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Yuichiro Kiyo

Publications and source records attributed to Yuichiro Kiyo.

At least 19 recordsLinked to original sources
arXiv2026

Scale dependence of top-quark cross section at $e^+e^-$ colliders near production threshold at NNNLO

The top-quark threshold cross section at $e^+e^-$ colliders near production threshold is investigated. We study the scale dependence of the cross section for $\sigma(e^+e^- \to ttX$) near $\sqrt{s} \simeq 2m_t$ and discuss the theoretical accuracy of the NNNLO prediction. We report that a threshold scan at an $e^+e^-$ collider would allow a determination of the top-quark mass with an accuracy of order 30 MeV.

hep-ph↗
arXiv2025

Probing Scalar-Mediator Quark Couplings via CLFV Lepton-Nucleon Scattering

We investigate charged lepton flavor violating (CLFV) deep-inelastic scattering, focusing on the gluon-initiated subprocess $\ell_i g \to \ell_j g$ via the gluon effective operator $ϕ\, G_{μν}^a G_a^{μν}$, and demonstrate how to probe the nature of the CLFV mediator $ϕ$, specifically its mass and interaction with quarks. We consider two benchmark scenarios for the mediator-quark coupling: (i) $h$-like scenario, in which the mediator couples to heavy quarks in proportion to their masses, and (i\hspace{-1pt}i) $b$-only scenario, where the coupling is restricted to bottom quark only. We demonstrate that these scenarios can be discriminated by examining the dependence of the differential cross section on the momentum transfer. Furthermore, we show that the peak position of the differential cross section exhibits a pronounced sensitivity to both the mass of the mediator and the coupling strengths with quarks.

hep-ph↗
arXiv2021

Charged lepton flavor violation associated with heavy quark production in deep inelastic lepton-nucleon scattering via scalar exchange

We study charged lepton flavor violation (CLFV) associated with heavy quark pair production in lepton-nucleon deep-inelastic scattering $\ell_i N \to \ell_j q\bar{q} X$. Here $\ell_i$ and $\ell_j$ denote the initial and final leptons; $N$ and $X$ are respectively the initial nucleon and arbitrary final hadronic system. We employ a model Lagrangian in which a scalar and pseudoscalar mediator generates the CLFV. We derive heavy quark structure functions for scalar and pseudoscalar currents and compute momentum distributions of the final lepton for the process. Our focus is on the heavy quark mass effects in the final lepton momentum distribution. We clarify the necessity of inclusion of the heavy quark mass to obtain reliable theory predictions for the CLFV signal searches in the deep-inelastic scattering.

hep-ph↗
arXiv2019

Determination of $α_s$ from static QCD potential: OPE with renormalon subtraction and lattice QCD

We determine the strong coupling constant $α_s$ from the static QCD potential by matching a theoretical calculation with a lattice QCD computation. We employ a new theoretical formulation based on the operator product expansion, in which renormalons are subtracted from the leading Wilson coefficient. We remove not only the leading renormalon uncertainty of $\mathcal{O}(Λ_{\rm QCD})$ but also the first $r$-dependent uncertainty of $\mathcal{O}(Λ_{\rm QCD}^3 r^2)$. The theoretical prediction for the potential turns out to be valid at the static color charge distance $Λ_{\rm \overline{MS}} r \lesssim 0.8$ ($r \lesssim 0.4$ fm), which is significantly larger than ordinary perturbation theory. With lattice data down to $Λ_{\rm \overline{MS}} r \sim 0.09$ ($r \sim 0.05$ fm), we perform the matching in a wide region of $r$, which has been difficult in previous determinations of $α_s$ from the potential. Our final result is $α_s(M_Z^2) = 0.1179^{+0.0015}_{-0.0014}$ with 1.3 % accuracy. The dominant uncertainty comes from higher order corrections to the perturbative prediction and can be straightforwardly reduced by simulating finer lattices.

hep-ph↗
arXiv2019

Determination of $α_s$ from static QCD potential with renormalon subtraction

We determine the strong coupling constant $α_s(M_Z)$ from the static QCD potential by matching a lattice result and a theoretical calculation. We use a new theoretical framework based on operator product expansion (OPE), where renormalons are subtracted from the leading Wilson coefficient. We find that our OPE prediction can explain the lattice data at $Λ_{\rm QCD} r \lesssim 0.8$. This allows us to use a larger window in matching, which leads to a more reliable determination. We obtain $α_s(M_Z)=0.1179^{+0.0015}_{-0.0014}$.

