New perspectives for testing electron-muon universality

Abstract

Intriguing results for tests of the universality of electrons and muons through measurements of rates of B → Kℓ+ℓ− and similar decays have been in the spotlight for years. The LHCb collaboration has recently reported new results which are in agreement with Lepton Flavour Universality, while the individual decay rates are found below their Standard Model predictions. In view of this new situation, we explore how much space is left for a violation of electron-muon universality. Considering new sources of CP violation and taking the new LHCb measurements into account, we show that significant differences between the short-distance coefficients for electronic and muonic final states are actually allowed by the current data. These patterns can be revealed through CP asymmetries in neutral and charged B → Kℓ+ℓ− decays. We obtain correlations between these observables and map them to the short-distance coefficients. This results in regions in New Physics parameter space with large differences between CP asymmetries of the decays with final-state electrons and muons, thereby leaving a lot of room for possible surprises in the future high-precision era.

CP violation in B decays: recent developments and future perspectives

“…A detailed study of the space left by the current data was performed in Ref. [83]. Interestingly, allowing for new CP-violating effects, it was demonstrated that significant differences between the Wilson coefficients of the electronic and muonic final states are still allowed.…”

“…In semileptonic decays of the kind B → K + − and B → K * + − , B s → φ + − , CP violation can also be explored through a variety of observables [76][77][78][79][80][81][82][83]. In these channels, we get contributions from cc resonances through intermediate J/ψ, ψ(2S), .…”

“…The new results are now in agreement with lepton flavour universality at the 1 σ level. On the other hand, using the most recent lattice QCD results for the relevant B → K from factors [89] and paying special attention to the input CKM parameters [17], the SM value of the CP-averaged B ± → K ± μ + μ − branching ratio is calculated as follows [83]:…”

Theoretical Highlights of CP Violation in $B$ Decays

“…As shown in the analysis in Ref. [33] due to new CP-violating effects, there is still significant room for violation of the electron-muon universality. More specifically, this analysis explores the CP-violating effects in the NP studies of rare decays making use of the muonic Wilson coefficients.…”