Challenging the Standard Model with suppressed b to d l+l- decays

Project Information

CLIMB

Grant agreement ID: 101077940

DOI

10.3030/101077940

EC signature date 21 December 2022

Start date 1 April 2023

End date 31 March 2028

Funded under

• European Research Council (ERC)

Total cost

€ 1 622 273,00

EU contribution

€ 1 622 273,00

Coordinated by

UNIVERSITAT DE BARCELONA
Spain

Periodic Reporting for period 1 - CLIMB (Challenging the Standard Model with suppressed b to d l+l- decays)

Reporting period: 2023-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

The Standard Model (SM) of particle physics is one of the most complete theories in science with a hugely successful predicting power. However, it is unable to explain critical observed phenomena, such as the dominance of matter over antimatter in the universe, and consequently needs to be extended.

Rare decays of b quarks, one of the heaviest quarks that form matter, are very suppressed in the SM. They are very sensitive to the existence of new particles or forces, generically referred to as New Physics (NP), that could change the properties of these decays. Recent measurements of the properties of rare b-quark decays to an s quark and two leptons (b→sℓ+ℓ−) show intriguing deviations with respect to the SM predictions that could be a hint of NP. In this project, we will explore the related and even more suppressed b-quark decays to a d quark and two leptons (b→dℓ+ℓ−), which are so far poorly known and even more sensitive to the existence of NP. Precise measurements of b→dℓ+ℓ− decays have the potential of shedding light on the existence of NP and its type, in combination with the existing measurements in b→sℓ+ℓ− processes.

For this purpose, we will develop innovative analysis tools and exploit the uniquely large sample of b hadrons from the LHCb experiment. The CLIMB project will address two specific questions:

1. Are the deviations observed in b→sℓ+ℓ− decays also present in b→dℓ+ℓ− transitions? This will be addressed by measuring differential decay probabilities and lepton universality ratios in b→dℓ+ℓ− decays for the first time. In the SM, b→sℓ+ℓ− and b→dℓ+ℓ− decays are related by the so-called quark-mixing matrix, the hierarchy of which is not fully understood. Some NP models aim to provide an explanation for the structure observed in nature. Knowing the properties of b→dℓ+ℓ− decays precisely is a critical input in this endeavour.

2. Are there new sources of matter-antimatter asymmetry beyond the SM in b→dℓ+ℓ− processes? This will be answered by measuring matter-antimatter asymmetries in b→dℓ+ℓ− decays with unprecedented precision, providing very strong constraints to NP models predicting an enhanced amount.

The main challenge of this programme lies in the study of very suppressed decays. Innovative reconstruction and selection techniques will be developed to access them.

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

During the the first two years, the project has made strong progress toward its main goals, even though some tasks are still ongoing and a few challenges have caused delays. The early results are encouraging and support the overall objectives.

The team is working to improve the measurements of two b→dℓ+ℓ− decays. Using the large datasets collected by the LHCb experiment in Run 2 (2015—2018) and Run 3 (2024), we have: prepared data samples and developed advanced selection methods using machine learning; focused on two specific decays: one involving a b meson and another involving a b baryon; made early estimates of how often these decays happen and how they behave, using simulations and data fitting techniques.

For the b-baryon decay, the analysis of the Run 2 dataset is very advanced, and we expect to observe many more events than in the Run 1 studies. This will allow for precise measurements of how often the decay occurs and whether it behaves differently for matter and antimatter. For the b-hadron decay, early results from the 2024 data suggest that the final analysis will be five times more precise than the previous studies with Run 1 data, meeting the project’s goal.

The team also aims to observe b→dℓ+ℓ− decays involving electrons for the first time and to test whether electrons and muons behave the same way in these decays—a principle known as Lepton Flavour Universality.

Progress includes preparing and calibrating the new Run 3 data, especially for detecting electrons; improving how the detector identifies electrons, using machine learning to recover lost energy and improve accuracy; analyzing a well-known decay involving electrons to test these methods. Early results are close to expected values, and further improvements are underway. We have also started studying rare decays of b hadrons involving electrons and have seen promising signs that we will have good chances of observing b→dℓ+ℓ− decays to electrons with the Run 3 dataset.

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 CLIMB project will dramatically change our knowledge of b→dℓ+ℓ− transitions with the observation of two b→de+e− decays, the measurement of several new observables highly sensitive to the existence of NP and a large improvement in the precision of the observables previously measured. The measurements will be performed in regions of the di-lepton invariant mass to enhance the sensitivity to NP, since different regions are sensitive to different NP effects.

The work on electron reconstruction and calibration will strongly benefit any further measurements involving electrons in the final state, not only in the processes of interest to the project but also in any other decay mode. Moreover, the knowledge acquired in this area will lay the foundations for the design of the electron reconstruction for the new ECAL detector for the future upgrade of the LHCb experiment.

CLIMB will also train a generation of researchers, who will acquire a deep understanding of the physics of rare b-hadron decays and of state-of-the-art analysis tools and reconstruction and calibration algorithms through their participation in the research programme. Moreover, the results and tools developed in CLIMB will be presented in international conferences and seminars to make them available to other research groups, contributing to the advancement of the European particle physics research programme.

In summary, CLIMB will provide unique tests of the SM and crucial input to strongly constrain and distinguish NP models through the precise measurement of b→dℓ+ℓ− decays. This will take the knowledge of rare b decays, currently focused on the more accessible b→sℓ+ℓ− transitions, one step forward. If a discovery was to be made, it would shape the future direction of Particle Physics and take us closer to the understanding of the building blocks of our Universe.