arXiv subjects

Hiroshi Ohki

Publications and source records attributed to Hiroshi Ohki.

At least 19 recordsLinked to original sources

arXiv2026

A novel view of the flavor-singlet spectrum from multi-flavor QCD on the lattice

SU(3) gauge theories with increasing number of light fermions are the templates of strongly interacting sectors and studying their low-energy dynamics and spectrum is important, both for understanding the strong dynamics of QCD itself, but also for discovering viable UV completions of beyond the Standard Model physics. In order to contrast many-flavors strongly interacting theories with QCD on a quantitative footing, we use Lattice Field Theory simulations. We focus on the study of the flavor-singlet spectrum in the scalar and pseudoscalar channels: this is an interesting probe of the dynamics of the strongly interacting sector, as reminded by the QCD case with the $f_0(500)$ ($σ$) and $η^\prime$ mesons. The hierarchy of the spectrum of a strongly coupled new gauge sector of the Standard Model defines the potential reach of future colliders for new physics discoveries. In addition to a novel hierarchy with light scalars, introducing many light flavors at fixed number of colors can influence the dynamics of the lightest flavor-singlet pseudoscalar. We present a complete lattice study of both these flavor-singlet channels on high-statistics gauge ensembles generated by the LatKMI collaboration with 4, 8, and 12 copies of light mass-degenerate fermions. We also present other hadron masses on the lightest ensemble for $N_f=8$ generated by the LatKMI collaboration and discuss the chiral extrapolation of the spectrum in this particular theory. We contrast the results to $N_f=4$ simulations and previous results of $N_f=12$ simulations.

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arXiv2026

Non-invertible Selection Rules from Generalized Discrete Gauging of Finite Non-Abelian Symmetries

We investigate non-invertible selection rules originating from the discrete $H$-gauging of theories with an underlying discrete global symmetry group $G$. To systematically describe these theories, we formulate a general framework for $H$-gauged models that incorporates generalized field transformations. Our approach naturally accommodates non-Abelian groups, for which multidimensional irreducible representations play an essential role. In such models with non-Abelian groups, the transformations induced by $H$ non-trivially mix the internal components of $G$-multiplets, potentially projecting out specific degrees of freedom. Consequently, conventional selection rules based on standard tensor product decompositions or conjugacy classes become insufficient. By analyzing the full semidirect product $G \rtimes H$, we introduce projected characters to derive necessary and sufficient conditions for non-vanishing $n$-point bare couplings. Furthermore, we demonstrate that the remaining field components obey an associative fusion-like algebra governed by their Clebsch-Gordan coefficients. Phenomenologically, these selection rules restrict allowed interactions and impose specific relations among coupling constants. We illustrate our results through concrete examples, including $Δ(54) \cong Δ(27)\rtimes \mathbb{Z}_2$ and $S_4 \cong A_4 \rtimes \mathbb{Z}_2$.

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arXiv2026

Calculation of neutron electric dipole moment from Lattice QCD

Experimental constraints on the neutron electric dipole moment (nEDM) may imply strong-CP problem in QCD, or unnatural smallness of the QCD theta angle. In this work, we present a novel determination of the neutron electric dipole moment (nEDM) $d_n$ sensitivity to theta term from nonperturbative QCD on a lattice with background electric field. Using Feynman-Hellmann theorem, we compute nEDM from the matrix element of local topological charge density between nucleon ground states spatially polarized by an electric field. These states have mixed spatial parity, and we construct them using variational analysis. We obtain statistically significant signal for the theta induced nEDM from lattices with 2+1 dynamical domain wall fermions corresponding to pion masses of 340, 420, and 576 MeV and lattice spacing $a\approx 0.11~\text{fm}$. After extrapolating to the physical point, we obtain $d_n=-0.0050(4)(8)\barθ$ e$\cdot$fm. Comparison with the current experimental bound on nEDM implies constraint $|\barθ|\lesssim 10^{-11}$, which confirms existence of the strong-CP problem in QCD. Our pioneering work demonstrates that neutron EDM can be reliably determined from the local density of topological charge with robust control of systematic effects, and can be directly extended to other CP-violating interactions.

