Searching for Charged Lepton Flavour Violation at LHCb and Long-Lived Particles with CODEX-b

Swallow, Paul ORCID: 0000-0003-2751-8515 (2024). Searching for Charged Lepton Flavour Violation at LHCb and Long-Lived Particles with CODEX-b. University of Birmingham. Ph.D.

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Abstract

The CODEX-b detector is an RPC-based experiment proposed to search for long-lived particles that may be produced in $\proton\proton$ collisions at the LHC. The first comprehensive investigation on the impact of misalignments of CODEX-b are presented alongside reports of hardware quality assurance tests of the readout electronics for this detector.

The mechanical precision <5 mm when constructing the RPC modules will be more than sufficient to ensure track reconstruction is unaffected by misalignments. The first major quality control process for the evaluation of CODEX-b’s RPC readout boards and its results are also presented, with more than 95% of the 440 tested boards validated for installation.

The first experimental upper limits of the branching fraction of the rare decay, \Lambda_b^0\rightarrow\Lambda e^\mp \mu^\pm, are presented based on analysis of data taken by the LHCb experiment between 2011 and 2018 (Run 1 & 2), corresponding to an integrated luminosity of 9 fb^{-1}.

Although the dataset remains blinded, estimated upper limits of the branching fraction of \Lambda_b^0\rightarrow\Lambda e^\mp \mu^\pm assuming no signal is present were obtained using a background-only proxy dataset derived from the data sidebands. This is evaluated for two scenarios, one with higher efficiency and one with lower background contributions (including a \Lambda_c decay veto), leading to results of

B(\Lambda_b^0\rightarrow\Lambda e^\mp \mu^\pm)_{std}\buildrel{\sim}\under< 2.09\times10^{-7} @ 95% CL,
B(\Lambda_b^0\rightarrow\Lambda e^\mp \mu^\pm)_{veto}\buildrel{\sim}\under< 6.45\times10^{-8} @ 95% CL,

respectively. These limits are comparable to that from a theoretical analysis of a proposed model-independent limit.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):