arXiv subjects

Wan-Zhe Feng

Publications and source records attributed to Wan-Zhe Feng.

At least 19 recordsLinked to original sources

arXiv2026

Scalar induced gravitational waves as probes of dark QCD

We investigate scalar induced gravitational waves (SIGWs) as probes of a dark QCD crossover. Motivated by twin Higgs and asymmetric twin baryon dark matter scenarios, we consider a dark QCD sector with a confinement scale approximately 5.5 times the Standard Model (SM) QCD scale. We construct the effective energy and entropy degrees of freedom for the SM supplemented by dark QCD sectors containing either three light dark quark flavors or all six dark quark flavors. The resulting equation of state parameter and sound speed are then used to solve the first-order scalar perturbations and the second-order SIGWs through the SM and dark QCD crossover epochs. For a monochromatic primordial curvature power spectrum, we first demonstrate that the realistic SM thermal history modifies the SIGW spectrum relative to the idealized radiation-dominated case. We then show that a dark QCD crossover generates an additional frequency-shifted distortion when the enhanced scalar mode reenters the horizon near the dark confinement scale. This distinctive feature can therefore serve as a characteristic signature of the dark QCD sector. Our results demonstrate that SIGWs provide a complementary cosmological probe of hidden confining sectors, with characteristic spectral features shifted to higher frequencies relative to the SM QCD imprint. The analysis developed in this work can also be extended to other well-motivated theories containing different dark confining sectors.

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arXiv2026

Supercool with PPO: Exploring Supercooled Phase Transitions via Reinforcement Learning

Gravitational waves from cosmological first-order phase transitions provide a powerful probe of hidden sectors and beyond the Standard Model physics. However, identifying phenomenologically relevant benchmark points remains computationally challenging, since viable and detectable signals typically occupy only a small fraction of the scanned parameter space. In this work, we introduce a reinforcement learning strategy based on Proximal Policy Optimization (PPO) to accelerate the search for gravitational wave signals from supercooled phase transitions in a minimal dark $U(1)_x$ sector. We construct a numerical reinforcement learning environment that maps the microscopic model parameters to the corresponding phase transition and gravitational wave observables, using a gauge-independent low-temperature formulation of the effective action. Several reward designs are developed to guide the agent toward parameter regions producing large gravitational wave amplitudes, broad frequency coverage, and detector sensitive benchmark points. We compare the PPO scans with conventional Monte Carlo scans in both narrow and broad windows of the $U(1)_x$ vacuum expectation value. Our results demonstrate that PPO provides an efficient goal-directed search strategy for gravitational wave phenomenology and offers a broadly applicable framework for learning-assisted exploration of high-dimensional scientific parameter spaces.

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arXiv2026

Gauge-independent gravitational waves from a minimal dark $U(1)$ sector with viable dark matter candidates

Searches for stochastic gravitational wave backgrounds generated by first-order phase transitions offer a powerful probe of hidden sectors, but quantitative predictions in gauge theories are obstructed by the gauge dependence of the finite-temperature effective potential and the associated tunneling action. We study a minimal gauged $U(1)$ dark sector containing a dark Higgs and a dark photon, optionally supplemented by a vectorlike dark fermion, coupled to the Standard Model through the Higgs portal or kinetic mixing. Using the Nielsen identity together with a controlled derivative expansion and power counting, we construct a gauge-independent effective action in the high- and low-temperature limits, enabling model-intrinsic nucleation dynamics and robust gravitational wave predictions. We perform dedicated Monte Carlo scans in both limits and map viable microscopic parameters to detector-facing peak frequencies and amplitudes, spanning bands relevant to pulsar timing arrays and planned space-based interferometers. In our scans, supercooled phase transitions typically produce much stronger signals and are more likely to fall within the sensitivity range of current and future gravitational wave detectors, whereas parametrically high-temperature phase transitions generally yield weaker signals. We further connect the phase transition phenomenology to viable dark matter candidates within the same minimal field content, providing benchmark targets for dark photon dark matter and dark fermion dark matter, and highlighting their complementarity with gravitational wave observables. Overall, our results provide an end-to-end, gauge-independent pipeline from a minimal hidden sector Lagrangian to gravitational wave spectra and cosmologically viable dark matter benchmarks, yielding the most reliable and concrete predictions to date for a minimal gauged $U(1)$ dark sector.

