arXiv subjects

Syuhei Iguro

Publications and source records attributed to Syuhei Iguro.

At least 19 recordsLinked to original sources

arXiv2026

$|V_{cb}|$ determinations from $\bar{B} \to D^{(*)} \ell \bar\nu$ decays within the SM and beyond

We investigate the $|V_{cb}|$ determinations from exclusive semi-leptonic $\bar{B} \to D^{(*)}\ell\bar\nu$ decays, together with comprehensive fit analyses of the $\bar{B} \to D^{(*)}$ transition form-factors, by taking into account recent updates of experimental distribution data and theoretical evaluations. Several commonly adopted form-factor parameterizations, including BSZ, BGL and HQET, have been considered under different fit scenarios. We compare the fitted values and study how the $|V_{cb}|$ determinations depend on the form-factor parameterizations and on the treatment of the experimental inputs. In particular, we reproduce the official PDG average of $|V_{cb}|$ with the BGL parameterization, while the HQET parameterization tends to give a smaller value of $|V_{cb}|$. We also consider new-physics effects that can contribute to $\bar{B} \to D^{(*)}\ell\bar\nu$, and examine whether non-zero new-physics contributions are still allowed by the current data.

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arXiv2026

Semileptonic sum rules in heavy-to-light charm decays

We investigate semileptonic sum rules in heavy-to-light charm decays, motivated by analogous relations in $b \to c$ and $b \to u$ transitions. Focusing on the $c \to d\overline{\ell}ν$ decays, $D \to π\overline{\ell}ν$, $D \to ρ\overline{\ell}ν$, and $Λ_c \to n\overline{\ell}ν$, we examine a relation among their lepton-flavor universality ratios $R_H^{μe}$. Although the charm sum rule is less precise than the relations in bottom-hadron decays, current low-energy and high-$p_T$ constraints on new physics restrict the actual deviation from the relation to below the percent level. The relation can therefore provide a useful consistency check of charm semileptonic measurements. As an application, we derive a prediction for the yet-unmeasured ratio $R_n^{μe}$ in $Λ_c \to n\overline{\ell}ν$.

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arXiv2026

$b \to c$ semileptonic sum rule: SU(3)$_{\rm{F}}$ symmetry violation

To clarify possible deviations in $b\to cτ\overlineν$ processes, the $b\to c$ semileptonic sum rule provides a valuable tool. This relation, derived based on heavy quark symmetry (HQS), offers a powerful consistency check among experimental results. In this work, we extend the previously proposed sum rule for $\{B\to D^{(*)} l\overlineν,\,Λ_b\to Λ_c l\overlineν\}$ to include $\{B_s\to D_s^{(*)} l\overlineν,\,Ξ_b\to Ξ_c l\overlineν\}$, thereby enabling more useful cross-checks. Although the relation is supported by HQS and SU(3) flavor symmetry, both symmetries are broken in reality, and the size of the violation needs to be quantified to assess the validity of the sum rule. While the violation is expected to be moderate based on chiral perturbation theory, we perform a numerical evaluation and compare it with future experimental sensitivities. We find that the violation remains smaller than the expected experimental uncertainty. Therefore another new physics agnostic and predictive sum rules are constructed to check the consistency.

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arXiv2026

$b \to c$ semileptonic sum rule: orbitally excited hadrons

We study semileptonic sum rules for $b \to c τ\overlineν$ transitions involving orbitally excited charm hadrons. Starting from the amplitude-level relation implied by the heavy quark symmetry, we construct sum rules relating these decays. We then examine deviations from the small-velocity limit. Our numerical analysis shows that the deviations generally increase once excited hadrons are involved, with tensor contributions often inducing sizable effects. At present, however, the relevant form factors are not yet sufficiently constrained. Further improvements in the hadronic inputs are essential for these sum rules to yield robust predictions for the corresponding lepton-universality ratios.

