Abstract
This thesis details work completed in the search for physics beyond the Standard Model. Thefirst half of the thesis focuses on the upcoming Phase II upgrade for the crystal electromagnetic
calorimeter of the Compact Muon Solenoid. Test beam data was analysed using available
Phase II prototype electronics and this study found that, in the absence of a magnetic field,
a cluster of a 3×3 crystal matrix provides better energy resolution than a single crystal or a
3×5 crystal matrix. An energy correction method to recover energy when an electron impacts
on the side of a crystal was also tested and found to return the resolution to close to that for
electrons impacting on the centre of the crystal. Algorithms that will cluster energy and reject
anomalous signals for the Phase II upgrade were also tested in software and on hardware using
an existing board that was designed for the L1 trigger. The clustering algorithm was tested on
1000 events in hardware with no discrepancies, and both anomalous signal rejection algorithms
were found to have the required 99.9% efficiency from 2 or 10 GeV, depending on the algorithm.
The second half of this thesis focuses on the ongoing search for a non-resonant Z’ boson
decaying to a dilepton pair in association with b-tagged jets, using 137 fb−1 of data collected
by the Compact Muon Solenoid experiment, as well as a test of lepton flavour universality. Two
models are considered for this search, both based on contact interactions. One considers the
interaction between two b quarks and two leptons, where the b quarks can be in the initial
and/or final state, and the second model considers the transition b → sℓℓ. The dilepton mass
distribution of both of these models depends on the energy scale of the new physics.
The dielectron part of this search requires the use of a dedicated high-energy electron/positron
identification criteria which was studied in detail to ensure that its behaviour was fully understood. For 2018, the efficiency of this dedicated criteria was found to be 86.94 ± 0.05% in data,
compared to an efficiency of 89.32 ± 0.07% in the CMS detector simulation. The scale factor
of 0.973 ± 0.001 (stat) ± 0.004 (sys) gives the ratio between the data and simulation and is
used as a correction in the Z
′ boson search. The flavour ratio is measured which compares the
differential cross sections as a function of dilepton mass for a quark-antiquark pair decaying
to a dimuon or a dielectron pair. For a data sample with no b-tagged jets, the flavour ratio
is in agreement with unity, which is consistent with lepton flavour universality as expected.
The predicted lepton flavour ratio for a sample with 1 b-tagged jet shows that for some energy
scales of new physics, there will be a deviation from unity at high mass. Once finished by the
Z
′ group, this analysis is expected to probe higher energy scale ranges than previous analyses.
The results are expected to either provide hints of physics beyond the Standard Model or allow
more stringent limits on new physics to be set.
| Date of Award | 3 Oct 2023 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Helen F Heath (Supervisor) & David Petyt (Supervisor) |