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Role of the possible $Σ^*(\frac{1}{2}^-)$ state in the $Λp \to Λp π^0$ reaction

The $Λp \to Λp π^0$ reaction near threshold is studied within an effective Lagrangian method. The production process is described by single-pion and single-kaon exchange. In addition to the role played by the $Σ^*(1385)$ resonance of spin-parity $J^P = 3/2^+$, the effects of a newly proposed $Σ^*$ ($J^P = 1/2^-$) state with mass and width around $1380$ MeV and $120$ MeV are investigated. We show that our model leads to a good description of the experimental data on the total cross section of the $Λp \to Λp π^0$ reaction by including the contributions from the possible $Σ^*(\frac{1}{2}^-)$ state. However, the theoretical calculations by considering only the $Σ^*(1385)$ resonance fail to reproduce the experimental data, especially for the enhancement close to the reaction threshold. On the other hand, it is found that the single-pion exchange is dominant. Furthermore, we also demonstrate that the angular distributions provide direct information of this reaction, hence could be useful for the investigation of the existence of the $Σ^*(\frac{1}{2}^-)$ state and may be tested by future experiments.