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arXiv2026

$b \to c$ semileptonic sum rule: exploring a sterile neutrino loophole

We investigate the $b\to c$ semileptonic sum rule in the presence of a massive sterile neutrino. Recent measurements of charged-current semitauonic $B$-meson decays exhibit a $\sim4\sigma$ deviation from the Standard Model predictions, whereas no such tension has been reported for the lowest-lying baryonic counterpart, the $\Lambda_b$ decay. Since these decay rates are related through the sum rule, accommodating such a mismatch beyond the level of uncertainties is nontrivial. We revisit this issue by considering dimension-six operators involving a massive sterile neutrino, and evaluate the resulting violation of the sum rule. We find that the induced effect remains negligible compared with the current experimental uncertainties, further strengthening the sum rule as a consistency check for the experimental data.

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arXiv2026

Chasing the two-Higgs-doublet model via electroweak corrections at $e^+e^-$ colliders

We present a comprehensive study of Higgs boson production associated with a neutrino pair at $e^+e^-$ colliders ($e^+ e^- \to h \, ν\barν$) at next-to-leading-order accuracy in both the Standard Model and the two-Higgs-doublet model. We show that these new physics effects will be observable in total and differential cross sections when compared with theoretical predictions that include electroweak corrections, even in the Higgs alignment limit. This highlights the potential of precision studies at future $e^+e^-$ colliders for searching new physics.

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arXiv2026

$b \to c$ semileptonic sum rule: Extension to angular observables

Lepton flavor universality is a key prediction of the Standard Model of particle physics and any violation of it immediately indicates the existence of new physics. Given the recent more than $4σ$ discrepancy in charged current semileptonic B meson decays and the absence of evident signals at the large hadron collider, independent cross-checks become invaluable. In this context, $b \to c$ semileptonic sum rules based on heavy quark symmetry are interesting since they allow us to check the consistency of experimental results. In this paper, we report newly found sum rules among angular observables of mesonic and baryonic $b\to c l \overlineν$ decays holding exactly in the large mass limit of heavy quarks. Moreover, we investigate corrections to the sum rule in realistic situations and discuss phenomenological implications.

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arXiv2026

$b \to c$ semileptonic sum rule: Current status and prospects

The $b \to c$ semileptonic sum rules provide relations between the decay rates of $B \to D^{(*)} τ\barν$ and $Λ_b \to Λ_c τ\barν$. Starting from the heavy quark and zero-recoil limits, we revisit the derivation of the sum rule for total decay rates. We then examine deviations from the limits and investigate corrections arising from realistic hadron masses and higher-order contributions to form factors, taking account of uncertainties. We show that these corrections are negligible compared to current experimental uncertainties, indicating that the sum rule is useful for cross-checking experimental consistency and testing the validity of the Standard Model predictions. In future, precise determinations of the form factors particularly for the tensor operator will be necessary to compare the sum rule predictions with $Λ_b \to Λ_c τ\barν$ data from the LHCb experiment and the Tera-Z projects.

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arXiv2026

Constructing heavy-quark sum rule for $b \to c$ meson and baryon decays

We study heavy-hadron semileptonic decays proceeding via $b \to c$ transition, such as $B \to D^{(*)}τ\barν_τ$ and $Λ_b \to Λ_cτ\barν_τ$. In the heavy-quark limit, where the heavy-quark symmetry holds, we provide a fundamental framework for heavy-quark sum rules among these decays based on the spin decomposition picture. The relation holds directly for the squared amplitudes without requiring phase-space integration. We then apply this relation to reproduce the sum rule among $B \to D^{(*)}τ\barν_τ$ and $Λ_b \to Λ_cτ\barν_τ$. Furthermore, we derive new sum rules for $Ω_b \to Ω_c^{(*)}$ transitions and those involving excited states, such as $B \to \{D_0^*,D_1^*\}$ and $B \to \{D_1,D_2^*\}$.