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arXiv2025

Electroweak right-handed neutrino portal dark matter

We study dark matter coupled to the Standard Model via electroweak scale right-handed neutrinos in a Type-I seesaw framework. We consider a minimal dark sector containing a fermion $\chi$ and a complex scalar $\phi$ whose only connection to the Standard Model is through renormalizable Yukawa interactions with right-handed Majorana neutrinos, thus realizing a neutrino portal after seesaw mixing. We discuss three representative realizations of electroweak right-handed neutrinos arising from the Type-I seesaw mechanism, spanning small, tiny, and ultraweak couplings to the Standard Model sector, so that the dark particles can either undergo secluded freeze-out or be produced via freeze-in. Instead of merely estimating the order of magnitude of the seesaw couplings, we use the Particle Swarm Optimization algorithm to obtain viable seesaw parameter sets consistent with neutrino data and other constraints, and then compute the coupled evolution of the dark particles and right-handed neutrinos, reproducing the observed dark matter relic abundance in representative benchmark scenarios. For freeze-out, dark matter depletion is controlled by coupled dark sector dynamics, requiring a full Boltzmann treatment for a reliable relic abundance. For freeze-in, internal dark interactions also alter the relic density: treating hidden particles as independent components with late decays added afterward can misestimate the abundance by $30\%$ or even $95\%$, depending on the interaction structure. Electroweak right-handed neutrino portal dark matter thus provides a robust and predictive framework that tightly connects neutrino physics, heavy neutral lepton phenomenology, and the cosmological dark matter relic density, offering a well-motivated benchmark for complementary collider, neutrino, and cosmological probes at the high energy frontier.

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arXiv2025

Gauge-independent Gravitational Waves from Cogenesis in a $B-L$ Conserving Universe

An analysis of baryogenesis and stochastic gravitational wave production is presented for an extension of the standard model where the dark sector consists of dark matter particles charged under a $U(1)_x$ gauge symmetry, while a subset of dark fields also carry lepton number but no $U(1)_x$ charge. We demonstrate that with CP violation induced by Yukawa couplings, equal and opposite lepton asymmetries are generated in the visible and hidden sectors. Subsequent evolution preserves lepton number separately in each sector, and sphaleron interactions partially convert the lepton asymmetry into baryon asymmetry near the temperature of the first-order phase transition. Further, we discuss stochastic gravitational wave background production for the first-order phase transition using a gauge-independent bubble nucleation dynamics which yields spectra also valid in the supercooled low-temperature regime with {$T_p/m_{A_x} \ll 1$} where $T_p$ is the percolation temperature and $m_{A_x}$ is the dark photon mass. A parameter-space scan identifies regions that simultaneously account for cogenesis of baryon asymmetry and dark matter and predict stochastic gravitational wave signals within reach of current (NANOGrav, EPTA, PPTA) and future detectors at higher frequencies, providing a unified framework for cogenesis and associated gravitational wave production.

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arXiv2024

Sub-GeV millicharge dark matter from the $U(1)_X$ hidden sector

We conduct a comprehensive study on the sub-GeV millicharge dark matter produced through the freeze-in mechanism. We discuss in general the mixing mechanism, encompassing both kinetic mixing and mass mixing, between the $U(1)_X$ hidden sector and the standard model, which can generate millicharge carried by the dark fermions from the hidden sector. We discuss in depth how such millicharge is generated, and clarify several misunderstandings regarding this subject in the literature. Without employing an effective field theory approach, where the photon field directly mixed with the additional $U(1)$, we analyze a general renormalizable model and investigate the complete evolution of the hidden sector particles. Due to the substantial self-interactions among hidden sector particles, the evolution of the hidden sector temperature plays a crucial role, which is addressed concurrently with the number densities of hidden sector particles by solving a set of coupled Boltzmann equations. We thoroughly examine eight benchmark models from six distinct cases. Some of our key findings from the analysis of these benchmark models may be generalizable and applicable to broader freeze-in scenarios. We also explore the possibility that the $\mathcal{O}$(keV) $U(1)_X$ dark photon is a viable dark matter candidate, even though it can contribute at most $\sim 5\%$ to the total observed dark matter relic density.

