arXiv subjects

Teppei Kitahara

Publications and source records attributed to Teppei Kitahara.

At least 55 records · Page 3Linked to original sources

arXiv2021

Novel approach to neutron electric dipole moment search using weak measurement

We propose a novel approach in a search for the neutron electric dipole moment (EDM) by taking advantage of signal amplification in a weak measurement, known as weak value amplification. Considering an analogy to the weak measurement that can measure the spin magnetic moment interaction, we examine an experimental setup with a polarized neutron beam through an external electric field with spatial gradient, where the signal is sensitive to the EDM interaction. In particular, a dedicated analysis of effects from impurities in pre- and post-selections is performed. We show that the weak value amplification occurs where the signal is enhanced by up to two orders of magnitude, and demonstrate a potential sensitivity of the proposed setup to the neutron EDM.

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arXiv2021

Muon g-2 vs LHC Run 2 in Supersymmetric Models

Supersymmetric models with sub-TeV charginos and sleptons have been a candidate for the origin of the long-standing discrepancy in the muon anomalous magnetic moment (g-2). By gathering all the available LHC Run 2 results, we investigate the latest LHC constraints on models that explain the anomaly by their chargino contribution to the muon g-2. It is shown that the parameter regions where sleptons are lighter than charginos are strongly disfavored. In contrast, we find that the models with $m_{\tildeμ_{\mathrm L}}\gtrsim m_{{\tildeχ}^{\pm}_1}$ are still widely allowed, where the lighter chargino dominantly decays into a W-boson and a neutralino.

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arXiv2020

Implications for new physics from a novel puzzle in $\bar{B}_{(s)}^0 \to D^{(\ast)+}_{(s)} \lbrace π^-, K^- \rbrace$ decays

Recently, the standard model predictions for the $B$-meson hadronic decays, $\bar{B}^0 \to D^{(\ast)+}K^-$ and $\bar{B}^0_s \to D^{(\ast)+}_s π^-$, have been updated based on the QCD factorization approach. This improvement sheds light on a novel puzzle in the $B$-meson hadronic decays: there are mild but universal tensions between data and the predicted branching ratios. Assuming the higher-order QCD corrections are not huge enough to solve the tension, we examine several new physics interpretations of this puzzle. We find that the tension can be partially explained by a left-handed $W^\prime$ model, which can be compatible with other flavor observables and collider bounds.

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arXiv2020

Probing $eμ$ flavor-violating ALP at Belle II

Recently, it was pointed out that the electron and muon g-2 discrepancies can be explained simultaneously by a flavor-violating axion-like particle (ALP). We show that the parameter regions favored by the muon g-2 are already excluded by the muonium-antimuonium oscillation bound. In contrast, those for the electron g-2 can be consistent with this bound when the ALP is heavier than 1.5 GeV. We propose to search for a signature of the same-sign and same-flavor lepton pairs and the forward-backward muon asymmetry to test the model at the Belle II experiment.

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arXiv2020

The electroweak effective field theory from on-shell amplitudes

We apply on-shell methods to the bottom-up construction of electroweak amplitudes, allowing for both renormalizable and non-renormalizable interactions. We use the little-group covariant massive-spinor formalism, and flesh out some of its details along the way. Thanks to the compact form of the resulting amplitudes, many of their properties, and in particular the constraints of perturbative unitarity, are easily seen in this formalism. Our approach is purely bottom-up, assuming just the standard-model electroweak spectrum as well as the conservation of electric charge and fermion number. The most general massive three-point amplitudes consistent with these symmetries are derived and studied in detail, as the primary building blocks for the construction of scattering amplitudes. We employ a simple argument, based on tree-level unitarity of four-point amplitudes, to identify the three-point amplitudes that are non-renormalizable at tree level. This bottom-up analysis remarkably reproduces many low-energy relations implied by electroweak symmetry through the standard-model Higgs mechanism and beyond it. We then discuss four-point amplitudes. The gluing of three-point amplitudes into four-point amplitudes in the massive spinor helicity formalism is clarified. As an example, we work out the $ψ^c ψZh$ amplitude, including also the non-factorizable part. The latter is an all-order expression in the effective-field-theory expansion. Further constraints on the couplings are obtained by requiring perturbative unitarity. In the $ψ^c ψZh$ example, one for instance obtains the renormalizable-level relations between vector and fermion masses and gauge and Yukawa couplings. We supplement our bottom-up derivations with a matching of three- and four-point amplitude coefficients onto the standard-model effective field theory (SMEFT) in the broken electroweak phase.

