Research History

Last Update: March 4, 2015

I have mainly studied on the following five topics (chronological):

1) Dynamical electroweak symmetry breaking (Technicolor models).
2) Dynamics of supersymmetric gauge theories and composite models.
3) Dynamics of supersymmetry breaking in supergravity models.
4) Fermion condensations in non-trivial backgrounds in string theories.
5) Particle physics models with D-branes in string theories.
6) Dynamics of D-branes in the system without supersymmetry.
7) Possible signatures of String Theory in cosmological observations.

These were the efforts to understand the origin of the mass hierarchy and flavor mixing of quarks and leptons.

Before LEP experiments have excluded simple technicolor models, I have studied (extended) technicolor models as a scenario of the fermion masses generation. The value of S and T parameters, which describe the radiative corrections to the propagators of weak bosons, has been strongly constrained by LEP experiments. The value of S parameter must be very small in comparison with the values predicted by simple technicolor models. I constructed with T.Yanagida a technicolor model which naturally gives small value of S and T parameters. We proceeded further to explain about 3-sigma deviation of the experimental value of R_b from the standard model prediction within this model. Although the model works very well, the experimental value of R_b was changed and the deviation had disappeared.

Since I thought GUT's could not give an explanation for fermion mass hierarchy (they include many parameters if we consider full Higgs sector), I changed my direction to composite models in which the Yukawa couplings for fermion masses should be dynamically generated. The dynamics of the supersymmetric gauge theories is a good candidate of the underlying dynamics of the composite model because of the remaining chiral symmetry for having massless fermion bound states. I constructed with N.Okada a model which includes necessary breaking of supersymmetry and electroweak symmetry. It is a toy model which includes only third generation, and its basic structure is the same of the model by A.E.Nelson and M.J.Strassler. I proceeded further to include other two generations. Although I constructed a concrete realistic model, it is very complicated.

Since I thought the construction of concrete models by just combining known mechanisms could not solve the problem, I changed my direction to study some important parts of the model. The topics that I chose was the dynamics of the supersymmetry breaking. It is very artificial to introduce an independent sector (gauge symmetry and matter) just for supersymmetry breaking. Almost all the proposed models have this structure. I constructed with N.Maru and N.Okada several concrete models of dynamical supersymmetry braking in which the gauged U(1)_R symmetry in supergravity theories is utilized. Because of the symmetry of supergravity theory, the U(1)_R gauge field must have Fayet-Iliopoulos term which can trigger the supersymmetry breaking. This fact had already been known, but there was no concrete model, since the cancellation of anomalies of gauge U(1)_R symmetry was considered to be very difficult. Although our mechanism is not the ideal one, because it includes an additional gauge group for supersymmetry breaking, the supersymmetry breaking due to the interplay between supergravity and gauge dynamics is a new mechanism.

There is a possibility that the supersymmetry is spontaneously broken by the fermion condensation in string theories. A concrete example of such system has already given in four-dimensional supergravity theories. In the system the gravitino condensate by some non-trivial space-time backgrounds breaks supersymmetry spontaneously. I made an effort to clarify whether such phenomena is possible or not in string theories, which may be a consistent theory of quantum gravity. I found that the tension of D3-brane in type IIB string theory in flat space-time is different from that in the flat space-time limit of the fivebrane background. This suggests the existence of some condensations due the the topological effect of the fivebrane background, since it can be shown that the value of tension is modified only by such condensations. It is probable that some operators composed by fermions condense. It is not clear whether such condensations have some relation to the half supersymmetry breaking by fivebrane backgrounds. I also have studied the explicit form of the fermion zero-modes in fivebrane backgrounds in low-energy effective theories of some string theories.

There are vary few researches on the fermion dynamics in string theory using the technique of the world-sheet conformal field theory. I have calculated the propagators of fermion fields (gravitino, dilatino and gaugino) in some string theories, and made an attempt to get some information of the fermion condensation. I have succeeded to evaluate the gaugino pair condensation in the fivebrane background of heterotic string theory. The value is proportional to the value of the B-field field strength in non-trivial four-dimensional space in the fivebrane background. Although it is possible that the condensation gives some contribution to the central charge in the ten-dimensional supersymmetry algebra (the central charge is required for the half supersymmetry breaking by the fivebrane background), the meaning and role of the condensation is still to be clarified.

After some formal works on string theory, I came back to the flavor problem of quarks and leptons. I recognized that so-called intersecting D-brane model can open new world in the phenomenological model building in string theory. The gauge symmetries are realized through the introduction of D-branes and the chiral fields are realized on the D-brane intersecting points in extra six dimensional space of type IIA or IIB string theory. Especially, the way to realize the gauge symmetry is very different from that in heterotic string models, and even the strong coupling gauge interactions in the visible sector can be realized. In the model buildings (without flux), I found a problem that we need too many D-branes to satisfy consistency conditions (Ramond-Ramond tadpole cancellation conditions as well as supersymmetry conditions). It means that we are forced to introduce some extra gauge symmetries in addition to the one in the standard model. It also means that we are forced to have rather many exotic particles which do not exist in nature. I proposed that these extra gauge interactions can be identified to the confining forces of preons which are the constituents of quarks and leptons. Namely, I proposed to introduce compositeness of quarks and leptons (and Higgs doublet fields). As a bonus we can have non-trivial Yukawa couplings among fermions (quark and leptons) and Higgs doublet fields, which is difficult in usual models. The "non-trivial Yukawa coupling" means that it can realize rather realistic hierarchical quark masses and their flavor mixings, which is difficult to be realized in usual models with elementary quarks and leptons (and Higgs doublet). The point is that the origin of the generation in composite models is different from that in the usual models. The multiple intersection of D-branes is the origin of the generation in usual models, while the D-brane splitting as well as the multiple intersection of D-branes are the origin of generation in composite models.

