北原 鉄平

We find that the phase appearing in the unitarity relation between $\mathcal{B}(K_L\rightarrow \mu^+\mu^-)$ and $\mathcal{B}(K_L\rightarrow \gamma\gamma)$ is equal to the phase shift in the interference term of the time-dependent $K\rightarrow \mu^+\mu^-$ decay. A probe of this relation at future kaon facilities constitutes a Standard Model test with a theory precision of about $2\%$. The phase has further importance for sensitivity studies regarding the measurement of the time-dependent $K\rightarrow \mu^+\mu^-$ decay rate to extract the CKM matrix element combination $\vert V_{ts} V_{td} \sin(\beta+\beta_s)\vert\approx A^2\lambda^5\bar\eta$. We find a model-independent theoretically clean prediction, $\cos^2\varphi_0 = 0.96 \pm 0.03$. The quoted error is a combination of the theoretical and experimental errors, and both of them are expected to shrink in the future. Using input from the large-$N_C$ limit within chiral perturbation theory, we find a theory preference towards solutions with negative $\cos\varphi_0$, reducing a four-fold ambiguity in the angle $\varphi_0$ to a two-fold one.

We investigate the di-Higgs productions at the Large Hadron Collider in the two-Higgs doublet model. In particular, we study the productions of an extra scalar, $\phi$ ($\phi=H,\,A$), in association with the Standard Model (SM)-like Higgs boson, $h$. We consider a scenario that the additional scalars have a large top-Yukawa interaction in the Higgs alignment limit. Then, the leading contribution to the production comes from the loop-induced gluon-fusion channel $gg \to h\phi$. Similar to the SM Higgs pair production, these $h\phi$ productions are sensitive to the Higgs potential as well as the Yukawa couplings. In this paper, we calculate these $h\phi$ production cross sections at the leading order, taking into account the theoretical constraints from the perturbative unitarity and vacuum stability bounds as well as the precision measurements from the Higgs and flavor physics. Furthermore, we scrutinize the di-Higgs production in the parameter spaces that can explain the excesses around 100$\,$GeV in di-$\tau$ and di-photon and/or the muon $g-2$ anomaly.

Abstract Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

Recently, the LHCb collaboration announced a preliminary result of the test of lepton flavor universality (LFU) in $B\to D^{(\ast)}$ semi-leptonic decays: $R_{D}^{\rm LHCb2022} = 0.441 \pm 0.089$ and $R_{D^{\ast } }^{\rm LHCb2022} = 0.281 \pm 0.030$ based on the LHC Run\,1 data. This is the first result of $R_{D}$ for the LHCb experiment, and its precision is comparable to the other $B$-factory data. Interestingly, those data prefer the violation of the LFU again. Including this new result, we update a circumstance of the $b \to c \tau \bar\nu$ measurements and their implications for new physics. A new wold average of the data by the BaBar, Belle, and LHCb collaborations is $R_{D} = 0.358 \pm 0.028$ and $R_{D^{\ast } } = 0.285 \pm 0.013$. Incorporating several recent developments of determinations of the $B \to D^{(\ast)}$ form factors in the Standard Model (SM), we observe a $4.1 \sigma$ deviation from the SM predictions. The general new physics formulae and the global fittings of the new physics parameters are also updated. Furthermore, precise measurements of the polarization observables in the $B\to D^{(\ast)}$ semi-leptonic decays at the Belle~II experiment, and the high-$p_{\rm T}$ flavored-tail searches at the LHC experiment will be able to indirectly distinguish different new physics models. We also discuss the LFU violation in $\Upsilon \to l^+ l^-$.

Fundamental physical constants are determined from a collection of precision measurements of elementary particles, atoms and molecules. This is usually done under the assumption of the Standard Model~(SM) of particle physics. Allowing for light new physics~(NP) beyond the SM modifies the extraction of fundamental physical constants. Consequently, setting NP bounds using these data, and at the same time assuming the CODATA recommended values for the fundamental physical constants, is not reliable. As we show in this Letter, both SM and NP parameters can be simultaneously determined in a consistent way from a global fit. For light vectors with QED-like couplings, such as the dark photon, we provide a prescription that recovers the degeneracy with the photon in the massless limit, and requires calculations only at leading order in the small new physics couplings. At present, the data show tensions partially related to the proton charge radius determination. We show that these can be alleviated by including contributions from a light scalar with flavor non-universal couplings.