# Bryn Roberts

I am a third year PhD student working on the ATLAS experiment at CERN. Along with third year PhD student Vangelis Vladimirov I am investigating measuring the $V_{cb}$ element of the CKM matrix in $t\bar{t}$ decays. I am currently on long term attachment in CERN until late March 2021.

#### Research

The $V_{cb}$ element of the CKM matrix has previously only been measured in the decays of B mesons where the decaying W boson is off-shell. In comparison $t\bar{t}$ decays are higher energy and so the W bosons are on-shell, this allows the measurement of $V_{cb}$ at a higher energy. There is also a tension between measurements made in exclusive and inclusive B meson decays and so making this measurement using a new method should offer insight into this tension. Since this measurement has never been made before it is also possible that there are surprises due to new physics contributions. This measurement will be made using the ATLAS detector at the LHC (Large Hadron Collider) using Run 2 data.

Further detail on the theory of this analysis can found in this paper by Paul and Van.

My work concentrates on looking for ways in which the statistics of the measurement could be improved, this currently consists of trying to use events with 5 or 6 jets rather than the 4 jets we expect in the simplest case.

#### Flavour Tagging

In addition to my research I am currently working on my qualification task which when completed will mean I am added to the ATLAS author list. Flavour tagging is the process by which we identify which flavour of quark caused a jet that we detect in our detector. We need to know how efficient this tagging is and how often the tagging mis-identifies a jet as a different flavour. This is done through a calibration process which also ensures consistency between the simulated data we use to prepare to make a measurement and the real data. This calibration process currently takes time and needs to be run manually through many different steps. My aim is to automate this process for b-jets such that the required parameters are set-up and then the entire calibration workflow runs without requiring any further input or action.

#### Contact Details

Warwick Office: P448a

Warwick Tel: +44 (0) 24761 51769

CERN Office: B591/R034

#### Supervisor

Prof. Paul Harrison