In the typical models using D-branes, like intersecting D-brane models, for example, supersymmetry is utilized to obtain stable configurations of D-brane. The field theory model which is expected to be realized as the low-energy effective theory is the minimal supersymmetric standard model. There is a problem that the mechanism of the supersymmetry breaking is difficult to incorporate explicitly in the string models from the beginning without assuming some non-perturbative effects. Since a certain special pattern of the supersymmetry breaking is necessary for the electroweak symmetry breaking in the minimal supersymmetric standard model, having the electroweak symmetry breaking, which is the origin of the masses of elementary particles, is a challenging problem in typical string models using D-branes. There is a possible framework towards obtaining stable non-supersymmetric models, which is so called "bottom-up" approach. The standard model might be realized on the D3-branes at an orbifold singularities in extra six-dimensional space. In case that the orbifold singularity is non-supersymmetric (or the six-dimensional orbifold space does not keep supersymmetry), we have models without supersymmetry. In this class of models the electroweak symmetry breaking can be triggered by the vacuum expectation value of the elementary Higgs doublet field through the non-trivial radiative corrections due to no supersymmetry. This is the radiative electroweak symmetry breaking, which is necessary and inevitable without naturalness problem. The scale of the electroweak symmetry breaking is determined by the scale of the string tension which should be TeV scale. I have proposed this scenario by constructing a concrete model, though some theoretical problems remain (the NS-NS tadpole problem, which is expected to be solved by "tadpole resummations", and the closed string tachyon problem, which is expected to be solved by constructing models based on Sagnotti's type 0'B string model). If this scenario is true, the string phenomena will be observed at LHC.

In case that the string scale is of the order of TeV scale, a peak in the dijet invariant mass distribution by string resonances should be observed at the LHC. Within the situation that some other possible physics beyond the standard model also predict such peak, I have pointed out some strategy to clarify whether the peak is string origin or not. The key aspect is that several string resonances of the same mass with different spins should contribute a peak simultaneously, and investigating the angular distribution of dijets from the resonance should solve the problem. I have carried out extensive Monte Carlo simulations of LHC experiments with M.Hashi, and future discovery potential of the resonance and the possibility of the angular analysis have been clarified.

The system of D-brane without supersymmetry generally has NS-NS tadpole problem, which indicates the assuming background space-time is not the solution of String Theory. The practical problem is that the open-string one-loop quantum corrections, which should be focused with much interest in the system without supersymmetry, diverge by the infrared singularity in the tree-level exchange of massless closed string states with tadpole couplings with D-branes. The procedure of Fischler and Susskind is famous to solve this problem, but unfortunately it is very difficult to do in practice. On the other hand, the procedure of "tadpole resummations" which was proposed by Dudas, Nicolosi, Pradici and Sagnotti, seems more practical way of doing. I have shown that it is really possible to do in String Theory, and obtained a further evidence of Sen's conjecture, namely, the tachyon condensation on D25-brane in bosonic string theory. I have also developed the formulation to calculate the one-loop corrections to the masses of scalar field towards clarifying the possibility of radiative gauge symmetry breaking in String Theory.

In case that the string scale is very large, it is impossible to observe the effect of String Theory in collider experiments. One may hope that such a high energy physics acts a role in very early universe, especially in the inflation of the universe. In a class of string model with "brane supersymmetry breaking" the scalar field which follows exponential potential is predicted. During the classical evolution of the universe from Big-Bang singularity, since the exponential potential is too steep, the scalar field is forced to climb the exponential potential and turn its motion to descend the potential. If we identify the scalar field as a inflaton, with some corrections to the potential for late-time inflation, it is possible that such a motion of scalar field leaves an imprint in the power spectrum of cosmic microwave background. I have examined with A.Sagnotti the possible deformations of the power spectrum by this "climbing scalar" and compare them with the observed power spectrum by WMAP and PLANCK collaborations.

The behavior of D-branes in the situation without supersymmetry is underway. I think that this is a key subject to understand spontaneous gauge symmetry breaking and moduli stabilization of extra six-dimensional compact space in superstring theory, which is necessary to connect String Theory to Particle Physics. I have tried with S.Kobayashi to understand spontaneous breaking of gauge symmetry with reducing its rank through the separation of D-branes in such way that they are identified by discrete symmetry of orbifold. I have also tried to propose an idea of volume moduli stabilization by the dynamics of D-branes. We need to know much more about the physics of D-branes in the situation without supersymmetry, though extensive works have been achieved about the supersymmetric systems of D-branes.