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

TL;DR

This paper proposes a new method to measure light-quark Yukawa couplings at lepton colliders using dihadron fragmentation and azimuthal modulations, providing a more sensitive probe of Higgs flavor structure.

Contribution

It introduces a theoretical framework leveraging azimuthal modulations in dihadron fragmentation to access light-quark Yukawa couplings at lepton colliders, sensitive to interference effects.

Findings

Potential to set limits on Yukawa couplings at the 10^{-4} level.

Disentangles up- and down-quark Yukawa contributions.

Establishes fragmentation dynamics as a new probe of Higgs flavor structure.

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, $h^\prime=\pi^\pm,K^\pm$, 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.