Motoi Endo

We investigate semileptonic sum rules in heavy-to-light charm decays, motivated by analogous relations in b → c and b → u transitions. Focusing on the c → d ℓ ¯ ν decays, D → π ℓ ¯ ν , D → ρ ℓ ¯ ν , and Λ c → n ℓ ¯ ν , we examine a relation among their lepton-flavor universality ratios R H μ e . 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 μ e in Λ c → n ℓ ¯ ν .

A bstract: We investigate the b → c semileptonic sum rule in the presence of a massive sterile neutrino. Recent measurements of charged-current semitauonic B -meson decays exhibit a ∼ 4 σ deviation from the Standard Model predictions, whereas no such tension has been reported for the lowest-lying baryonic counterpart, the Λ _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.

A bstract: We study lepton-number-violating interactions at dimension seven in the Standard Model effective field theory that contribute to the meson decays B → Kνν and K → πνν . Such interactions could washout the baryon asymmetry of the Universe and also contribute to the neutrinoless double beta decay, even though the interactions involve a change in down-type quark flavors. We clarify conditions under which excesses in meson decay rates over the Standard Model predictions can be successfully observed. We also show that, although these interactions contribute to neutrino masses at the two-loop level, the Weinberg operator can be introduced consistently without spoiling the scenario.

We study heavy-hadron semileptonic decays proceeding via b → c transition, such as B → D ( * ) τ ν ¯ τ and Λ b → Λ c τ ν ¯ 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 → D ( * ) τ ν ¯ τ and Λ b → Λ c τ ν ¯ τ . Furthermore, we derive new sum rules for Ω b → Ω c ( * ) transitions and those involving excited states, such as B → { D 0 * , D 1 * } and B → { D 1 , D 2 * } .

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.