arXiv subjects

Teppei Kitahara

Publications and source records attributed to Teppei Kitahara.

At least 19 recordsLinked to original sources

arXiv2026

On-Shell Amplitudes and Black-Hole Perturbations: Exact Reissner-Nordstr\"om Mixing

Can flat-space on-shell amplitudes determine the channel basis of a coupled black-hole perturbation problem? We address this question for electromagnetic and gravitational perturbations of a Reissner-Nordstr\"{o}m (RN) black-hole. We organize the minimally coupled photon-graviton tree amplitudes off a heavy charged source into a $2\times 2$ channel-space matrix and perform a parity-resolved Jacob-Wick partial-wave projection. For every radiative multipole $\ell \geq 2$ and in both parity sectors, we show that the trace-free fixed-source partial-wave matrix is exactly proportional to the trace-free Moncrief coupling matrix, and therefore selects the same constant spectral projectors. Through a first-Born matching, the amplitudes determine the same eigenspaces in the leading $r^{-3}$ weak-field potential, but not the complete radial potentials. Using the exact classical RN potentials as independent curved-background input, we show that these projectors persist throughout the full radial domain. We also explicitly retain finite-mass effects through $\mathcal{O}(\omega/m)$, finding a nonvanishing commutator with the Moncrief coupling matrix, which shows that the RN-projector alignment is spoiled by genuine two-body recoil effects. As a first step toward rotation, we further extract the representation-independent linear-spin term from a minimally coupled Dirac amplitude. We find that the complete tree-level channel matrix factorizes with a single linear-spin dressing, while the formal $J=2$ block fails to preserve the unchanged RN projectors. This restricted result does not constitute a test of Kerr-Newman separability, but it indicates that a rotating generalization must account for spin-induced angular-mode mixing. We expect that this on-shell method can be extended to more general long-range scattering systems with two asymptotic channels.

hep-th↗

arXiv2026

New Directions in Kaon Physics: Interference in $K^0\to\mu^+\mu^-$ as a New Golden Mode

The rare decays $K^0_L \to\mu^+\mu^-$ and $K^0_S \to\mu^+\mu^-$ have long been regarded as difficult channels to extract short-distance physics because they are dominated by long-distance contributions via two-photon exchanges. A qualitatively new feature arises once the interference between $K_L^0$ and $K_S^0$ is taken into account. The interference term is sensitive to genuine direct $CP$ violation in $s \to d \mu^+ \mu^-$ processes and turns this channel into a clean probe of the $CP$-violating short-distance physics. In this contribution, I summarize the basic mechanism of the $K_L^0$--$K_S^0$ interference, the flavor-tagging strategy at LHCb-like setup, and the projected sensitivities for a kaon-unitarity-triangle parameter combination $|A^2\lambda^5\bar\eta|$ and also for the sign ambiguity of the $K_L^0 \to \gamma \gamma$ three-point amplitude. As a result, it is expected that the CKM parameter $|A^2\lambda^5\bar\eta|$ could be constrained by LHCb at the level of about $35\%$ of its Standard Model (SM) value, and the discrete ambiguity in $\mathcal{B}(K_L^0 \to \mu^+\mu^-)_{\rm SM}$ could be resolved at more than $3\sigma$ by the end of the high luminosity LHC.

hep-ph↗

arXiv2026

Baryon-Meson Sum Rule for $b \to s \nu\bar\nu$

We derive a robust sum rule among the branching fractions of $\Lambda_b \to \Lambda \nu \bar\nu$ and $B \to K^{(\ast)} \nu\bar\nu$, assuming that right-handed neutrinos are decoupled. Despite the presence of 18 independent Wilson coefficients in the effective Hamiltonian, this relation remains exact. Remarkably, it is found that the coefficients of this baryon-meson sum rule are numerically identical to those of the $b\to c$ semileptonic sum rule among the branching fractions of $\Lambda_b \to \Lambda_c \tau \bar\nu$ and $B \to D^{(\ast)}\tau\bar{\nu}$. Once the decay rate of $B \to K^{\ast} \nu \bar\nu$ is measured, the decay rate of $\Lambda_b \to \Lambda \nu\bar\nu$ can be determined in a model-independent manner for new-physics scenarios involving only left-handed neutrino interactions. This clearly demonstrates that observables in baryonic and mesonic $b \to s \nu \bar{\nu}$ transitions will serve as a powerful probe for discriminating among new-physics scenarios.

