Towards a Better Understanding of the Short-range Repulsive Nuclear Force

One of the central problems in nuclear physics concerns the origin of the short-range repulsive nuclear force. It is suggested that this repulsive force can be explained based on the spin-spin interaction between quarks and the Pauli-forbidden states at the quark level. In particular, it is necessary to explore what is known as the “10-plet quark–Pauli forbidden state” in the flavor SU(3) symmetry.

The Σ⁺p scattering channel is considered one of the best channels for studying the 10-plet state. Specifically, the strength of the repulsive interaction is related to the differential cross-section for this scattering. However, theoretical predictions of the differential scattering cross-section in the momentum region above 400 MeV/c gives varying results. It is, therefore, necessary to determine this scattering cross-section experimentally.

Recently, an international team of researchers conducted a high-statistics Σ⁺p scattering experiment at the Japan Proton Accelerator Research Complex (J-PARC) Hadron Experimental Facility. First, Σ⁺ particles were generated in a liquid hydrogen (or LH₂) target. The Σ⁺p scattering events were then produced by colliding the Σ⁺ particles with the LH₂ target. To identify these events, the team used the magnetic spectrometer systems to reconstruct the incident momentum of Σ⁺ particles and the CATCH detector system to detect the recoil protons of the scattering events.

The team identified around 2400 events, 80 times more than that recorded in past experiments, in the momentum range of (0.44 – 0.8) GeV/c, and estimated the differential scattering cross-section for three separate momentum regions. Compared to past experiments, their results had lower uncertainties and significantly better quality. The calculated differential cross section was smaller than those predicted by theoretical models.

Further, the team estimated the phase shift of the ³S₁ channel, which is related to the quark Pauli effect, for the first time. This provided a strong constraint on the strength of the repulsive force.

Overall, this study will provide a deeper understanding of the repulsive nuclear force, and, hopefully, further our understanding of matter and the Universe as a whole.