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arXiv2020

New physics implications of recent search for $K_L \to π^0 ν\barν$ at KOTO

The KOTO experiment recently reported four candidate events in the signal region of $K_L\to π^0 ν\barν$ search, where the standard model only expects $0.10\pm 0.02$ events. If confirmed, this requires physics beyond the standard model to enhance the signal. We examine various new physics interpretations of the result including these: (1) heavy new physics boosting the standard model signal, (2) reinterpretation of "$ν\barν$" as a new light long-lived particle, or (3) reinterpretation of the whole signal as the production of a new light long-lived particle at the fixed target. We study the above explanations in the context of a generalized new physics Grossman-Nir bound coming from the $K^+ \to π^+ν\barν$ decay, bounded by data from the E949 and the NA62 experiments.

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arXiv2019

Addendum to "Impact of polarization observables and $B_c\to τν$ on new physics explanations of the $b\to c τν$ anomaly"

In this addendum to arXiv:1811.09603 we update our results including the recent measurement of ${\cal R}(D)$ and ${\cal R}(D^*)$ by the Belle collaboration: ${\cal R}(D)_{\rm Belle} = 0.307\pm0.037\pm0.016$ and ${\cal R}(D^*)_{\rm Belle}=0.283\pm0.018\pm0.014$, resulting in the new HFLAV fit result ${\cal R}(D) = {0.340\pm0.027 \pm 0.013}$, ${\cal R}(D^*) = {0.295\pm0.011 \pm 0.008 }$, exhibiting a $3.1\,σ$ tension with the Standard Model. We present the new fit results and update all figures, including the relevant new collider constraints. The updated prediction for ${\cal R}(Λ_c)$ from our sum rule reads ${\cal R}(Λ_c)= \mathcal{R}_{\rm SM}(Λ_c) \left( 1.15 \pm 0.04 \right) = 0.38 \pm 0.01 \pm 0.01$. We also comment on theoretical predictions for the fragmentation function $f_c$ of $b\to B_c$ and their implication on the constraint from $B_{u/c}\toτν$ data.

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arXiv2019

SMEFT top-quark effects on $ΔF=2$ observables

We investigate model independent top-quark corrections to $ΔF = 2$ processes for the down-type quarks within the framework of the Standard Model Effective Field Theory. Dimension-six $ΔF = 1$ operators contribute to them through renormalization group evolutions and matching conditions. We provide a complete one-loop matching formula from the top quarks for $ΔF=2$ transitions. We also demonstrate these corrections on $ΔM_{B_s}$ in the left-right symmetric model, which are compared with the conventional calculation.

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arXiv2019

Impact of polarization observables and $ B_c\to τν$ on new physics explanations of the $b\to c τν$ anomaly

The combined analysis of the BaBar, Belle, and LHCb data on $B\to Dτν$, $B\to D^*τν$ and $B_c\to J/Ψτν$ decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of $F_L(D^*)$ and of the $τ$ polarizations, and especially on the constraint from the branching fraction ${\rm BR}(B_c\toτν)$. We critically review this constraint and do not support the aggressive limit of ${\rm BR}(B_c\toτν)<10\%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${\rm BR}(B_c\toτν)$ constraint has a significant impact: depending on whether one uses a limit of $60\%$, $30\%$ or $10\%$, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative $60\%$ limit a scenario with scalar operators gives the best fit to data, while for an aggressive $10\%$ limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of $B\to Dτν$, $B\to D^*τν$ and $Λ_b\to Λ_cτν$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${\rm BR}(Λ_b\to Λ_cτν)$ over its SM prediction by $(24\pm 6)\%$ for the current $\mathcal{R}(D^{(*)})$ data.

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arXiv2019

$D^{\ast}$ polarization vs. $R_{D^{(\ast)}}$ anomalies in the leptoquark models

Polarization measurements in $\bar{B} \to D^{(\ast)} τ\overlineν$ are useful to check consistency in new physics explanations for the $R_{D}$ and $R_{D^{\ast}}$ anomalies. In this paper, we investigate the $D^{\ast}$ and $τ$ polarizations and focus on the new physics contributions to the fraction of a longitudinal $D^{\ast}$ polarization ($F_{L}^{D^{\ast}}$), which is recently measured by the Belle collaboration $F_{L}^{D^{\ast}} = 0.60 \pm 0.09$, in model-independent manner and in each single leptoquark model (${\rm R}_2$, ${\rm S}_1$ and ${\rm U}_1$) that can naturally explain the $R_{D^{(\ast)}}$ anomalies. It is found that $\mathcal{B}(B_c^{+} \to τ^{+} ν)$ severely restricts deviation from the Standard Model (SM) prediction of $F_{L, \textrm{SM}}^{D^{\ast}} = 0.46 \pm 0.04$ in the leptoquark models: [0.43, 0.44], [0.42, 0.48], and [0.43, 0.47] are predicted as a range of $F_{L}^{D^{\ast}}$ for the ${\rm R}_2$, ${\rm S}_1$, and ${\rm U}_1$ leptoquark models, respectively, where the current data of $R_{D^{(\ast)}}$ is satisfied at $1\,σ$ level. It is also shown that the $τ$ polarization observables can much deviate from the SM predictions. The Belle II experiment, therefore, can check such correlations between $R_{D^{(\ast)}}$ and the polarization observables, and discriminate among the leptoquark models.

