New Physics in Rare B Decays after Moriond 2021 (2103.13370v3)

Abstract: The anomalies in rare $B$ decays endure. We present results of an updated global analysis that takes into account the latest experimental input -- in particular the recent results on $R_K$ and BR$(B_s \to \mu^+\mu^-)$ -- and that qualitatively improves the treatment of theory uncertainties. Fit results are presented for the Wilson coefficients of four-fermion contact interactions. We find that muon specific Wilson coefficients $C_9 \simeq -0.73$ or $C_9 = -C_{10} \simeq -0.39$ continue to give an excellent description of the data. If only theoretically clean observables are considered, muon specific $C_{10} \simeq 0.60$ or $C_9=-C_{10} \simeq -0.35$ improve over the Standard Model by $\sqrt{\Delta \chi^2} \simeq 4.7\sigma$ and $\sqrt{\Delta \chi^2} \simeq 4.6\sigma$, respectively. In various new physics scenarios we provide predictions for lepton flavor universality observables and CP asymmetries that can be tested with more data. We update our previous combination of ATLAS, CMS, and LHCb data on BR$(B_s \to \mu^+\mu^-)$ and BR$(B^0\to \mu^+\mu^-)$ taking into account the full two-dimensional non-Gaussian experimental likelihoods.

• The paper updates Wilson coefficient analyses in rare B decays to account for LFU anomalies and branching ratio discrepancies.
• It employs an effective Hamiltonian framework to robustly test NP scenarios, notably favoring muon-specific shifts in coefficients (C9 or C9 = –C10).
• The findings underscore the need for refined LHC measurements to further probe potential New Physics beyond the Standard Model.

Overview of New Physics in Rare $B$ Decays after Moriond 2021

This paper by Wolfgang Altmannshofer and Peter Stangl offers an updated analysis of rare $B$ decays, considering the latest experimental results postulated during the Moriond 2021 conference. The persistent anomalies in rare $B$ decays, particularly in the $b \to s \ell \ell$ transitions, challenge the robustness of the Standard Model (SM) and hint at the existence of New Physics (NP). The authors employ these discrepancies to scrutinize the Wilson coefficients of four-fermion contact interactions, thus proposing viable NP scenarios that could reconcile these differences.

Key Experimental Data and Observations

The paper prominently investigates the lepton flavor universality (LFU) violation through the ratios $R_K$ and $R_{K^*}$, which exhibit significant deviations from SM predictions. The SM predicts these ratios to be near unity, yet LHCb's findings suggest a value of $R_K = 0.846$, with an uncertainty reduced by approximately 30% from previous measurements, elevating the tension to $3.1\sigma$.

Moreover, the paper evaluates the branching ratios $\text{BR}(B_s \to \mu^+\mu^-)$ and $\text{BR}(B^0 \to \mu^+\mu^-)$, leveraging non-Gaussian experimental likelihoods to refine the world averages. The $B_s \to \mu^+\mu^-$ measurement is particularly insightful due to its simplicity in theoretical calculation, relying mainly on a single hadronic parameter—its decay constant—as known from lattice QCD.

Methodological Enhancements

A notable methodological advancement is the treatment of theoretical uncertainties accommodating potential NP effects. The authors assess these using the effective Hamiltonian framework, parameterizing NP contributions via the Wilson coefficients of dimension-6 operators. This approach allows for a robust comparison between theoretical predictions and experimental data when considering NP scenarios.

The fit results show considerable preference for specific muon-related Wilson coefficients, namely $C_9 \simeq -0.73$ or $C_9 = -C_{10} \simeq -0.39$, both providing substantial alignment with the data. Such findings hint that these coefficients could be manifestations of NP, adjusting the observed anomalies in LFU ratios and branching fractions.

Speculative NP Scenarios

The authors explore various NP scenarios by adjusting these coefficients and find that LFU-specific $C_{10}$ or the relation $C_9 = -C_{10}$ could address SM discrepancies with substantial statistical significance ($4.6\sigma$ to $5.6\sigma$). Scalar Wilson coefficients also receive attention, showing a preference for negative values, suggesting suppressed $B_s \to \mu^+\mu^-$ branching ratios matching current experimental evidence.

Future Directions

The implications of this work are manifold. Continued refinement of theoretical predictions, as well as more precise measurements of LFU ratios and CP asymmetries—as projected following LHCb's Run 3—present promising arenas for detecting NP. The potential discovery of NP would necessitate a reevaluation of the SM, emphasizing the need for coordinated efforts to further probe these intriguing patterns.

Overall, Altmannshofer and Stangl's paper underscores the vital interplay between theoretical foundation and experimental validation, presenting a compelling case for the existence and discovery of NP through rare $B$ decay processes. The cumulative insights lend credence to the broader effort to unravel the deeper structures underlying particle physics beyond the SM.