decays and neutral meson mixings.

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arXiv2019

$t\to cg$ and $t\to cZ$ in Universal Extra Dimensional Models

In this work we perform a complete one-loop calculation of the flavor-changing top quark decays $t\to cg$ and $t\to cZ$ in the universal extra dimensional models. We find that the branching ratios of these decays in the minimal scenario remain unaltered from the Standard Model expectations for currently allowed values of the inverse compactification radius. In the non-minimal setup, the branching ratios can be enhanced from the Standard Model due to the presence of boundary localized terms which modify the mass spectrum and couplings in the theory in a non-trivial way. We also check the compatibility of the parameter choices that result in enhancements in these rare decays with other experimental observations.

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arXiv2019

A variant two-Higgs doublet model with a new Abelian gauge symmetry

We consider a two-Higgs doublet model extended with a broken Abelian gauge symmetry under which all Standard Model (SM) quarks, fourth generation fermions and a new SM-singlet scalar boson are charged. Such a setup is shown to be able to accommodate the muon anomalous magnetic dipole moment while being consistent with existing constraints of flavor-violating decays of charged leptons and Z boson. The new scalar boson offers a suitable dark matter candidate that interacts with the SM particles via the Higgs portal and the $Z'$ boson associated with the new gauge symmetry. The dark matter direct detection bound is found to impose a strong constraint on the new gauge coupling.

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arXiv2019

Dark Matter, Muon $g-2$, Electric Dipole Moments and $Z\to \ell_i^+ \ell_j^-$ in a One-Loop Induced Neutrino Model

We study a simple one-loop induced neutrino mass model that contains both bosonic and fermionic dark matter candidates and has the capacity to explain the muon anomalous magnetic moment anomaly. We perform a comprehensive analysis by taking into account the relevant constraints of charged lepton flavor violation, electric dipole moments, and neutrino oscillation data. We examine the constraints from lepton flavor-changing $Z$ boson decays at one-loop level, particularly when the involved couplings contribute to the muon $g-2$. It is found that $\text{BR}(Z\to μτ)\simeq (10^{-7}$ - $10^{-6})$ while $\text{BR}(τ\toμγ)\lesssim 10^{-11}$ in the fermionic dark matter scenario. The former can be probed by the precision measurement of the $Z$ boson at future lepton colliders.

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arXiv2018

Revisiting electroweak phase transition in the standard model with a real singlet scalar

We revisit the electroweak phase transition in the standard model with a real scalar, utilizing several calculation methods to investigate scheme dependences. We quantify the numerical impacts of Nambu-Goldstone resummation, required in one of the schemes, on the strength of the first-order electroweak phase transition. We also employ a gauge-independent scheme to make a comparison with the standard gauge-dependent results. It is found that the effect of the Nambu-Goldstone resummation is typically $\sim1\%$. Our analysis shows that both gauge-dependent and -independent methods give qualitatively the same result within theoretical uncertainties. In either methods, the scale uncertainties in the ratio of critical temperature and the corresponding Higgs vacuum expectation value are more than 10%, which signifies the importance of higher-order corrections.

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