arXiv subjects

Yuan-Zhen Li

Publications and source records attributed to Yuan-Zhen Li.

7 recordsLinked to original sources

arXiv2026

New Particles at the Z-Pole: Tera-Z factories as discovery and precision machines

Several proposed future lepton colliders are capable of producing trillions of Z-bosons, including FCC-ee, CEPC, LEP3 and LEP-Z. Such Tera-Z factories can discover new elementary particles with couplings to the Z-boson that are orders of magnitude smaller than current bounds. For couplings near the currently excluded parameter regions, they could produce sufficiently large samples to study the new particles' properties in detail, thereby serving as both a discovery and precision machine in one. Using simple analytic estimates, we quantify the dependence of the expected event yield in long-lived particle searches on the number of produced Z-bosons and on the detector dimensions. From this, we derive estimates for both the discovery reach and the measurement precision attainable at such facilities. While the precision of such estimates of course falls short of proper simulations, the analytic approach is suitable for a quick assessment of the sensitivity for a given design. We illustrate this with two examples, heavy neutral leptons and axion-like particles. Under optimistic assumptions, these could be produced in the millions and billions, respectively, effectively turning future lepton colliders into exotics factories.

hep-ph↗

arXiv2026

Machine Learning Does It and Does It Better: Unearthing Primordial Dark-Matter Velocities from the Matter Power Spectrum

One effective way of learning about the production and properties of dark matter in the early universe is by extracting information about the primordial dark-matter phase-space distribution from the matter power spectrum. Several years ago a simple empirical formula was introduced which successfully reproduces most of the salient features of the primordial dark-matter phase-space distribution from the matter power spectrum -- even in situations in which this distribution is non-thermal, multi-modal, or exhibits other complicated features. Continuing this line of research, we investigate the extent to which machine-learning techniques can improve upon this analytic approach. Interestingly, we find that a one-dimensional convolutional neural network not only succeeds in reconstructing the dark-matter phase-space distribution with greater accuracy, but can also be applied to a broader range of matter power spectra.

astro-ph.CO↗

arXiv2026

Does the Electron EDM Preclude Electroweak Baryogenesis ?

Electroweak baryogenesis (EWBG) constitutes a theoretically compelling and experimentally testable mechanism for explaining the origin of the baryon asymmetry of the universe (BAU). New results for the electric dipole moment (EDM) of the electron place significant constraints on the beyond Standard Model CP-violation needed for successful EWBG. Using a specific model illustration, we show how new developments in EWBG quantum transport theory that include CP-violating sources first order in gradients imply more relaxed EDM constraints -- and thereby greater EWBG viability -- than implied by previous approximation formulations. We also illustrate how these developments enable a more realistic treatment of CP-conserving interactions that can also have a decisive impact on the predicted BAU.

hep-ph↗

arXiv2025

Primordial lepton asymmetries: neutrino transport, spectral distortions and cosmological constraints

The primordial neutrino asymmetry leaves profound imprints on the evolution history of the universe, which can be constrained by cosmological observations, including Big Bang Nucleosynthesis (BBN), Cosmic Microwave Background (CMB), and Large-Scale Structure (LSS). We present comprehensive analysis on implications and constraints of the primordial neutrino asymmetry, based on a precise treatment of neutrino decoupling by solving the complete (anti)neutrino quantum kinetic equations in the Closed-Time-Path formalism. { Assuming the same primordial asymmetry $ξ_ν$ for all neutrino flavors,} the effective number of neutrinos $N_{\rm eff}$ and (anti)neutrino spectral distortions are calculated, and we find that the non-instantaneous decoupling correction is given by $δN_{\rm eff} = 0.0440 + 0.0102 \, ξ_ν^2$. Then we perform the state-of-the-art calculation for the abundance of light elements including (anti)neutrino spectral distortions, which indicate a positive asymmetry $0.032 \leq ξ_nu \leq 0.052$ from EMPRESS data. The implications of the neutrino asymmetry for the CMB and LSS are studied in detail, and we find that the Baryon Acoustic Oscillations (BAO) are also significantly affected by $ξ_νin addition to the sum of neutrino masses. A combined analysis with EMPRESS BBN, Planck CMB and BOSS BAO data yields a tighter constraint $ξ_ν= 0.024 \pm 0.012$, which provides constraints on UV models capable of producing large asymmetries.

hep-ph↗

arXiv2025

Neutrinophilic $\mathbfΛ$CDM Extension for EMPRESS, DESI and Hubble Tension

A number of recent cosmological observations have indicated the presence of new physics beyond the $\mathbfΛ$CDM model. Combining observations from EMPRESS on helium abundance and DESI on baryon acoustic oscillations with Hubble tension, we show that all of them can be explained concurrently with a extension of the $\mathbfΛ$CDM model with primordial neutrino asymmetry $ξ_ν$ and additional contribution to the effective number of neutrinos $δN_{\rm eff}$. Based on the accurate treatments of neutrino decoupling and BBN processes, we present state-of-the-art constraints on neutrino asymmetry for the fixed or varying $N_{\rm eff}$. Comparing different extensions of the $\mathbfΛ$CDM model, we show that the neutrinophilic $\mathbfΛ$CDM extension with $ξ_ν = 0.056 \pm 0.017 $ and $δN_{\rm eff} = 0.41 \pm 0.16$ is preferred by current observations, while the Hubble tension in this model is also alleviated to be $2.2 σ$.

hep-ph↗

arXiv2024

Distinguishing thermal histories of dark matter from structure formation

It is important to understand the implications of current observational constraints and potential signatures on the thermal history of dark matter. In this paper, we build the connection between the present-day velocities and the production mechanism of dark matter and find that the current observation on structure formation can be imposed to constrain the decoupling temperatures and the phase-space distribution of dark matter. We further explore the potential of distinguishing different possible thermal histories of dark matter with hypothetical future observational data. Using the freeze-in/-out scenarios as templates, we find that future precision data may uniquely identify the allowed parameter spaces for freeze-in and freeze-out, or even completely rule out one of the scenarios. This method can be more generally applied to other scenarios.

hep-ph↗

arXiv2022

Revisiting Dark Matter Freeze-in and Freeze-out through Phase-Space Distribution

We revisit dark-matter production through freeze-in and freeze-out by solving the Boltzmann equations at the level of the phase-space distribution $f(p,t)$. Using the $2\to2$ annihilation and the $1\to2$ decay processes for illustration, we compare the resulting dark-matter relic abundance with that from the number-density approach. In the transition regime between freeze-in and freeze-out, we find the difference can be quite significant, or even by orders of magnitude if the annihilation of dark-matter particles or the decaying mediator is neglected. The freeze-in production in the $2\to2$ and the $1\to 2$ processes can also result in non-thermal phase-space distributions, or even multi-modal ones with out-of-equilibrium decay, which can potentially affect structure formation at late times. We also investigate how elastic scatterings can distort such non-thermal distributions.

hep-ph↗