Speaker
Description
In the nucleon-nucleon ($NN$) interaction, realistic nuclear force models have been established with $NN$ scattering data. On the other hand, there are relatively large uncertainties due to limited $\Lambda N$ scattering data in case of the $\Lambda N$ interaction. The spectroscopic studies of $\Lambda$ hypernuclei have played an important role in knowledge of the effective $\Lambda n$ interaction.
An $nn\Lambda$ is a neutral baryon system which consists of two neutrons and a $\Lambda$. The study of the pure $\Lambda$-neutron system such as $nn\Lambda$ is expected to give us information on the $\Lambda n$ interaction. Although HypHI group at GSI reported measuring events which were a possibility of the bound $nn\Lambda$ [1], existence of the $nn\Lambda$ was not confirmed due to the limited significance. Since search for the $nn\Lambda$ with independent experiment is important, we measured the ${\rm ^3 H}(e,e'K^+)nn\Lambda$ reaction spectroscopy (E12-17-003) at Jefferson Lab (JLab) Hall A in 2018.
In this experiment, the $nn\Lambda$ and $\Lambda$ quasi-free ($\Lambda$-QF) production would be produced by a cryogenic tritium gas target with the thickness of 84.8 mg/cm$^2$ and a high intensity and high energy primary electron beam (${\rm I_{e}}=$22.5 $\mu$A, ${\rm E_{e}=}$4.32 GeV). The missing mass was obtained by measuring momenta of $K^+$ ($p_K=$1.8 GeV/c) and scattered electrons ($p_{e'}=$2.2 GeV/c) with two High Resolution Spectrometers (HRSs) which are the standard equipment at JLab Hall A. Validity of the missing mass was studied with the $\Lambda$ and $\Sigma^0$ mass in the ${\rm H}(e,e'K^+)\Lambda/\Sigma^0$ reaction. However, we could not find any clear peaks on the missing mass spectrum [2].
$\Lambda$-QF events in the ${\rm ^3 H}(e,e'K^+)X$ reaction were observed in this experiment. The $\Lambda$-QF distribution has an event excess around the $nn\Lambda$ threshold region. A similar excess was already found by the previous experiment that measured $\Lambda$-QF productions with the ${\rm ^3 He}(e,e'K^+)X$ reaction and it gave important information about $\Lambda n$ final state interaction (FSI) [3]. Ref. [3] successfully reproduced the excess on the missing mass by using an effective range ($r$) and a scattering length ($a$) of several effective $\Lambda N$ potential models. Since the measured system in this experiment is the pure neutral system, the $\Lambda$-QF distribution has important information about the $\Lambda n$ FSI. We fitted the $\Lambda$-QF distribution by changing the effective range and scattering length so that fitting chi-square becomes minimum.
I will present this experimental overview and the analysis result of the $\Lambda n$ FSI from the $\Lambda$-QF distribution.
[1] C. Rappold et al., Physical Review C 88, 041001 (2013).
[2] K.N. Suzuki et al., Prog. Theor. Exp. Phys. 2022, 013D01 (2022).
[3] F. Dohrmann et al., Phys. Rev. C 76, 054004 (2007).
