Periodic Reporting for period 4 - FLAY (Flavor Anomalies and the origin of the Yukawa couplings)
Okres sprawozdawczy: 2024-03-01 do 2024-08-31
One of the greatest mysteries in particle physics is why we observe three generations (or flavours) of quarks and leptons, i.e. the fundamental constituents of matter, and why their masses are so different. The purpose of this project was to shed light on this fundamental question. Solving it is of paramount importance for a better understanding the microscopic laws of Nature. This question is also at the basis of our own existence: the stability of atomic nuclei, as well as structure and properties of atoms, strongly depend on the precise values of the elementary particle masses, in particular the mass of the electron (the lightest charged lepton) as well as those of the up and down quarks.
The search for an underlying explanation for the observed pattern of quark and lepton masses is a long-standing issue in particle physics, often referred to as the “flavour problem.” What made this project original, compared with previous attempts to address the flavour problem, was the attempt to solve it by postulating new dynamics that can differentiate the different generations near the electroweak scale, that is, by postulating new fundamental forces that distinguish different generations at energy scales accessible by experiments. From a theoretical point of view, this attempt was motivated by the idea of a link between the flavour problem and the other key open question of the Standard Model (SM), namely, the stability of the Higgs sector. From a phenomenological point of view, this attempt was motivated by the so-called “flavour anomalies”, i.e. by a few hints of deviations from Standard Model predictions reported, in recent years, by experiments in selected flavour-changing processes.
The search for an underlying explanation for the observed pattern of quark and lepton masses is a long-standing issue in particle physics, often referred to as the “flavour problem.” What made this project original, compared with previous attempts to address the flavour problem, was the attempt to solve it by postulating new dynamics that can differentiate the different generations near the electroweak scale, that is, by postulating new fundamental forces that distinguish different generations at energy scales accessible by experiments. From a theoretical point of view, this attempt was motivated by the idea of a link between the flavour problem and the other key open question of the Standard Model (SM), namely, the stability of the Higgs sector. From a phenomenological point of view, this attempt was motivated by the so-called “flavour anomalies”, i.e. by a few hints of deviations from Standard Model predictions reported, in recent years, by experiments in selected flavour-changing processes.
The project was articulated along three complementary lines: 1) the development of precise predictions for flavour-changing observables; 2) the development of Effective Field Theory (EFT) approaches to analyse the consistency of general theoretical hypotheses addressing the origin of the flavour hierarchies, and other open problems of the SM, with experimental data; 3) the constructions of ultraviolet (UV) complete models addressing both the origin of the flavour hierarchies and the recent flavour anomalies. We made substantial progress on all these lines.
Line 1 has been an essential step to improve, both in quantity and in quality, the extraction of theoretical information from experimental data. In this context, we have presented the first systematic analysis of QED corrections in B→Kℓ+ℓ- decays at the double-differential level, reducing the theoretical uncertainty on the Lepton Flavour Universality ratios below the 1% level. We have also developed a novel method that allow us to extract precise information about the underlying short-distance dynamics from any hadronic transition of the type b→sℓ+ℓ-, including processes with many hadrons in the final state.
Line 2 has been a key ingredient to facilitate the interpretation of data and, most important, to test general hypotheses about the underlying theory. We have developed the innovative concept of an EFT describing generic extensions of the SM where the new dynamics couples mainly to the third generation. We have shown how this effective theory represents a well-defined limit of the so-called Standard Model EFT (or SMEFT). By a detailed comparison with both low- and high-energy data, we have also shown that, within this EFT, the scale of physics beyond the SM can be as low as 1.5 TeV without additional model-dependent assumptions. This is a key result that corroborates one of the key hypotheses of this project, namely the existence of a connection between the flavour problem and the hierarchy problem. We have also shown how this EFT can be embedded into UV-complete extensions of the SM featuring flavour non-universal gauge groups, which are particularly interesting in view of line 3, and we have uncovered a series of interesting effects arising at the quantum in such models.
The most original part of the project is the last line, where we introduced and developed the concept of “flavour deconstruction”. By this term we mean extensions of the SM in which one or more of the three fundamental forces, associated with a given (flavour-universal) gauge group, arises as the low-energy limit of a UV theory with manifestly non-universal flavour forces. We have shown that via this hypothesis, postulating the first layer of flavour non-universality at the TeV scale, it is possible to construct consistent SM extensions that can address both the origin of the flavour hierarchies and the Higgs hierarchy problem. We have also shown how flavour deconstruction in four dimensions can be viewed as the limit of a five-dimensional theory, with a compact fifth dimension and an appropriate set of branches along this dimension.
Line 1 has been an essential step to improve, both in quantity and in quality, the extraction of theoretical information from experimental data. In this context, we have presented the first systematic analysis of QED corrections in B→Kℓ+ℓ- decays at the double-differential level, reducing the theoretical uncertainty on the Lepton Flavour Universality ratios below the 1% level. We have also developed a novel method that allow us to extract precise information about the underlying short-distance dynamics from any hadronic transition of the type b→sℓ+ℓ-, including processes with many hadrons in the final state.