hep-ph↗
arXiv2017

Determination of $m_c$ and $m_b$ from quarkonium 1S energy levels in perturbative QCD

We update determination of the $\overline{\rm MS}$ masses of the charm and bottom quarks, from comparisons of the masses of the charmonium and bottomonium $1S$ states with their perturbative predictions up to next-to-next-to-next-to-leading order in $\varepsilon$ expansion and using the $\overline{\rm MS}$ masses. Effects of non-zero charm-quark mass in the bottomonium masses are incorporated up to next-to-next-to-leading order. We obtain $\overline m_c=1246\pm 2 (d_3) \pm 4 (α_s) \pm 23 (\text{h.o.} )~{\rm MeV} $ and $\overline m_b=4197\pm 2 (d_3) \pm 6 (α_s) \pm 20 (\text{h.o.} )\pm 5 (m_c)~ {\rm MeV} $, which agree with the current Particle Data Group values.

hep-ph↗
arXiv2016

On enhanced corrections from quasi-degenerate states to heavy quarkonium observables

It is well known that in perturbation theory existence of quasi-degenerate states can rearrange the order counting. For a heavy quarkonium system, naively, enhanced effects ($l$-changing mixing effects) could contribute already to the first-order and third-order corrections to the wave function and the energy level, respectively, in expansion in $α_s$. We present a formulation and note that the corresponding (lowest-order) corrections vanish due to absence of the relevant off-diagonal matrix elements. As a result, in the quarkonium energy level and leptonic decay width, the enhanced effects are expected to appear, respectively, in the fifth- and fourth-order corrections and beyond.

hep-ph↗
arXiv2014

Study of top quark pair production near threshold at the ILC

We report on a study of top pair production at the International Linear Collider (ILC) around center of mass energy (E$_{\rm CM}$) = 350 GeV using an ILD detector simulator based on the Detailed Baseline Design (DBD) configuration. Here we will report on a result of 6-Jet final state, $t\ bar{t} \rightarrow bWbW \rightarrow bqqbqq$. A result for the 4-Jet final state, $t\ bar{t} \rightarrow bWbW \rightarrow bqqblν$, which has almost the same statics as that of the 6-Jet final state will be included in the future. For an energy scan of 11 center of mass energy points (340 - 350GeV) and two beam polarization combinations (P($e^+,\ e^-$) = ($\pm$0.3, $\mp$0.8)) with 10 fb$^{-1}$ each, the statistical errors on the top quark Yukawa coupling, its mass and width are estimated. The results are $δy_t$ = 4.2%, $δm_t$ = 16 MeV in potential subtracted scheme (PS), and $δΓ_t$ = 21 MeV.

hep-ex↗
arXiv2014

Leptonic decay of the Upsilon(1S) meson at third order in QCD

We present the complete next-to-next-to-next-to-leading order short-distance and bound-state QCD correction to the leptonic decay rate Gamma(Upsilon(1S)->l+l-) of the lowest-lying spin-1 bottomonium state. The perturbative QCD prediction is compared to the measurement Gamma(Upsilon(1S)->e+e-)=1.340(18) keV.

hep-ph↗
arXiv2013

The real photon structure functions in massive parton model in NLO

We investigate the one-gluon-exchange ($αα_s$) corrections to the real photon structure functions $W_{TT} $, $W_{LT}$, $W_{TT}^{a} $ and $W_{TT}^τ$ in the massive parton model. We employ a technique based on the Cutkosky rules and the reduction of Feynman integrals to master integrals. We show that a positivity constraint, which is derived from the Cauchy-Schwarz inequality, is satisfied among the unpolarized and polarized structure functions $W_{TT}$, $W_{TT}^a$ and $W_{TT}^τ$ calculated up to the next-to-leading order in QCD.

hep-ph↗
arXiv2012

The polarized photon structure function $g_1^γ(x,Q^2)$ in massive parton model in NLO

