Interpreting Hints for Lepton Flavor Universality Violation (1704.05435v2)

Abstract: We interpret the recent hints for lepton flavor universality violation in rare $B$ meson decays. Based on a model-independent effective Hamiltonian approach, we determine regions of new physics parameter space that give a good description of the experimental data on $R_K$ and $R_{K^*}$, which is in tension with Standard Model predictions. We suggest further measurements that can help narrowing down viable new physics explanations. We stress that the measured values of $R_K$ and $R_{K^*}$ are fully compatible with new physics explanations of other anomalies in rare $B$ meson decays based on the $b \to s \mu\mu$ transition. If the hints for lepton flavor universality violation are first signs of new physics, perturbative unitarity implies new phenomena below a scale of $\sim 100$ TeV.

• The paper reveals potential new physics as deviations in R_K and R_K* exceed 4σ, indicating pronounced lepton flavor universality violation.
• It employs an effective Hamiltonian framework to scrutinize key Wilson coefficients, notably C9^μ and C10^μ, against Standard Model values.
• The findings suggest that NP scenarios like light Z' bosons or leptoquarks may account for the observed anomalies in rare B meson decays.

An Analysis of Lepton Flavor Universality Violation in Rare B Meson Decays

The paper "Interpreting Hints for Lepton Flavor Universality Violation" by Altmannshofer, Stangl, and Straub provides a detailed examination of recent experimental data suggesting possible deviations from the Standard Model (SM) in rare $B$ meson decays. Utilizing a model-independent effective Hamiltonian framework, the authors aim to interpret the observed anomalies in lepton flavor universality (LFU) in the context of new physics (NP).

Intricacies of LFU Violation in $B \to K$ and $K^*$ Decays

The principal focus of this investigation is on the measured ratios $R_K$ and $R_{K^*}$, which indicate possible LFU violation in the $b \to s \ell \ell$ transitions. The calculated SM predictions for these ratios exhibit negligible deviation from unity due to the dominance of lepton Yukawa couplings. However, experimental measurements reveal that $R_K$ and $R_{K^*}$ exhibit significant deviations, posing a challenge to the SM framework.

Effective Hamiltonian Analysis and Numerical Results

The paper employs an effective Hamiltonian formalism, incorporating four-fermion contact interactions to pinpoint potential regions of NP parameter space that align with the anomaly observables. Key Wilson coefficients—$C_9^\ell$, $C_{10}^\ell$, along with their primed counterparts—were scrutinized. Notably, the best-fit scenarios suggest substantial deviations in these coefficients from their SM values. A particularly salient finding is the pronounced compatibility between NP in the left-handed quark current coefficients and recent data. The authors present that a negative $C_9^\mu$ and a positive $C_{10}^\mu$ can cogently account for the observed discrepancies in $R_K$ and $R_{K^*}$, each exhibiting significant statistical pulls exceeding 4$\sigma$.

Implications and Future Directions

The theoretical implications of the findings are noteworthy, signaling that if these deviations are genuine indicators of NP, new phenomena must exist below a scale of approximately 100 TeV, barring perturbative unitarity violations. The results thus underscore the necessity of considering NP models incorporating light $Z^\prime$ bosons or leptoquarks, which could generate such effective interactions.

Future investigations are posited to leverage precision observables, particularly LFU differences in angular distributions, such as $D_{P_4'}$ and $D_{P_5'}$, in $B \to K^* \ell^+ \ell^-$ decays. These measurements are poised to further refine the understanding of the chiral structure of the lepton currents implicated in LFU violation, distinguishing between diverse NP scenarios.

Conclusion

Altmannshofer et al.'s paper presents a meticulous analysis of LFU violation hints, elucidating the potential presence of NP in $B$ meson decays. The compatibility of their findings with existing rare decay anomalies reinforces the plausibility of beyond-standard interactions, thereby advancing opportunities to explore minimal extensions to the SM. Their paper establishes a foundation for future experimental and theoretical work directed towards uncovering the nature of LFU violation and its underlying causes within the particle physics landscape.