Towards the discovery of new physics with lepton-universality ratios of b→sℓℓ decays

Abstract

Tests of lepton-universality as rate ratios in b → sll transitions can be predicted very accurately in the Standard Model. The deficits with respect to expectations reported by the LHCb experiment in muon-toelectron ratios of the B → K ðÃÞ ll decay rates thus point to genuine manifestations of lepton nonuniversal new physics. In this paper, we analyze these measurements in the context of effective field theory. First, we discuss the interplay of the different operators in R K and R K à and provide predictions for R K à in the Standard Model and in new-physics scenarios that can explain R K . We also provide approximate numerical formulas for these observables in bins of interest as functions of the relevant Wilson coefficients. Secondly, we perform frequentist fits to R K and R K à . The Standard Model disagrees with these measurements at 3.7σ significance. We find excellent fits in scenarios with combinations ofwith pulls relative to the Standard Model in the region of 4σ. An important conclusion of our analysis is that a lepton-specific contribution to O 10 is important to understand the data. Under the hypothesis that new-physics couples selectively to the muons, we also present fits to other b → sμμ data with a conservative error assessment and comment on more general scenarios. Finally, we discuss new lepton universality ratios that, if new physics is the origin of the observed discrepancy, should contribute to the statistically significant discovery of new physics in the near future.

Flavour anomalies after the R K ∗ measurement

“…As shown in figure 2, away from the Standard Model our errors are still of a few percent, in agreement with refs. [35,36]. However, other groups [37] find a much larger theoretical error in the presence of New Physics, due to a more conservative treatment of the form factor uncertainties.…”

On the importance of electroweak corrections for B anomalies

“…A [78] and therefore, at leading order in the NP contribution, we have 58) with the running scale µ replaced by a constituent massμ ≈ 1 GeV. On the other hand, the experimental data and the SM prediction are in the following ratio [78] σ exp.…”

“…The above setup is the most natural one to accommodate simultaneously and in a correlated way charged-and neutral-current anomalies. Moreover, it is favoured by global fit analyses of b → s + − data [16][17][18][19][20][21][22] including the very recent experimental result for R µ/e K * [54][55][56][57][58][59][60][61][62]. At the electroweak scale m EW , additional operators will arise from Lagrangian (2.3), due to the well-known phenomenon of operator mixing.…”

Theory overview on FCNC B-decays

“…Refs. [2,3,4,5]). In particular, a good fit to the differential observable P 5 [6], together with the hints on LFU violation in R K and R K * [7,8,9], is obtained by considering a new physics contribution to the C bsµ ≡ C Dµ 32 coefficient in…”