New physics searches at kaon and hyperon factories.

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.

Self-Consistent Extraction of Spectroscopic Bounds on Light New Physics.

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 Committee on Data of the International Science Council 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 nonuniversal couplings.

New Physics Implications of Recent Search for K_{L}→π

The KOTO experiment recently reported four candidate events in the signal region of K_{L}→π^{0}νν[over ¯] search, where the standard model only expects 0.10±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 "νν[over ¯]" 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^{+}→π^{+}νν[over ¯] decay, bounded by data from the E949 and the NA62 experiments.

Soft-Photon Corrections to B[over ¯]→Dτ

We evaluate long-distance electromagnetic (QED) contributions to B[over ¯]^{0}→D^{+}τ^{-}ν[over ¯]_{τ} and B^{-}→D^{0}τ^{-}ν[over ¯]_{τ} relative to B[over ¯]^{0}→D^{+}µ^{-}ν[over ¯]_{µ} and B^{-}→D^{0}µ^{-}ν[over ¯]_{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 ∼4% and ∼3%, respectively, for the soft-photon energy cut in the range 20-40 MeV.

Direct CP Violation in K→µ

A rare decay K_{L}→µ^{+}µ^{-} has been measured precisely, while a rare decay K_{S}→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}→µ^{+}µ^{-} standard-model predictions at O(60%). We also stress that an unknown sign of A(K_{L}→γγ) can be determined by a measurement of the interference, which can much reduce a theoretical uncertainty of B(K_{L}→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.

Supersymmetric Explanation of CP Violation in K→ππ Decays.

Recent progress in the determination of hadronic matrix elements has revealed a tension between the measured value of ε_{K}^{'}/ε_{K}, which quantifies direct CP violation in K→ππ decays, and the standard-model prediction. The well-understood indirect CP violation encoded in the quantity ε_{K} typically precludes large new-physics contributions to ε_{K}^{'}/ε_{K} and challenges such an explanation of the discrepancy. We show that it is possible to cure the ε_{K}^{'}/ε_{K} anomaly in the minimal supersymmetric standard model with squark masses above 3 TeV without overshooting ε_{K}. This solution exploits two features of supersymmetry: the possibility of large isospin-breaking contributions (enhancing ε_{K}^{'}) and the Majorana nature of gluinos (permitting a suppression of ε_{K}). Our solution involves no fine-tuning of CP phases or other parameters.

Singlino resonant dark matter and 125 GeV Higgs boson in high-scale supersymmetry.

We consider a singlino dark matter (DM) scenario in a singlet extension model of the minimal supersymmetric standard model, which is the so-called the nearly minimal supersymmetric standard model. We find that with high-scale supersymmetry breaking the singlino can obtain a sizable radiative correction to the mass, which opens a window for the DM scenario with resonant annihilation via the exchange of the Higgs boson. We show that the current DM relic abundance and the Higgs boson mass can be explained simultaneously. This scenario can be fully probed by XENON1T.