We investigate the one-gluon-exchange ($αα_s$) corrections to the polarized real photon structure function $g_1^γ(x,Q^2)$ in the massive parton model. We employ a technique based on the Cutkosky rules and the reduction of Feynman integrals to master integrals. The NLO contribution is noticeable at large $x$ and does not vanish at the threshold of the massive quark pair production due to the Coulomb singularity. It is found that the first moment sum rule of $g_1^γ$ is satisfied up to the NLO.

hep-ph↗
arXiv2011

Measuring the top Yukawa coupling at the ILC at sqrt(s) = 500 GeV

We report on the feasibility of the direct measurement of the top Yukawa coupling g_t at the International Linear Collider (ILC) during its first phase of operation with a center-of-mass energy of 500 GeV. The signal and background models incorporate the non-relativistic QCD corrections which enhance the production cross section near the t-tbar threshold. The e+e- -> t tbar H signal is reconstructed in the 6-jet + lepton and the 8-jet modes. The results from the two channels are combined. The background processes considered are e+e- -> t bbar W- / tbar b W+ (which includes e+e- -> t tbar), e+e- -> t tbar Z, and e+e- -> t tbar g* -> t tbar b bar. We use a realistic fast Monte-Carlo detector simulation. Signal events are selected using event shape variables, through jet clustering, and by identifying heavy flavor jets. Assuming a Higgs mass of 120 GeV, polarized electron and positron beams with (Pe-,Pe+) = (-0.8,+0.3), and an integrated luminosity of 1 ab-1, we estimate that the e+e- -> t tbar H events can be seen with a statistical significance of 5.2 sigma, corresponding to the relative top Yukawa coupling measurement accuracy of |Delta g_t / g_t| = 10%.

hep-ph↗
arXiv2010

Improved determination of inclusive electromagnetic decay ratios of heavy quarkonium from QCD

We consider a different power counting in potential NRQCD by incorporating the static potential exactly in the leading order Hamiltonian. We compute the leading relativistic corrections to the inclusive electromagnetic decay ratios in this new scheme. The effect of this new power counting is found to be large (even for top). We produce an updated value for the $η_b$ decay to two photons. This scheme also brings consistency between the weak coupling computation and the experimental value of the charmonium decay ratio.

hep-ph↗
arXiv2001

CP Violation in Kaon System in Supersymmetric SU(5) Model with Seesaw-Induced Neutrino Masses

CP violations in the kaon system are studied in supersymmetric SU(5) model with right-handed neutrinos. We pay a special attention to the renormalization group effect on the off-diagonal elements of the squark mass matrices. In particular, if the Yukawa couplings and mixings in the neutrino sector are sizable, off-diagonal elements of the right-handed down-type squark mass matrix are generated, which affect CP and flavor violations in decay processes of the kaon. We calculate supersymmetric contributions to epsilon (as well as Delta m_K), Br(K_L -> pi^0 nu \bar{nu}), and epsilon'/epsilon in this framework. We will see that the supersymmetric contribution to the epsilon parameter can be as large as (and in some case, larger than) the experimentally measured value. We also discuss its implication to future tests of the unitarity triangle of the Kobayashi-Maskawa matrix.

hep-ph↗
arXiv2000

Azimuthal angular dependence of decay lepton in e+ e- --> t tbar

We discuss top quark production and its subsequent decay for searching new physics at lepton colliders. The angular dependence of the decay leptons is calculated including both QCD corrections and anomalous gamma Z - t tbar couplings. The off-diagonal spin basis for the top and anti-top quarks is shown to be useful to probe the anomalous couplings.

hep-ph↗
arXiv1998

The Small $x$ Behavior of $g_1$ in the Resummed Approach

The double logarithmic terms $α_{s} \ln^{2}x $ are important to predict precisely the small $x$ behavior of the spin structure function $g_{1}$. We numerically analyze the evolution of the flavor non-singlet $g_{1}$ including the all-order resummed effect of these terms. It is pointed out that the next-to-leading logarithmic corrections produce an unexpectedly large suppression factor over the experimentally accessible range of $x$ and $Q^{2}$. This implies that the next-to-leading logarithmic contributions are very important in order to obtain a definite prediction.

hep-ph↗