Possibility of standard model and new physics

We have a theory called standard model in particle physics at present. This is a gauge field theory based on SU(3)�~SU(2)�~U(1) gauge group. It includes quark and lepton (fermion), and Higgs (boson). It has been confirmed that this model is consistent to all the experimental results at present. It is the best model in the particle physics we have at present. However, the standard model is not considered to be the final theory, because there are various unsolved theoretical problems in it. Therefore, it is believed that, instead of the standard model, New Physics describes the phenomena at higher energy scale than that achievable by present experiments.

Scale of standard model

One of the problem is related to the vacuum in the standard model. The vacuum in the standard model is determined from the minimum of the Higgs potential. Since Higgs field has the vacuum expectation value at that point, electroweak symmetry with SU(2)�~U(1) gauge group breaks down and weak boson, quark and lepton obtain the mass. Therefore, the scale in the standard model is characterized by that of the vacuum expectation value or of the mass of weak boson, and it is determined to be about 100GeV from experiments. This energy scale is almost the maximum energy in the present high energy experiments.

Gauge hierarchy problem

As a characteristic scale, we know the Planck scale besides the scale of the standard model. This is the scale in which very weak gravitational force becomes comparable to the electroweak interaction and the strong interaction. This scale is surprisingly seventeen degrees higher than that of the standard model. Although the standard model does not include gravity, it might be natural to expect the existence of the quantum theory, which describes both the standard model and the gravitational interaction consistently at high energy around the Planck scale. Then, a question arises as to why these two scales are so different. This is called gauge hierarchy problem.

Instability of vacuum

We cannot explain the reason why Higgs field has the vacuum expectation value at all, in other words, the origin of the electroweak symmetry breaking. This is because we introduce the convenient Higgs potential from the begining. Moreover, there exists an important problem with the Higgs potential and the vacuum determined from it, and this problem is related to the gauge hierarchy problem. It is the problem of vacuum instability. It is known that, if we take the quantum effects in the standard model into consideration, the Higgs potential strongly depends on the scale of New Physics which is expected to exist at higher energy scale. It is natural to think that the scale of the standard model is not so different that of New Physics due to this property. It is known that the standard model is realized as a consequence of a terrible fine tuning of parameters if the scale of New Physics is much higher than that of the standard model (for example, the Planck scale which we have mentioned above), In other words, the vacuum in the standard model is unstable against the quantum effects. Many exparts expect the existence of New Physics which resolves this problem.

Supersymmetry

Supersymmetry is a prominent candidate for New Physics. This is because this peculiar symmetry, namely, supersymmetry cancels the quantum effect of scalar potential so that it guarantees the stability of vacuum. By the way, it is known that supersymmetry is broken at low energy, since the evidence of supersymmetry has not been observed yet. The natural supersymmetry breaking scale is estimated to be about 1TeV from the effect of this breaking scale on the quantum correction to Higgs potential.

Brane World

Recent years, higher-dimensional scenario called `Brane World' is argued frequently as another possibility of New Physics except supersymmetry. In typical scenario the Planck scale of the higher-dimensional theory is about 1TeV, which is near that of the standard model, and the problem of vacuum instability does not exist. The higher-dimensional theory has the structure in which the Planck scale in 4-dimensions seems effectively to be considerably large due to its geometrical feature. In any case, the scale of New Physics related to the instability of vacuum in the standard model is expected to be about 1TeV.

Other problems

The standard model contains difficult problems in addition to above-mentioned problems. We can give the hierarchy problem of the mass of quarks and leptons as an example. In the standard model, all the now known mass of fermions and flavor mixing matrix elements are parameters determined only from experiments, and there is no explanation in the framework of the standard model. It is expected that New Physics exists in higher scale and it gives clear explanation.

Research subjects in theory group

Phenomenological particle physics is the academic studies, such as the theoretical investigation of the above-mentioned New Physics and the research in the structure of the standard model in more detail. The research is always deeply connected with experiments. We aim to reveal higher scale physics than the standard model, based on the facts which have been demonstrated by experiments until now. In the KEK theory group, faculty members, postdocs and graduate students as a whole investigate widespread research field in the phenomenological particle physics -- particle physics model such as supersymmetric theory and Brane World scenario, flavor physics, perturbative QCD, particle cosmology and so on.

Cooperation with experiment group

At present the flavor structure of both of the quarks and leptons is becoming clear from B-Factory and Neutrino Experiment. Experimental particle physics groups in Japan have accomplished excellent achievement in this direction. Especially, B-Factory and K2K Neutrino Experiment are carried out in KEK, and the KEK theory group carries on the investigation while maintaining close relationship to the KEK experiment group. It is quite possible that New Physics such as supersymmetry is indirectly verified by the experiments of flavor physics, and we put a special emphasis on the theoretical investigation into the effect of New Physics. For the physics at the high energy frontier, we plan to start the operation of the proton-proton colliding type large Hadron accelerator (LHC) in 2007, It is highly expected that Higgs particles are detected and New Physics such as supersymmetric model and Brane World scenario are directly verified. Researchers all over the world including many Japanese people are planing to construct electron-positron linear colliders at present. Both the theory group and the experiment group in KEK cooperate with each other in carring on the investigation of the possibility that, at the linear collider, we can verify New Physics, such as the Higgs physics, the supersymmetric model and the Brane World scenario. We also carry on the investigaton of long baseline neutrino experiments in the future.

Finally

It is expected that phenomenological particle physics develops remarkably in next five to ten years. In the KEK theory group we go ahead with our investigation vogorously hereafter.