03 September 2026

A discrete, anomaly-free $\mathbb Z_9$ gauge symmetry may persist to the deep infrared, exactly forbidding proton decay. This $\mathbb Z_9$ can emerge from Higgsing a lepton-flavour non-universal $U(1)_X$ that generates realistic neutrino masses and mixings through a high-scale seesaw and predicts an additional light Goldstone. Adding an anomaly-free chiral heavy-quark sector turns this mode into the QCD axion. The same selection rules that ban proton decay also forbid PQ-violating operators below a suitably high dimension, $d_{\rm PQ}\gtrsim10$, including operators involving the heavy quarks that radiatively match onto the scalar potential. We construct explicit models viable when PQ breaking occurs before or after inflation; in the latter case, avoiding stable heavy relics and eliminating the string-wall network tightly constrain the heavy-quark spectrum. We identify restricted regions of parameter space where thermal leptogenesis remains viable and, for $d_{\rm PQ}\geq13$, can coexist with a high-quality axion constituting all of the dark matter.

Dirac leptogenesis offers a qualitatively different route to the baryon asymmetry: total lepton number is conserved, but equal and opposite asymmetries are stored in the Standard Model and right handed neutrino sectors, with only the visible component processed by electroweak sphalerons. We present the first dedicated study of how evaporating primordial black holes (PBHs) modify this mechanism. In a minimal charged-scalar mediator setup, we solve the coupled Boltzmann system including thermal production, Hawking emission of the mediator $X$ and of right handed neutrinos, the PBH contribution to the expansion rate, and entropy injection. We find that Hawking emission can populate $X$ in regions where thermal production is inefficient, allowing its subsequent CP-violating decays to enhance the baryon asymmetry. For representative benchmarks, PBHs extend the successful region from mediator masses around $10^9\,{\rm GeV}$ down to about $10^6\,{\rm GeV}$ and can compensate for smaller effective CP asymmetries. This effect is not universal: where thermal Dirac leptogenesis is already efficient, PBHs can instead reduce the final asymmetry. Their evaporation also deposits energy in the right handed neutrino sector and contributes to $ΔN_{\rm eff}$. In the PBH parameter scan, part of the region that reproduces the observed baryon asymmetry lies below the Planck reference level $ΔN_{\rm eff}=0.284$ and above the projected CMB-S4 sensitivity $ΔN_{\rm eff}=0.06$.

The concept of quantum information has provided new ways to investigate the theoretical structure of particle interactions. In particular, it has been shown that extrema of quantum entanglement in scattering and decay processes can be related to symmetries and characteristic properties of particles. We focus on a more specific question: whether the minimization of entanglement entropy generated in scattering processes is systematically related to the suppression of interactions that violate global symmetries or selection rules. We conjecture that the minimization of the entanglement entropy can select the symmetry-preserving point when a new interaction opens a symmetry-violating scattering channel. We investigate the plausibility of this conjecture by considering interactions beyond the Standard Model (BSM) that violate global symmetries or the corresponding selection rules, including lepton number violation, baryon number violation, lepton flavor violation, and flavor-changing neutral currents. We show that, when symmetry-violating interactions open final-state sectors that are orthogonal to the symmetry-preserving sector, the symmetry-preserving point becomes a local minimum of the entanglement entropy. Our results support the possibility that the minimization of entanglement provides a common information-theoretic principle underlying the suppression of BSM phenomena relevant to symmetry-violating interactions.

Neutrino self-interactions mediated by a light scalar offer a compelling resolution to cosmological tensions and may naturally arise in neutrino mass generation mechanisms. When the scalar also couples to a light dark-sector fermion, supernova neutrinos can resonantly annihilate with the cosmic neutrino background (C$ν$B) into invisible dark radiation, depleting the flux en route to Earth. We use this depletion to constrain the neutrino-scalar coupling from the SN1987A data. Within a Bayesian framework we analyze the data for two supernova neutrino emission models, a parameterized model and a 2D hydrodynamic simulation, and for two coupling types, a mass-independent one and a mass-proportional one. Our results show that new-physics limits from SN1987A cannot be quoted independently of the heavy-flavor emission, the flavor conversion, or the coupling structure. Finally, we forecast that a future high-statistics burst recorded by Hyper-Kamiokande would restore a meaningful upper bound even when flavor conversion is included.