hep-ph↗

arXiv2026

Full Three-Loop Electroweak Multiplet Contributions to the Electron Electric Dipole Moment

Experimental sensitivity to the electric dipole moment (EDM) of the electron has improved remarkably in recent years. Consequently, future prospects could probe new physics whose contribution to the electron EDM first arises at three-loop order. Additional SU(2)$_L$ multiplets with CP-violating Yukawa interactions, which contribute to the electron EDM at three-loop level, is one such testable new physics scenario. In this scenario, the electron EDM is radiatively induced from two contributions: the CP-odd trilinear $W$-boson coupling, called the electroweak-Weinberg operator, and the CP-odd dipole operator of electron. The former and the latter operators are generated at two-loop and three-loop levels, respectively, after integrating out the SU(2)$_L$ multiplets. Within the same models, according to an analysis based on the Standard Model Effective Field Theory (SMEFT), we previously found that the contribution to the electron EDM from the electroweak-Weinberg operator can be probed in future experiments. However, the one-loop matching condition between the electron EDM and the electroweak-Weinberg operator does not receive a large logarithmic enhancement because the associated anomalous dimension is zero. The CP-odd dipole operator of the electron would contribute to the electron EDM at the same three-loop order as the contribution through the electroweak-Weinberg operator. In this paper, we directly calculate the electron EDM induced by the CP-violating Yukawa interactions of the SU(2)$_L$ multiplets at full three-loop level. A central result is that the full three-loop calculation is a factor of three larger than that of the electroweak-Weinberg operator alone.

hep-ph↗

arXiv2025

Complete one-loop QED corrections to $D_s^+$ leptonic decays and impact on the CKM unitarity test

Recently, a violation of the CKM unitarity condition has been reported in the latest charm-meson data and the latest lattice results, once the universal electroweak correction is taken into account. In this article, we analytically derive for the first time the complete one-loop electroweak (EW) and QED corrections to the $D_{s}^+ \to \ell^+ \nu_\ell$ decays for $\ell = \mu, \tau$. Our analysis incorporates both short-distance EW-QED corrections, which are beyond the leading-logarithmic approximation (the so-called Sirlin factor), and long-distance soft-photon corrections depending on the maximum total energy of undetected photons with their resummation. Although the inclusive photon QED corrections to the meson leptonic decays are well known, they do not match the actual measurement circumstances in $D_s^+ \to \mu^+ \nu_\mu$. We find $ |V_{cs}|_{D_s} = 0.991 \pm 0.007 $ from the latest data on $D_s^+$ leptonic decays. We show that properly including these radiative corrections is essential to bring the second-column CKM unitarity tests into agreement with the Standard Model expectation. The study emphasizes that the current limiting factor in confirming CKM unitarity is the precision of QED corrections, and it points out that improving lattice simulations, taking the QED corrections into account, would be desirable for a more robust confirmation.

hep-ph↗

arXiv2025

CP violation in $K\to\mu^+\mu^-$ with and without time dependence through a tagged analysis

We point out that using current knowledge of ${\cal B}(K^0_L\to\mu^+\mu^-)$ and $ {\cal B}(K^0_L\to \gamma\gamma)$, one can extract short-distance information from the combined measurement of the time-integrated CP asymmetry, $A_{\rm CP}(K^0\to\mu^+\mu^-)$, and of ${\cal B}(K^0_S\to\mu^+\mu^-)$. We discuss the interplay between this set of observables, and demonstrate that determining ${\rm sign}[A_{\rm CP}(K^0\to\mu^+\mu^-)]$ would eliminate the discrete ambiguity in the Standard Model prediction for ${\cal B}(K^0_L\to\mu^+\mu^-)$. We then move on to feasibility studies within an LHCb-like setup, using both time-integrated and time-dependent information, employing $K^0$ and $\overline K{}^0$ tagging methods. We find that, within an optimistic scenario, the short-distance amplitude, proportional to the CKM parameter combination $|A^2\lambda^5\bar\eta|$, could be constrained by LHCb at the level of about $35\%$ of its Standard Model value, and the discrete ambiguity in ${\cal B}(K^0_L\to\mu^+\mu^-)_{\rm SM}$ could be resolved at more than $3\sigma$ by the end of the high luminosity LHC.

hep-ph↗

arXiv2025

Discriminating Tauphilic Leptoquark Explanations of the $B$ Anomalies via $K\to \pi\nu\bar\nu$ and $B\to K\nu\bar\nu$