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arXiv2026

The Neutron Electric Dipole Moment from Lattice QCD using a Background Electric Field

We present the calculation of the neutron electric dipole moment (nEDM) $d_n$ using 2+1 flavor domain wall fermion ensembles with fixed lattice spacing $a\approx 0.11\,\text{fm}$ and pion masses of 340, 420, and 576 MeV. We show that the neutron electric dipole moment can be extracted from the energy shift induced by a static uniform external background electric field in the presence of the CP-violating QCD theta-term, $\barθQ_{top}$. Motivated by the Feynman-Hellmann theorem, we employ sampling of the topological charge $q_\text{top}(t)$ on a single time-slice rather than the global topological charge $Q_\text{top}=\int q_\text{top}(t) \, dt$, which dramatically improves the statistical precision of the $θ$-induced nEDM. Key to our method is to calculate the forward matrix element of the topological charge density in the nucleon deformed by a background electric field. We find that calculation with the traditional positive parity-projected nucleon operator is subject to large excited-state contamination. To remove the contamination, we construct the ground state of the deformed nucleon by solving a non-Hermitian generalized eigenvalue problem. With this approach, we find consistent values for the nEDM when using different nucleon interpolating operators, regardless of whether they are covariant or non-covariant under chiral transformations. Finally, after extrapolating to the physical point, we obtain $d_n=-0.0050(4)^\text{stat}(8)^\text{sys}\barθ$ $e$ fm, where the systematic uncertainty includes excited-state effects estimated as variation with the Euclidean-time fits and the dependence on the strength of the electric field applied to the neutron. Conventional systematic errors like discretization, finite-volume, and chiral extrapolation effects will be addressed in future work.

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arXiv2026

Unified Origin of Dirac Neutrino and Asymmetric Dark Matter Masses via a Dirac-Type Leptogenesis

We propose a simple and unified framework that simultaneously explains the origins of light Dirac neutrino masses, asymmetric dark matter (ADM), and the baryon asymmetry of the Universe. The model is based on an extended $U(1)_X$ Froggatt-Nielsen--like mechanism, which naturally generates suppressed Yukawa couplings and realizes a Dirac seesaw for neutrino masses. An additional $\mathbb{Z}_4$ symmetry stabilizes the dark sector, where chiral fermions charged under $\mathbb{Z}_4$ serve as ADM candidates. Leptogenesis occurs through the out-of-equilibrium decays of heavy Dirac neutrinos, where the generated asymmetry is shared between the visible and dark sectors due to exact lepton-number conservation. The same suppression mechanism that explains the smallness of neutrino masses also determines the GeV-scale ADM mass. Numerical studies demonstrate that a fully asymmetric DM scenario is realized, consistent with relic abundance, Big Bang nucleosynthesis, and direct detection constraints. This framework provides an experimentally testable connection between neutrino physics, dark matter, and baryogenesis within an anomaly-free setup.

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arXiv2024

CP-like Symmetry with Discrete and Continuous Groups and CP Violation/Restoration

We study physical implications of general CP symmetry including CP-like symmetry. Various scattering amplitudes of CP asymmetry are calculated in CP-like symmetric models. We explicitly show that the CP-like transformation leads to a specific relation between different CP asymmetries. The resultant relation is similar to the one obtained in GUT baryogenesis and sphaleron processes, where we also obtain a required condition for generating particle number asymmetry in CP-like symmetric models. In addition, we propose a generalization of a CP-like transformation for continuous symmetry groups. Since the CP transformation is an outer automorphism, which depends on the internal symmetry group, it turns out that the physical CP and CP-like symmetries can be mutually converted through the spontaneous symmetry breaking (SSB) of the internal symmetry. We investigate properties of physical CP asymmetry in both CP and CP-like symmetric phases, and find that the spontaneous CP violation and restoration can be observed even in models with continuous groups. We demonstrate that CP-like symmetric models with continuous Lie groups can be naturally realized in physical CP symmetric models through the SSB.

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arXiv2023

The calculations of Nucleon Electric Dipole Moment using background field on Lattice QCD

Measurements of nucleon and nuclei Electric Dipole Moments (EDMs) play an important role in probing CP violation and exploring physics beyond the Standard Model. We extract the neutron EDM by measuring the energy shift of the nucleon two-point correlation function in the presence of a background field. The UV divergence of the topological charge density operator is mitigated using gradient flow, and the diffusion effect induced by the gradient flow process is included into the fit ansatz. Our calculations were carried out on two 2+1 DWF fermion, Iwasaki, gauge field ensembles generated by the RBC/UKQCD collaborations with inverse lattice spacing 1.73 GeV and pion masses of about 340 and 420 MeV.

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arXiv2023

All-mode Renormalization for Tensor Network with Stochastic Noise

In usual (non-stochastic) tensor network calculations, the truncated singular value decomposition (SVD) is often used for approximating a tensor, and it causes systematic errors. By introducing stochastic noise in the approximation, however, one can avoid such systematic errors at the expense of statistical errors which can be straightforwardly controlled. Therefore in principle, exact results can be obtained even at finite bond dimension up to the statistical errors. A previous study of the unbiased method implemented in tensor renormalization group (TRG) algorithm, however, showed that the statistical errors for physical quantity are not negligible, and furthermore the computational cost is linearly proportional to a system volume. In this paper, we introduce a new way of stochastic noise such that the statistical error is suppressed, and moreover, in order to reduce the computational cost we propose common noise method whose cost is proportional to the logarithm of volume. We find that the method provides better accuracy for the free energy compared with the truncated SVD when applying to TRG for Ising model on square lattice. Although the common noise method introduces systematic error originated from a correlation of noises, we show that the error can be described by a simple functional form in terms of the number of noises, thus the error can be straightforwardly controlled in an actual analysis. We also apply the method to the graph independent local truncation algorithm and show that the accuracy is further improved.

