Xin-Kai Wen

The color-octet (CO) mechanism is a cornerstone of nonrelativistic QCD, yet knowledge of its long-distance matrix elements remains limited, preventing stringent tests of the theory. We demonstrate that the Artru-Collins asymmetry in hadronic decays of the P -wave bottomonium state χ b 2 provides a direct probe of CO dynamics. At leading order, the asymmetry arises exclusively from the CO decay channel, whereas the color-singlet (CS) contribution affects only the unpolarized rate, so that a nonzero signal constitutes unambiguous evidence of the CO mechanism. This observable provides a novel way to extract the ratio ρ 8 between CO and CS matrix elements. Focusing on e + e − → ϒ ( 2 S ) → γ χ b 2 at Belle, we show that the asymmetric beam configuration preserves the asymmetry in the laboratory frame and avoids the strong suppression present in the center-of-mass frame. With the Belle II dataset, ρ 8 could be determined with sufficient precision to address the long-standing discrepancy between the lattice calculations and phenomenological determinations.

Electromagnetic dipole interactions of light quarks induce distinct spin correlations in quark pairs produced at lepton colliders, favoring entangled spin-triplet state aligned along the z ^ axis or spin-singlet state. These correlations lead to unique cos ⁡ ( ϕ 1 − ϕ 2 ) azimuthal asymmetries in inclusive π + π − -dihadron pair production and in back-to-back hadron pairs ( π π , K π , K K ), which are absent in the SM. Using published Belle and BaBar measurements together with projected sensitivities based on ratios of azimuthal asymmetries, we demonstrate that these measurements provide significant constraints on light-quark dipole couplings, with a reduced dependence on poorly known nonperturbative fragmentation functions and free from contamination by other new physics effects. This approach offers a clean and novel probe of light-quark dipole interactions in collider experiments.

Directly probing light-quark Yukawa couplings and their flavor structure remains a major challenge due to their smallness and overwhelming QCD backgrounds. In this Letter, we propose a theoretical framework to access these couplings at lepton colliders through transverse spin dependent azimuthal modulations in dihadron fragmentation. These modulations arise from the interference between Higgs mediated and standard model amplitudes in e − e + → q q ¯ Z , producing angular structures that are linearly sensitive to the Yukawa couplings y q , in contrast to conventional observables that scale as y q 2 . By combining channels with an identified accompanying single hadron, h ′ = π ± , K ± , and p / p ¯ , this approach cleanly disentangles the up- and down-quark Yukawa contributions, yielding typical limits at the O ( 10 − 4 – 10 − 3 ) level and establishing fragmentation dynamics as a novel and complementary probe of the Higgs flavor structure.

We propose to probe light-quark dipole interactions at lepton colliders using the azimuthal asymmetry of a collinear dihadron pair ( h 1 h 2 ) produced in association with another hadron h ′ . This asymmetry, arising from quantum interference in the quark spin space, is exclusively sensitive to dipole interactions at the leading power of the new physics scale and simultaneously probes both the real and imaginary components of the dipole couplings. By combining all possible channels of h ′ , this method allows for disentangling the up and down quark dipole moments and has the potential to significantly strengthen current constraints by one to two orders of magnitude.