Line 2 has been a key ingredient to facilitate the interpretation of data and, most important, to test general hypotheses about the underlying theory. We have developed the innovative concept of an EFT describing generic extensions of the SM where the new dynamics couples mainly to the third generation. We have shown how this effective theory represents a well-defined limit of the so-called Standard Model EFT (or SMEFT). By a detailed comparison with both low- and high-energy data, we have also shown that, within this EFT, the scale of physics beyond the SM can be as low as 1.5 TeV without additional model-dependent assumptions. This is a key result that corroborates one of the key hypotheses of this project, namely the existence of a connection between the flavour problem and the hierarchy problem. We have also shown how this EFT can be embedded into UV-complete extensions of the SM featuring flavour non-universal gauge groups, which are particularly interesting in view of line 3, and we have uncovered a series of interesting effects arising at the quantum in such models.
The most original part of the project is the last line, where we introduced and developed the concept of “flavour deconstruction”. By this term we mean extensions of the SM in which one or more of the three fundamental forces, associated with a given (flavour-universal) gauge group, arises as the low-energy limit of a UV theory with manifestly non-universal flavour forces. We have shown that via this hypothesis, postulating the first layer of flavour non-universality at the TeV scale, it is possible to construct consistent SM extensions that can address both the origin of the flavour hierarchies and the Higgs hierarchy problem. We have also shown how flavour deconstruction in four dimensions can be viewed as the limit of a five-dimensional theory, with a compact fifth dimension and an appropriate set of branches along this dimension.
Two results of the project are beyond the state of the art. One of them was also totally unexpected before the start of the project.
1. The development of theories based on the concept of “flavour deconstruction”.
We have demonstrated that the observed hierarchies among fermion masses emerge naturally from the hypothesis of a flavour non-universal gauge structure at high energies, i.e. from fundamental forces acting differently on the different families. In this class of theories, the gauge group of the Stannard Model emerges naturally as low-energy limit, and flavour universality is an accidental low-energy property. While this possibility was known before our investigation, such theories were largely ignored and not considered of phenomenological interest. Thanks to our work, they now represent one of the promising candidates to extend the SM at high energies. We have shown how such class of theories not only describe in a natural way the observed mass hierarchies but could also address the stability of the Higgs sector and explain the persisting anomalies observed in semi-leptonic B meson decays. Given the experimental evidence of the flavour anomalies remain uncertain, we could not single out a single preferred candidate to extend the SM: we classified and analysed which options are possible. Only a handful of them are consistent with present data. If our hypotheses are correct, future data will be able to tell which of these options is chosen by Nature.
Although this goal was partly expected considering the preliminary results obtained by the PI prior to the start of the project, the impact and generality of the results obtained at the end of the project are greater than initially assumed and go far beyond the state of the art.
2. The development of a public code able to match generic SM extensions into the SMEFT. A very interesting methodological output, which was not foreseen at the start of the project, has been the development of a public code able to match generic SM extensions into an appropriate EFT containing only SM fields. This innovative tool can compute, automatically, all the point-like interactions among SM fields generated by integrating out arbitrary new particles. The package, documentation, and example notebooks are publicly available at https://gitlab.com/matchete/matchete(odnośnik otworzy się w nowym oknie). This project was carried out in full autonomy by young personnel hired within the project and, as anticipated, was completely beyond the initial scope of the project.
1. The development of theories based on the concept of “flavour deconstruction”.
We have demonstrated that the observed hierarchies among fermion masses emerge naturally from the hypothesis of a flavour non-universal gauge structure at high energies, i.e. from fundamental forces acting differently on the different families. In this class of theories, the gauge group of the Stannard Model emerges naturally as low-energy limit, and flavour universality is an accidental low-energy property. While this possibility was known before our investigation, such theories were largely ignored and not considered of phenomenological interest. Thanks to our work, they now represent one of the promising candidates to extend the SM at high energies. We have shown how such class of theories not only describe in a natural way the observed mass hierarchies but could also address the stability of the Higgs sector and explain the persisting anomalies observed in semi-leptonic B meson decays. Given the experimental evidence of the flavour anomalies remain uncertain, we could not single out a single preferred candidate to extend the SM: we classified and analysed which options are possible. Only a handful of them are consistent with present data. If our hypotheses are correct, future data will be able to tell which of these options is chosen by Nature.
Although this goal was partly expected considering the preliminary results obtained by the PI prior to the start of the project, the impact and generality of the results obtained at the end of the project are greater than initially assumed and go far beyond the state of the art.
2. The development of a public code able to match generic SM extensions into the SMEFT. A very interesting methodological output, which was not foreseen at the start of the project, has been the development of a public code able to match generic SM extensions into an appropriate EFT containing only SM fields. This innovative tool can compute, automatically, all the point-like interactions among SM fields generated by integrating out arbitrary new particles. The package, documentation, and example notebooks are publicly available at https://gitlab.com/matchete/matchete(odnośnik otworzy się w nowym oknie). This project was carried out in full autonomy by young personnel hired within the project and, as anticipated, was completely beyond the initial scope of the project.