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arXiv2021

$\ell N$ inclusive scattering cross sections on the lattice

Utilizing the approach recently proposed for the $\ell N$ inclusive scattering cross section on the lattice, we compute the differential scattering cross section for the charged current process $\ell p \rightarrow νn $ for various kinematical channels. The simulation is carried out on the 2+1 flavor $16^3 \times 32 $ ensemble with Iwasaki and Domain Wall Fermion action. The lattice results are compared with MINER$ν$A result for the equivalent process.

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arXiv2021

Hybrid stochastic method for the tensor renormalization group

We propose a hybrid stochastic method for the tensor renormalization group (TRG) approach. TRG is known as a powerful tool to study the many-body systems and quantum field theory on the lattice. It is based on a low-rank approximation of the tensor using the truncated singular value decomposition (SVD), whose computational cost increases as the bond dimension increases, so that efficient cost reduction techniques are highly demanded. We use noise vectors for the low-rank approximation with the truncated SVD, by which the truncation error is replaced with a statistical error due to noise, and an error estimation could be improved. We test this method in the classical Ising model and observe a better accuracy than TRG. We also discuss a cross contamination issue in a multiple use of the same noise vectors, and to remove this systematic error we consider position-dependent noise vectors.

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arXiv2021

Modular Origin of Mass Hierarchy: Froggatt-Nielsen like Mechanism

We study Froggatt-Nielsen (FN) like flavor models with modular symmetry. The FN mechanism is a convincing solution to the flavor puzzle in quark sector. The FN mechanism requires an extra $U(1)$ gauge symmetry which is broken at high energy. Alternatively, in the framework of modular symmetry the modular weights can play the role of the FN charges of the extra $U(1)$ symmetry. Based on the FN-like mechanism with modular symmetry we present new flavor models for quark sector. Assuming that the three generations have a common representation under modular symmetry, our models simply reproduce the FN-like Yukawa matrices. We also show that the realistic mass hierarchy and mixing angles, which are related each other through the modular parameters and a scalar vev, can be realized in models with several finite modular groups (and their double covering groups) without unnatural hierarchical parameters.

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arXiv2020

Towards fully non-perturbative computation of inelastic $\ell N$ scattering cross sections from lattice QCD

We propose a fully non-perturbative method to compute inelastic lepton-nucleon ($\ell N$) scattering cross sections using lattice QCD. The method is applicable even at low energies, such as the energy region relevant for the recent and future neutrino-nucleon ($νN$) scattering experiments, for which perturbative analysis is invalidated. The basic building block is the forward Compton-scattering amplitude, or the hadronic tensor, computed on a Euclidean lattice. Total cross section is constructed from the hadronic tensor by multiplying a phase space factor and integrating over the energy and momentum of final hadronic states. The energy integral that induces a sum over all possible final states is performed implicitly by promoting the phase space factor to an operator written in terms of the transfer matrix on the lattice. The formalism is imported from that of the inclusive semileptonic B meson decay [P. Gambino, S. Hashimoto, arXiv:2005.13730]. It can be generalized to compute the $\ell N$ scattering cross sections and their moments, as well as the virtual correction to the nuclear $β$-decay. Necessary quark-line contractions for two current insertions corresponding to the Compton amplitude to be computed on the lattice are summarized.

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arXiv2020

Modular Flavor Symmetry on Magnetized Torus

We study the modular invariance in magnetized torus models. Modular invariant flavor model is a recently proposed hypothesis for solving the flavor puzzle, where the flavor symmetry originates from modular invariance. In this framework coupling constants such as Yukawa couplings are also transformed under the flavor symmetry. We show that the low-energy effective theory of magnetized torus models is invariant under a specific subgroup of the modular group. Since Yukawa couplings as well as chiral zero modes transform under the modular group, the above modular subgroup (referred to as modular flavor symmetry) provides a new type of modular invariant flavor models with $D_4 \times \mathbb{Z}_2$, $(\mathbb{Z}_4 \times \mathbb{Z}_2) \rtimes \mathbb{Z}_2$, and $(\mathbb{Z}_8 \times \mathbb{Z}_2) \rtimes \mathbb{Z}_2$. We also find that conventional discrete flavor symmetries which arise in magnetized torus model are non-commutative with the modular flavor symmetry. Combining both two symmetries we obtain a larger flavor symmetry, where the conventional flavor symmetry is a normal subgroup of the whole group.

