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| Main Authors: | , , |
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| Format: | Preprint |
| Published: |
2024
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2406.09381 |
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Table of Contents:
- In this work we advocate a set of novel azimuthal-angle-related observables associated with exclusive hadron production from two-photon fusion at $e^+ e^-$ colliders, taking the $γγ\to ππ$ as a benchmark process. As a direct consequence of the linearly polarized quasi-real photons emitted off the electron and positron beams, the $\cos 2ϕ$ azimuthal asymmetry in dipion production is predicted within the transverse-momentum-dependent (TMD) factorization framework. In numerical analysis, we take the helicity amplitudes of $γγ\to ππ$ determined from the partial wave solutions in dispersion relation as input, and find that the predicted $\cos2ϕ$ azimuthal modulation may reach 40\% for the typical kinematical setup of {\tt Belle 2} and {\tt BESIII} experiments. Future accurate measurement of this azimuthal asymmetry may facilitate the direct extraction of the relative phase between two helicity amplitudes with photon helicity configurations $++$ and $+-$. This knowledge provides a valuable input for the dispersive determination of the hadronic light-by-light (Hlbl) contributions.