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arXiv2024

Annihilating to the Darker: Thermal Relic Dark Matter with an Ultraweak Portal to the Standard Model

Thermal relic dark matter has been severely constrained in recent years by direct and indirect dark matter searches, as well as multi-messenger probes of dark sectors. At the current level of experimental precision, it has become difficult for many thermal dark matter models to deplete their abundance sufficiently through freeze-out to reproduce the observed relic density. We study the possibility that thermal dark matter couples only ultraweakly to the Standard Model (SM), and therefore remains effectively undetectable in current experiments, while interacting much more strongly with a darker sector that controls its freeze-out history. Hence, the dominant annihilation channels of a thermal relic may proceed primarily into the darker sector rather than into SM particles. We first summarize the general classes of portal interactions that may connect the SM, a hidden sector, and a darker concealed sector, together with the corresponding experimental constraints. We then illustrate the mechanism in two representative realizations. The first is a prototype $U(1)_x \times U(1)_c$ setup with kinetic and mass mixing between the hidden and concealed gauge sectors. The second is a more motivated $U(1)_{B-L}\times U(1)_c$ construction, in which the $U(1)_{B-L}$ gauge interaction is strongly constrained and the hidden--concealed connection is mediated primarily by a real scalar. In both frameworks, we identify two qualitatively distinct scenarios: assisted depletion and darker conversion. By solving the full set of coupled Boltzmann equations and presenting benchmark models for dark matter masses in the 1--200~GeV range, we show that electroweak scale thermal relic dark matter may remain viable even when its direct portal to the SM is ultraweak, provided that sufficiently strong hidden--concealed interactions govern the cosmological evolution.

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arXiv2024

Cosmologically Consistent Analysis of Gravitational Waves from hidden sectors

Production of gravitational waves in the early universe is discussed in a cosmologically consistent analysis within a first order phase transition involving a hidden sector feebly coupled with the visible sector. Each sector resides in its own heat bath leading to a potential dependent on two temperatures, and on two fields: one a standard model Higgs and the other a scalar arising from a hidden sector $U(1)$ gauge theory. A synchronous evolution of the hidden and visible sector temperatures is carried out from the reheat temperature down to the electroweak scale.The hydrodynamics of two-field phase transitions, one for the visible and the other for the hidden is discussed, which leads to separate tunneling temperatures, and different sound speeds for the two sectors. Gravitational waves emerging from the two sectors are computed and their imprint on the measured gravitational wave power spectrum vs frequency is analyzed in terms of bubble nucleation signature, i.e., detonation, deflagration, and hybrid. It is shown that the two-field model predicts gravitational waves accessible at several proposed gravitational wave detectors: LISA, DECIGO, BBO, Taiji and their discovery would probe specific regions of the hidden sector parameter space and may also shed light on the nature of bubble nucleation in the early universe. The analysis presented here indicates that the cosmologically preferred models are those where the tunneling in the visible sector precedes the tunneling in the hidden sector and the sound speed $c_s$ lies below its maximum, i.e., $c^2_s<\frac{1}{3}$. It is of interest to investigate if these features are universal and applicable to a wider class of cosmologically consistent models.

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arXiv2024

Darker matter generating from the dark

The non-detection of dark matter may be attributed to the dark matter residing in a darker hidden sector. We explore the possibility that a hidden sector produced through the freeze-in mechanism, can further generate an even more hidden sector via an additional freeze-in process. Such a two-step freeze-in process produces dark matter coupled weaker-than-ultraweakly to the standard model particles, and is thus referred to as the "darker matter". To illustrate the two-step freeze-in process, we study a model featuring two $U(1)$ hidden sectors. The first $U(1)$ sector is directly coupled to the standard model with feeble interactions, while the second $U(1)$ sector is directly coupled to the first $U(1)$ sector and thus only indirectly to the standard model, rendering it darker. Remarkably, darker matter candidates residing in the second darker $U(1)$ sector, generated from the two-step freeze-in process, can account for almost the entire observed dark matter relic density. The darker matter, interacted with standard model particles through ultraweak couplings, can exhibit velocity-dependent self-interacting cross-sections, which potentially provides an explanation for addressing problems associated with cosmic small-scale structures. Additionally, the dark photon darker matter residing in the darker hidden sector can be responsible for the galactic 511 keV photon signal, consistent with various dark matter density profiles.

