Beyond Standard Model scenarios addressing the flavor puzzle and the hierarchy problem generally predict dominant new physics couplings with fermions of the third generation. In this Letter, we explore the collider and astrophysical signatures of new light scalar and pseudoscalar particles dominantly coupled to the $\tau$-lepton. The best experimental prospects are expected at Belle II through the $e^+e^-\to\tau^+\tau^-\gamma\gamma$, $\tau^+\tau^-\gamma$, $3\gamma$, mono-$\gamma$ processes, and the $\tau$ anomalous magnetic moment. The correlated effects in these searches can unambiguously point toward the underlying new physics dynamics. Moreover, we study astrophysics bounds - especially from core-collapse supernovae and neutron star mergers - finding them particularly effective and complementary to collider bounds. We carry out this program in the well-motivated context of axion-like particles as well as generic CP-even and CP-odd particles, highlighting possible ways to discriminate among them.

We propose a unique topological portal between quantum chromodynamics (QCD) and a dark sector characterized by a global symmetry breaking, which connects three QCD to two dark pions. When gauged, it serves as the leading portal between the two sectors, providing an elegant, self-consistent scenario of light thermal inelastic dark matter. The inherent antisymmetrization leads to diminished annihilations at later times and suppressed direct detection. However, novel collider signatures offer tremendous prospects for discovery at Belle II.

Motivated by the recent anomalies observed in semileptonic decays of B-mesons, the class of models denoted as"4321"is introduced and analysed. The importance of calculating next-to-leading-order (NLO) effects in performing precise compatibility tests of data with 4321 predictions is discussed. On the one hand, the coupling of the $U_1 \sim (\textbf{3},\textbf{1})_{2/3}$ leptoquark appearing in the spectrum to the third-generation fermions is large, giving rise to sizable NLO corrections in the relevant semileptonic operators. On the other hand, the new source of flavor violation is introduced with the vector-like fermions, resulting in the processes whose relevance can only be quantified at the loop order.