What is the world made of? One of the main goals of elementary particle physics is to answer this question. Nowadays people believe that all the matter we see around us is made of quarks and leptons, which are interacting via four fundamental forces (gravity, electromagnetic, strong and weak forces). In 2004, through the study of gauge/string duality, we encountered a clue to a new possibility which looks completely different from this traditional view of world.

According to the standard theory of elementary particles, a baryon is made of three quarks, meson is made of a quark-antiquark pair, and the strong interaction among them is described by QCD. However, we found some evidence that string theory in a certain curved background may also describe the same physics. In this description, mesons appear as strings and baryons are a kind of soliton called D-branes. We analyzed properties of hadrons based on string theory and found that various physical quantities roughly agree well with the experimental values. Although there are still a lot of things to clarify, we think this is a very interesting possibility and expect fruitful developments in the future.