Leptoquark models are prime candidates for new physics (NP) explanations of the long-standing anomalies in semi-leptonic $B$ decays; $b\to c \tau \bar\nu$ (encoded in $R(D^{(\ast)})$) and $b\to s\ell\bar\ell (\ell=e,\mu)$ transitions. Furthermore, Belle II and NA62 reported weaker-than-expected limits on $B^+ \to K^+ \nu\bar\nu$ and $K^+ \to \pi^+ \nu \bar\nu$, respectively. While the $R(D^{(\ast)})$ and $b\to s\ell \bar\ell$ measurements can be explained with NP contributions at the $O(10\%)$ level, the neutrino channels suggest that the NP effect could be comparable in size to the Standard Model one. In this context, we consider the two types of leptoquark models with minimal sets of the couplings that can best describe the semi-leptonic $B$ anomalies and lead at the same time to effects in the neutrino modes, the singlet-triplet scalar leptoquark model ($S_1+S_3$) and the singlet vector leptoquark model ($U_1$). More specifically, the neutrino channels pose non-trivial constraints on the parameter space, and we find that large effects (i.e., accounting for the current central value) in $B\to K^{(*)}\nu\bar\nu$ are only possible in the $S_1+S_3$ setup, while both models can account for the central value of $K^+\to \pi^+\nu\bar\nu$.

hep-ph↗

arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

hep-ph↗

arXiv2025

Two-loop corrections to QCD $\theta$ angle from evanescent operator in the BMHV scheme

We study the renormalization of the QCD $\theta$ angle at the two-loop level focusing on divergent and finite $CP$-violating contributions from evanescent operators, using dimensional regularization with the BMHV scheme. When one considers the Lagrangian in $d$-dimensional space-time instead of four dimensions in dimensional regularization, evanescent operators that break the chiral symmetry are induced. Consequently, $T$-odd and $P$-odd fermion loops in the BMHV scheme generate evanescent contributions to the QCD $\theta$ angle. We carefully classify the evanescent contributions into two types: one originating from the evanescent operators at the one-loop level and the other directly produced by two-loop calculations. We show that renormalization of the parameters in the BMHV scheme keeps removing those unphysical contributions to the QCD $\theta$ angle at any scale at the two-loop level. We also discuss how the rephasing invariance of the radiative correction to the QCD $\theta$ angle is realized in the BMHV scheme.

hep-ph↗

arXiv2025

Light scalar beyond the Higgs mixing limit

We explore the possibility that the interactions of a light scalar singlet, which mixes with the Standard Model~(SM) Higgs, also receive other UV contributions of comparable size. We focus, in particular, on the flavor aligned limit, where couplings of the light scalar to the SM are almost flavor diagonal, but not necessarily proportional to the Higgs Yukawa couplings. The phenomenology of such a general flavor aligned light scalar differs from both the Higgs-mixed scalar, as well as from a general axion-like particle. We explore this for light scalar masses below a few hundred MeV, such that they can be produced in kaon decays, and in decays of $\eta$ and $\eta'$ mesons, and the transitions described using chiral perturbation theory. We then derive constraints on the light scalar interactions, assuming that light scalar decays are either just into photons or are invisible. We also discuss several UV examples of such light scalar models: a two-Higgs doublet model extended by a light scalar, a light dilaton from the dark sector, and a SM extended by heavy vector-like quarks and a light scalar. For the latter we also performed matching onto low energy theory at one-loop.

hep-ph↗

arXiv2024

Impact of the Electroweak Weinberg Operator on the Electric Dipole Moment of Electron

Recent progresses in the measurements of the electric dipole moment (EDM) of the electron using the paramagnetic atom or molecule are remarkable. In this paper, we calculate a contribution to the electron EDM at three-loop level, introducing the CP-violating Yukawa couplings of new SU(2)$_L$ multiplets. At two-loop level, the Yukawa interactions generate a CP-violating dimension-six operator, composed of three SU(2)$_L$ field strengths, called the electroweak Weinberg operator. Another one-loop diagram with the operator inserted induces the electron EDM. We find that even if new SU(2)$_L$ particles have masses around TeV-scale, the electron EDM may be larger than the Standard Model contribution to the paramagnetic atom or molecule EDMs. We also discuss the relation between the Barr-Zee diagram contribution at two-loop level and the three-loop one, assuming that the SM Higgs has new Yukawa interactions with the SU(2)$_L$ multiplets.

