Test of Nambu Theory and Kobayashi-Maskawa Theory
 Figure 1 : Charge conjugation. |
 Figure 2 : Parity transformation. |
 Figure 3 : Fermi National Accelerator Laboratory. |
 Figure 4 : CDF detector. (Installation process of the silicon tracking detector into the center portion.) |
 Figure 5 : Candidate event of the top quark production observed at CDF. |
 Figure 6 : Six species of quarks and leptons, and gauge bosons. |
 Figure 7 : Schematic drawing of “the spontaneous symmetry breaking”. |
As everyone knows, the Nobel Prize in Physics in 2008 was awarded to Yoichiro Nambu (Emeritus Professor at the University of Chicago and Osaka City University), Makoto Kobayashi (Emeritus Professor at the High Energy Accelerator Research Organization), and Toshihide Maskawa (Emeritus Professor at Kyoto University). It is truly delightful that Japanese research is recognized worldwide. The research achievements of these three individuals are extremely important physical theories that form the backbone of particle physics and are supported by numerous experimental verifications. Osaka City University has also been heavily involved in these verification experiments. This time, we would like to introduce these experimental verifications.
Let’s start with the Kobayashi-Maskawa theory. The reason for their award is the “discovery of the origin of the breaking of CP symmetry.” CP symmetry is a symmetry that appears when performing charge conjugation (C transformation) and parity transformation (P transformation) simultaneously. Charge conjugation refers to replacing “particles” with “antiparticles” (Figure 1), and parity transformation refers to reversing space (x → –x, y → –y, z → –z), which means transforming the world into a mirror image (Figure 2). It has been confirmed that in electromagnetic and strong interactions, both C and P transformations independently preserve symmetry. However, in the case of weak interactions involved in beta decay, it was proposed by T. D. Lee and C. N. Yang in 1956 that P symmetry is not preserved (it is broken), and this was confirmed experimentally by C. S. Wu the following year. At that time, it was “common sense” to believe that the laws of physics do not change even if space is reversed, so this discovery caused a great shock. However, even if P transformation is broken, it was thought that symmetry in physical laws would be restored if P transformation is combined with C transformation to make a CP transformation, which corresponds to the transformation of “matter” and “antimatter.” However, in 1964, it was discovered that this CP symmetry is also slightly broken through the observation of the decay of neutral K mesons by J. W. Cronin and V. L. Fitch, which again caused a great shock in the physics community. The question then became what the cause of this CP symmetry breaking was. Many theoretical models were proposed to explain this phenomenon, one of which was the Kobayashi-Maskawa theory. Their theory suggested that by introducing three generations of SU(2) doublets, totaling six quarks (up quark (u), down quark (d), strange quark (s)), into the three types of quarks known at the time, it is possible to include complex numbers in the coupling constants between quarks, thereby allowing the breaking of CP symmetry. Subsequently, the fourth quark, the charm quark (c), was discovered in 1974 by S. C. C. Ting and B. Richter, and the fifth quark, the bottom quark (b), was discovered in 1977 by L. M. Lederman and others, making the three-generation prediction of the Kobayashi-Maskawa theory more realistic. Finally, the sixth quark, the top quark (t), was discovered in 1995 by the CDF experiment group at Fermilab (in which Osaka City University’s High Energy Physics Laboratory participates) and its competitor, the DØ experiment group (Figures 3 and 4: CDF detector). It is now understood that there are three generations each of quarks and leptons, as shown in Figure 6. Furthermore, the Kobayashi-Maskawa theory predicted a much larger CP symmetry breaking in neutral B mesons than in neutral K mesons, which led to the initiation of the Belle experiment at the High Energy Accelerator Research Organization (KEK) and the BaBar experiment at the Stanford Linear Accelerator Center (SLAC). To date, it has been confirmed with very high precision that this prediction is correct. Osaka City University’s Cosmic and Particle Physics Experiment Laboratory is participating in the Belle experiment.
Next is the Nambu theory, which is the “discovery of the spontaneous symmetry breaking”. When a system with a certain symmetry in nature transitions to a state with lower energy by intentionally becoming asymmetrical, this phenomenon or process is called “spontaneous symmetry breaking.” A commonly cited example is placing a marble at the center of a rotationally symmetric Mexican hat-shaped slope, as shown in Figure 7. Assume gravity acts vertically. In this case, if the marble can be placed precisely at the center of the peak, the forces on the marble balance out, and it can remain stationary. This state retains the symmetry of the system when the central axis is rotated. However, if the marble is given even the slightest disturbance, it can no longer maintain equilibrium and will roll down the slope into the depression. Energetically, this is a much more stable state. However, in this state, rotating the central axis changes the position of the marble, so the rotational symmetry of the system is lost. Similarly, in nature, there are cases where symmetry is spontaneously lost to achieve a more stable state, and this often holds the key to solving unexplained phenomena. In condensed matter physics, the mechanism of superconductivity was precisely this. In particle physics, the “Higgs mechanism,” which gives rise to the Higgs particle that our laboratory is searching for, is one such example. In the Higgs mechanism, it is thought that the potential of the quantum field called the Higgs field has the shape shown in Figure 7, and the breaking of symmetry by the marble rolling down the slope corresponds to particles acquiring mass that initially had no mass. In the CDF experiment, in which our laboratory participates, we are currently searching for this Higgs particle, and if it is discovered, it will further support the Nambu theory. The data collected in the CDF experiment, which began in 2001, has now increased in statistics and is beginning to reach the sensitivity to the production frequency predicted by the theory. We may soon be able to announce the discovery of the Higgs particle.