Investigation of Nuclei Which Have “Strangeness” by a Tool of Electron Beam

An ordinal nucleus consists of nucleons such as a proton and a neutron. Each nucleon is composed of a combination of three of up and down quarks, and a particle that is made by three quarks called a baryon. In our world, a matter which contains other types of quarks does exist.

A baryon which has a strange quark is called a hyperon, and a nucleus which has a hyperon is called a “hypernucleus”. Hyperons are not stable and decayed with a typical lifetime of a few hundred picoseconds in an environment of the earth. That’s why we do not see any hyperons and hyperon matter around us. However, it is believed that hyperons should exist in a very dense nuclear matter such as a neutron star in nature.

The strong interaction is one of fundamental forces that we currently know. The nucleons interact with each other by the strong interaction to form a nucleus. Our interest is how the strong interaction works among other types of baryons particularly between a Lambda hyperon and a nucleon (Lambda-N interaction). The Lambda is the lightest hyperon which consists of a combination of u, d, and strange (s) quarks.

The nucleon interaction has been described by the Meson Exchange Model that was suggested by Dr. Yukawa (Nobel Prize 1949) and some other phenomenological models such as the Chiral Effect Field Theory, the Quark Cluster Model and so on. Recently, theoretical calculations based on the QCD (quantum chromodynamics) which is the first principle calculation of the strong force are rapidly being developed due to improvement of computer performance. The nucleon (NN) interaction is understood much better in contrast to the YN interaction. The reason is simple. Experimental data for the YN interaction study are deadly scarce. What we aim is to investigate the Lambda-N interaction by means of the precise energy measurement of Lambda hypernuclei.

We use one of state-of-the-art accelerators CEBAF at Thomas Jefferson National Accelerator Facility (JLab, https://www.jlab.org/), Newport News, VA, US. CEBAF can provide a high quality electron beam which meets our experimental requirements to realize a high precision measurement of Lambda hypernuclei. There are grand scale magnetic spectrometers (HES, HKS) that were designed and built for our physics goals at JLab.

We have been carrying out our experimental projects collaborating with international members: e.g. researchers and students from Kyoto University (Japan), Tohoku University (Japan), Hampton University (US), Florida International University (US), INFN (Italy) etc. Our group (Kyoto group) is one of the core groups that leads the hypernuclear projects at JLab. We are happy if you are interested in our research activities and feel free to contact us anytime.