Unveiling Light-Quark Yukawa Flavor Structure via Dihadron Fragmentation at Lepton Colliders

Abstract

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 $e^-e^+\to q\bar{q}Z$, 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, $h^\prime=π^\pm,K^\pm$, and $p/\bar{p}$, this approach cleanly disentangles the up- and down-quark Yukawa contributions, yielding typical limits at the $\mathcal{O}(10^{-4})$ level and establishing fragmentation dynamics as a novel and complementary probe of the Higgs flavor structure.

Figures (3)

• Figure 1: Representative diagrams for the interference between the Higgs signal and continuum background with identified fragmentation hadrons in Eq. \ref{['eq:sia']}. Gray bubbles indicate additional fragmentation products. Quark chiralities (L,R) are shown, and the Yukawa coupling induces a chirality flip.
• Figure 2: Kinematic configuration of $e^-e^+\to q\bar{q}Z$ with quark $q$ fragmenting into a dihadron ($h_1h_2$) and antiquark $\bar{q}$ producing a single hadron $h'$.
• Figure 3: Expected 68% C.L. limits on light-quark Yukawa couplings from the channels $(\pi^+\pi^-)\pi^\pm$ (red), $(\pi^+\pi^-)K^\pm$ (blue), and $(\pi^+\pi^-)p/\bar{p}$ (green), with their combination in gray, assuming $CP$ conservation and extracted from the azimuthal asymmetries $A_{LR, UD}^{h'}$.