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arXiv2023

Holographic Operator Product Expansion of Loop Operators in $\mathcal{N}=4$ $SO(N)$ Super Yang-Mills Theory

In this paper, we compute the correlation functions of Wilson(-'t~Hooft) loops with chiral primary operators in $\mathcal{N}=4$ supersymmetric Yang-Mills theory with $SO(N)$ gauge symmetry, which has a holographic dual description of Type IIB superstring theory on the $AdS_{5}\times\mathbf{RP}^{5}$ background. Specifically, we compute the coefficients of the chiral primary operators in the operator product expansion of Wilson loops in the fundamental representation, Wilson-'t Hooft loops in the symmetric representation, Wilson loops in the anti-fundamental representation and the spinor representation. We also compare these results to the $\mathcal{N}=4$ $SU(N)$ super Yang-Mills theory.

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arXiv2023

Twin Cogenesis

We investigate a cogenesis mechanism within the twin Higgs setup which can naturally explain the nature of dark matter, the cosmic coincidence puzzle, little hierarchy problem, leptogenesis and the tiny neutrino masses. Three heavy Majorana neutrinos are introduced to the standard model sector and the twin sector respectively, which explain the tiny neutrino masses and generate the lepton asymmetry and the twin lepton asymmetry at the same time. The twin cogenesis mechanism applies to any viable twin Higgs model without an explicit $\mathbb{Z}_2$ breaking in the leptonic sector and evading the $ΔN_{\rm eff}$ constraint. We illustrate the twin cogenesis mechanism using the neutrino-philic twin two Higgs doublet model, a newly proposed model to lift the twin neutrino masses with spontaneous $\mathbb{Z}_2$ breaking. The dark photon with a Stueckelberg mass $\mathcal{O}(10)$ MeV ensures the energy in the twin sector as well as the symmetric component of twin sector particles can be depleted. The lightest twin baryons are the dark matter candidates with masses approximately 5.5 GeV, which explain naturally the amount of dark matter and visible matter in the Universe are of the same order. We also demonstrate twin cogenesis in the fraternal twin Higgs setup, in which the dark matter candidate is the twin bottom bound state $Ω^\prime_{b^\prime b^\prime b^\prime}$.

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arXiv2023

Explaining the $W$ boson mass anomaly and dark matter with a $U(1)$ dark sector

The $W$ boson mass recently reported by the CDF collaboration shows a deviation from the standard model prediction with an excess at $7σ$ level. We investigate two simple extensions of the standard model with an extra $U(1)$ dark sector. One is the $U(1)_x$ extension, where the $U(1)_x$ gauge field mixes with the standard model through gauge kinetic terms. The other is a general $U(1)_{\mathbf{A} Y+\mathbf{B} q}$ extension of the standard model. Fitting various experimental constraints we find the $U(1)_x$ extension with only kinetic mixing can enhance the $W$ boson mass for at most 10~MeV. While the $U(1)_{\mathbf{A} Y+\mathbf{B} q}$ extension can easily generate 77~MeV enhancement of the $W$ boson mass and also offer a viable dark matter candidate with mass ranging from several hundred GeV to TeV, which may be detected by future dark matter direct detection experiments with improved sensitivities.

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arXiv2022

TF08 Snowmass Report: BSM Model Building

We summarize the state of Beyond the Standard Model (BSM) model building in particle physics for Snowmass 2021, focusing mainly on several whitepaper contributions to BSM model building (TF08) and closely related areas.

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arXiv2021

Hidden sectors and a multi-temperature universe

A variety of supergravity and string based models contain hidden sectors which can play a role in particle physics phenomena and in cosmology. In this note we discuss the possibility that the visible sector and the hidden sectors in general live in different heat baths. Further, it is entirely possible that dark matter resides partially or wholly in hidden sectors in the form of dark Dirac fermions, dark neutralinos or dark photons. A proper analysis of dark matter and of dark forces in this case requires that one deals with a multi-temperature universe. We discuss the basic formalism which includes the multi-temperature nature of visible and hidden sectors in the analysis of phenomena observable in the visible sectors. Specifically we discuss the application of the formalism for explaining the velocity dependence of dark matter cross sections as one extrapolates from galaxy scales to scales of galaxy clusters. Here the dark photon exchange among dark fermions can produce the desired velocity dependent cross sections consistent with existing galactic cross section data indicating the existence of a new fifth (dark) force. We also discuss the possibility that the dark photon may constitute a significant portion of dark matter. We demonstrate a realization of this possibility in a universe with two hidden sectors and with the visible sector and the hidden sectors in different heat baths which allows a satisfaction of the constraints that the dark photon have a lifetime larger than the age of the universe and that its relic density be consistent with Planck data. Future directions for further work are discussed.

