DOE OSTI2014
In 2009 (August-November), the E05-115 experiment was carried out at JLab to investigate L hypernuclei in the wide mass region up to A = 52 ($7\atop{Λ}$He, $10\atop{Λ}$Be, $12\atop{Λ}$B and $52\atop{Λ}$V) with the (e,e'K + ) reaction. This is the first attempt to investigate a medium heavy L hypernucleus with the (e,e'K + ) reaction. Experimentally, it is difficult to measure heavier L hypernuclei as background rates of particles which originate from electromagnetic processes are roughly in proportion to Z2 (Z: target proton number) in the (e,e'K + ) experiment. To perform the experiment, many experimental techniques have been developed and introduced such as optimization of the electron spectrometer configuration (tilt method), clean kaon identification, particle tracking under high multiplicity environment, precise energy scale calibration and so on. In the present thesis, experimental results of the elementary process of p(e,e'K + )L, L hypernuclei of $7\atop{Λ}$He, $10\atop{Λ}$Be, $12\atop{Λ}$B and $52\atop{Λ}$V are shown. Elementary processes of the electroproduction of L and Σ 0 , p(e,e'K + )L, Σ 0 were used for the absolute energy scale calibration of our spectrometer systems. A careful Monte Carlo simulation shows that the binding energy can be obtained with a systematic error of 0.11 MeV with our energy scale calibration method. A study of the elementary process of L is important to understand L hypernuclei as it is essential for theoretical calculations of L hypernuclei. The differential cross section of the p(e,e'K + )L reaction at the small K + scattering angle (theta-CM/gamma-K approx. 15.5°), the small Q 2 (approx 0.01 [GeV/c] 2 ) and the total energy of W = 1.92 GeV, where no experimental data exists was obtained to be 235 ± 13$+28\atop{-24}$ nb/sr. The ground state (1/2 + ) binding energy of $7\atop{Λ}$He was already measured in JLab E01-011 (2005). In the present work, the binding energy of 1/2 + state was determined to be B Λ = 5.55 ± 0.10 ±0.11 MeV with five times more statistic and smaller systematic errors than those of the previous experiment. The ground state binding energy is important to test the phe- nomenologically introduced CSB (Charge Symmetry Breaking) LN interaction for A = 7, T = 1 hypernuclear systems. In addition, a peak which is interpreted as 3/2 + and 5/2 + states was measured to be B Λ = 3.65 ± 0.20 ±0.11 MeV with sufficient statistic for the first time. Only three events of the ground state of $10\atop{Λ}$ Be had been observed in the emulsion experiments. The present experiment is the first spectroscopic measurement of 10/L-Be, and the detailed structures have been successfully measured for the first time. About three times better energy resolution was achieved in the present experiment (0.78 MeV in FWHM) than that of the mirror L hy- pernucleus, $10\atop{Λ}$B (2.2 MeV in FWHM) which was measured in the (π + ,K + ) experiment at KEK. The result of the ground state binding energy was obtained to be B Λ = 8.55 ± 0.07 ± 0.11 MeV which serves also to discuss about the CSB effect in the LN interaction. $12\atop{Λ}$B has been measured with the world best energy resolution of 0:5 MeV (FWHM) among the reaction spectroscopy of L hypernuclei. The results of $12\atop{Λ}$B are compared with the experimental results in the previous experiments to confirm the consistency. Furthermore, the obtained ground state binding energies of $12\atop{Λ}$B (B Λ = 11.38 ± 0.02 ± 0.11 MeV) and $52\atop{Λ}$V (B Λ = 21.88 ± 0.59 ± 0.11 MeV) indicate that the reported value of 12/L-C which has been used as a reference of binding energy measurements for the (π + ,K + ) experiments would be shallower by ~ 0.5 MeV. A pilot study for investigation in the medium-heavy mass region with the (e,e'K + ) experiment was performed by measuring $52\atop{Λ}$V. The ground state binding energy of $52\atop{Λ}$V has been measured, overcoming high multiplicity environment. The results are discussed with the ex- perimental results of 51/L-V measured at KEK. The present result is the first measurement of L's binding energy of the ground state without the emulsion reference in the medium-heavy mass region, which could be a substantial improvement in the information needed for understanding the single particle potential of L. In the present experiment, L hypernuclear measurement with a small systematic error of ~ 0.1 MeV by the (e,e'K + ) reaction has been established. Moreover, the present work opened a door to the heavier L hypernuclear measurement with the (e,e'K + ) reaction in the future.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