Hong-Hao Zhang

Leptophilic dark matter (DM) is a well-motivated thermal weakly interacting massive particle framework that can evade stringent nuclear-recoil searches while remaining testable via DM-induced heating of neutron stars (NSs). In this work, we study leptophilic scalar DM in a U ( 1 ) L μ − L τ gauge extension of the Standard Model, which provides a common leptophilic portal for all scenarios considered. To reproduce the observed relic abundance while suppressing direct-detection signals, we investigate three benchmark realizations: (i) a secluded DM scenario in which the relic density is set by annihilation into U ( 1 ) L μ − L τ gauge bosons and two pseudo-Nambu-Goldstone boson (pNGB) DM models based on (ii) an SO(4) symmetry and (iii) an SO(3) symmetry. In the SO(4) pNGB model, the DM mass arises at tree level from a soft breaking term, while the elastic scattering amplitude is suppressed by a symmetry-protected cancellation. In the SO(3) pNGB model, the DM mass is generated radiatively at one loop via the U ( 1 ) L μ − L τ gauge interaction, and we show that this gauging preserves the same cancellation mechanism, maintaining compatibility with direct-detection null results. We perform a systematic parameter scan imposing relic density, direct and indirect detection, and neutrino trident constraints and identify viable sub-TeV to TeV DM candidates. Under the optimistic maximal-heating assumption that the capture rate reaches the geometric limit and that the captured DM population efficiently thermalizes and attains capture-annihilation equilibrium inside NSs, we find that the remaining parameter space can be tested by near-infrared observations of old NSs, providing sensitivity complementary to terrestrial searches in regions that are currently weakly constrained.

Symmetry serves as a fundamental cornerstone of modern physics research, with group theory being the core mathematical tool for characterizing symmetries. This paper aims to explore how to systematically teach group theory concepts using Maxwell's electromagnetic theory as the main thread in the course “Group Theory in Physics”. Maxwell's theory serves as an ideal pedagogical example, containing a rich variety of symmetry structures: it encompasses both finite groups and Lie groups, spacetime symmetries and internal symmetries, as well as traditional 0-form symmetries and generalized 1-form symmetries. Following the pedagogical principle of progressing from simple to complex, we first review the various formulations of Maxwell's equations (differential equations, covariant form, differential forms) and the action principle. Subsequently, we systematically elucidate the various symmetries of Maxwell's theory from both the equation and action perspectives: (1) Poincaré symmetry—including spacetime translations and Lorentz transformations, corresponding to energy-momentum conservation; (2) U(1) gauge symmetry—an internal symmetry acting on the electromagnetic potential, corresponding to charge conservation; (3) discrete symmetries—charge conjugation (C), parity (P), time reversal (T), and their combined CPT transformation; (4) conformal symmetry—scale transformations and special conformal transformations; (5) electromagnetic duality symmetry—discrete or continuous symmetries exchanging electric and magnetic fields, and generalized duality transformations with the introduction of magnetic monopoles. Through this systematic pedagogical design, students can not only gain a deep understanding of the mathematical structure of group theory and its physical significance, but also closely connect abstract group-theoretic concepts with concrete electromagnetic phenomena, experiencing the intellectual evolution from classical physics to modern gauge field theory. The course content starts from the fundamental Lorentz group and gradually delves into generalized symmetries at the forefront of contemporary physics, both reinforcing students' understanding of classical electrodynamics and laying a solid group-theoretic foundation for subsequent courses such as quantum field theory and condensed matter physics.

A bstract: We present a pragmatic approach to lower down the mass scale of right-handed neutrinos in leptogenesis by introducing a scalar decaying to right-handed neutrinos. The key point of our proposal is that the out-of-equilibrium decays of the scalar provide an additional source for right-handed neutrinos and hence the lepton asymmetry. This mechanism works well at low temperatures when the washout of the generated lepton asymmetry is suppressed. Thus, the lepton asymmetry can be effectively produced despite the washout effect is strong or not. Through a comprehensive analysis, we demonstrate that such a scalar-assisted leptogenesis can typically decrease the viable right-handed neutrino mass scale by one to three orders of magnitude.

We review the derivation of the Boltzmann equation and its cosmological applications in this paper. A novel derivation of the Boltzmann equation, especially the collision term, is discussed in detail in the language of quantum field theory without any assumption of a finite temperature system. We also discuss the integrated Boltzmann equation, incorporating the temperature parameter as an extension of the standard equation. Among a number of its cosmological applications, we mainly target two familiar examples, the dynamics of the dark matter abundance through the freeze-out/in process and a baryogenesis scenario. The formulations in those systems are briefly discussed with techniques in their calculations.