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arXiv2018

Recent developments on direct $CP$ violation in the kaon system and connection to $K \to πν\barν$ measurements

The first lattice result from the RBC and UKQCD Collaborations and improved perturbative calculations of $\varepsilon^{\prime}_K / \varepsilon_K$ have implied that the Standard-Model (SM) expectation deviates from measured values at the $2.8\,σ$ level. Since $\varepsilon^{\prime}_K / \varepsilon_K$ comes from $CP$-violating FCNC and is significantly suppressed in the SM, the discrepancy can be explained easily in several new physics (NP) models. In this contribution, it is shown that correlations with the other rare decays, especially $K\to πν\overlineν$ and $K_S \to μ^+ μ^-$, are crucial for discrimination of the NP models. These channels can be probed precisely in the future by the NA62 and KOTO experiments for $K\to πν\overlineν$ and LHCb experiment for $K_S \to μ^+ μ^-$.

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arXiv2018

Soft-Photon Corrections to $\bar{B} \to D τ^{-} \barν_τ$ Relative to $\bar{B} \to D μ^{-} \barν_μ$

We evaluate long-distance electromagnetic (QED) contributions to $\bar{B}{}^0 \to D^+ τ^{-} \barν_τ$ and $B^- \to D^0 τ^{-} \barν_τ$ relative to $\bar{B}{}^0 \to D^+ μ^{-} \barν_μ$ and $B^- \to D^0 μ^{-} \barν_μ$, respectively, in the standard model. We point out that the QED corrections to the ratios $R(D^{+})$ and $R(D^{0})$ are not negligible, contrary to the expectation that radiative corrections are almost canceled out in the ratio of the two branching fractions. The reason is that long-distance QED corrections depend on the masses and relative velocities of the daughter particles. We find that theoretical predictions for $R(D^{+})^{τ/μ}$ and $R(D^{0})^{τ/μ}$ can be amplified by $\sim4\%$ and $\sim3\%$, respectively, for the soft-photon energy cut in range $20$-$40$ MeV.

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arXiv2018

Probing SUSY effects in $K_S^0\rightarrowμ^+μ^-$

We explore supersymmetric contributions to the decay $K_S^0\rightarrowμ^+μ^-$, in light of current experimental data. The Standard Model (SM) predicts $\mathcal{B}(K_S^0\rightarrowμ^+μ^-)\approx5\times 10^{-12}$. We find that contributions arising from flavour violating Higgs penguins can enhance the branching fraction up to $\approx 35\times 10^{-12}$ within different scenarios of the Minimal Supersymmetric Standard Model (MSSM), as well as suppress it down to $\approx 0.78\times 10^{-12}$. Regions with fine-tuned parameters can bring the branching fraction up to the current experimental upper bound, $8\times 10^{-10}$. The mass degeneracy of the heavy Higgs bosons in MSSM induces correlations between $\mathcal{B}(K_S^0\rightarrowμ^+μ^-)$ and $\mathcal{B}(K_L^0\rightarrowμ^+μ^-)$. Predictions for the $CP$ asymmetry in $K^0\rightarrowμ^+μ^-$ decays in the context of MSSM are also given, and can be up to eight times bigger than in the SM.

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arXiv2018

Gluino-mediated electroweak penguin with flavor-violating trilinear couplings

In light of a discrepancy of the direct $CP$ violation in $K\toππ$ decays, $\varepsilon'/\varepsilon_K$, we investigate gluino contributions to the electroweak penguin, where flavor violations are induced by squark trilinear couplings. Top-Yukawa contributions to $ΔS = 2$ observables are taken into account, and vacuum stability conditions are evaluated in detail. It is found that this scenario can explain the discrepancy of $\varepsilon'/\varepsilon_K$ for the squark mass smaller than 5.6 TeV. We also show that the gluino contributions can amplify $\mathcal{B}(K \to πν\overlineν)$, $\mathcal{B}(K_S \to μ^+ μ^-)_{\rm eff}$ and $ΔA_{\rm CP}(b\to sγ)$. Such large effects could be measured in future experiments.

