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arXiv · 2606.08230

Resonance Femtoscopy Beyond the On-Shell Approximation

Liang Zhang·Tianhao Shao·Song Zhang·Kai-Jia Sun·Yu-Gang Ma
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Abstract

The observed shift of the $\Delta(1232)$ resonance peak in $\pi$-$p$ femtoscopic correlations challenges the conventional Breit-Wigner description of resonances in femtoscopy. We revisit the Koonin-Pratt framework by formulating femtoscopy in the momentum-space representation. By employing the T-matrix approach to disentangle on-shell and off-shell contributions, we show that the finite spatial extent of the emission source naturally induces sensitivity to off-shell scattering dynamics. Using a Friedrichs-Lee model constrained by low-energy $\pi$-$p$ scattering data, we numerically demonstrate that this off-shell sensitivity leads to a peak shift accompanied by a dip on the high-momentum side of the peak. The predicted high-momentum side dip is absent in the data, pointing to source properties beyond the simple Gaussian approximation.

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Liang Zhang, Tianhao Shao, Song Zhang, Kai-Jia Sun, Yu-Gang Ma. 2026-06-06. Resonance Femtoscopy Beyond the On-Shell Approximation. https://arxiv.org/abs/2606.08230

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