Experimental goals and the requirement

The goal of the experimental program is to investigate $\Lambda$ hypernuclear structure by measuring precise $\Lambda$ hypernuclear mass spectra from light to medium-heavy mass region by taking advantages of the (e,e'K$^+$) reaction.

The proposed experiment will improve the best energy resolution of the $\Lambda$ hypernuclear reaction spectroscopy ever achieved and also extend the (e,e'K$^+$) reaction spectroscopy to the medium-heavy mass region. The energy resolution of 300 keV and high statistical accuracy of excitation spectra will provide key information on the single-particle behavior of a $\Lambda$ hyperon in a medium mass hypernuclear system, and allow precise studies of the effective $\Lambda$N interaction.

Immediate goals of the E01-011 experiments can be summarized as follows:

1. Excitation spectra of medium-heavy $\Lambda$ hypernuclei will be studied using the $^{28}$Si(e,e'K$^+$) $^{28}_{\Lambda }$Al reaction. It will measure precise binding energies and widths of various hypernuclear states for A=28. The new structures and/or spin-orbit splittings indicated by the recent $(\pi ^+,K^+)$ reaction spectra in the medium-heavy hypernuclei will be investigated with the unprecedented energy resolution.

2. Precise measurement of $^{12}$C(e,e'K$^+$) $^{12}_{\Lambda }$B will be used in order to study the detailed structure of a typical $p$-shell $\Lambda$ hypernucleus in a qualitative way. In particular, a high quality $^{12}_{\Lambda }$B spectrum can be compared to its controversial mirror-symmetric hypernucleus, $^{12}_{\Lambda }$C, which was well studied by the $(\pi ^+,K^+)$ reaction.

The successful pioneering experiment, E89-009 gave us confidence to explore the (e,e'K$^+$) hypernuclear study and was thoroughly examined in order to improve the experimental conditions significantly.

Although the basic configuration is similar to that of E89-009, two key experimental conditions are improved for E01-011 as follows:

• A new high resolution kaon spectrometer (HKS) was designed to have 20 msr solid angle with the splitter magnet and simultaneously to achieve a momentum resolution of $2 \times 10^{-4}$ (FWHM). The HKS spectrometer is under construction.
• A new experimental configuration will be employed to maximize hypernuclear production rates. The main background, 0-degree bremstrahlung electrons, should be avoided while the scattered electrons at a sufficiently forward angle should be measured. The ``Tilt method'', in which the Enge electron spectrometer is tilted by 4-5 degrees vertically to the splitter dispersive plane, will enable us to accept a beam current as high as a few tens $\mu$A. The "Tilt method" inherits advantageous aspects of the "0-degree tagging method" employed in the E89-009 experiment. At the same time, it suppresses the huge background of bremstrahlung electrons at the focal plane of the scattered electron spectrometer. The new configuration allows us to measure hypernuclear spectra even with higher Z targets such as $^{28}$Si and $^{51}$V.

Employing the proposed new experimental configuration and with the high-resolution kaon spectrometer (HKS), we expect more than an order of magnitude higher hypernuclear yields, and an order of magnitude better signal-to-accidental ratio compared to the previous E89-009 experiment.