J-PARC E10 experiment
Investigation of unknown neutron-rich Lambda hypernuclei
An atomic nucleus is composed of multiple protons and neutrons (both of them are members of baryons). The protons and neutrons are held together by a strong attractive force that acts between them, and move in accordance with quantum dynamics. There is some degree of freedom in the combinations of proton and neutron numbers in an atomic nucleus, and many types of atomic nuclei exist in the natural world. An even large number of nucleus types can be created artificially.
It is also possible to artificially create a different type of atomic nucleus called a hypernucleus. The difference between a hypernucleus and an atomic nucleus is that in addition to protons and neutrons, a hypernucleus also contains a type of baryon called a hyperon. There are several types of hyperons, which have been given names such as Lambda particle (Λ), Sigma particle (Σ), Xi particle (Ξ), and Omega particle (Ω). Of these, the Lambda particle has the lowest mass (approximately 20% more than the mass of a proton or neutron), and is the most stable of the hyperons, making it suitable as a particle for creating hypernuclei. A hypernucleus which contains this Lambda particle is called a "Lambda hypernucleus", and even though it is the most extensively researched of the hypernuclei, our knowledge is still in the early stages compared with research into atomic nuclei which has been conducted for more than a century.
Further progress in hypernucleus research:
towards creation of a neutron-rich Lambda hypernucleus
One of the purposes of Lambda hypernucleus research is investigating the force which acts between the Lambda particles and the protons and neutrons. The fact that a Lambda hypernucleus exists shows that there is a strong attractive force acting between the lambda particles and the protons and neutrons. (Without sufficient attractive force, the Lambda particles would immediately escape from the atomic nucleus.) It is known that in order to investigate the properties of this force in more detail, we should perform precise measurements of the energy when the Lambda hypernucleus is in an excited state. In recent years, it has become possible to precisely measure the energy of the gamma rays that are emitted from a hypernucleus in an excited state, and we are in the process of gathering information about the size of the force, which depends on the combination of spins (angular momentum unique to particles) of the Lambda particles, protons, and neutrons.
Although not yet as successful with hypernuclei as with atomic nuclei, there have been studies concerning Lambda hypernuclei with proton-excess or neutron-excess. We are now conducting research using a type of hadron reaction called a double charge exchange reaction to create neutron-rich Lambda hypernuclei. This double charge exchange reaction converts two protons in an atomic nucleus to one neutron and one Lambda particle. Because the reaction decreases the number of protons and increases the number of neutrons in the nucleus, it is possible to produce a Lambda hypernucleus with neutron-excess.
J-PARC E10 experiment:
producing neutron-rich Lambda hypernuclei by using
a double charge exchange reaction
There are two types of double charge exchange reactions that are believed promising for experiments. The first is a reaction between a negatively charged pion (π-) and the atomic nucleus that produces a positively charged kaon (K+). The second is a reaction between a negatively charged kaon (K-) and the atomic nucleus that produces a positively charged pion (π+). We have proposed research using the former of these two reactions.
Because the pions which we use in our research require extra energy when generating hyperons, particles with high momentum of 1.2 GeV/c are used. (This represents approximately 1 GeV of kinetic energy, and is larger than the rest mass M of a proton or neutron converted to energy by Mc2.) In the natural world, only a very small number of these pions are produced by a reaction between primary cosmic rays and atoms and molecules in the top of Earth's atmosphere, and these cannot be used in experiments. Therefore in actual research, protons are accelerated to high energy by an accelerator (for example to kinetic energy of 30 GeV – in other words, more than 30 times the rest mass energy of a proton or neutron), and then collided with atomic nuclei to produce high-momentum pions.
In Japan, a facility which can produce pions by this method exists at Tokai-Mura in Ibaraki Prefecture, and is known as the Japan Proton Accelerator Complex (or J-PARC). We are carrying out research using the accelerator at J-PARC to produce neutron-rich Lambda hypernuclei in experiments that are called J-PARC E10. (The name bears the usual method of adding serial numbers in the order the experiments were proposed: E01, E02, E03, …)
Expected research results:
boundary of stability of hypernuclei
and matter at the core of neutron stars
Research into atomic nuclei with proton excess or neutron excess has become highly active in recent years. One objective is to find the boundary of stability of atomic nuclei. When this boundary is exceeded, the excessive protons or neutrons in the nucleus escape. Searching for this boundary of stability is also a very interesting theme in research concerning neutron-rich Lambda hypernuclei. With hypernuclei, there is an effect known as the "glue-like role of Λ hyperon," which is believed to allow the production of neutron-rich Lambda hypernuclei beyond the boundary for normal atomic nuclei.
Hypernucleus research is also closely related to research concerning astronomical compact objects which have been studied extensively in recent years. A typical example of a compact object is a neutron star. A neutron star is believed to be composed primarily of neutrons, and can be considered to be a giant neutron-rich atomic nucleus (hence, the close relationship to researches of atomic nuclei). At the center of a neutron star, gravity applies intense pressure and it is possible that some of the neutrons may change to hyperons. Therefore the center of a neutron star could be described as a giant neutron-rich hypernucleus. Although some information about neutron star mass has been obtained by observation, it is not possible to measure the radius directly. It is believed that research conducted in laboratories here on earth concerning neutron-rich Lambda hypernuclei is an important means of learning the properties of the matter at the center of a neutron star, which cannot be investigated directly by astronomical observations.
J-PARC E10 and E22 Experiment