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v3.3.2-pre
The rare semileptonic Bd → K∗μ+μ− decay has been extensively studied within the Standard Model (SM) and beyond. In the last two releases, measurements provided by the LHCb experiment suggested a discrepancy found between the theoretical SM prediction and the experimental value of the angular observable P′5 at low q^2. We critically reassess the theoretical uncertainties in the Standard Model calculation of the Bd → K∗l+l− angular observables, focusing on this kinematic region. We point out that even optimized observables are affected by sizable uncertainties coming from several sources. First, departures from the infinite mass limit are numerically sizable and cannot be reduced by redefining the soft functions. Second, hadronic contributions generated by current-current operators with charm are difficult to estimate, especially for q^2 ~ 4 mc^2. Finally, charmonium contributions are not power suppressed and are expected to increase the uncertainties close to the hadronic resonances. Taking these uncertainties into account, we perform a detailed numerical analysis and present both predictions and fit results obtained using different sets of data, showing agreement between theory and experiment.