Abstract:The LHCb collaboration has recently presented their result on R K = B (B + → K + µ + µ − ) /B (B + → K + e + e − ) for the dilepton invariant mass bin m 2 = 1 − 6 GeV 2 ( = µ, e). The measurement shows an intriguing 2.6 σ deviation from the Standard Model (SM) prediction. In view of this, we study model independent New Physics (NP) explanations of R K consistent with other measurements involving b → s + − transition, relaxing the assumption of lepton universality. We perform a Bayesian statistical fit to the NP Wilson Coefficients and compare the Bayes Factors of the different hypotheses in order to quantify their goodness-of-fit. We show that the data slightly favours NP in the muon sector over NP in the electron sector.
The BABAR Collaboration has recently reported the measurement of the ratio of the branching fractions of " B ! DðD Ã Þ À " to " B ! DðD Ã Þ' À " ' which deviates from the Standard Model prediction by 2 (2:7). This deviation goes up to 3:4 level when the two measurements in the D and D Ã modes are taken together and could indicate new physics. Using an effective Lagrangian for the new physics, we study the implication of these results and calculate other observables that can shed light on the nature of the new physics. We show that the measurements of the forward-backward asymmetries and the and D Ã polarization fractions can be distinguished among the various couplings of the new physics operators.
Recent LHCb results on R K * , the ratio of the branching fractions of B → K * μ + μ − to that of B → K * e + e − , for the dilepton invariant mass bins q 2 ≡ m 2 = [0.045-1.1] GeV 2 and [1.1-6] GeV 2 show approximately 2.5σ deviations from the corresponding Standard Model prediction in each of the bins. This, when combined with the measurement of R K (q 2 = [1 − 6] GeV 2 ), a similar ratio for the decay to a pseudo-scalar meson, highly suggests lepton non-universal new physics in semi-leptonic B meson decays. In this work, we perform a model independent analysis of these potential new physics signals and identify the operators that do the best job in satisfying all these measurements. We show that heavy new physics, giving rise to q 2 independent local 4-Fermi operators of scalar, pseudo-scalar, vector or axial-vector type, is unable to explain all the three measurements simultaneously, in particular R K * in the bin [0.045-1.1], within their experimental 1σ regions. We point out the possibility to explain R K * in the low bin by an additional light ( 20 MeV) vector boson with appropriate coupling strengths to (b s) and (ē e).
The measurement of R D (R D * ), the ratio of the branching fraction of B → Dτν τ (B → D * τν τ ) to that of B → Dlν l (B → D * lν l ), shows 1.9σ (3.3σ) deviation from its Standard Model (SM) prediction. The combined deviation is at the level of 4σ according to the Heavy Flavour Averaging Group (HFAG). In this paper, we perform an effective field theory analysis (at the dimension 6 level) of these potential New Physics (NP) signals assuming SU(3) C × SU(2) L × U(1) Y gauge invariance. We first show that, in general, R D and R D * are theoretically independent observables and hence, their theoretical predictions are not correlated. We identify the operators that can explain the experimental measurements of R D and R D * individually and also together. Motivated by the recent measurement of the τ polarisation in B → D * τν τ decay, P τ (D * ) by the Belle collaboration, we study the impact of a more precise measurement of P τ (D * ) (and a measurement of P τ (D)) on the various possible NP explanations. Furthermore, we show that the measurement of R D * in bins of q 2 , the square of the invariant mass of the lepton-neutrino system, along with the information on τ polarisation and the forward-backward asymmetry of the τ lepton, can completely distinguish the various operator structures. We also provide the full expressions of the double differential decay widths for the individual τ helicities in the presence of all the 10 dimension-6 operators that can contribute to these decays.
We perform a comprehensive study of the impact of new-physics operators with different Lorentz structures on decays involving the b → sµ + µ − transition. We examine the effects of new vector-axial vector (VA), scalar-pseudoscalar (SP) "' and tensor (T) interactions on the differential branching ratios and forward-backward asymmetries (A F B 's) ofB T , and the longitudinal-transverse asymmetry A LT . We identify the Lorentz structures that would significantly impact these observables, providing analytical arguments in terms of the contributions from the individual operators and their interference terms. In particular, we show that while the new VA operators can significantly enhance most of the asymmetries beyond the Standard Model predictions, the SP and T operators can do this only for A F B inB
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