The LUX-ZEPLIN (LZ) experiment has reported one nuclear recoil event reconstructed at $E_R=248\pm23_{\rm stat}\pm23_{\rm sys}$\,keV in a search extending to $E_R\simeq270$\,keV. We investigate an inelastic electroweak (EW) doublet interpretation in which spontaneous Peccei--Quinn (PQ) breaking leaves a residual parity that stabilizes the lighter neutral state and generates the Majorana splitting. A PQ-charged singlet fermion acquires its Majorana mass from the PQ-breaking vacuum expectation value and, after being integrated out, induces the dimension-five operator that splits a vectorlike EW doublet. A minimal KSVZ colored sector supplies the QCD anomaly. For a representative PQ-breaking scale $v_S=4.4\times10^{11}$\,GeV and Majorana mass $M_N\simeq50$\,TeV, the relation $M_N=y_Sv_S/\sqrt{2}$ gives $y_S\simeq1.6\times10^{-7}$, while the splitting relevant for LZ requires a Higgs--doublet Yukawa coupling $y\simeq0.023$. In a standard thermal history, the EW doublet constitutes only a subcomponent of the dark matter. A rate estimate probing the high velocity tail gives representative benchmarks with $M_D\simeq380$--$420$\,GeV, $\dchi\simeq323$--$330$\,keV, and $ξ_χ\simeq0.12$--$0.15$ yielding an order-one LZ event count. The remaining abundance is supplied by the QCD axion through vacuum misalignment for suitable $v_S$ and initial angle. The excited state has a radiative lifetime of order $3\times10^{-2}$\,s and a decay length of order $20$\,km, leaving the LZ topology as a single nuclear recoil while enabling a complementary luminous signal after terrestrial upscattering.

It is known that CP violation occurs in flavor physics, and the CKM matrix is complex. We investigate the Yukawa sector of a four-Higgs model based on D_5 symmetry. To understand all possible sources of CP violation in the Yukawa sector, we systematically analyze the impact of all possible representation assignments of the left-handed and right-handed quark fields under the D_5 group. After imposing the conditions of non-block-diagonal CKM matrix and the absence of massless quarks, we find that the only viable representation consists of one D_5 doublet and one singlet. In this case, two distinct types of quark mass models emerge in the quark sector. For each model, we introduce the most general vacuum that allows spontaneous CP violation (SCPV) into the mass matrices. Due to the specific structure of the D_5 symmetry, the vacuum expectation values (vevs) satisfy particular relations such that, for both quark models, the up- and down-quark mass squared matrices (i.e., M_d M_d^\dagger and M_u M_u^\dagger) can be simultaneously block-diagonalized by the same unitary transformation, forcing the CKM matrix to be block-diagonal and thus yielding vanishing mixing angles and CP violation. This result rules out the possibility of explaining experimental observations within the D_5 4HDM under the SCPV framework.

Lepton flavor violation (LFV) processes are forbidden in the standard model (SM), hence the observation of LFV transitions would represent a clear signal of new physics beyond the SM. In this work, we investigate the muon anomalous magnetic dipole moments (MDM) and LFV processes $l_{j}^{-}\to l_{i}^{-}γ$ and $l_{j}^{-}\to l_{i}^{-}l_{i}^{-}l_{i}^{+}$ in the extension of the SM with $U(1)_F$ local gauge symmetry (FDM). The muon anomalous MDM is one of the most precisely measured quantities in particle physics, which can be used to constrain the contributions from new couplings in the FDM. The newly incorporated $U(1)_F$ charges are correlated with the flavor properties of fermions, hence the newly added Yukawa couplings make contributions to these LFV processes. Considering the latest experimental constraints on the muon anomalous MDM, the new interactions in the FDM can make significant contributions to the LFV processes $l_{j}^{-}\to l_{i}^{-}γ$ and $l_{j}^{-}\to l_{i}^{-}l_{i}^{-}l_{i}^{+}$, the predicted branching ratios can well reach the future experimental sensitivity.

The 248-keV nuclear-recoil-like event reported by LZ can be interpreted as near-threshold upscattering of 1.08-TeV thermal Higgsino dark matter. A neutral-Higgsino splitting of order $0.3$--$0.35$ MeV places the fixed weak-interaction cross section close to the observed LZ interval and implies electroweak gauginos at ${O}(10^7)$ GeV. This spectrum is compatible with gravity-mediated high-scale supersymmetry and a Kim--Nilles Peccei--Quinn sector. Although the GUT scale is predicted to be lower than the conventional one, proton decay is suppressed due to the high SUSY scale which also solves the conventional CP, flavor, gravitino and moduli problems. Indeed, proton decay may be probed in the future. The PQ symmetry lowers the Higgsino mass below the SUSY scale and also yields a QCD axion that solves the strong-$CP$ problem. If the axion and Higgsino constitute mixed dark matter, the halo-profile-dependent H.E.S.S. limit can be relaxed.

