Optical Calibration For Jefferson Lab HKS Spectrometer

Jefferson Lab HKS experiment aims at obtaining high resolution hypernuclear spectroscopy by $(e,e'K)$ reaction. The calibration of the HKS spectrometer is crucial in order to obtain the proposed missing mass resolution. This calibration procedure is complicated due to the use of a zero degree on-target splitter magnet. We have developed a high precision optical calibration method, making use of the known masses of $\Lambda$,$\Sigma^0$ hyperons produced from hydrogen in CH$_2$ target and the narrow width of $^{12}_\Lambda$B hypernuclear ground state from $^{12}$C target. To deal with the high accidental background, we have implemented the Expectation-Maximization parameter estimation method. In this talk, the procedure of the calibration will be described and the preliminary results of the calibration will be presented.