Probes of Lepton Flavor Universality in b → u Transitions

Abstract

Anomalies recently observed in semileptonic b → c ℓ − ν ¯ ℓ and b → s ℓ + ℓ − transitions point to violation of Lepton Flavour Universality. Strategies for new analyses of different modes are required, in particular for the modes induced by the b → u transition. We describe the purely leptonic B decay, the B ¯ → π ℓ − ν ¯ ℓ channel and the B semileptonic modes to ρ ( 770 ) and a 1 ( 1260 ) in extensions of the Standard Model involving Lepton Flavour Universality violating b → u operators. In particular, we review the observables in the four-dimensional angular B ¯ → ρ ( π π ) ℓ − ν ¯ ℓ and B ¯ → a 1 ( ρ π ) ℓ − ν ¯ ℓ distributions, suitable to pin down deviations from the Standard Model. We discuss the complementarity among the various modes for New Physics searches.

Impact of $$b \rightarrow s \ell \ell $$ anomalies on rare charm decays in non-universal $$Z'$$ models

“…The long-distance contributions in these decay modes are predicted to be ∼ 10 −16 [52]. For B c → B + ν ν mode, the short-distance SM contribution is ∼ 1.5×10 −16 whereas the long-distance contribution is estimated to be ∼ 10 −16 [77]. Therefore for unambiguous signature of new physics in these decay modes, the short-distance branching ratio should be enhanced well above 10 −16 .…”

“…In this regard, the decays induced by the quark level transition c → uν ν are particularly interesting as the long-distance effects are relatively smaller in comparison to their charged dileptons counterparts. Hence in c → u sector, dineutrino modes can be considered to provide genuine null-tests of SM [38,[75][76][77][78]. The c → uν ν transition induces several exclusive decay modes such as D + → π + ν ν, D 0 → π 0 ν ν and B + c → B + ν ν.…”

“…It was shown that the branching ratio of D + → π + ν ν can be enhanced up to ∼ 10 −6 which falls in the same ballpark as the model-independent upper bound. In [77], it was shown that the branching ratio of B + c → B + ν ν in the 331 model can be enhanced up to ∼ 10 −11 . It would be interesting to see whether such large enhancements is allowed by the current b → s and neutrino trident data in the non universal Z model considered in this work.…”

One colorful resolution to the neutrino mass generation, three lepton flavor universality anomalies, and the Cabibbo angle anomaly

“…From our example solution, the rate of b → uµν could deviate from the SM one by ∼ 40% due to the smallness of V ub and the interference between the NP and the SM weak interaction. More insights of the intriguing flavor problem are expected if the better experimental measurements on the b → ulν transitions are available [157].…”

Null test searches for BSM physics with rare charm decays

“…The experimental and theoretical status of rare charm decays is presented in Ref. [5] and increasing and ongoing research interest [1,2,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] manifests the importance of progress in the field.…”