New measurement system for the spin correlation coefficients in deuteron-proton elastic scattering

Recent years have seen a growing need for high-precision data on spin observables in nucleon-deuteron (Nd) elastic scattering below the pion production threshold. Such data are expected to play a crucial role in advancing the understanding of the three-nucleon force (3NF) and in establishing its description within the framework of chiral effective field theory ( $\chi$ EFT). This paper presents a new measurement system developed at the RIKEN RI Beam Factory to address such demand, featuring a solid-state polarized proton target and the KuJyaku detector for deuteron-proton (d-p) elastic scattering experiments. In conjunction with the polarized deuteron beams provided by the polarized ion source at RIKEN, the system enables measurement of the spin correlation coefficients-for which data remain scarce-along with the deuteron and proton analyzing powers in d-p elastic scattering. Performances of the newly developed target and detector systems were evaluated under realistic beam conditions through deuteron-polarized proton scattering experiment at 135 MeV/nucleon. The angular distribution of the extracted relative differential cross-section and proton analyzing power exhibit close agreement with existing data, validating the identification of d-p elastic events using the KuJyaku detector. The absolute polarization of the solid-state polarized proton target was determined to be $0.034\pm 0.{004}_{\mathrm{stat}.}\pm 0.{001}_{\mathrm{sys}.}$ , with its stability under beam irradiations confirmed throughout the experiment. Discussions are presented on the estimated uncertainties of the spin correlation coefficients to be measured. The results are compared with the latest sensitivity studies on the low-energy constants in the 3NF sector of $\chi$ EFT at the fifth order (N $_{}^4$ LO), demonstrating the feasibility of spin-observable measurements using the new experimental apparatus as a crucial step toward establishing the high-precision 3NF potential.