hep-ph↗

arXiv2024

Theoretical point of view on Cabibbo angle anomaly

We present the current situation of the determinations of the first-row CKM components and show the Cabibbo angle anomaly corresponding to a deficit in the first-row CKM unitarity condition at the $3\sigma$ level. In this contribution, we show two new physics interpretations: heavy vector-like quark models and a MeV scale sterile neutrino models. The super tau-charm facility will directly probe the other CKM unitarity conditions related to $V_{cd}$.

hep-ph↗

arXiv2024

Global fit to $b \to c\tau\nu$ anomalies as of Spring 2024

Recently, several new experimental results of the test of lepton flavor universality (LFU) in $B\to D^{(\ast)}$ semi-leptonic decays were announced: the first result of $R_{D}$ from the LHCb Run 1 data, the first results of $R_{D}$ and $R_{D^\ast}$ from the LHCb Run 2 data, and the first result of $R_{D^\ast}$ from the Belle II collaboration. Including these new data, a global analysis still prefers the violation of the LFU between the tau and light leptons. A new world average of the data from the BaBar, LHCb, Belle, and Belle II collaborations is $R_{D} = 0.342 \pm 0.026$ and $R_{D^{\ast}} = 0.287 \pm 0.012$. Including this new data, we update a circumstance of the $b \to c \tau \overline\nu$ measurements and their implications for new physics (NP). Incorporating recent developments for the $B \to D^{(\ast)}$ form factors in the Standard Model (SM), we observe a $4.4 \sigma$ deviation from the SM prediction. Our updates also include; model-independent NP formulae for the related observables; and the global fittings of parameters for leptoquark scenarios as well as single NP operator scenarios. Furthermore, we show future potential to indirectly distinguish different NP scenarios with the use of the precise measurements of the polarization observables in $B\to D^{(\ast)}\tau \overline\nu$ at the Belle II and the high-$p_{\rm T}$ flavored-tail searches at the LHC. We also discuss an impact on the LFU violation in $\Upsilon \to l^+ l^-$.

hep-ph↗

arXiv2023

Closer look at the matching condition for radiative QCD $\theta$ parameter

In this paper, we scrutinize a radiatively generated QCD $\theta$ parameter at the two-loop level based on both full analytical loop functions with the Fock-Schwinger gauge method and the effective field theory approach, using simplified models. We observe that the radiatively generated $\theta$ parameters at the low energy scale precisely match between them. It provides validity to perturbative loop calculations of the QCD $\theta$ parameter with the Fock-Schwinger gauge method. Furthermore, it is also shown that the ordinary Fujikawa method for the radiative $\theta$ parameter by using $\bar\theta = - arg det M_q^{loop}$ does not cover all contributions in the simplified models. But, we also find that when there is a scale hierarchy in $CP$-violating sector, evaluation of the Fujikawa method is numerically sufficient. As an application, we calculate the radiative $\theta$ parameter at the two-loop level in a slightly extended Nelson-Barr model, where the spontaneous $CP$ violation occurs to solve the strong $CP$ problem. It is found a part of the radiative $\theta$ parameters cannot be described by the Fujikawa method.

hep-ph↗

arXiv2023

MeV Sterile Neutrino in light of the Cabibbo-Angle Anomaly

A modified neutrino sector could imprint a signature on precision measurements of the quark sector because many such measurements rely on the semi-leptonic decays of the charged currents. Currently, global fits of the determinations of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements point to a $3\sigma$-level deficit in the first-row CKM unitarity test, commonly referred to as the Cabibbo-angle anomaly. We find that a MeV sterile neutrino that mixes with the electron-type neutrino increases the extracted $|V_{ud}|$, accommodating the Cabibbo-angle anomaly. This MeV sterile neutrino affects the superallowed nuclear $\beta$ decays and neutron decay, but it barely modifies the other measurements of the CKM elements. While various constraints may apply to such a sterile neutrino, we present viable scenarios within an extension of the inverse seesaw model.

