Measurement of $Λ$ Polarization in the $π^{-}p \to K^{0} Λ$ Reaction at $p_{π^{-}}=1.33$ GeV/$c$ toward a New $Λp$ Scattering Experiment

TL;DR

This work delivers high-precision measurements of $\Lambda$ polarization in the $\pi^- p \to K^0 \Lambda$ reaction at $p_{\pi^-}=1.33$ GeV/$c$, using J-PARC E40 with $K^0$ and $\Lambda$ reconstruction via $K^0\to\pi^+\pi^-$ and $\Lambda\to p\pi^-$ decays. The polarization is extracted from the angular distribution of the decay proton, $\frac{dN}{d\cos\theta_p} = \frac{N_0}{2}\left[1+\alpha P_{\Lambda}\cos\theta_p\right]$, after careful background subtraction, acceptance corrections, and a chi-square–driven bin optimization. The result, $\langle P_{\Lambda} \rangle = 0.932 \pm 0.058\,\text{(stat)} \pm 0.028\,\text{(syst)}$ in $0.60<\cos\theta^{K0}_{CM}<0.85$, demonstrates a strongly polarized $\Lambda$ and provides essential input for dynamical coupled-channel (DCC) partial-wave analyses and for planning a future polarized $\Lambda p$ scattering program (E86). The observed large polarization is interpreted as constructive interference between spin-flip and non-spin-flip amplitudes, underscoring the role of multi-channel resonance dynamics in hyperon production and offering a stringent benchmark for theoretical models.

Abstract

This paper presents high-precision experimental data of the polarization of the $Λ$ hyperon in the $π^{-}p \to K^{0} Λ$ reaction, measured in the angular range $0.6<\cos θ^{CM}_{K0}<1.0$ with a fine bin width of $d\cos θ^{CM}_{K0}=0.05$. The data were obtained from the J-PARC E40 experiment at the K1.8 beamline in the J-PARC Hadron Experimental Facility. The observed average polarization of $Λ$ in the range $0.60<\cos θ^{CM}_{K0}<0.85$ was $0.932 \pm 0.058 \,(\text{stat}) \pm 0.028 \,(\text{syst})$, demonstrating the successful extraction of precise polarization observables. This result provides essential experimental input for partial wave analysis (PWA) of dynamical coupled-channel (DCC) models, which aim to uncover the underlying mechanisms of $N^{*}$ resonances that emerge in intermediate states of $πN$ and $γN$ interactions. Besides, it indicates the feasibility of a strongly polarized $Λ$ beam suitable for future $Λp$ scattering experiments (e.g., J-PARC E86).

Figures (24)

• Figure 1: Schematic view of the experimental setup in J-PARC E40. After the K1.8 beamline spectrometer, composed of QQDQQ magnets, a cylindrical detector cluster (CATCH Akazawa) surrounding the liquid hydrogen (LH$_2$) target, and a forward magnetic spectrometer (KURAMA) were installed.
• Figure 2: Correlation between the mass squared ($m^{2}$) and momentum measured using the KURAMA spectrometer, taken in E40 data. The region with $-0.1<m^{2}<0.15$ and $p<1.1$ was selected as $\pi^{+}$.
• Figure 3: Correlation between energy deposit measured using CFT ($\Delta E$) and total energy measured using BGO ($E$), taken in E40 data. The region surrounded by the red solid lines was selected as $\pi^{-}$.
• Figure 4: Missing mass of the $\pi^{-} p\to K^{0} X$ reaction, $M_{\pi^{-} p\to K^{0} X}$. The red solid line represents the fitting function of two Gaussians and a third-order polynomial, and the blue shade represents the estimated background structure. The $\pm3\sigma$ interval of the first Gaussian (1.07-1.16 GeV/$c^{2}$) was defined as the $\Lambda$ range for later analyses. The total yield of $\Lambda$ was estimated to be $6.73\times10^{4}$.
• Figure 5: Correlation between $\Delta p$ and $\cos{\Delta\theta}$, taken in the simulation data.