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Yuichi Koga

Publications and source records attributed to Yuichi Koga.

7 recordsLinked to original sources
arXiv2026

Maximal Enhancements in Eight-Dimensional Non-Supersymmetric Heterotic Strings

We classify maximally enhanced, rank-preserving non-supersymmetric heterotic strings in eight dimensions by studying orbifolds of supersymmetric heterotic strings on $T^2$. We first review the construction in nine dimensions, where it reproduces the $95$ maximally semisimple enhancement points obtained from previous extended-Dynkin analyses. We then start from the maximally enhanced supersymmetric Narain points in eight dimensions and use Kac's theorem to enumerate candidate order-two inner actions, retaining only those that lift to consistent Narain-lattice shifts. This gives $1210$ maximal enhancement points, of which $71$ are tachyon-free, and $25$ have neither tachyons nor shifted-sector massless scalars. For each entry we determine the gauge algebra and the low-lying scalar and fermion spectra, and for the tachyon-free cases we evaluate the one-loop cosmological constant. For the 25 subcases without shifted-sector massless scalars, we further compute the Hessian of the one-loop potential and test the refined de Sitter swampland conjecture.

hep-th↗
arXiv2025

More on 8d non-supersymmetric branes and heterotic strings

We determine the maximal gauge groups arising in $E_8$ heterotic string theory on $T^2$. Our analysis proceeds along two approaches. First, we start the moduli space of the supersymmetric heterotic string theory on $T^2$ focusing on points of maximal gauge enhancement. At these special points, the charge lattice can exhibit a $\mathbb{Z}_2$ outer automorphism corresponding to the bulk gauge symmetry. By orbifolding the worldsheet theory by it with the fermion parity, we obtain the maximal gauge group of the $E_8$ theory. Second, we directly study the toroidal compactification of 10d $E_8$ heterotic string. Both approaches agree, yielding a classification of $22$ maximal gauge groups. For each case, we present the corresponding massless spectrum. In light of the no global symmetry/cobordism conjecture in quantum gravity, our result also offer a classification of non-supersymmetric branes in 8d supersymmetric heterotic string theories.

hep-th↗
arXiv2025

Classification of Modular Symmetries in Non-Supersymmetric Heterotic String theories

We study modular symmetries in non-supersymmetric heterotic string theories on toroidal backgrounds with Wilson line modulus, constructed by stringy Scherk-Schwartz compactification. In particular, we focus on a subgroup of the T-duality group $O(D+16,D,\mathbb{Z})$ with $D=2$ given by an outer automorphism of the Narain lattice, which can be mapped to the Siegel modular group $\mathrm{Sp}(4,\mathbb{Z})$. We classify the modular symmetries and a $CP$-like symmetry on $T^2$ and its orbifolds with symmetric and asymmetric orbifold twists. It turns out that the non-supersymmetric heterotic string theories only enjoy a part of modular symmetries in contrast to supersymmetric ones. Furthermore, the gauge symmetry is maximally enhanced at fixed points of modular symmetries on $T^2$ on which we analyze the vacuum structure of eight-dimensional tachyon-free vacua.

hep-th↗
arXiv2025

Classification of Modular Symmetries in Type IIB Flux Landscape

In this work, we study modular symmetries in type IIB flux landscape by investigating symplectic basis transformations of period vectors on toroidal orbifolds. To fix explicit cycles of a third-cohomology basis regarding the untwisted complex structure modulus, which is necessary to construct the period vectors, we find that the following two symmetries are required for the period vectors: (i) ``Scaling duality '' which is a generalized $S$-transformation of $PSL(2, \mathbb{Z})$ and (ii) the modular symmetries to be consistent with symmetries derived from mass spectra of the closed string in type IIB string theory. Furthermore, by considering flux quanta on the cycles, we explore type IIB flux vacua on toroidal orientifolds and flux transformations under the modular symmetries of the period vectors.

hep-th↗
arXiv2023

Interpolation and Exponentially Suppressed Cosmological Constant in Non-Supersymmetric Heterotic Strings with General $\mathbb{Z}_{2}$ Twists

We study general non-supersymmetric heterotic string models, including so-called interpolating models, $d$-dimensionally compactified with the arbitrary number of freely acting $\mathbb{Z}_{2}$ twisted directions. Taking the limits of the compactified radii to zero and infinity (the endpoint limits), we show some examples of the various interpolation patterns in the $d=2$ (8-dimensional) case. In the region where supersymmetry is asymptotically restored, we derive the formula for the one-loop cosmological constant of $(10-d)$ dimensional non-supersymmetric heterotic string models with general $\mathbb{Z}_{2}$ twists, which does not depend on all the other endpoints and find out the points in the moduli space where the cosmological constant is exponentially suppressed. The moduli stability of the cosmological constant is also analyzed.

hep-th↗
arXiv2022

Target Space Duality of Non-Supersymmetric String Theory

T-dualities of the non-supersymmetric string models, which are constructed by twisted compactifications, are investigated. We show that the T-duality groups of such models are obtained by imposing congruence conditions on $O\left( d_{L},d_{R},\mathbb{Z}\right) $, that is, the non-supersymmetric string models are invariant under congruence subgroups of $O\left( d_{L},d_{R},\mathbb{Z}\right)$. We also point out that the transitions among the non-supersymmetric models can be induced by acting $O\left( d_{L},d_{R},\mathbb{Z}\right) $ transformations.

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