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arXiv2017

Direct $CP$ Violation in $K \to μ^+ μ^-$

A rare decay $K_L \to μ^+ μ^- $ has been measured precisely, while a rare decay $K_S \to μ^+ μ^- $ will be observed by an upgrade of the LHCb experiment. Although both processes are almost CP-conserving decays, we point out that an interference contribution between $K_L$ and $K_S$ in the kaon beam emerges from a genuine direct CP violation. It is found that the interference contribution can change $K_S \to μ^+ μ^-$ standard-model predictions at $\mathcal{O}(60\%)$. We also stress that an unknown sign of $\mathcal{A}(K_L \to γγ)$ can be determined by a measurement of the interference, which can much reduce a theoretical uncertainty of $\mathcal{B}(K_L \to μ^+ μ^-)$. We also investigate the interference in a new physics model, where the $ε'_K / ε_K$ tension is explained by an additional $Z$-penguin contribution.

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arXiv2017

$K\to πν\overlineν$ in the MSSM in Light of the $ε^{\prime}_K/ε_K$ Anomaly

The Standard-Model (SM) prediction for the CP-violating quantity $ε_K^{\prime}/ε_K$ deviates from its measured value by 2.8 $σ$. It has been shown that this tension can be resolved within the Minimal Supersymmetric Standard Model (MSSM) through gluino-squark box diagrams, even if squarks and gluinos are much heavier than 1 TeV. The rare decays $K_L \to π^0ν\barν$ and $K^+ \to π^+ν\barν$ are similarly sensitive to very high mass scales and the first one also measures CP violation. In this article, we analyze the correlations between $ε^{\prime}_K/ε_K$ and $B(K_L \to π^0ν\barν)$ and $B(K^+ \to π^+ν\barν)$ within the MSSM aiming at an explanation of $ε_K^{\prime}/ε_K$ via gluino-squark box diagrams. The dominant MSSM contribution to the $K \to πν\barν$ branching fractions stems from box diagrams with squarks, sleptons, charginos, and neutralinos, and the pattern of the correlations is different from the widely studied $Z$-penguin scenarios. This is interesting in light of future precision measurements by KOTO and NA62 at J-PARC and CERN, respectively. We find $B(K_L \to π^0ν\barν)/B^{SM} (K_L \to π^0ν\barν)\lesssim 2\,(1.2)$ and $B(K^+ \to π^+ν\barν)/B^{SM}(K^+ \to π^+ν\barν) \lesssim 1.4\,(1.1)$, if all squark masses are above 1.5 TeV, gaugino masses obey GUT relations, and if one allows for a fine-tuning at the $1\%\,(10\%)$ level for the gluino mass. Larger values are possible for a tuned CP violating phase. Furthermore, the sign of the MSSM contribution to $ε_K^{\prime}$ imposes a strict correlation between $B(K_L \to π^0ν\barν)$ and the hierarchy between the masses $m_{\bar{U}}$, $m_{\bar{D}}$ of the right-handed up-squark and down-squark: sgn$[B(K_L \to π^0ν\barν)-B^{SM} (K_L \to π^0ν\barν)] = $sgn$(m_{\bar{U}}-m_{\bar{D}}) $.

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arXiv2017

Correlations of $ε^{\prime}_K/ε_K$ with $K \to πν\overlineν$ in Models of New Physics

Recent calculations have pointed to a 2.8 $σ$ tension between data on $ε^{\prime}_K / ε_K$ and the standard-model (SM) prediction. Several new physics (NP) models can explain this discrepancy, and such NP models are likely to predict deviations of $\mathcal{B}(K\to πν\overlineν)$ from the SM predictions, which can be probed precisely in the near future by NA62 and KOTO experiments. We present correlations between $ε^{\prime}_K / ε_K$ and $\mathcal{B}(K\to πν\overlineν)$ in two types of NP scenarios: a box dominated scenario and a $Z$-penguin dominated one. It is shown that different correlations are predicted and the future precision measurements of $K \to πν\overlineν$ can distinguish both scenarios.

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arXiv2017

Revisiting Kaon Physics in General $Z$ Scenario

New physics contributions to the $Z$ penguin are revisited in the light of the recently-reported discrepancy of the direct CP violation in $K\toππ$. Interference effects between the standard model and new physics contributions to $ΔS = 2$ observables are taken into account. Although the effects are overlooked in the literature, they make experimental bounds significantly severer. It is shown that the new physics contributions must be tuned to enhance $\mathcal{B}(K_L \to π^{0} ν\barν)$, if the discrepancy of the direct CP violation is explained with satisfying the experimental constraints. The branching ratio can be as large as $6 \times 10^{-10}$ when the contributions are tuned at the 10 % level.

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