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arXiv2025

Discriminating Tauphilic Leptoquark Explanations of the $B$ Anomalies via $K\to πν\barν$ and $B\to Kν\barν$

Leptoquark models are prime candidates for new physics (NP) explanations of the long-standing anomalies in semi-leptonic $B$ decays; $b\to c τ\barν$ (encoded in $R(D^{(\ast)})$) and $b\to s\ell\bar\ell (\ell=e,μ)$ transitions. Furthermore, Belle II and NA62 reported weaker-than-expected limits on $B^+ \to K^+ ν\barν$ and $K^+ \to π^+ ν\barν$, 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 $B\to K^{(*)}ν\barν$ are only possible in the $S_1+S_3$ setup, while both models can account for the central value of $K^+\to π^+ν\barν$.

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arXiv2025

On sum rules for semi-leptonic $b \to c$ and $b \to u$ decays

The semi-leptonic $b \to c l ν$ processes are receiving a lot of attention, as the lepton flavor universality violation has been hinted by the measured ratios $R_{D^{(*)}} = Γ(B \to D^{(*)} τν)/Γ(B \to D^{(*)} \ellν)$ for $\ell = e,μ$. Recently, it has also been pointed out that the baryonic counterpart, $R_{Λ_c} = Γ(Λ_b \to Λ_c τν)/Γ(Λ_b \to Λ_c \ellν)$, has a strong correlation with $R_{D^{(*)}}$, referred to as the R ratio sum rule in this paper. The correlation is almost independent of the new physics (NP) contributions and hence can predict $R_{Λ_c}$ from the measured $R_{D^{(*)}}$. On the other hand, we have fewer measurements and/or theoretical studies of the semi-leptonic $b \to u l ν$ processes, although the same arguments can be applied to the ratios $R_π$, $R_ρ$, and $R_p$ as above. Since these processes are measurable at the ongoing LHCb run-3 and/or Belle~II experiments, precise studies on them are important as well. In this paper, we obtain the semi-analytic formulae for all the aforementioned $R_X$ ratios in the presence of model-independent NP contributions by using the available lattice QCD and/or light-cone sum rule fits to the form factors. Two novel points are highlighted: (i) We evaluate uncertainties of $R_X$ including both the Standard Model (SM) and NP terms, inherited from the form factor fits, and discuss how the uncertainties affect the $R$ ratio sum rules. (ii) We obtain the R ratio sum rule among the semi-leptonic $b \to u l ν$ processes for the first time, which provides a complementary motivation for observing these processes. In addition, based on our model-independent results, we investigate how the different NP scenarios work in the $b\to c$ and $b\to u$ sectors and perform a combined study in the framework of SM effective field theory with specific flavor symmetries.

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arXiv2025

Searching for Di-Higgs Signatures of Light Charged Scalars

The excess in $t\to b\overline{b}c$ observed by ATLAS points towards a charged Higgs boson with a mass around 130$\,$GeV, consistent with the expectations from the $B$ anomalies, i.e.$~R_{D^{(*)}}$ and $b\to s\ell^+\ell^-$ data. As a non-minimal flavour structure is required for an explanation of these observables, this points towards a two-Higgs-doublet model with generic Yukawa couplings. Such a scenario predicts a sizable cross section for the pair production of the charged Higgs at the Large Hadron Collider, which can be tested by recasting SM di-Higgs searches. While the predicted event rate is even higher than the one of SM Higgs pair production, the smaller efficiency (w.r.t.$~$SM Higgs pair production) reduces the signal yield. Nonetheless, dedicated searches can probe most of the interesting parameter space and lead to a discovery with Run-3 or High-Luminosity LHC data.

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arXiv2025

Heavy quark symmetry behind $b \to c$ semileptonic sum rule

Lepton flavor universality violations in semileptonic $b \to c$ transitions have garnered attention over a decade. For $R_{H_c}={\rm{BR}}(H_b\to H_c τ\barν_τ)/{\rm{BR}}(H_b\to H_c \ell\barν_\ell)$ with $\ell$ being $e,\, μ$, a sum rule among $R_{D}$, $R_{D^*}$ and $R_{Λ_c}$ was proposed to check consistency in the experimental results independently of new physics models. We revisit this relation from the perspective of the heavy quark symmetry. We derive a sum rule holding exactly in the heavy quark limit and clarify how model-dependent corrections are introduced in a realistic situation.