hep-ph↗

arXiv2023

Electric Dipole Moments as Probes of $B$ Anomaly

The measurements of the lepton flavor universality (LFU) in $\mathcal{B}({\bar{B}} \to D^{(\ast)} l \bar{\nu})$ indicate a significant deviation from the standard model prediction at a 3-4$\sigma$ level, revealing a violation of the LFU ($R_{D^{(\ast)}}$ anomaly). It is known that the $R_{D^{(\ast)}}$ anomaly can be easily accommodated by an $SU(2)_L$-singlet vector leptoquark (LQ) coupled primarily to third-generation fermions, whose existence is further motivated by a partial gauge unification. In general, such a LQ naturally leads to additional $CP$-violating phases in the LQ interactions. In this paper, we point out that the current $R_{D^{(\ast)}}$ anomaly prefers the $CP$-violating interaction although $\mathcal{B}({\bar{B}} \to D^{(\ast)} l\bar{\nu})$ are $CP$-conserving observables. The $CP$-violating LQ predicts a substantial size of the bottom-quark electric dipole moment (EDM), the chromo-EDM, and also the tau-lepton EDM. Eventually at low energy, the nucleon and electron EDMs are radiatively induced. Therefore, we conclude that the $R_{D^{(\ast)}}$ anomaly with the $SU(2)_L$-singlet vector LQ provides unique predictions: neutron and proton EDMs with opposite signs and a magnitude of $\mathcal{O}(10^{-27})\,e$cm, and suppressed electron EDM. Furthermore, we show that a similar EDM pattern is predicted in an $SU(2)_L$-doublet scalar LQ scenario that can accommodate the $R_{D^{(\ast)}}$ anomaly as well. These EDM signals could serve as crucial indicators in future experiments.

hep-ph↗

arXiv2023

Current Status of the Muon g-2 Interpretations within Two-Higgs-Doublet Models

In this article, we review and update implications of the muon anomalous magnetic moment (muon $g-2$) anomaly for two-Higgs-doublet models (2HDMs), which are classified according to imposed symmetries and their resulting Yukawa sector. In the minimal setup, the muon $g-2$ anomaly can be accommodated by the type-X (lepto-philic) 2HDM, flavor-aligned 2HDM (FA2HDM), muon-specific 2HDM ($\mu$2HDM), and $\mu\tau$-flavor violating 2HDM. We summarize all relevant experimental constraints from high-energy collider experiments and flavor experiments, as well as the theoretical constraints from the perturbative unitarity and vacuum stability bounds, to these 2HDMs in light of the muon $g-2$ anomaly. We clarify the available parameter spaces of these 2HDMs and investigate how to probe the remaining parameter regions in future experiments. In particular, we find that, due to the updated $B_s\to\mu^+ \mu^-$ measurement, the remaining parameter region of the FA2HDM is almost equivalent to the one of the type-X 2HDM. Furthermore, based on collider simulations, we find that the type-X 2HDM is excluded and the $\mu$2HDM scenario will be covered with the upcoming Run 3 data.

hep-ph↗

arXiv2023

Asymmetric di-Higgs signals of the next-to-minimal 2HDM with a $U(1)$ symmetry

The two-Higgs-doublet model with a $U(1)_H$ gauge symmetry (N2HDM-$U(1)$) has several advantages compared to the ``standard'' $Z_2$ version (N2HDM-$Z_2$): It is purely based on gauge symmetries, involves only spontaneous symmetry breaking, and is more predictive because it contains one parameter less in the Higgs potential, which further ensures $CP$ conservation, i.e., avoiding the stringent bounds from electric dipole moments. After pointing out that a second, so far unknown version of the N2HDM-$U(1)$ exists, we examine the phenomenological consequences for the Large Hadron Collider (LHC) of the differences in the scalar potentials. In particular, we find that while the N2HDM-$Z_2$ predicts suppressed branching ratios for decays into different Higgs bosons for the case of the small scalar mixing (as suggested by Higgs coupling measurements), both versions of the N2HDM-$U(1)$ allow for sizable rates. This is particularly relevant in light of the CMS excess in resonant Higgs-pair production at around $650\,$GeV of a Standard Model Higgs boson subsequently decaying to photons and a new scalar with a mass of $\approx90\,$GeV subsequently decaying to bottom quarks (i.e., compatible with the CMS and ATLAS $\gamma\gamma$ excesses at $95\,$GeV and $\approx 670\,$GeV). As we will show, this excess can be addressed within the N2HDM-$U(1)$ in case of a nonminimal Yukawa sector, predicting an interesting and unavoidable $Z+ b\bar b$ signal and motivating further asymmetric di-Higgs searches at the LHC.

hep-ph↗