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arXiv2021

A multi-temperature universe can allow a sub-MeV dark photon dark matter

An analysis of sub-MeV dark photon as dark matter is given which is achieved with two hidden sectors, one of which interacts directly with the visible sector while the second has only indirect coupling with the visible sector. The formalism for the evolution of three bath temperatures for the visible sector and the two hidden sectors is developed and utilized in solution of Boltzmann equations coupling the three sectors. We present exclusion plots where the sub-MeV dark photon can be dark matter. The analysis can be extended to a multi-temperature universe with multiple hidden sectors and multiple heat baths.

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arXiv2021

Self-interacting hidden sector dark matter, small scale galaxy structure anomalies, and a dark force

The short distance behavior of dark matter (DM) at galaxy scales exhibits several features not explained by the typical cold dark matter (CDM) with velocity-independent cross-section. We discuss a particle physics model with a hidden sector interacting feebly with the visible sector where a dark fermion self-interacts via a dark force with a light dark photon as the mediator. We study coupled Boltzmann equations involving two temperatures, one for each sector. We fit the velocity-dependent DM cross-section to the data from scales of dwarf galaxies to clusters consistent with relic density constraint.

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arXiv2020

Expanding the parameter space of natural supersymmetry

SUSY/SUGRA models with naturalness defined via small $μ$ are constrained due to experiment on the relic density and the experimental limits on the WIMP-proton cross-section and WIMP annihilation cross-section from indirect detection experiments. Specifically models with small $μ$ where the neutralino is higgsino-like lead to dark matter relic density below the observed value. In several works this problem is overcome by assuming dark matter to be constituted of more than one component and the neutralino relic density deficit is made up from contributions from other components. In this work we propose that the dark matter consists of just one component, i.e., the lightest neutralino and the relic density of the higgsino-like neutralino receives contributions from the usual freeze-out mechanism along with contributions arising from the decay of hidden sector neutralinos. The model we propose is an extended MSSM model where the hidden sector is constituted of a $U(1)_X$ gauge sector along with matter charged under $U(1)_X$ which produce two neutralinos in the hidden sector. The $U(1)_X$ and the hypercharge $U(1)_Y$ of the MSSM have kinetic and Stueckelberg mass mixing where the mixings are ultraweak. In this case the hidden sector neutralinos have ultraweak interactions with the visible sector. Because of their ultraweak interactions the hidden sector neutralinos are not thermally produced and we assume their initial relic density to be negligible. However, they can be produced via interactions of MSSM particles in the early universe, and once produced they decay to the neutralino. For a range of mixings the decays occur before the BBN producing additional relic density for the neutralino. Models of this type are testable in dark matter direct and indirect detection experiments and at the high luminosity and high energy LHC.

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arXiv2020

A long-lived stop with freeze-in and freeze-out dark matter in the hidden sector

In extended supersymmetric models with a hidden sector the lightest $R$-parity odd particle can reside in the hidden sector and act as dark matter. We consider the case when the hidden sector has ultraweak interactions with the visible sector. An interesting phenomenon arises if the LSP of the visible sector is charged in which case it will decay to the hidden sector dark matter. Due to the ultraweak interactions, the LSP of the visible sector will be long-lived decaying outside the detector after leaving a track inside. We investigate this possibility in the framework of a $U(1)_X$-extended MSSM/SUGRA model with a small gauge kinetic mixing and mass mixing between the $U(1)_X$ and $U(1)_Y$ where $U(1)_Y$ is the gauge group of the hypercharge. Specifically we investigate the case when the LSP of MSSM is a stop which decays into the hidden sector dark matter and has a lifetime long enough to traverse the LHC detector without decay. It is shown that such a particle can be detected at the HL-LHC and HE-LHC as an $R$-hadron which will look like a slow moving muon with a large transverse momentum $p_T$ and so can be detected by the track it leaves in the inner tracker and in the muon spectrometer. Further, due to the ultraweak couplings between the hidden sector and the MSSM fields, the dark matter particle has a relic density arising from a combination of the freeze-out and freeze-in mechanisms. It is found that even for the ultraweak or feeble interactions the freeze-out contribution relative to freeze-in contribution to the relic density is substantial to dominant, varying between 30\% to 74\% for the model points considered. It is subdominant to freeze-in for relatively small stop masses with relatively larger stop annihilation cross-sections and the dominant contribution to the relic density for relatively large stop masses and relatively smaller stop annihilation cross-sections.

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