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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.

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arXiv2025

$\rm{SU}(3)_F$ sum rules for CP asymmetry of $D_{(s)}$ decays

Charge-parity (CP) asymmetries in charm decays are extremely suppressed in the Standard Model and may well be dominated by new physics contributions. The LHCb collaboration reported the results of direct CP asymmetry measurements in $D^0\to K^+ K^-$ and $D^0\to π^+π^-$ decays with unprecedented accuracy: $a_{\rm{CP}}(K^+ K^-)=(7.7\pm5.7)\times 10^{-4}$ and $a_{\rm{CP}}(π^+π^-)=(23.2\pm6.1)\times 10^{-4}$, with the latter quantity inferred from the precise measurement of $Δa_{\rm{CP}} =\, a_{\rm{CP}}(K^+ K^-) -a_{\rm{CP}}(π^+π^-) =\, (-15.7\pm2.9)\times 10^{-4}$. When interpreted within the Standard Model, these values indicate a breakdown of the approximate $U$-spin symmetry of QCD. If, however, this symmetry holds and the data stem from new physics, other CP asymmetries should be enhanced as well. We derive CP asymmetry sum rules based on $\rm{SU}(3)$ flavor symmetry for $D$ meson decays into a pair of pseudoscalar mesons as well as a pair of a pseudoscalar and a vector meson for two generic scenarios, with $ΔU=0$ and $|ΔU|=1$ interactions, respectively. The correlations implied by the sum rules can be used to check the consistency between different measurements and to discriminate between these scenarios with future data. For instance, we find $a_{\mathrm{CP}}(π^{+}K^{* 0}) + a_{\mathrm{CP}}(K^{+}\overline{K}^{* 0}) = 0$ for $ΔU=0$ new physics and the opposite relative sign for the $|ΔU|=1$ case. One sum rule, connecting four decay modes, holds in both scenarios. We further extend our sum rules to certain differences of CP asymmetries from which the $D$ production asymmetries drop out.

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arXiv2024

Cosmology-Independent Constraints on Irreducible Magnetic Monopole Background

We present a novel mechanism for the irreducible production of magnetic monopoles from interactions of cosmic rays and interstellar medium (ISM). Resulting monopoles drain energy from galactic magnetic fields, disrupting their formation and sustainability. We generalize conventional Parker bounds to monopoles with extended energy spectrum and, considering cosmic ray ISM monopole production, set novel constraints from disruption of Milky Way Galactic magnetic fields and their seeds. Further, we set first constraints on disruption of galactic magnetic fields and their seeds of Andromeda galaxy, with results being competitive with distinct existing bounds. Unlike Parker limits of previous works that relied on cosmological monopoles, our constraints are independent of cosmological monopole production or their primordial abundance. Besides, we estimate new constraints on dipole magnetic moments generated from cosmic ray ISM interactions. We discuss implications for monopoles with generalized magnetic charges.

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arXiv2024

Global fit to the 2HDM with generic sources of flavour violation using \textsf{GAMBIT}

We perform a global statistical analysis of the two-Higgs-doublet model with generic sources of flavour violation using \textsf{GAMBIT}. This is particularly interesting in light of deviations from the Standard Model predictions observed in $b\to cτ\bar ν$ and $b\to s\ell^+\ell^-$ transitions as well as the indications for a charged Higgs with a mass of 130\,GeV in top quark decays. Including all relevant constraints from precision, flavour and collider observables, we find that it is possible to simultaneously explain both the charged and neutral current $B$ anomalies. We study the impact of using different values for the $W$ mass and the Standard Model prediction for $g-2$ of the muon and provide predictions for observables that can probe our model in the future such as lepton flavour violation searches at Belle II and Higgs coupling strength measurements at the high-luminosity LHC.

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arXiv2024

Global fit to $b \to cτν$ 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 $b \to c τ\overlineν$ 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 $4.4 σ$ 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 $B\to D^{(\ast)}τ\overlineν$ 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 $Υ\to l^+ l^-$.

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