Rare decays

Rare decays of beauty hadrons constitute an ideal laboratory for the search for physics phenomena beyond the Standard Model (SM). These decays are Flavour Changing Neutral Currents (FCNC) and are thus forbidden at tree level in the SM. Rare, loop-mediated processes allow indirect access to quantum corrections from degrees of freedom at larger scales and provide excellent complementarity to direct searches of New Physics (NP).

Our group is one of the main players in the study of bsll processes, mainly focused on two aspects: the study of the angular distribution of B0K*0μμ decays, and the test of lepton flavour universality in B0K*0ll decays. The main feature of these two types of analysis is that the results so far are showing tensions with the SM predictions, providing tantalizing hints of NP phenomena: on one hand, the analysis of the angular distribution of the B0K*0μμ with LHCb Run I data shows a deviation of 3.4σ with respect to the SM prediction, and, on the other, the study of the ratio of branching fractions of B+K+μμ and B+K+ee,  and B0K*0μμ and B0K*0ee decays show deviations of 2.2-2.7σ with respect to the expected value.

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Figure 1. Measurements of P5 performed by LHCb using 2011 data (blue) and the full Run I dataset (black), compared to the same measurements performed by Belle (green) and to the SM theoretical predictions (orange).
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Figure 2. Comparison of the LHCb RK* measurements with the SM theoretical predictions from CDHMV, EOS, flav.io, and JC. The predictions are displaced horizontally for presentation.

Our group led the effort in measuring the angular distribution of the B0K*0μμ with LHCb Run I and is currently working on an improved and updated version of this measurement. Additionally, work is ongoing to study lepton flavour universality in the angular distributions of B0K*0μμ and B0K*0ee decays, which is of key importance to confirm the anomalies observed in the ratios of branching fractions.

In addition to these activities, our group has been involved in the search for long-lived scalar particles, which provide access to many different hidden sector models, and lepton flavour violation in tau decays, using data from Run I in both cases. Moreover, we are currently involved in the study of radiative decays of beauty hadrons, which can be used to constrain NP models with large right-handed currents.