Experimental goals and the requirement

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

The proposed experiment will provide $\Lambda$ hypernuclear spectra with the best energy resolution ($\sim$ 300 keV) ever achieved in hypernuclear reaction spectroscopy and also make it possible to realize (e,e'K$^+$) reaction spectroscopy in the medium-heavy mass region. The energy resolution of a few hundred keV and high statistical accuracy of the level structure will yield information on the single-particle behavior of a $\Lambda$ hyperon in a medium mass hypernuclear system, and also allow precise studies of the effective $\Lambda$-N interaction.

Immediate goals of E01-011 are the following:

1. Excitation spectra of $\Lambda$ hypernuclei beyond the p-shell region will be studied in
   the $^{28}$Si(e,e'K$^+$) $^{28}_{\Lambda }$Al reaction. The precision mass spectrum will provide binding energies and widths of the single particle states in the A=28 hypernuclear system. New hypernuclear structures and/or spin-orbit splittings suggested by the recent $(\pi^+,K^+)$ reaction spectrum in the medium-heavy hypernuclei can be investigated with the unprecedented energy resolution.

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

This experimental program can now be undertaken with deeper confidence thanks to the pioneering and successful E89-009 experiment. In designing the E01-011 experiment, the E89-009 results were thoroughly examined in order to significantly improve the experimental conditions. Although the basic configuration is similar to that of E89-009, the two key experimental conditions which we will employ in E01-011 are as follows,

• A new high resolution kaon spectrometer (HKS) was designed to have 3 times greater solid angle than the SOS when they are used with the Splitter magnet and simultaneously to achieve a momentum resolution of $2 \times 10^{-4}$. Currently the HKS spectrometer is under construction.
• A new experimental configuration, ``Tilt method'', that will maximize hypernuclear production rates has been proposed. In this configuration, the Enge electron spectrometer is tilted vertically to the splitter dispersive plane so as to choose electron scattering angles of 4-5 degrees. This avoids the intense 0-degree bremsstrahlung electrons but still allows detection of the scattered electrons at a sufficiently forward angle. Therefore, the ``Tilt method'' inherits the advantageous aspects of the ``0 degree tagging method'' employed in the E89-009 experiments, but it is relatively free from the huge background rate of bremsstrahlung electrons at the focal plane of the electron spectrometer. This will allow us to use beam currents as high as a few tens $\mu$A. The new configuration makes it possible to measure hypernuclear spectra even with higher Z targets.

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