Studies of spin-dependent interactions in hyperon-proton
scattering [E452] with a newly developed track-detector
SCITIC [T485, T498, T511].
In order to study the hyperon-nucleon spin-orbit interactions, a new track detector named
SCITIC (scintillating track image camera) has been developed, and asymmetries of the polarized Σ+ elastic scattering on protons have been measured.
The polarized hyperons, Σ+ and Λ, were produced through the p(π + ,K+ ) reactions at pπ =1.6 GeV/c in a liquid-scintillator active target, and tracks of the hyperon scattering eventswere recorded with the SCITIC (Fig.1(a) )[1]. The track image in the liquid scintillator wasfocussed on a photocathode of an image intensiﬁer (IIT) through an optical lens system, andafter multiplications of the photoelectrons in 4-stages of IIT, the picture was recorded with aCCD camera. Two or three sets of SCITICs were used to take pictures from diﬀerent angles
to reproduce a three-dimensional event pattern (Fig.1(b),(c)). The hyperon-scattering eventswere selectively stored with trigger signals from a spectrometer indicating the kaons. Roughly,800,000 pictures have been accumulated, and about 1/10 of them have so far been analyzed.
Results of the analyses are summarized in Fig.2. Major motivation of the present experiment [E452] was to compare the eﬀects of spin-orbit interactions on the left/right asymmetry(NL - NR )/(NL + NR ) in the elastic scattering of polarized Σ+ and Λ hyperons. The Σ+
hyperon was known to be polarized as large as 0.8 from previous experiments[2] as shown inFig.2(a). The polarization of Λ determined from the present analysis was also large (Fig.2(c)).
Data on the left/right asymmetries for both Σ+ p- and Λp-scatterings are shown in Fig.2(b)and (d), respectively. Results from a reanalysis of the E289 data are also included.
Although analyses of 10-times more data with more sophisticated data-taking systems andmore elaborated analyzing tools are under way, data from the early stage of this experimentindicated a large Σ+ p spin-orbit interaction in contrast to the case of the Λp.
The smalless of ΛN spin-orbit interaction has been indicated in the old Λ-hypernuclear experiment by the Heidelberg-Saclay collaboration at CERN in 1978[3], and conﬁrmed by recent
experiments at KEK and BNL with better precision. Based on a naive quark-model picture,
H. Pirner[4] has pointed out that, in contrast to the ΛN system, the spin-orbit interaction
ΛN
ΣNNN
must be large in the ΣN system (VLS 0, VLS= 43 VLS). K. Yazaki[5] has also indicated apossible large spin-orbit interaction for the ΣN system, while the ΛN LS-interaction is small
due to cancellation between anti-symmetric and symmetric LS-terms. The present results
support these theoretical suggestions.

References[1] A. Sato, PhD Thesis, TUS-PN-0010.
[2] D.J. Candlin et al., Nucl. Phys. B226 (1983 )1.
J. Haba et al., Nucl. Phys. B299 (1988 )627.
[3] W. Bruckner et al., Phys. Lett. 79B (1978 )157.
[4] H.J. Pirner, Phys. Lett. 85B (1979 )190.
[5] K. Yazaki, Nucl. Phys. A479 (1988 )217c.

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Figure 1: (a) Schematic illustration of SCITIC. (b) Typical pictures of hyperon scattering
seen by two sets of SCITIC arranged as shown in (c).

2Figure 2: Hyperon polarizations following the (π + ,K+ ) reactions[2], and asymmetries of thepolarized hyperon scattering on protons: (a) Σ+ and (b) Λ.

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