Gauge-invariant implications of the LHCb measurements on lepton-flavor nonuniversality

Abstract

We study the implications of the recent measurements of RK and RK * by the LHCb collaboration. We do that by adopting a model-independent approach based on the Standard Model Effective Field Theory (SMEFT), in which the dominant new physics effects are encoded in the coefficients of dimension-six operators respecting the full Standard Model (SM) gauge symmetry. After providing simplified expressions for RK and RK * , we determine the implications of the recent LHCb results for these observables on the coefficients of the SMEFT operators at low and high energies. We also take into account all b → s data, which combined lead to effective New Physics (NP) scenarios with SM pulls in excess of 5 σ. Thus the operators discussed in this paper would be the first dimension-six terms in the SM Lagrangian to be detected experimentally. Indirect constraints on these operators are also discussed. The results of this paper transcend the singularity of the present situation, and set a standard for future analyses in b → s transitions when the NP is assumed to lie above the electroweak scale.

The B → πK puzzle revisited

“…These include (i) R K (LHCb [7]) and (ii) R K * (LHCb [8]), where R K,K * ≡ B(B +,0 → K +, * 0 µ + µ − )/B(B +,0 → K +, * 0 e + e − ), (iii) the angular distribution of B → K * µ + µ − (LHCb [9, 10], Belle [11], ATLAS [12] and CMS [13]), and (iv) the branching fraction and angular distribution of B 0 s → φµ + µ − (LHCb [14,15]). Recent analyses of these discrepancies [16][17][18][19][20][21][22][23][24][25][26] combine constraints from all measurements and come to the following conclusions: (i) there is indeed a significant disagreement with the SM, somewhere in the range of 4-6σ, and (ii) the most probable explanation is that the NP primarily affects b → sµ + µ − transitions. Arguably the simplest NP explanation is that its contribution to b → sµ + µ − comes from the tree-level exchange of a Z boson that has a flavor-changing coupling tosb, and also couples to µ + µ − .…”

“…Following its announcement, a number of papers appeared [16][17][18][19][20][21][22][23][24][25][26] computing the size of the discrepancy with the SM, and determining the general properties of the NP required to explain the results. Combining constraints from all measurements, the general consensus is that there is indeed a significant disagreement with the SM, somewhere in the range of 4-6σ (this large range is due to the fact that different groups deal with the theoretical uncertainties in different ways).…”

On the importance of electroweak corrections for B anomalies

“…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.…”

Explaining the $$R_K$$ R K and $$R_{K^*}$$ R K ∗

“…While the deviations from the Standard Model (SM) in the individual ratios are only at the level of 2.2σ -2.5σ , the a e-mail: diptimoy.ghosh@weizmann.ac.il combined deviation (the exact number depends on how one combines the 3 results) is large enough to look for NP explanations. 1 For recent studies, see [4,[13][14][15][16][17][18][19].…”