Principle and configuration

The previous E89-009 experimental setup accepted reaction electrons and kaons at angles including zero degrees. It was found that the event accumulation rate was limited by the accidental background rate from bremstrahlung electrons at 0 degree. We, therefore, proposed to tilt the electron spectrometer by a small angle sufficient to exclude electrons from the bremstrahlung process.

A plan view of the E01-011 geometry, which consists of a splitter magnet, the scattered electron spectrometer (Enge spectrometer) and the high-resolution kaon spectrometer (HKS), is shown in Figure 4. Both the HKS spectrometer and the Enge spectrometer are positioned at angles as forward as possible, but avoiding 0-degree electrons and positrons. The HKS spectrometer, having a QQD configuration, was designed for the kaon arm. It has a momentum resolution of 2 $\times$ 10$^{-4}$ at 1.2 GeV/$c$, and a large solid angle of 20 msr, when used with the splitter (Table 1).

For the design of the E01-011 experiment, the data taken in E89-009 experiment were fully studied. The singles rates of electrons, positrons, pions and protons in each arm were extracted from the analyzed data. The measured hadron rates were compared with the EPC code calculations and the normalization factors were derived. Assuming the obtained normalization factor at the forward angles, pion and proton singles rates for each spectrometer in the present E01-011 setup were evaluated as listed in Table 8.

In the present experimental setup, the Enge split-pole spectrometer used for the E89-009 experiment will be adopted for the scattered electrons. However, the spectrometer is to be vertically tilted by 4.5 degrees so that a major part of bremstrahlung electrons and the M$\phi$ller scattered electrons do not enter the spectrometer acceptance. Since the optics of the splitter + Enge spectrometer is modified due to the tilt, it becomes necessary to measure not only position but also incident angle of the particle trajectory at the focal plane to determine its momentum precisely. The focal plane detector system needs to be redesigned and will be explained later.

The splitter magnet has the same geometry as that used in E89-009 but the gap will be widened so that it matches the larger geometrical acceptance of the HKS spectrometer.

The configuration and specification of the E01-011 hypernuclear spectrometer system is summarized in Table 1.

表 1: Experimental condition and specification of the E01-011 hypernuclear spectrometer system

Beam condition
Beam energy	1.8 GeV
Beam momentum stability	1 $\times$ 10$^{-4}$
General configuration	Splitter+Kaon spectrometer+Enge spectrometer
Kaon spectrometer
Configuration	QQD and horizontal bend
Central momentum	1.2 GeV/$c$
Momentum acceptance	$\pm$ 12.5 %
Momentum resolution ($\Delta p/p$)	2 $\times$ 10$^{-4}$ (FWHM)
	(beam spot size 0.1mm assumed)
Solid angle	20 msr with the splitter
	(30 msr without the splitter)
Kaon detection angle	Horizontal : 7 degrees
Enge split-pole spectrometer
Central momentum	0.3 GeV/$c$
Momentum acceptance	$\pm$ 20 %
Momentum resolution ($\delta p/p$)	4 $\times$ 10$^{-4}$ (FWHM)
Electron detection angle	Horizontal : 0 degrees
	Vertical : 4.5 degrees