Probing the Core of Nuclear Structure through the $πN$ Scattering at an Electron-Positron Collider

Abstract: Short-range correlation pairs (SRCs) -- core of nuclear structure, composed of highly off-shell nucleons -- are mostly studied via electron-nucleon scattering, leaving a gap in meson-based probes. We propose probing SRC off-shell nucleons via quasielastic $π<sup>+$-bound proton scattering ($π⁺ p \to π⁺ p$) at electron-positron colliders, of which the beryllium-based ($<sup>{9}$Be) beam pipe of the BESIII experiment operating at BEPCII, addresses a key gap and enables meson-beam investigations of SRCs. We point out that off-shellness of SRC nucleons yields measurable signatures: accumulated missing energy ($\sim0.1$\,GeV), shifted proton effective mass (0.7-0.8\,GeV), and cross-section differences from free scattering or with only Fermi motion. As an estimate, we find that BESIII's high luminosity and $π<sup>+$ yield support $\sim10<sup>4$ scattering events, while STCF ($50\times$ higher luminosity) will greatly enhance this number. This first meson-beam SRC study at an electron-positron collider fills $<sup>{9}$Be research gaps and advances understanding of nuclear structure core and nonperturbative QCD.