New physics in $b\to s$ transitions after LHC run 1 (1411.3161v4)

Abstract: We present results of global fits of all relevant experimental data on rare $b \to s$ decays. We observe significant tensions between the Standard Model predictions and the data. After critically reviewing the possible sources of theoretical uncertainties, we find that within the Standard Model, the tensions could be explained if there are unaccounted hadronic effects much larger than our estimates. Assuming hadronic uncertainties are estimated in a sufficiently conservative way, we discuss the implications of the experimental results on new physics, both model independently as well as in the context of the minimal supersymmetric standard model and models with flavour-changing $Z'$ bosons. We discuss in detail the violation of lepton flavour universality as hinted by the current data and make predictions for additional lepton flavour universality tests that can be performed in the future. We find that the ratio of the forward-backward asymmetries in $B \to K^* \mu^+\mu^-$ and $B \to K^* e^+e^-$ at low dilepton invariant mass is a particularly sensitive probe of lepton flavour universality and allows to distinguish between different new physics scenarios that give the best description of the current data.

• The paper identifies significant tensions in angular observables and branching ratios of b→s decays compared to Standard Model predictions.
• The analysis rigorously evaluates hadronic uncertainties and employs model-independent fits, highlighting significant negative NP contributions to the Wilson coefficient C9.
• The study underscores potential lepton flavor universality violation, prompting the need for targeted future measurements to decisively probe these anomalies.

Overview of New Physics Investigations in $b\to s$ Transitions After LHC Run 1

The paper by Altmannshofer and Straub presents a comprehensive analysis of new physics (NP) in the rare decays mediated by the $b \to s$ flavor-changing neutral current (FCNC) transitions. These transitions have recently been highlighted by tensions observed between the Standard Model (SM) predictions and experimental data from LHCb after its first run. The authors perform global fits using all relevant experimental data on rare $b \to s$ decays and scrutinize potential sources of theoretical uncertainties to address these discrepancies.

Key Findings and Analysis

1. Tensions with the Standard Model: The authors identify significant tensions between SM predictions and observed data, particularly in angular observables of $B \to K^* \mu^+\mu^-$ and suppression in the branching ratio of $B_s \to \phi \mu^+\mu^-$. They emphasize the longstanding acknowledgment of such tensions since the first indications by LHCb in 2013. Primarily, these issues could stem from unaccounted hadronic effects potentially larger than currently estimated within the SM.
2. Investigation of Hadronic Uncertainties: The paper critically evaluates potential underestimations in hadronic uncertainties and concludes that violations of QCD factorization and duality might mimic new physics effects in certain observables but cannot fully account for all tensions. This analysis provides an essential foundation for assessing whether these discrepancies could be due to SM physics alone or hint at NP.
3. Model-Independent and Model-Dependent Analyses: The paper extends to model-independent scenarios involving Wilson coefficients, notably indicating significant negative NP contributions to $C_9$ and highlighting their necessity for reconciling tensions. The authors also explore these scenarios in the context of the minimal supersymmetric standard model (MSSM) and models featuring flavor-changing $Z'$ bosons, aligning with the most substantial discrepancies.
4. Lepton Flavor Universality Violation: The paper dedicates substantial focus to the emerging indication of lepton flavor universality violation, as suggested by the $R_K$ anomaly. The analysis suggests that scenarios affecting muonic modes specifically (and weakly affecting electronic ones) align better with the experimental data. These findings emphasize the urgent need for future tests on lepton flavor universality.
5. Implications and Predictions: The authors assume a conservative estimation of theoretical uncertainties and use the derived conclusions to discuss potential implications for NP, both broadly and within specific models like the MSSM. In doing so, the paper extensively elaborates on the constraints these anomalies impose on possible NP candidates and what future measurements could decisively establish or refute.

Future Directions

The authors suggest contributions to future explorations, indicating that further experimental data, particularly from refined measurements at LHCb and future lepton flavor universality tests, will be vital in demystifying these tensions. The paper highlights specific ratios and observables that can serve as litmus tests for NP, thus guiding both experimental physicists and theorists in prioritizing the most promising avenues for resolving the current enigmas in $b \to s$ transitions.

In summary, the authors provide a detailed analysis within the broad context of $b \to s$ transitions, deftly navigating the multifaceted implications of current data and theory. The paper remains a critical resource for addressing the discrepancies observed in $b \to s$ decays and stimulates further inquiry into these charming transitions and their potential to unveil new physics.