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Exploring New Phenomena through Flavour Changing Neutral Currents with LHCb

Final Report Summary - NP IN FCNCS AT LHCB (Exploring New Phenomena through Flavour Changing Neutral Currents with LHCb)

The objective of the project "NP in FCNCs at LHCb" is the search for processes beyond the Standard Model (SM) through the study of rare flavour changing neutral current (FCNC) decays. While the SM is very successful in the description the fundamental building blocks of matter and their interactions, it is known to be incomplete. Physics models that address the open questions of the SM, so called New Physics (NP), generally introduce new heavy particles. The search for these new particles is a central objective of particle physics.

The proposal pursues the search for NP through precision measurements of rare FCNC decays, that are forbidden at lowest perturbative order in the SM and only allowed as quantum fluctuations. Rare decays are therefore heavily suppressed and NP contributions can have a comparably large impact, affecting the rates and angular distributions of rare decays. The proposal concentrates on semileptonic transitions of a b quark into an s quark and two muons that are well accessible with the LHCb experiment.

As first part of this project, the first full angular analysis of the b->s decay B0->K*0 mu+mu- was performed using the data taken by the LHCb experiment during LHC Run 1. This decay is of particular importance since it allows access to many angular observables that are extremely sensitive to NP and that furthermore give information to differentiate between different NP models. Preliminary results were presented by the researcher at the international Moriond Electroweak conference in 2015 and were published as conference note (LHCb-CONF-2015-002, available at The following paper, published in the journal of high energy physics (JHEP 02 (2016) 104) is one of the major results of the project. It also provides measurements of additional angular observables, most notably the CP asymmetries of the angular distributions. The results gained considerable attention in the flavour physics community since one angular observable, denoted as P5', deviates from its SM prediction. The significance of this deviation corresponds to 3.4 standard deviations globally. A direct fit of the Lorentz structure of the decay to further improve the sensitivity to NP is currently in preparation.

The second part of the project was the angular analysis of the related decay Bs0->phi mu+mu- and a determination of its branching fraction. This analysis comprises the world's most precise measurements of these quantities and was published as JHEP 09 (2015) 179. While the angular observables are found to be in good agreement with SM predictions, the branching fraction is found to be in tension with the SM prediction at around 3 standard deviations.

The third part of the project is a test of lepton universality by comparing the rate of the decay Bs0->phi mu+mu- with the corresponding decay mode with electrons in the final state.
This test is of particular interest due to recent hints of non-lepton universality in decays of heavy flavour. The work is ongoing and progressing well. It is performed together with a PhD student the researcher is supervising, and will enter the collaboration internal review process soon.

To interpret the observed tensions with the SM predictions, the published results are combined in global fits of all data on rare decays. The overall agreement with the SM is at the level of 3-4 standard deviations. Consistent NP explanations in the form of new heavy gauge bosons or leptoquarks have been proposed that can explain the observed tensions. However, also underestimated hadronic uncertainties of the SM prediction could be responsible. Both possibilities and ways to distinguish between them are currently under intense discussion in the community.

In summary, the project has been very successful: Important new results on rare decays have been published that show tensions with SM predictions that might be first hints of NP. The results have been disseminated at major high energy physics conferences and received considerable attention in the flavour physics community. Furthermore, the results have motivated already ongoing and future work, both in theory and in experiment.