Searching for charged lepton flavour violation in top quark Interactions with the ATLAS detector

George, William ORCID: 0000-0003-3674-7475 (2024). Searching for charged lepton flavour violation in top quark Interactions with the ATLAS detector. University of Birmingham. Ph.D.

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Abstract

A search for charged lepton flavour violating (CLFV) $\tau\mu tq$ interactions is presented, where $u_{k} = \{ u, c\}$ , using the ATLAS Run 2 dataset at $\sqrt{s}=$ 13.0 TeV. Observation of such a process, which is strictly prohibited in the Standard Model (SM) of particle physics, would be a clear indicator of new physics. The search targets $\tau\mu tq$ interactions in both single top quark production and $t\bar{t}$ decay, considering events with two muons, a hadronically-decaying tau lepton and at least one jet, of which exactly one must be $b$ -tagged. In absence of a significant excess above the SM background, an exclusion limit is placed on the CLFV branching ratio $B(t \rightarrow \mu\tau q)<8.7 \times 10^{-7}$ at 95\% confidence level (CL). Constraints are placed on SM effective field theory Wilson coefficients ranging from $c_{\ell equ}^{3(2313)}/\Lambda^{2} < 0.10~\mathrm{TeV}^{-2}$ for $\mu\tau ut$ to $c_{\ell equ}^{1(2323)}/\Lambda^{2} < 1.8~\mathrm{TeV}^{-2}$ for $\mu\tau ct$ (95\% CL). The $gu_{k}\to\Ptop\mu\tau$ process could be mediated by a scalar leptoquark. Assuming a fixed hierarchy of couplings to lepton-quark pairs, exclusion limits are placed on leptoquark coupling strengths $1.12 < \lambda < 3.29$ for masses between 0.5 and 2.0 TeV. The sensitivity of the search is statistically limited and would benefit greatly from the inclusion of future ATLAS datasets. The new ATLAS ITk detector will be crucial to recording high quality data after Run 3; quality assurance measurements are presented which validate the technologies employed by the ITk strip sensors.