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arXiv2020

Computing Nucleon Electric Dipole Moment from lattice QCD

Electric dipole moments (EDMs) of nucleons and nuclei are actively considered as direct evidence of the CP violation. Calculations of nucleon EDMs on lattice are required to connect the quark- and hadron- level effective CP violating interactions within QCD or other CP violating sources in new physics beyond the standard model. Among them, the theta-induced nucleon EDM, that is the only such renormalizable interaction, has widely been investigated on a lattice. In the report, we review recent developments of the lattice calculations of nucleon EDM induced QCD theta term.

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arXiv2019

Gravitational Waves from Walking Technicolor

We study gravitational waves from the first-order electroweak phase transition in the $SU(N_c)$ gauge theory with $N_f/N_c\gg 1$ ("large $N_f$ QCD") as a candidate for the walking technicolor, which is modeled by the $U(N_f)\times U(N_f)$ linear sigma model with classical scale symmetry (without mass term), particularly for $N_f=8$ ("one-family model"). This model exhibits spontaneous breaking of the scale symmetry as well as the $U(N_f)\times U(N_f)$ radiatively through the Coleman-Weinberg mechanism $\grave{a}$ la Gildener-Weinberg, thus giving rise to a light pseudo dilaton (techni-dilaton) to be identified with the 125 GeV Higgs. This model possess a strong first-order electroweak phase transition due to the resultant Coleman-Weinberg type potential. We estimate the bubble nucleation that exhibits an ultra supercooling and then the signal for a stochastic gravitational wave produced via the strong first-order electroweak phase transition. We show that the amplitude can be reached to the expected sensitivities of the LISA.

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arXiv2018

Novel $|V_{us}|$ Determination Using Inclusive Strange $τ$ Decay and Lattice HVPs

We propose and apply a new approach to determining $|V_{us}|$ using dispersion relations with weight functions having poles at Euclidean (space-like) momentum which relate strange hadronic $τ$ decay distributions to hadronic vacuum polarization functions (HVPs) obtained from lattice QCD. We show examples where spectral integral contributions from the region where experimental data have large errors or do not exist are strongly suppressed but accurate determinations of the relevant lattice HVP combinations remain possible. The resulting $|V_{us}|$ agrees well with determinations from $K$ physics and 3-family CKM unitarity. Advantages of this new approach over the conventional hadronic $τ$ decay determination employing flavor-breaking sum rules are also discussed.

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arXiv2018

Nucleon charges with dynamical overlap fermions

We calculate the scalar and tensor charges of the nucleon in 2+1-flavor lattice QCD, for which the systematics of the renormalization of the disconnected diagram is well controlled. Numerical simulations are performed at a single lattice spacing a = 0.11 fm. We simulate four pion masses, which cover a range of $m_π\sim$ 290 - 540 MeV, and a single strange quark mass close to its physical value. The statistical accuracy is improved by employing the so-called low-mode averaging technique and the truncated solver method. We study up, down, and strange quark contributions to the nucleon charges by calculating disconnected diagrams using the all-to-all quark propagator. Chiral symmetry is exactly preserved by using the overlap quark action to avoid operator mixing among different flavors, which complicates the renormalization of scalar and tensor matrix elements and leads to possibly large contamination to the small strange quark contributions. We also study the nucleon axial charge with contribution from the disconnected diagram. Our results are in reasonable agreement with experiments and previous lattice studies.

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arXiv2017

Flavor-singlet spectrum in multi-flavor QCD

Studying SU(3) gauge theories with increasing number of light fermions is relevant both for understanding the strong dynamics of QCD and for constructing strongly interacting extensions of the Standard Model (e.g. UV completions of composite Higgs models). In order to contrast these many-flavors strongly interacting theories with QCD, we study the flavor-singlet spectrum as an interesting probe. In fact, some composite Higgs models require the Higgs boson to be the lightest flavor-singlet scalar in the spectrum of a strongly interacting new sector with a well defined hierarchy with the rest of the states. Moreover, introducing many light flavors at fixed number of colors can influence the dynamics of the lightest flavor-singlet pseudoscalar. We present the on-going study of these flavor-singlet channels using multiple interpolating operators on high-statistics ensembles generated by the LatKMI collaboration and we compare results with available data obtained by the Lattice Strong Dynamics collaboration. For the theory with 8 flavors, the two collaborations have generated configurations that complement each others with the aim to tackle the massless limit using the largest possible volumes.

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