Flavour Anomalies with advanced particle Identification MEthods

Project Information

FAIME

Grant agreement ID: 884719

DOI

10.3030/884719

EC signature date 10 June 2020

Start date 1 October 2020

End date 30 September 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 732 275,00

EU contribution

€ 2 446 810,75

Coordinated by

INSTITUT JOZEF STEFAN
Slovenia

Periodic Reporting for period 3 - FAIME (Flavour Anomalies with advanced particle Identification MEthods)

Reporting period: 2023-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

The project is about studying phenomena observed in particle physics that we do not (yet) fully understand, and that could have a very important impact on our understanding of the way the Universe has evolved since its origin in the Big Bang. The Standard Model of particle physics, formulated nearly half a century ago, describes the fundamental matter particles and their interactions. Almost all experimental results confirm the validity of the Standard Model to good precision. There are, however, some gaps in its description of our universe, as well as new theories and experiments that point to potential violations of some of its laws. Lepton universality refers to a key assumption underlying the Standard Model, which states that all three species of charged leptons (three of the 12 basic – indivisible particles, with different masses; the best known of those is the electron) are believed to interact in the same way with other particles. Thus, they should be created equally in particle transformations or ‘decays’ when differences in their masses are considered. However, growing evidence suggests that this may not be the case.

This project plans to provide a definite demonstration with experiments involving rare decays, exploiting highly sensitive detection technology and sophisticated analyses. We will investigate how one sort of elementary particles (leptons) interacts with the other (quarks) by using the data that the Belle II Collaboration is collecting with the Belle II detector at the SuperKEKB electron-positron collider in Tsukuba, Japan. For example, one wishes to measure the rate of decays of B mesons (heavy particles, composed of quarks) into final states with an electron on one hand, and with a muon or a tau lepton (the other two species of charged leptons) on the other hand.
Within the research program, novel, highly advanced identification methods for charged particles are being developed. They will be of crucial importance to suppress backgrounds arising from other, much more abundant decays in measurements of rare processes where the sensitivity to a possible contribution of New Physics (i.e. phenomena, not described in the Standard Model) is largest. One should note that there are processes that are up to a hundred thousand more abundant than the ones one tries to measure.

Some of our findings are also of importance in other fields; notably, an application has been awarded an ERC Proof-of-Concept project for advances in medical imaging methods.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project consists of three interconnected activities: developing advanced identification methods of particles in the Belle II detector, studying departures from Lepton Flavour Universality (a theoretical prediction that all species of charged leptons interact uniformly with quarks) in decays of B and D mesons, and exploring processes completely forbidden within the current theoretical description of the elementary particle interactions (Standard Model).

The developing methods for identifying leptons, pions, and kaons has been the primary research focus in the initial project phase as it lays the foundation for advancements in the other two packages. Notably, a new method has been developed for separating low-momentum electrons, muons, and pions, utilizing Machine Learning techniques, specifically a Convolutional Neural Network (CNN), to analyze patterns in the electromagnetic calorimeter. Results of a detailed study of the Cherenkov light radiated by pions and kaons has been conducted were incorporated into an algorithm that improves particle type differentiation.

The core research theme of the project is the study of departures from Lepton Flavour Universality. The enhanced particle identification capabilities were used in developing B and D mesons tagging and reconstruction methods. Several reports have been accepted for publication, including novel methods for identifying the production flavor of neutral charmed and beauty mesons. B-meson decays were studied that involve a tau lepton and a light meson (a pion or a rho meson) in the final state, optimizing event selection criteria, and comparing simulated and measured data.

We have also started a promising new avenue for checking lepton flavor universality through the measurement of the rates of decays of B mesons into any hadron, composed of a charm quark, and either an electron, muon or a tau lepton. The initial feasibility study showed promising results, and a new postdoctoral researcher joined the team to strengthen this challenging measurement.

Within the third activity, research is being conducted on exotic processes, forbidden in the Standard Model, where lepton flavor number or baryon number are not conserved. Here we searched for a B meson decay into a kaon, accompanied by an electron and a tau lepton (lepton flavour number not conserved). A preparatory study for decays in which baryon number (number of heavy particles composed of three quarks like a proton) is not conserved has also been carried out.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The following results of the project can be considered breakthroughs beyond the state of the art: the electron/muon/pion separation by using the information from the electromagnetic calorimeter, a pioneering use of convolutional neural networks in analyzing the patterns (images) left by different particle species; particle identification improvement in separating pions from kaons in both Cherenkov-based detectors of the Belle II spectrometer through a very detailed description of their interaction with the detector material; the best measurement so far of the upper limit on the probability for the transition of a B meson to a kaon and two leptons of different types as a pioneering effort in several similar studies that we plan to carry out.

The study of departures from Lepton Flavour Universality will remain the core research theme of the project. We will finalize the measurement of B-meson decays that involve a tau lepton and a light meson (a pion or a rho meson) in the final state, and look for departures from Lepton Flavour Universality by comparing its probability to the probability of a related B-meson decay with a light lepton instead of the tau lepton in the final state. We will continue our research along the promising new avenue for checking lepton flavor universality through the measurement of the rates of decays of B mesons into any hadron, composed of a charm quark, and either an electron, muon or a tau lepton. The study of a very interesting transition of a B-meson to a kaon and a pair of tau leptons B0→K0S τ+τ− will be finalized.

Within the third activity, research on exotic processes, forbidden in the Standard Model, we will continue our search for B-meson decays in which baryon number (number of heavy particles composed of three quarks like a proton) is not conserved; such a transition could signal the existence of a new sort of dark matter particles.

The results of our studies are expected to have a significant impact in the coming years, both on the progress in particle physics as well as on our understanding of Nature in general.

An event, the result of an electron-positron collision in the Belle II spectrometer

PArticle identification detector for the forward region of the Belle II spectrometer