Enhancement of direct CP asymmetry in Z′ models

Abstract

We consider CP-violating Z′ models to account for the anomalies in b→sℓℓ decays. Using the updated constraints from lepton flavor universality violating ratios, b→sμμ CP-conserving and CP-violating observables, Bs−B¯s mixing and neutrino trident we obtain the favored parameter space of two classes of Z′ models generating the new physics scenarios with Re[C9μNP]<0 and [C9μNP=−C10μNP,C9eNP=−C10eNP]. We study the predictions of direct CP asymmetry ACP in B+→K+μμ decays and CP asymmetric angular observables in these models. The favored 1σ parameter space of Z′ models generating scenario Re[C9μNP]<0 allows for an enhancement in the integrated ACP in q2=[8,9]  GeV2 bin up to ±25%, while the [C9μNP=−C10μNP,C9eNP=−C10eNP] scenario allows only positive values of ACP. However, these ACP values flip sign depending on the choice of J/ψ phase, hence distinguishing these two Z′ models through a measurement in this bin requires a more reliable determination of the J/ψ phase. The prediction of ACP in the q2=[16,17]  GeV2 bin are promising as they allow for an enhancement only in the positive direction for [C9μNP=−C10μNP,C9eNP=−C10eNP] scenario, irrespective of the choice of strong phase, while both positive and negative values are allowed for the scenario Re[C9μNP]. We also find that a future more precise measurement of CP-asymmetric angular observables A8 and A9 in the low-q2 bins can provide distinguishing signatures of these two Z′ models and help constrain new physics CP-violating phases. Published by the American Physical Society 2024