Investigation of high-precision Λ hypernuclear spectroscopy via the (e,e'K<sup>+</sup>) reaction

Abstract

The study of Λ hypernuclear structure is very interesting in point of the understanding of the interaction between Λ and nucleon (Λ-N interaction) and its strange structure itself due to the containment of a Λ hyperon which has a strangeness as a new degree of freedom. In the several way to study the Lamda hypernuclei, the (e,e'K+) reaction spectroscopy is a powerful tool for the precise investigation of Λ hypernuclear structure. The purpose of the preset thesis is the establishment of the experimental design with the efficient data analysis method for the (e,e'K+) hypernuclear spectroscopic experiment in the wide mass region (from A=7 to A=52). It is very challenging to perform the (e,e'K+) spectroscopic experiment with such a heavy target, because of the huge electron background due to the bremsstrahlung process. In the experiment, it is required to obtain the necessary hypernuclear yield, suppressing the background event ratio. We achieved these requirements by newly constructing the high resolution electron spectrometer (HES) and splitter magnet (SPL) dedicated to the (e,e'K+) spectroscopic experiment. The HES consists of two quadrupole magnets and a dipole magnets (Q-Q-D) with a momentum resolution of dp/p = 3x10-4 at p = 0.84 GeV/c. It was used being vertically tilted by 6.5 degree so as to optimize signal to noise ratio and hypernuclear yield. The SPL is a dipole magnet. The experimental target was placed at the entrance of this magnet. The role of the SPL is to separate four kind of particles; scattered kaons, photons created by the bremsstrahlung, the post beam and scattered electrons. In addition, since the SPL is a part of the kaon and electron spectrometers. We designed the magnet shape carefully considering these points. The experiment was performed with 2.344 GeV/c electron beam from CEBAF at Jefferson Lab. The experimental setup consists of the HES, SPL and HKS (high momentum resolution kaon spectrometer). The HKS is also a Q-Q-D type spectrometer with the momentum resolution of dp/p = 2x10-4 at p = 1.2 GeV/c. In the data analysis, the particle momentum calibration was the most important procedure. At the initial point, the particle momentum was obtained from the calculated magnetic field map of the spectrometer whose accuracy is an order of 10-2. The initial momentum was calibrated by two step, the the magnetic field map improvement and the calibration with known masses of Λ/σ0 which were observed by the CH2 target data. As a result of the calibration, the momentum resolutions of HKS and HES were estimated as 4x10-4 and 6x10-4, respectively. Though these values are the double of the designed value, it was achieved to obtain the Λ/σ0 peaks with the same order of the designed energy from the original calculated magnetic field. The cross section was calculated with the several estimated factors. The averaged p(γ*, K+)Λ cross section in the HKS acceptance, (0.90 < cos(θ^CM_K+) < 1.0) was calculated as 227 ± 12 ±26 [nb/sr], which is consistent within the error bar with the other experiment results of p(γ, K+)Λ. The obtained yield of the peak was almost same as the designed value with the considered detector efficiencies. The observed hypernuclear spectrum of 12Λ B was also consistent with the other experimental results. These analysis result represents that the experimental setup including the newly constructed HES and SPL worked and the calibration procedure of this unique experimental setup is basically established.