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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - NIOBE (Non-Leptonic Three-Body B Decays: Theory and Phenomenology)

Teaser

Summary

\"Non-leptonic decays of B mesons have been studied extensively since the 90s, when experimental progress made it possible to produce B mesons in large numbers and controlled conditions. These studies have been essential in testing and understanding the flavor and CP sector of the Standard Model. However, theory predictions are difficult to produce. The experimental program at LHC and Belle-II opens now the door to a more systematic study of exclusive non-leptonic B decays with more than 2 particles in the final state. The theoretical study of such decay modes is the subject of this project. The objective has been to put in place a framework and a set of calculations that will contribute to the theoretical understanding of these decay modes and to the interpretation of the wealth of experimental data.

This framework relies on the property of \"\"factorization\"\" of short and long-distance physics, and allows to write decay amplitudes in terms of simpler non-perturbative matrix elements. One of the main outputs of the research performed here has been the calculation of the non-perturbative matrix elements genuine of three-body non-leptonic decays in the borders of the kinematic phase space (B--> 2-meson form factors). One of the conclusions is that the method of light-cone sum rules with B-meson distribution amplitudes is the ideal approach so far. These calculations have been performed for the pi-pi and the K-pi systems, and it has been identified that the correction due to the width of the K* in B decays with K* mesons in the final states is as large as 20% at the level of the decay rate. This conclusion seems very relevant in applications to semileptonic rare decays, and will also be the case in non-leptonic decays as soon as experimental precision is improved. In addition, contributions from resonances at higher K-pi invariant masses are also found to be relevant for the determination of the B-->K* form factors. Another new element that we have introduced in this project is the possibility to use LHCb measurements at high K-pi invariant masses to gain knowledge on the K*.

Non-leptonic B decays are also used systematically to determine CKM parameters, which are fundamental constants related to flavor. As another output of this project, it has been realized that in the situation of generic physics beyond the Standard Model (SM), when new degrees of freedom are above the electroweak scale, the way we use non-leptonic B decays to gain access to these fundamental parameters has to be revised. We have presented a proposal for CKM determinations beyond the SM, and the avenue of revisiting the role of non-leptonic decays has been opened. Regarding the predictions for CP violation, the challenge remains in gaining control over different partial waves, and on the structure of CP-violating asymmetries far from the edges of the kinematic phase space. Thus, more work is needed in this direction, where hopefully future experimental studies will shed some light.

Finally, a symbiotic relationship between non-leptonic and rare B decays has been explored, in the quest towards a theoretical approach to \"\"non-local\"\" form factors. The conclusion is that a detailed experimental study of non-leptonic B decays such as B-->K* X(1--), where X(1--) is a generic hadronic final state with spin 1, and negative parity and charge conjugation, will be a strong handle for predictions of semileptonic FCNC decays.\"

Work performed

During the course of the project, important explorative work has been done as well as several specific studies that have been published. First, an assessment of the global plan has been carried out and notes on all the different areas in the work plan have been written. Subsequently, detailed work on the form factors has been conducted and published (Phys.Rev. D96 (2017) no.5, 051901, JHEP 1912 (2019) 083), as well as a generic study of CP violation in the three-pion mode, which has also been published (JHEP 1710 (2017) 117). Tangential work on rare decays has also been published (Eur.Phys.J. C79 (2019) no.8, 714), which also contributes to the deeper understanding of the methods that can be applied subsequently to the non-leptonic decays, as well as work on the extraction of CKM elements in the presence of New Physics (JHEP 1905 (2019) 172). We note that CKM physics has a large overlap with the topic of the project. In addition, a detailed analytic calculation of two-loop contributions to off-shell photonic b-->s penguins has been performed, which allows to study the analyticity properties of FCNC processes (arXiv:1912.09099 [hep-ph]).

New avenues have opened up as a result of the published research. These new avenues have translated into a new project that the fellow is conducting as a new permanent staff member at the University of Barcelona, a position that was envisaged as one of the main `impact\' goals of this fellowship.

Dissemination has been performed through an extensive program of seminars at conferences (14) and invited seminars at universities (13). The fellow has also organized two workshops, one specific of Three-Body Non-Leptonic B decays (https://indico.mitp.uni-mainz.de/event/177/), and the other on computing tools for SMEFT phenomenology (https://indico.cern.ch/event/787665/).

Final results

\"The results achieved in the course of this project open up interesting future perspectives in the study of B decays into final states which include two mesons with low invariant mass, such as non-leptonic three-body B decays in the edges of phase space or semileptonic FCNC B decays into vector mesons (e.g. B-->K*ll). These perspectives also include the inter-relation among these different types of B decays, and the idea of moving towards more \"\"global\"\" phenomenological analyses where all the information is treated simultaneously. An example is provided in (JHEP 1912 (2019) 083), where the form factors relevant for decays such as B --> Kpipi are related to rate distributions in semi-leptonic tau decays and constrained by LHCb measurements of B--> K+pi+mu+mu in the high K-pi mass window. Members of the LHCb collaboration have already shown great interest in our findings. Extension of this work is already in progress and includes a deep study of two-meson states in an S-wave configuration.

Another perspective has to do with revisiting the strategies for extraction of CKM elements from non-leptonic decays in the presence of generic heavy Beyond-The-SM Physics, as discussed in (JHEP 1905 (2019) 172). This will become a serious deal as soon as the tremendous amount of data that will be collected by the Belle-2 experiment is analyzed.

Yet another perspective has been opened by the results published in (Eur.Phys.J. C78 (2018) no.6, 451), where the study of non-local effects in exclusive b-->sll decays has been given a new twist. Our results have been the subject of posterior studies by members of the LHCb collaboration studying the potential of the method confronting the amount of data that will be available within the next decade. This work has also been complemented by an analytic OPE matching calculation at NLO in (arXiv:1912.09099 [hep-ph]), which allows for the first time to apply the program of (Eur.Phys.J. C78 (2018) no.6, 451) consistently.\"