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Insertion of STC into TRT at the Department of Physics, Oxford
Credit: CERN

Mr Luke Scantlebury-Smead

Graduate student

Research theme

  • Fundamental particles and interactions

Sub department

  • Particle Physics

Research groups

  • LHCb
luke.scantlebury-smead@physics.ox.ac.uk
Telephone: 73382
Denys Wilkinson Building, room 653
  • About

Lepton Flavour Universality Violation

Standard Model predictions of the ratios between the branching ratios of B-meson semitauonic decays relative to decays involving lighter lepton families such as

\(\mathcal{R}(D^{(*)}) \equiv \frac{\mathcal{B}(B \rightarrow D^{(*)} \tau^\pm \nu_\tau)}{\mathcal{B}(B \rightarrow D^{(*)} \mu^\pm \nu_\mu )}\)

are known to a percent level uncertainty. A key objective in flavour physics is to measure these quantities experimentally to the same level of precision since they are particularly sensitive to beyond the Standard Model theories with flavour-dependent couplings such as an extended Higgs sector or leptoquarks.

A two plot of RD and RD* results compared to the SM predictions for these values.

Measurements of  \(\mathcal{R}(D) \) and  \(\mathcal{R}(D^*) \) performed at Belle, BaBar, and LHCb have produced results that are in tension with the Standard Model to up to 4\(\sigma\). The plot above shows these various measurements along with their average and the average of Standard Model predictions. These experimental results show consistency across different experimental apparatus and analysis techniques, including different final states, but the average shows a tension with the Standard Model of over 3\(\sigma\).

I am analysing the angular structure of the  \(B^0 \rightarrow D^{*-} \tau^+ \nu_\tau\) decay, using angular observables to search for New Physics contributions to their values. This will add another data point to the plot above but will be introducing the measurement of 12 angular coefficients, previously unmeasured for this decay.

This decay is particularly challenging to select as it has very high track multiplicities which result in large combinatorial backgrounds. Also, the lepton flight results in similar topologies to events of the type  \(B^0 \rightarrow D^{*-} D_s (X)\), where \(X\) includes one or more particles produced in the decay that may not be reconstructed. These events are a significant background to the analysis. Since neutrinos are produced in both the tau production and decay, the tau and B momentum are not fully reconstructed. To reduce these backgrounds, multivariate analysis is used to separate the signal from the background.

Research interests

Particle Physics, Lepton Universality, LHCb

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