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Closing in on New Physics in light-quark flavour transitions: from tau decays to the LHC

Periodic Reporting for period 1 - Tau-SYNERGIES (Closing in on New Physics in light-quark flavour transitions: from tau decays to the LHC)

Reporting period: 2017-07-01 to 2019-06-30

In spite of its success, there are theoretical and observational reasons to think that the Standard Model (SM) of Particle Physics is part of a larger framework. Efforts at three basic frontiers are carried out to search for new phenomena, i.e. Beyond the Standard Model (BSM) effects. The Energy Frontier uses powerful colliders to discover heavy new particles, and it is currently led by the CERN Large Hadron Collider (LHC). The Intensity Frontier uses precision measurements to search for BSM physics through quantum effects. Last, the Cosmic Frontier uses the universe as a laboratory to search for new phenomena.

This action studies the synergy of the Energy and the Intensity Frontiers, with the focus on non-standard effects affecting light quarks (u,d,s) and any lepton flavour (e,μ,τ). The goal is follow a model-independent approach whose results can be easily applied to specific BSM scenarios. Such general approach is based on an Effective Field Theory (EFT) framework where the effects of heavy fields are parameterised by Wilson Coefficients that multiply high-dimensional operators.

The case of hadronic τ decays is particularly interesting, as it represents the only low-energy handle on non-standard duτν or suτν interactions. Inclusive decays have been the object of enormous experimental and theory efforts in the last decades, providing precise determinations of fundamental parameters such as the strong coupling constant. On the other hand, exclusive decays are also interesting BSM probes thanks to the impressive precision of Lattice QCD (LQCD) calculations. Although this makes these processes interesting BSM probes, they have received very little attention from this point of view. Finally, the interplay of these low-energy studies with direct searches at the LHC has not been studied either.

The Tau-SYNERGIES action proposes a model-independent study of current and future high-precision low-energy measurements. It includes also the study of the interplay with collider searches. The main research objectives are the following:
- An unprecedented model-independent analysis of hadronic tau decays including BSM effects, and their complementarity with LHC searches;
- A comprehensive EFT global fit of low-energy processes involving light quarks (u,d,s) and any lepton flavour (e,μ,τ).

These objectives were achieved during the realization of this action, and they were published through several scientific articles.
The first results obtained in this project were presented in JHEP 1708 (2017) 123. In that article we carried out the first flavor-general model-independent analysis of a long list of low-energy measurements, as well as an analysis of their interplay with direct searches at the LHC. This work was followed by a dedicated study of nuclear beta decays as BSM probes, which was presented in the invited review article Prog.Part.Nucl.Phys. 104 (2019) 165-223.

These studies focused on processes involving light leptons (electrons, muons, and their associated neutrinos) and light quarks. The next step was a novel study of hadronic tau decays as BSM probes, which was published in Phys.Rev.Lett. 122 (2019) no.22 221801. These processes represents the only low-energy handle on non-standard interactions between tau leptons and light quarks.

The next works focused on flavor-changing processes. They involve additional complications because of the appearance of many SM parameters: CKM factors in the case of quarks and PMNS factors in the case of leptons. Our articles (JHEP 1905 (2019) 172 and JHEP 1905 (2019) 173) presented a method to overcome these complications and to carry out consistent analyses of flavor-changing processes in the presence of generic new phenomena.

The results obtained in this project were published in several scientific articles, and presented in well-known conferences and workshops. They were also presented through invited seminars at leading research institutions, and they were communicated to the general public through various outreach activities and media appearances.
The results presented in JHEP 1708 (2017) 123 showed that it is possible to carry out model-independent EFT analyses with a general flavor structure. Moreover, we made our results public so that other researchers can study the implications of high-precision low-energy measurements for any BSM model. The results have indeed be used by other colleagues, as illustrated by more than 50 citations in 2 years.

I wrote an invited review article on nuclear beta decays [Prog.Part.Nucl.Phys. 104 (2019) 165-223] in collaboration with two leading experimentalists, Prof. Naviliat-Cuncic and Prof. Severijns. The impact of this work is illustrated by invited plenary talks at major conferences, and by almost 50 citations in 1.5 years.

One of the main results of this action was the comprehensive and unprecedented study of hadronic tau decays as BSM probes, which was published in the prestigious journal Phys. Rev. Lett. As in the previous works, the numerical results were made fully public (i.e. correlation matrices were provided), so that other researchers can trivially benefit from our work. Moreover, it has opened unexpected research opportunities, such as the use of the channel tau → pi pi nu to obtain competitive BSM bounds. This work showed that hadronic tau decays are a competitive BSM probe thanks to the impressive experimental and theory precision, as illustrated in the attached figure [from Phys.Rev.Lett. 122 (2019) no.22 221801].
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