Periodic Reporting for period 1 - B2DTauNu (Model-independent angular analysis of semileptonic B meson decays involving tau leptons) Reporting period: 2020-10-01 to 2022-09-30 Summary of the context and overall objectives of the project The overarching goal of particle physics research is to understand the nature of fundamental particles and their interactions. All of the visible matter in our universe, ranging from tiny bacteria all the way up to stars and galaxies, is formed from the finite set of atoms appearing in the periodic table. Atoms themselves are composed from an even smaller set of fundamental particles, namely the electron, the up quark, and the down quark, all of which are held together by force-carrying particles such as photons and gluons. In the earliest moments of our universe immediately after the Big Bang, quarks and leptons from two heavier families were also present in abundance, as were their antimatter partners. Why our universe consists of thee particle generations, and what happened to all of the antimatter present in our early universe, remain central questions in particle physics today. The field of flavour physics, within which this project falls, tries to address these questions and more by studying the properties of quarks and leptons in all three families. By studying the interactions of different quarks and leptons, flavour physicists hope to achieve a complete understanding of the Standard Model (SM), our most successful theory to date for explaining the universe at tiny distance scales. In attempting to pin down the SM, chinks in its considerable armoury could also appear. Indeed, hints of deviations from the SM, which may indicate the presence of new particles and forces, have previously been observed in past experiments and at the CERN LHCb experiment. This project aims to extend our understanding of these deviations, which are collectively known as “flavour anomalies”, via an angular analysis of beauty quarks decaying to a charm quark plus a tau lepton and a neutrino in LHCb data.The b -> c tau nu process is understudied historically, given the relatively small numbers of beauty quarks produced in previous experiments and the difficultly with which tau leptons can be identified in experimental data. At the LHCb experiment, however, trillions of beauty quarks have thus far been produced during Run 1 and 2, and great advances have been made in the reconstruction and identification of beauty hadron decays to tau final states. Leveraging these developments, this project makes use of the full LHCb Run 1 + 2 dataset to study B0 -> D* tau nu decays, in order to measure a set of 13 physical observables which further describe the nature of the b -> c tau nu quark transition. These observables will help theoretical physicists interrogate the SM with additional rigour, in order to determine whether the flavour anomalies are genuine hallmarks of New Physics or simply a result of statistical fluctuations or systematic experimental issues. For society as a whole, the project thus contributes considerably to our fundamental knowledge of the universe and the behaviour of its smallest constituents. 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 has been undertaken using LHCb data collected between 2011 and 2018, in addition to Monte Carlo (MC) simulated samples of B0 -> D* tau nu signal decays and a wide variety of background processes. The data and MC samples have been processed in an identical fashion, in order to reconstruct candidate signal decays. Due to the nature of B0 -> D* tau nu decays, a great many similar-looking processes contribute to the total LHCb dataset, and the background level must be reduced prior to a dedicated measurement of the signal properties. To this end, a set of machine learning binary classifiers are used, which help to identify which candidates in data are most likely to be B0 -> D* tau nu decays. The classifiers employed used state of the art XGBoost algorithms deployed at scale across several terabytes of data and simulation. In addition to other selection requirements, such as the flight distance of tau leptons from their initial production point, the classifiers help reduce the total background level in LHCb data by multiple orders of magnitude.Following the candidate selection process, a set of detailed studies are performed in order to align the MC simulation with data. These control studies are performed with subsets of data and simulation that can be directly compared, and help to identify aspects of the simulation which do not sufficiently agree with the data. The discrepancies found are corrected using a statistical method known as cubic spline fitting, which provides a set of functions that can be applied to all simulated samples used in the analysis to improve the agreement with data. This work is crucial to any successful analysis of B0 -> D* tau nu decays, and has been performed in a more rigorous fashion than in previous experimental measurements.After improving the agreement between data and simulation, an angular analysis of the B0 -> D* tau nu decay is performed. The purpose of this analysis is to measure a set of 12 angular coefficients, which together with the total decay rate fully characterise the B0 -> D* tau nu process. The angular analysis is performed using a maximum-likelihood fit method, where the LHCb data is described as a sum of the signal and several background contributions each modelled using MC. A significant complication in this approach is the fact that the neutrinos produced in the B0 -> D* tau nu decay cannot be detected in LHCb: the result is that the decay angles measured experimentally are significantly distorted compared to their true forms. However, a novel histogram-based fitting method developed by the project leader has enabled an angular analysis to be performed regardless of the missing neutrino information.The main results of the project are derived directly from the histogram-based angular fit, and include the 12 angular coefficients of the B0 -> D* tau nu decay and a measurement of the total decay rate via an observable known as R(D*). All of the observables are measured with both statistical and systematic uncertainties, which together reflect the limited precision with which measurements can be made given the current LHCb dataset size, as well as finite knowledge of the MC simulations used and backgrounds present in the data. The 12 angular coefficient results are the first of their kind, while the measurement of R(D*) builds upon similar measurements made previously at LHCb. 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) This project delivers a world-first angular analysis of the B0 -> D* tau nu decay, and the measurement of 13 physical observables related to the b -> c tau nu quark transition. This process is one of the most poorly understood in flavour physics, and as such the project results move the field well beyond the current state of the art. The 13 observable measurements will appear in a journal publication after peer review, and will be made available open access for use within the theoretical flavour physics community. These new measurements will provide a rich set of additional information in the comparison of experimental results and SM predictions, helping to further clarify the flavour anomalies picture. The methods applied in this project also demonstrate the feasibility of semileptonic angular analysis in B hadron decays, which will revolutionise future efforts to study such processes both at LHCb and elsewhere. Related documents signal_topo.png
