High Resolution Spectroscopy of the ${\mathrm{}}_{\Lambda }^{12}\mathrm{B}$ Hypernucleus Produced by the $(e,e^{\prime }{K}^{+})$ Reaction

High-energy, cw electron beams at new accelerator facilities allow electromagnetic production and precision study of hypernuclear structure, and we report here on the first experiment demonstrating the potential of the $(e,e^{\prime }{K}^{+})$ reaction for hypernuclear spectroscopy. This experiment is also the first to take advantage of the enhanced virtual photon flux available when electrons are scattered at approximately zero degrees. The observed energy resolution was found to be $\approx 900\mathrm{k}\mathrm{e}\mathrm{V}$ for the ${\mathrm{}}_{\Lambda }^{12}\mathrm{B}$ spectrum, and is substantially better than any previous hypernuclear experiment using magnetic spectrometers. The positions of the major excitations are found to be in agreement with a theoretical prediction and with a previous binding energy measurement, but additional structure is also observed in the core excited region, underlining the future promise of this technique.

Figure 1

The experimental plan view showing both the kaon spectrometer (SOS) and the electron spectrometer (ENGE). The SOS is a $\mathrm{Q}\mathrm{D}\overline{\mathrm{D}}$ spectrometer with Q an entrance quadrupole, and $\mathrm{D}\overline{\mathrm{D}}$ two dipoles bending in opposite directions, providing large momentum acceptance but reducing dispersion.

Figure 2

The missing mass spectrum obtained from a ${\mathrm{C}\mathrm{H}}_x$ target at an incident electron energy of 1864 MeV. The solid histogram is the accidental background.

Figure 3

The summed ${\mathrm{}}_{\Lambda }^{12}\mathrm{B}$ missing mass spectrum for both energies. The solid histogram is the measured accidental background, and the curve is a theoretical calculation, spread by 900 keV and overlayed on, not fit to, the data.