Search for charged-lepton flavour violation in top-quark interactions using eµ final states with the ATLAS detector

Stampekis, Alexios ORCID: 0000-0002-7666-7544 (2024). Search for charged-lepton flavour violation in top-quark interactions using eµ final states with the ATLAS detector. University of Birmingham. Ph.D.

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Abstract

A search for charged-lepton-flavour violation in top-quark production and decay is presented.
The data analysed corresponds to 139 inverse femtobarns of proton-proton collisions at a centre-of-mass energy of 13 TeV delivered by the LHC and recorded by the ATLAS detector.
Two processes are studied; the first is the production of a semileptonically decaying single top quark in association with a pair of opposite-sign different-flavour leptons, the second is the production of a top-quark pair, where one top quark decays into a pair of opposite-sign different-flavour leptons and a light up-type quark, while the other decays into a leptonically decaying W boson and a b-quark according to the Standard Model.
The signal signature requires the presence of three leptons defined as electrons or muons,
and is thus characterised by the presence of one b-jet, up to one light-quark jet, and either two electrons and one muon, or one electron and two muons.
In the absence of a signal, an observed (expected) upper limit on the branching ratio of
BR(t -> eμq) < 1.6 (1.7) * 10^{-7} is set at the 95 % confidence level. An EFT interpretation is also discussed.

During Run 3 of LHC,
the elevated luminosity and pileup conditions call for advanced electronic systems to efficiently
trigger the physics objects.
Among the new processing units,
of high importance are the Feature Extractor (FEX) modules, responsible for identifying leptons and jets.
For the commissioning of these modules,
the design, testing and commissioning of a separate module, the FEX Test Module was needed.
In this thesis, the process of testing and commissioning this FPGA-based digital system is discussed.
Tests concerning the interpretation of incoming electrical signals as localised energy deposits in the ATLAS detector
by the electron FEX module firmware are also discussed briefly.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):