Abstract
Flavor physics experiments allow to probe the accuracy of the Standard Model (SM) description at low energies, and are sensitive to new heavy gauge bosons that couple to quarks and leptons in a relevant way. The apparent anomaly in the ratios of the decay of B-mesons into D-mesons and different lepton flavors, {R}_{D^{left(*right)}}=mathrm{mathcal{B}}left(Bto {D}^{left(ast right)}tau nu right)/mathrm{mathcal{B}}left(Bto {D}^{left(ast right)}ell nu right) is particularly intriguing, since these decay processes occur at tree-level in the SM. Recently, it has been suggested that this anomaly may be explained by new gauge bosons coupled to right-handed currents of quarks and leptons, involving light right-handed neutrinos. In this work we present a well-motivated ultraviolet complete realization of this idea, embedding the SM in a warped space with an SU(2)L ⊗ SU(2)R ⊗ U(1)B − L bulk gauge symmetry. Besides providing a solution to the hierarchy problem, we show that this model, which has an explicit custodial symmetry, can explain the {R}_{D^{left(*right)}} anomaly and at the same time allow for a solution to the {R}_{K^{left(*right)}} anomalies, related to the decay of B-mesons into K-mesons and leptons, {R}_{K^{left(*right)}}=mathrm{mathcal{B}}left(Bto {K}^{left(ast right)}mu mu right)/mathrm{mathcal{B}}left(Bto {K}^{left(ast right)}eeright) . In addition, a model prediction is an anomalous value of the forward-backward asymmetry AFBb, driven by the Z{overline{b}}_R{b}_R coupling, in agreement with LEP data.
Highlights
These decay processes occur at tree-level in the Standard Model (SM), and can only be affected in a relevant way by either light charged gauge bosons, or heavy ones strongly coupled to the SM fermion fields
The experimental measurements of RD(∗) show significant deviations from the SM values, a surprising result due to the tree-level nature of this process in the SM. Possible resolutions of this anomaly face significant constraints from the excellent agreement of flavor physics observables with the values predicted within the SM
We have presented an explicit realization of the solution to the RD(∗) anomaly based on the contribution of righthanded currents of quarks and leptons to this process
Summary
Neutrinos localized on the IR brane, as is the case with the righthanded neutrinos ντR, couple in a relevant way to the Higgs and tend to acquire masses of the same order as the charged lepton masses This can be seen from the fact that the Yukawa couplings in eq (2.32) will provide a Dirac mass to the third generation neutrinos mDνLνR+h.c. in order to obtain realistic masses we will assume a double seesaw scenario [36]. In principle the anomaly in the branching ratio B(B → D(∗)τRνR) might give rise to a large contribution to the branching fraction B(Ds → τ ν) 0.05 from the process sc → τR+νR, which is mediated by the KK modes WRn. since cR and sR are in the bulk, and in different SU(2)R doublets, they couple to WRn only via mixing with the third generation quarks. The value of Cτ obtained above to explain RD(∗) can only slightly ameliorate this anomaly, and one should wait for more accurate experimental measurements of R(J/Ψ) before further discussion of this issue
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