Master Thesis defense by Daniel Hans Bruk

Title: B-Physics and Gradient Boosted Decision Trees


This thesis is part of the Rare decays subgroup of the ATLAS B-Physics and Light States Working Group at CERN who aims to measure the RK⇤0 double ratio, which through Lepton Flavor Universality (LFU) can indicate if there are physics Beyond the Standard Model present in the B0 ! K⇤0``-decay. The work of the thesis is related to the electron channel where `` is an Electron-proton pair with the focus of separating the two species of signal: B0 and its antiparticle: B0 from the background. This is achieved using Gradient Boosting Decision Trees (GBDTs) trained on a mix of Monte Carlo-generated data and ATLAS data.

After hyperparameter optimization and feature engineering, the GBDT model ended up with a total B0 signal efficiency of 72 ± 3% in Monte Carlo and B0-mass sidebands. As the B0-mass region in the low q2-bin (q2 2 [1.1, 6.0]GeV2/c4) is blinded at the current stage of the analysis, the signal yield for calculating the RK⇤0 -ratio is extracted using a c2-fit in the high q2-bin (q2 2 [6.0, 11.0]GeV2/c4). The fit used is a composite probability function consisting of two Gaussian probability distributions and a Bukin distribution, giving a signal yield of NSig(B0) = 1853 ± (45) on Period K, Run 2 ATLAS data with a signal significance of 20.0 ± 0.4.

This shows GBDTs are a viable approach in separating B0 and B0 from the background, and the hope is that the contribution of this thesis will benefit the Rare decays group in the measurement of the RK⇤0 -ratio.