η and η ′ decays into lepton pairs

Abstract

In this work, we calculate the branching ratios for the η(η ) → decays, where = e, µ. These processes have tiny rates in the Standard Model due to spin flip, loop and electromagnetic suppressions, which could make them sensitive to New Physics effects. In order to provide a reliable input for the Standard Model, we exploit the general analytical properties of the amplitude. For that purpose, we invoke the machinery of Canterbury approximants, which provides a systematic description of the underlying hadronic physics in a data-driven fashion. Given the current experimental discrepancies, we discuss in detail the role of the resonant region and comment on the reliability of χPT calculations. Finally, we discuss the kind of new physics which we think would be relevant to account for them.

Lepton flavor (universality) violation in rare kaon decays

“…At the two-loop level, we have estimated [10] the size of the shift to be ∆χ µ µ − ∆χ ee = −2.8, in line with explicit calculations based on Canterbury approximants [17]. Table 2: Values of the contact term χ(M ρ ) extracted from the measured K L → e + e − and K L → µ + µ − rates.…”

Radiative corrections to double-Dalitz decays revisited

“…[5], which contains four different diagrams (and four additional exchange diagrams for identical leptons). Such a contribution is UV divergent for a constant TFF and would require the same counterterm appearing in P →ll decays [32][33][34][35]. In the following, we consider a nonconstant TFF that can be decomposed into massive-like photon propagators (see Appendix C) and refer to Appendix E for the case of a constant TFF.…”

“…Consequently, such effects would be as small as ðm π =2ΛÞ 4 , where Λ is expected to be the order of M V π 0 (see Appendix C and Refs. [24,33,42]). In addition, since the process peaks at low energies, the double-virtual region is-experimentally-less populated.…”

“…This was extended to the double-virtual case in Refs. [24,33,42] and involves the use of Canterbury approximants. The simplest approach 27 reads…”

Updated pseudoscalar contributions to the hadronic light-by-light of the muon (g − 2)

“…48 For a detailed description, examples and performance of the method, the reader is referred to the Appendix of Ref. 49 and Ref. 50.…”