Periodic Reporting for period 3 - FLAY (Flavor Anomalies and the origin of the Yukawa couplings)
Période du rapport: 2022-09-01 au 2024-02-29
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 is 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” (the observed mass hierarchies are denoted “flavour hierarchies”). What makes this project both original and with concrete chances of success, with respect to previous attempts to address this problem, is that recent experimental data hint at deviations from the Standard Model predictions in flavour physics (i.e. in processes where quarks decays into lighter quarks and leptons of a different flavour). As proposed by the PI and collaborators before the start of the project, these "flavour anomalies" could be the first hints of a new fundamental force acting differently on the different generations of quarks and leptons, linked to an extension of the Standard Model addressing the origin of quark and lepton masses.
At the beginning of the project the evidence of these anomalies was rather weak from a statistical point of view, but the overall picture was quite encouraging. Interestingly enough, the significance of these anomalies has grown in the last two years, reinforcing the main hypothesis of the PI, and allowing significant step forwards in the identification of the underlying extension of the Standard Model addressing the origin of quark and lepton masses.
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” (the observed mass hierarchies are denoted “flavour hierarchies”). What makes this project both original and with concrete chances of success, with respect to previous attempts to address this problem, is that recent experimental data hint at deviations from the Standard Model predictions in flavour physics (i.e. in processes where quarks decays into lighter quarks and leptons of a different flavour). As proposed by the PI and collaborators before the start of the project, these "flavour anomalies" could be the first hints of a new fundamental force acting differently on the different generations of quarks and leptons, linked to an extension of the Standard Model addressing the origin of quark and lepton masses.
At the beginning of the project the evidence of these anomalies was rather weak from a statistical point of view, but the overall picture was quite encouraging. Interestingly enough, the significance of these anomalies has grown in the last two years, reinforcing the main hypothesis of the PI, and allowing significant step forwards in the identification of the underlying extension of the Standard Model addressing the origin of quark and lepton masses.
The project is articulated along three main, complementary lines:
1) The development of new precise predictions of flavor-changing observables.
This step is essential to improve, both in quantity and in quality, the extraction of theoretical information from the experimental data.
2) The development of Effective Field Theory (EFT) approaches to analyse the data, and correspondingly build simplified models.
This step is essential both to facilitate the interpretation of data and, most important, to test general hypotheses about the underlying model using all available data (not only those from the flavour-physics experiments, such as the LHCb experiment at CERN, but also those of high-energy experiments ATLAS and CMS).
3) Built ultraviolet complete models addressing both the origin of the flavour hierarchies and the recent flavour anomalies.
In the first two and half years of the project we achieved substantial progress on all these three lines.
Concerning the line 1), a true breakthrough has been achieved developing a novel method that allow us to extract precise information about the underlying short-distance dynamics from any transition of the type b→sℓ+ℓ-, including processes with many hadrons in the final state (that before our work where considered useless from this point of view). This result allows us to put together many more experimental data, facilitating a possible confirmation of the anomalies beyond any reasonable doubt in a very short time (ideally by the end of the project).
Concerning the line 2), we have performed a series of systematic studies that have significantly narrowed down the options of viable models addressing the anomalies. Interestingly enough, the original proposal of the PI that the anomalies are explained by the exchange of a vector leptoquark filed, resulting from the unification of quarks and leptons of different generations at different energy scales, remains one of the few viable options. Our studies have also clearly demonstrated that this option leads to high-energy signatures that should be observed by the existing high-energy experiments (ATLAS and CMS).
Concerning the line 3), a major breakthrough has been the identification of a model based on an extra space-time dimension that not only address the origin of the flavour hierarchies and the recent flavour anomalies, but it also explain the lightness of the Higgs boson mass. In other words, we identified a connection between the flavour anomalies and the other longs-standing open issues in particle physics, namely the stabilisation of the Higgs sector.
1) The development of new precise predictions of flavor-changing observables.
This step is essential to improve, both in quantity and in quality, the extraction of theoretical information from the experimental data.
2) The development of Effective Field Theory (EFT) approaches to analyse the data, and correspondingly build simplified models.
This step is essential both to facilitate the interpretation of data and, most important, to test general hypotheses about the underlying model using all available data (not only those from the flavour-physics experiments, such as the LHCb experiment at CERN, but also those of high-energy experiments ATLAS and CMS).
3) Built ultraviolet complete models addressing both the origin of the flavour hierarchies and the recent flavour anomalies.
In the first two and half years of the project we achieved substantial progress on all these three lines.
Concerning the line 1), a true breakthrough has been achieved developing a novel method that allow us to extract precise information about the underlying short-distance dynamics from any transition of the type b→sℓ+ℓ-, including processes with many hadrons in the final state (that before our work where considered useless from this point of view). This result allows us to put together many more experimental data, facilitating a possible confirmation of the anomalies beyond any reasonable doubt in a very short time (ideally by the end of the project).
Concerning the line 2), we have performed a series of systematic studies that have significantly narrowed down the options of viable models addressing the anomalies. Interestingly enough, the original proposal of the PI that the anomalies are explained by the exchange of a vector leptoquark filed, resulting from the unification of quarks and leptons of different generations at different energy scales, remains one of the few viable options. Our studies have also clearly demonstrated that this option leads to high-energy signatures that should be observed by the existing high-energy experiments (ATLAS and CMS).
Concerning the line 3), a major breakthrough has been the identification of a model based on an extra space-time dimension that not only address the origin of the flavour hierarchies and the recent flavour anomalies, but it also explain the lightness of the Higgs boson mass. In other words, we identified a connection between the flavour anomalies and the other longs-standing open issues in particle physics, namely the stabilisation of the Higgs sector.
As pointed out above, the developments along the lines 1) and 3) are major breakthroughs, that go beyond the state of the art.
On the other hand, despite the increase in statistical significance, the experimental status of the anomalies is not yet convincing beyond any reasonable doubt.
In the remaining years we plan to further work along the three lines outlined above to refine the structure of the model we have identified (several options are still open), and especially identify new signatures that could unambiguously lead to a confirmation of this picture with the help of more data (both at low and high energies).
The ultimate goal is still the development of a new theory of fundamental interactions. This high-gain objective cannot be guaranteed at this stage, but it remains a possible outcome of this project.
On the other hand, despite the increase in statistical significance, the experimental status of the anomalies is not yet convincing beyond any reasonable doubt.
In the remaining years we plan to further work along the three lines outlined above to refine the structure of the model we have identified (several options are still open), and especially identify new signatures that could unambiguously lead to a confirmation of this picture with the help of more data (both at low and high energies).
The ultimate goal is still the development of a new theory of fundamental interactions. This high-gain objective cannot be guaranteed at this stage, but it remains a possible outcome of this project.