Testing leptOn Flavour Universality in excited strange mesons

Project Information

TOFU

Grant agreement ID: 896384

DOI

10.3030/896384

Project terminated on 30 September 2021

EC signature date 26 February 2020

Start date 1 May 2020

End date 30 April 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 175 572,48

EU contribution

€ 175 572,48

Coordinated by

STICHTING NEDERLANDSE WETENSCHAPPELIJK ONDERZOEK INSTITUTEN
Netherlands

Periodic Reporting for period 1 - TOFU (Testing leptOn Flavour Universality in excited strange mesons)

Reporting period: 2020-05-01 to 2022-04-30

Summary of the context and overall objectives of the project

This project revolves around testing one of the key assumptions of the Standard Model which describes all known particles and their interactions: lepton flavour universality. This assumption states that leptons (electrons and their heavier siblings, muons and taus) all have the exact same couplings to the forces of nature. However, recent measurements of three different experiments have shown hints that this assumption may not hold, which has major implications to the field of particle physics: it could even lead to the discovery of a new force of nature.

For this project, I have been working on a measurement to study decays of strange B mesons (Bs) into excited strange D mesons (Ds*). In particular, I have compared of such decays with muons to those with taus in a ratio called R(Ds*). Compared to the previously performed analyses, this one has the advantage of having fewer backgrounds from excited states, which form a major uncertainty on the existing measurements.

Another part of the project concerns the development of a validation framework to test the reconstruction software of the upgraded LHCb detector. In the upgraded detector, which is currently being commissioned, the hardware trigger is removed and replaced by a software trigger. In addition, the alignment and calibration of the detector is done in real-time, such that more stringent requirements can be placed on the data selection. Hence, the software must be very well tested before commissioning; otherwise valuable data will be lost.

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

For the software validation framework, I have developed a dashboard showing daily tests of reconstruction software to track the development of software and its impact on reconstruction quantities. This dashboard is monitored frequently and will also be used to select the most relevant quantities to monitor during data taking. I have presented this dashboard and other advances in data quality monitoring at the LHCP conference in 2021. Moreover, I have coordinated the writing of the trigger lines that are necessary to select interesting data for future lepton flavour universality tests and other analyses with semileptonic decays.

Before making the measurement of the ratio R(Ds*), I was working on a measurement of the decay distribution of Bs -> Ds* mu nu decays, which is a necessary input for this measurement, and also a proof of concept that we can make this measurement with reconstruction of the soft photon in the Ds*->Ds gamma decay. In addition, it provides input for theorists to make predictions of R(Ds*). This measurement was ongoing at the start of my Marie Sklodowksa Curie project, and published during the first year. It yielded multiple citations from theorists who indeed made the calculations necessary as input for this project.

The measurement of the ratio R(Ds*) itself is in an advanced state. Most of the backgrounds have been studied and included in the measurement, and the data selection to reduce these backgrounds has been finalised. A framework to fit the data is set up, and needs further validation. Most of the data calibration is done, and remaining studies are ongoing. All of these will be finalised in 2022, after which the analysis will undergo internal review before its results become publicly available.

I have disseminated the outcome of lepton flavour universality measurements to a non-expert audience. I have given guest lectures at university and high schools, as well as a talk on lepton flavour universality to a general audience at the Nikhef open day.

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)

Continuous integration tests for LHCb’s reconstruction software are now expanded such that the reconstruction runs on a daily basis over 10 different data samples and produce a few hundred histograms on a webpage, organised per topic. Different software builds can be easily compared, by overlaying them, or showing the differences or ratios of histograms produced with these builds. An example of this can be seen in the attached images, where three different decay types are shown to exaggerate the visible differences. The algorithms to create the histograms are updated regularly and will also be used to monitor the data quality during data taking.

The measurement of the decay rate of Bs -> Ds* tau nu decays, as well as the corresponding ratio R(Ds*), can only be performed with the LHCb experiment. It allows to understand the backgrounds from excited states better than with the existing measurements alone. Combined with the large data set collected with the upgraded detector, high-precision measurements of lepton flavour universality are possible.

The research described in this project proposal concerns the understanding of the fundamental building blocks of the universe. It is motivated by the human desire to understand the world around them, and has no direct socio-economic impact, nor wider societal implications. Nevertheless, the outcome of this project may have a major impact on the field of particle physics. The violation of lepton flavour could possibly even lead to the discovery of a new force of nature. The societal impact is reached by giving outreach talks to a general audience and especially to kids.

Unfolded normalised differential decay rate for the Bs->Ds* mu nu decay, overlayed with fits

Screenshot of the reconstruction dashboard, showing track quantities for three different decay types