The fundamental particles observed in nature are classified into two groups; hadrons and leptons. Hadrons are further classified into two groups named baryons (such as proton and neutron) and mesons (such as pion predicted by Yukawa). Leptons include electron and neutrino. Hadrons are composite particles made of more fundamental entities called quarks. Hadrons interact with each other via the strong interaction, which ordinates in the color charges carried by the quarks. The strong interaction is described by a gauge theory named quantum chromodynamics (QCD).
The force mediated by the strong interaction is, as its name itself is telling, extraordinarily strong. Due to this fact, it is generally difficult to calculate or understand the dynamics of QCD. However, about 10 years ago, the gauge/gravity correspondence (duality) has been discovered. It states that the strong coupling limit of supersymmetric gauge theories is equivalent to the weak coupling limit of the string theory on curved spacetime or gravitational theories. Through this correspondence, it becomes possible to calculate observables in gauge theories by calculating the corresponding quantities in gravitational theories.
The important issue is thus how to generalize this correspondence to more general classes of gauge theory. In particular, it has been a long-standing open question whether there exits the string/gravity dual of QCD, the gauge theory of our realistic strong force. The model proposed by Dr. Sakai and Dr. Sugimoto is the first string model that corresponds to our QCD. Not only their model qualitatively explains the spontaneous breaking of the chiral symmetry which is the origin of the masses of the elementary particles, but also it quantitatively reproduces the masses of the mesons and various experimental data with good precision.
This study has attracted a great deal of attention as a pioneering work providing a powerful tool to cope with the formidable task in the QCD dynamics. Their model is employed in the analysis of the RHIC experiment at Brookhaven National Laboratory. Their model is also used in the studies of the QCD phase structure, as a practical method to predict or analyze new phases under high temperature/density realized in the neutron stars. The model is now widely used under the name of "Sakai-Sugimoto model". The citations of the two papers published in Progress of Theoretical Physics are reaching 500 times, which shows the incredible impact of this work.