Hajime Aoki

Revisiting the Naturalness Problem -- Who is afraid of quadratic divergences? --

It is widely believed that quadratic divergences severely restrict natural constructions of particle physics models beyond the standard model (SM). Supersymmetry provides a beautiful solution, but the recent LHC experiments have excluded large parameter regions of supersymmetric extensions of the SM. It will now be important to reconsider whether we have been misinterpreting the quadratic divergences in field theories. In this paper, we revisit the problem from the viewpoint of the Wilsonian renormalization group and argue that quadratic divergences, which can always be absorbed into a position of the critical surface, should be simply subtracted in model constructions. Such a picture gives another justification to the argument that the scale invariance of the SM, except for the soft-breaking terms, is an alternative solution to the naturalness problem. It also largely broadens possibilities of model constructions beyond the SM since we just need to take care of logarithmic divergences, which cause mixings of various physical scales and runnings of couplings.

Ginsparg-Wilson relation, topological invariants, and finite noncommutative geometry

We show that the Ginsparg-Wilson (GW) relation can play an important role to define chiral structures in {\it finite} noncommutative geometries. Employing GW relation, we can prove the index theorem and construct topological invariants even if the system has only finite degrees of freedom. As an example, we consider a gauge theory on a fuzzy two-sphere and give an explicit construction of a noncommutative analog of the GW relation, chirality operator and the index theorem. The topological invariant is shown to coincide with the 1st Chern class in the commutative limit.

Chiral Fermions and the Standard Model from the Matrix Model Compactified on a Torus

It is shown that the IIB matrix model compactified on a six-dimensional torus with a nontrivial topology can provide chiral fermions and matter content close to the standard model on our four-dimensional spacetime. In particular, generation number three is given by the Dirac index on the torus.

Chiral anomaly on a fuzzy 2-sphere

We investigate chiral anomaly for fermions in fundamental representation on noncommutative (fuzzy) 2-sphere. In spite that this system is realized by finite dimensional matrices and no regularization is necessary for either UV or IR, we can reproduce the correct chiral anomaly which is consistent with the calculations done in flat noncommutative space. Like the flat case, there are ambiguities to define chiral currents. We define chiral currents in a gauge-invariant way and a gauge-covariant way, and show that the corresponding anomalous chiral Ward-Takahashi identities take different forms. The Ward-Takahashi identity for the gauge-invariant current contains explicit nonlocality while that for the covariant one is given by a local expression.

Orbifold matrix model

We study a matrix model describing type IIB superstring in orbifold backgrounds. We particularly consider a {\bf C}^3/{\bf Z}_3 orbifold model whose six dimensional transverse space is orbifolded by {\bf Z}_3 discrete symmetry. This model is chiral and has d=4 {\cal N}=1 supersymmetry of Yang-Mills type as well as an inhomogeneous supersymmetry specific to matrix models. We calculate one-loop effective action around some backgrounds, and the result can be interpreted as interactions mediated by massless particles in IIB supergravity in orbifold background, if the background is in the Higgs branch. If the background is in the Coulomb branch, the dynamics is governed by the reduced model of d=4 super Yang-Mills theory, which might be interpreted as exchange of massless particles in the twisted sector. But the perturbative calculation cannot reproduce the supergravity result. We also show that this model with a large Higgs vacuum expectation value becomes IIB (IKKT) matrix model.Abstract We study a matrix model describing type IIB superstring in orbifold backgrounds. We particularly consider a C3/Z3 orbifold model whose six-dimensional transverse space is orbifolded by Z3 discrete symmetry. This model is chiral and has d=4 N =1 supersymmetry of Yang–Mills type as well as an inhomogeneous supersymmetry specific to matrix models. We calculate one-loop effective action around some backgrounds, and the result can be interpreted as interactions mediated by massless particles in IIB supergravity in orbifold background, if the background is in the Higgs branch. If the background is in the Coulomb branch, the dynamics is effectively governed by the reduced model of d=4 super Yang–Mills theory, which might be interpreted as exchange of massless particles in the twisted sector. But the perturbative calculation cannot reproduce the supergravity result. We also show that this model with a large Higgs vacuum expectation value becomes IIB (IKKT) matrix model.

Probability distribution of the index in gauge theory on 2d non-commutative geometry

We investigate the effect of non-commutative geometry on the topological aspects of gauge theory using a non-perturbative formulation based on the twisted reduced model. The configuration space is decomposed into topological sectors labeled by the index

The index of the overlap Dirac operator on a discretized 2d non-commutative torus

\nu

Dominance of a single topological sector in gauge theory on non-commutative geometry

of the overlap Dirac operator satisfying the Ginsparg-Wilson relation. We study the probability distribution of

Realizing three generations of the Standard Model fermions in the type IIB matrix model

\nu

Probability of the Standard Model Appearance from a Matrix Model

by Monte Carlo simulation in the U(1) gauge theory on 2d non-commutative torus. To our surprise, the distribution turns out to be asymmetric under