Inclusive spectrum of the $d(\pi^+, K^+)$ reaction at 1.69 GeV/c

TL;DR

This study measured the inclusive spectrum of the $d(\pi^+, K^+)$ reaction at 1.69 GeV/c, revealing known features and new deviations such as a $\Sigma N$ cusp and a shifted hyperon resonance bump, enhancing understanding of hyperon production.

Contribution

First detailed analysis of the inclusive $d(\pi^+, K^+)$ spectrum at this energy, identifying specific deviations from a simple quasi-free model related to hyperon resonances and $\Sigma N$ cusp effects.

Findings

Observation of a $\Sigma N$ cusp at 2.13 GeV/$c^2$

Shift of hyperon resonance bump by about 22 MeV/$c^2$

Spectrum largely explained by quasi-free model with notable deviations.

Abstract

We have measured an inclusive missing-mass spectrum of the $d(\pi^+, K^+)$ reaction at the pion incident momentum of 1.69 GeV/$c$ at the laboratory scattering angles between 2$^\circ$ and 16$^\circ$ with the missing-mass resolution of 2.7 $\pm$ 0.1 MeV/$c^2$ (FWHM) at the missing mass of 2.27 GeV/$c^{2}$. In this Letter, we first try to understand the spectrum as a simple quasi-free picture based on several known elementary cross sections, considering the neutron/proton Fermi motion in deuteron. While gross spectrum structures are well understood in this picture, we have observed two distinct deviations; one peculiar enhancement at 2.13 GeV/$c^2$ is due to the $\Sigma N$ cusp, and the other notable feature is a shift of a broad bump structure, mainly originating from hyperon resonance productions of $\Lambda(1405)$ and $\Sigma(1385)^{+/0}$, by about 22.4 $\pm$ 0.4 (stat.) $^{+2.7}_{-1.7}$ (syst.) MeV/$c^2$ toward the low-mass side, which is calculated in the kinematics of a proton at rest as the target.