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HYP BETA DEC Report Summary

Project ID: 330458
Funded under: FP7-PEOPLE
Country: Germany

Final Report Summary - HYP BETA DEC (Standard and non-standard hyperon beta decays)

With the discovery of the Higgs boson, the LHC program for the search of new physic (NP) is now in full swing. Any model eventually superseding the standard model (SM) and introducing new dynamics with effective masses of the order of the energies at reach is expected to contribute to low-energy flavor observables. In fact, although no new particles have been identified yet, anomalies in flavor-changing transitions of hadrons have started to appear.

The central purpose of the “Hyp-beta” project is to assess the potential that a high precision program of semileptonic hyperon-decay (SHD) measurements could have in terms of the discovery of NP at the teraelectronvolt (TeV) energy scale. This involves identifying prominent observables particularly sensitive to non-standard effects and evaluating the experimental sensitivity needed to perform critical tests of the s-quark to u-quark flavor transitions in the SM.

In a paper appeared in Phys. Rev. Lett. in 2015 we reported that the SHD decay modes are, indeed, very sensitive to the exchange of new particles with masses as heavy as a few TeV. This allows one to put constraints on (or rule out) a broad class of NP models using the very old SHD data base and in competition with searches using the modern LHC data. Later, in 2016, we published a comprehensive, and completely model-independent statistical study of all the available data on the s->u and d->u (semi)leptonic transitions. This analysis allows one to assess the relative merits of the the different physical processes to discover NP and emphasized the potential interest that a renewed experimental program on hyperon decays could have. Indeed, new studies and measurements of hyperon decays are being considered by the LHCb and NA62 collaborations at CERN. In addition, our work has also triggered interest in the LQCD community as it provides a clear physics case for the more precise calculation of form factors, and a systematic framework to translate their results directly into bounds of physics at very high energies. .

In fact, an important aspect of this program has been to assess our knowledge of the QCD form factors, which are essential to extract experimentally the parameters of the elementary flavor transition in the hadronic weak decays. Great progress has been made in the last decade in LQCD or in ChPT, which is an effective theory of QCD at low energies. In case of hyperon decays, besides its application to searches of NP at TeV scales, the form factors provide interesting information related to the internal structure of the baryons (electromagnetic structure, masses, etc) and contain elements to the understanding of many of the crucial properties of the ordinary or more exotic (e.g. neutron stars) matter, or can appear modulating the interaction between weakly interacting dark-matter particles and nuclei.

In a series of papers appeared in 2014, we studied some of these form factors. The first one is an article published in Phys.Rev.Lett. where we propose a new method using an exact equation of QCD to obtain an accurate determination of prominent (scalar) form factors. In two other papers published in Phys. Rev. D, we tackled the two most important form factors entering in the SHD, which are crucial to understand the SM decay rates, the vector and axial hyperon couplings, f1(0) and g1(0). The former is central in the determination of the CKM matrix element Vus and we calculated, for the first time volume and chiral corrections which are an important systematic effect in LQCD computations. For the latter, we gave a complete calculation of the form factor in ChPT with a satisfactory perturbative convergence. Among other things, this allows one to understand theoretically the relations among the form factors in different SHD channels.

Finally, the project has had important spin-offs when some of its conceptual principles have been applied to the decays of the b-mesons. We first published in Phys.Rev.Lett. an article in which we provided an efficient parameterization of the NP in b->s rare decays using only symmetry principles. This allowed us to interpret unambiguously a recent possible signal of lepton-universality violation in an observable that can be constructed with these decays. The importance of this work was recognized by the editors of Phys. Rev. Lett. with an “Editor suggestion”, and it was featured in the web-site of the journal, appearing later in some on-line scientific news hubs. Furthermore, we published in Phys. Rev. Lett. a new proposal to measure the cross-section of the resonant production of Bs* mesons in lepton-lepton scattering. This is very clean, from the point of view of our knowledge of the corresponding hadronic matrix elements, and it can unambiguously confirm or refute the different claims of discovery of NP in B rare decays that have been recently made in the literature. Finally , we also proposed a new analysis of the angular distributions of the B->D^(*) tau nu decays (published in PRD) which also shows promising tensions with the SM.

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