Experimental techniques and performance of Λ-hypernuclear spectroscopy with the (e,e′

Abstract

The missing-mass spectroscopy of Λ hypernuclei via the (e, e ′ K + ) reaction has been developed through experiments at JLab Halls A and C in the last two decades. For the latest experiment, E05-115 in Hall C, we developed a new spectrometer system consisting of the HKS and HES; resulting in the best energy resolution (∆E ≃ 0.5-MeV FWHM) and BΛ accuracy (∆BΛ ≤ 0.2 MeV) in Λhypernuclear reaction spectroscopy. This paper describes the characteristics of the (e, e ′ K + ) reaction compared to other reactions and experimental methods. In addition, the experimental apparatus, some of the important analyses such as the semi-automated calibration of absolute energy scale, and the performance achieved in E05-115 are presented. II. MISSING-MASS SPECTROSCOPY WITHTHE (e, e ′ K + ) REACTION Λ hypernuclei were first found in nuclear emulsions that were exposed to cosmic rays [19]. Later, the Λ binding energies B Λ , defined in Eq. (17), of A ≤ 15 hypernuclei were obtained in experiments with nuclear emulsions exposed to mesic beams such as K − [20]. A typical accuracy on the B Λ determination is approximately ∆B Λ ≤ 0.1 MeV including systematic errors. Except for a few cases, emulsion experiments were able to determine only the ground-state B Λ as they derived the B Λ by tracing weak decay processes of Λ hypernuclei which take a longer time than deexcitations emitting γ-rays and neutrons. In addition, with the emulsion technique, the complexity of decay sequences from heavier hypernuclei prevented B Λ measurements with the mass numbers larger

The cross-section measurement for the $^3{\textrm H}(e,e'K^+)nnΛ$ reaction

“…The measurable quantity in our experiment was the momentum. Therefore, a correction for the momentum is a more practical option than the energy-loss correction in the analysis as was done in the past hypernuclear experiment [17]. The momentum-loss correction was applied event by event depending on y and z mean T for e and K + .…”

“…Major contributions to the systematic uncertainty on B Λ were obtained from (1) the calibration method and (2) the correction of momentum loss in materials particularly for the target cell. The uncertainty that comes from the calibration method using the Λ and Σ 0 productions was evaluated to be about ±0.1 MeV in the previous hypernuclear experiment [17], which was performed following the same method. We analysed the p(e, e K + )Λ peak from the tritium gas target, in which a small percentage of hydrogen contamination took place.…”

“…Missing-mass spectroscopy is a powerful tool for the investigation of the nnΛ state because it allows a measurement for both the bound and resonant states. The missing-mass spectroscopy of Λ hypernuclei with the (e, e K + ) reaction was developed at the Jefferson Lab (JLab), and the high resolution hypernuclear data were successfully published [15][16][17][18][19]. However, a tritium target which requires strict regulations to be followed concerning safety issues, is necessary for the (e, e K + )-reaction spectroscopy, because a proton gets converted into a Λ in the reaction.…”

Spectroscopy of $A=9$ hyperlithium by the $(e,e^{\prime}K^{+})$ reaction

“…We present new binding energy data of 9 Λ Li which is compared with that of the mirror hypernucleus 9 Λ B. We performed a series of Λ binding energy measurements for several p-shell hypernuclei with a new magnetic spectrometer system HKS-HES at JLab Hall C (Experiment JLab E05-115) [20], and results for 7 Λ He [21], 10 Λ Be [22] and 12 Λ B [23] were published. We acquired data with a 9 Be target to produce 9 Λ Li in the same experimental period.…”

“…Peaks of other coincidence reactions such as (e ′ ,π + ) and (e ′ ,p) are located at different positions from that of the (e ′ ,K + ) peak because of the wrong assumptions of particle masses for π + s and protons. The other coincidence events and most of accidental coincidence events could be removed by a coincidence time selection with a time gate of ±1 ns width for the real (e ′ , K + ) coincidence peak [20]. With KID-2 and 3, survival ratios for π + and proton were suppressed down to 0.047% and 0.019%, respectively with maintaining > 80% of K + survival ratio [29].…”