The LUX-ZEPLIN (LZ) experiment has reported a single candidate event in the high-energy nuclear recoil window $248\pm32.5\ \mathrm{keV}_{\mathrm{nr}}$ with an exposure of $2.84\ \mathrm{ton}\cdot\mathrm{yr}$, while the low-energy spectrum remains consistent with background expectations. We demonstrate that this excess can be naturally explained by the neutral-current absorption of fermionic dark matter on xenon nuclei. For a dark matter mass $m_χ\simeq 247\ \mathrm{MeV}$, the coherent absorption process produces a monoenergetic nuclear recoil at $E_R \simeq 248\ \mathrm{keV}_{\mathrm{nr}}$. At this momentum transfer, the absorption process enters the incoherent regime, where scattering off individual nucleons produces a broad recoil spectrum extending from $\sim 200\ \mathrm{keV}$ to $100.2\ \mathrm{MeV}$. We show that a single effective field theory coupling can simultaneously produce one event in the $248\pm32.5\ \mathrm{keV}_{\mathrm{nr}}$ window while remaining consistent with the non-observation of events in neighboring energy regions. The required single-nucleon absorption cross section is $σ_{χN}^{\mathrm{NC}} = 1.07\times10^{-46}\ \mathrm{cm}^2$, corresponding to an effective field theory scale $Λ\simeq 11.5\ \mathrm{TeV}$. However, a recasting analysis of KamLAND data on the neutron-emission channel $χ+{}^{12}\mathrm{C} \to ν+ n + {}^{11}\mathrm{C}^*$ excludes this benchmark parameter space, establishing a significant tension between the LZ excess interpretation and existing constraints from large-volume scintillator detectors. We discuss the implications of this tension and prospects for resolving it with future dedicated high-energy analyses.

The Higgsino dark matter of the minimal supersymmetric standard model is one of the most minimal and canonical WIMP candidates. We demonstrate that this classic candidate can account for both the high-energy nuclear-recoil excess reported by the LZ experiment and the mild preference in the 14 years of FermiLAT Galactic-center data, with the relic abundance fixing the Higgsino dark matter mass near 1.1 TeV and the recoil spectrum selecting a mass splitting of a few hundred keV. Together with the 10-year IceCube solar-neutrino constraint, the Higgsino interpretation further pins down a viable region of parameter space. It predicts a Galactic-center gamma-ray line and endpoint signal near the present H.E.S.S. sensitivity and within CTAO's projected reach, making this classic candidate testable in the near future.

We propose Higgsino dark matter as a potential interpretation of the $248~\mathrm{keV}$ nuclear-recoil event of interest reported by the LUX-ZEPLIN (LZ) experiment. LZ studied several rare background processes and detector effects in detail, but did not identify any as a likely explanation of the event. A nearly pure Higgsino with mass $m_{\widetilde H}\sim1~\mathrm{TeV}$ naturally realizes inelastic dark matter through the off-diagonal $Z$ coupling of two nearly degenerate neutral Majorana states separated by a mass splitting $δ$. The same electroweak interaction fixes the inelastic Higgsino-nucleon scattering cross section, rather than leaving it as a free parameter. We show that the predicted Higgsino inelastic scattering cross section approaches the published LZ two-sided 90% confidence interval for $δ\sim350~\mathrm{keV}$.

Higgsino dark matter remains one of the most convincing dark matter candidates. It can exist in a ``quasi-Dirac" regime, when the splitting between two Majorana components is on the order of $O(\rm few~\,MeV)$ or less. Owing to the strong gravitational pull of the Sun, the DM particles accelerate to up to $\sim 1400$\,km/sec in the solar core, which allows for inelastic scattering on heavy elements and very effective capture onto solar-bound trajectories. We evaluate the expected flux of the neutrinos from the expected $χχ\to W^+W^-,ZZ$ annihilation for the cosmologically preferred mass, $m_χ\simeq 1.08$\,TeV while keeping the mass splitting $δ$ as a free parameter. Confronting it with the non-observation of high-energy neutrinos from the Sun by the IceCube experiment, we obtain a robust limit, $δ>566$\,keV. These limits exclude the interpretation of the recent LZ event in terms of endothermic inelastic scattering of Higgsino dark matter on xenon nuclei.