Probing the Core of Nuclear Structure through the $\pi N$ Scattering at an Electron-Positron Collider

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 ${\pi }^{+}$-bound proton scattering (${\pi }^{+}p\to {\pi }^{+}p$) at electron-positron colliders, of which the beryllium-based (${}^9$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 ($\sim 0.1$,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 ${\pi }^{+}$ yield support $\sim {10}^4$ 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 ${}^9$Be research gaps and advances understanding of nuclear structure core and nonperturbative QCD.