Family non-universal Z' models with protected flavor-changing interactions

Abstract: We define a new class of $Z'$ models with neutral flavor-changing interactions at tree level in the down-quark sector. They are related in an exact way to elements of the quark mixing matrix due to an underlying flavored $U(1)'$ gauge symmetry, rendering these models particularly predictive. The same symmetry implies lepton-flavor non-universal couplings, fully determined by the gauge structure of the model. Our models allow to address presently observed deviations from the SM and specific correlations among the new physics contributions to the Wilson coefficients $C_{9,10}^{(\prime) \ell}$ can be tested in $b \to s \ell^+ \ell^-$ transitions. We furthermore predict lepton-universality violations in $Z'$ decays, testable at the LHC.

• The paper introduces non-universal Z′ models that generate tree-level FCNCs in the down-quark sector linked to CKM matrix elements.
• Researchers employ a flavored U(1)′ gauge symmetry to tightly constrain model parameters and predict lepton universality violations in rare B decays.
• The study provides clear predictions for anomalous b → sℓ⁺ℓ⁻ transitions, offering testable signals for new physics at the LHC and beyond.

Family Non-Universal $Z^{\prime}$ Models with Protected Flavor-Changing Interactions

The paper presented by Alejandro Celis et al. explores a novel class of $Z^{\prime}$ models characterized by neutral flavor-changing interactions at the tree level in the down-quark sector, mediated by an underlying flavored $\mathrm{U(1)}^{\prime}$ gauge symmetry. These models are significant in their predictive capacity, linking flavor-changing neutral currents (FCNCs) directly to elements of the quark mixing, or CKM, matrix. The implications are substantial for deviations in current measurements from the Standard Model (SM) predictions, particularly in semileptonic $b \to s \ell^+ \ell^-$ transitions.

Model Construction

The authors extend the SM by introducing a $\mathrm{U(1)}^\prime$ symmetry, which results in new FCNCs constrained to the down-quark sector. The symmetry enforces specific quark and lepton charge distributions, thereby constraining the model’s parameter space and enhancing its predictive nature. This gauge symmetry also breaks lepton flavor universality, predicting violations that can be experimentally tested, for instance, in $Z^{\prime}$ decay channels at the LHC.

Phenomenological Implications

The $Z^{\prime}$ models predict significant correlations among new physics contributions to the Wilson coefficients $C_{9,10}^{(\prime) \ell}$. These predictions are crucial given the anomalies observed in recent LHCb measurements. For example, the deviation in the $R_K$ ratio (indicative of lepton universality violation) and discrepancies in angular distributions from $B \rightarrow K^*\mu^+\mu^-$ decays can be addressed within this framework. The $Z^{\prime}$ boson’s coupling structure is distinctive due to its dependence on CKM matrix elements, which gives rise to naturally suppressed FCNCs.

Predictions and Testing

The models offer a concrete testable prediction: lepton-universality violations in particular $Z^{\prime}$ decay channels, measureable at the LHC. Additionally, due to the specific charge assignments and symmetry constraints, the $Z^{\prime}$ boson contributions to rare $B$ decays exhibit defined patterns, allowing for signals of new physics beyond the SM. Notably, these contributions are predicted to alter processes with naturally suppressed SM contributions, such as $B_s$ mixing and lepton flavor ratios in meson decays.

Constraints and Theoretical Developments

The gauge structure inherent in the $Z^{\prime}$ models tightly constrains the parameter space, with current data from $B$ physics providing a stringent testing ground. Constraints from electroweak precision tests, atomic parity violation, and neutrino trident production are considered, ensuring that $Z^{\prime}$ and scalar contributions remain consistent with precision measurements. Furthermore, anomaly cancellation conditions are satisfied without resorting to exotic new particles, maintaining minimal model extensions.

Conclusion and Future Prospects

This detailed exploration into $Z^{\prime}$ models emerges as an important theoretical development in new physics models, aiming to resolve current deviations in $b \to s \ell^+\ell^-$ processes. The paper sets a clear direction for future investigations and experimental validations at particle colliders such as the LHC. The models provide not only a conceptual framework for understanding current discrepancies but offer paths for new discoveries in flavor physics and tests of lepton universality. Continued refinement of these predictions and further experimental data will serve as decisive tests of the $Z^{\prime}$ models and their underlying theoretical assumptions.