Principle and experimental configuration

The previous E89-009 experimental setup accepted reaction electrons and kaons at angles including zero degrees. In this configuration, the event accumulation rate was limited by the accidental background rate from bremsstrahlung electrons at 0 degrees. Therefore, we proposed to tilt the electron spectrometer by a small angle sufficient to exclude electrons from the bremsstrahlung process.

A plan view of the E01-011 geometry, which consists of a splitter magnet (Splitter), 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 the 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 about 16 msr, when used with the Splitter. Experimental condition and specification of the HKS spectrometer is summarized in 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 normalization factors with respect to this code were derived. Assuming the obtained normalization factor at the forward angles, pion and proton singles rates of each counter in the present E01-011 setup were evaluated as listed in table 10.

In the present experimental setup, the Enge split-pole spectrometer used for the E89-009 experiment will be employed again as the scattered electron spectrometer. However, the spectrometer will be vertically tilted to accept only the electrons with an scattering angle larger than 4 degrees so that a major part of bremsstrahlung electrons and the M$\phi$ller scattered electrons do not enter the spectrometer acceptance. Since the optics of the splitter + Enge spectrometers are modified due to the tilt, it becomes necessary to measure not only positions but also incident angle of the particle trajectory at the focal plane to determine the momentum with sufficient precision. Therefore, the focal plane detector system was redesigned. Details of the detector will be explained later.

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

Table 1 summarizes the configuration and specification of the E01-011 hypernuclear spectrometer system.

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 % ($1.05 - 1.35$ GeV/$c$)
Momentum resolution ($\Delta p/p$)	2 $\times$ 10$^{-4}$
	(beam spot size 0.05 mm assumed)
Solid angle	16 msr with a splitter
	(30 msr without Splitter)
Kaon detection angle	Horizontal : 7 degrees ($0 - 14^\circ$)
Enge split-pole spectrometer
Central momentum	0.316 GeV/$c$
Momentum acceptance	$\pm$ 30 % ($221 - 411$MeV/$c$)
Momentum resolution ($\delta p/p$)	4 $\times$ 10$^{-4}$
Tilt angle	Vertical : 7.75 degrees $^*$
Angular acceptance	$3.7 - 5.7$ degrees

$^*$ The 7.75 degrees tilt of the spectrometer corresponds to the acceptance of electrons with scattering angles larger than 3.7 degrees.