Hadron Spectroscopy

At J-PARC (Japan Proton Accelerator Research Complex), we can use kaons which contain a strange quark (or antiquark) as a high intensity beam. There is no other facility like this. We can create Ξ particles or Ω particles which contain two or three strange quarks by using this kaon beam.

Two kinds of light quarks (u,d) are responsible for the strong interaction². This strong interaction prevents matter from breaking. Since this interaction is too strong, we need tremendous energy in order to break a hadron apart. What will happen if we put a strange quark in such a system? The interaction relatively becomes weaker because a strange quark is several times as heavy as a light quark.

We examine features reflected in how to combine together when we dare to put a particle which experiences weak interaction in a system where strong interaction is dominant. We study how quarks are bound together in a hadron, specifically what is the true degree of freedom to make hadrons.

Only mesons made of one quark and one antiquark and baryons made of three quarks were known as hadrons. In theory, however, it is possible that there is a composite particle made of four or more quarks. Hadrons containing four quarks are called tetraquarks and ones containing five quarks are called pentaquarks. Hadrons made of six quarks are called dibaryons as they are considered as two baryons.

Recently such exotic hadrons have been observed. We also carried out a pentaquark search experiment (J-PARC E19 experiment) by using high intensity pion beams. Unfortunately, we could not discover pentaquarks, but they are searched for in the LEPS2 experiment at SPring-8.

1. hadron:
   The general term for mesons made of one quark and one antiquark (e.g. pions and kaons) and baryons made of three quarks (e.g. protons and neutrons).
2. strong interaction:
   There are four fundamental forces in the world. They are strong, electromagnetic, weak and gravitational forces. It is the strong force that is the strongest out of all of them. They are carried by elementary particles. For example, electrically charged particles interact with each other by means of exchange of photons. Such an action is called interaction. The strong interaction is mediated by the exchange of massless particles called gluons that act between quarks. This is detailed in the theory of quantum chromodynamics (QCD) which is the theory of quark-gluon interactions.