Modelling uncertainties for measurements of the H → γγ Channel with the ATLAS Detector at the LHC
dc.contributor.advisor
Clark, Philip
dc.contributor.advisor
Leonidopoulos, Christos
dc.contributor.advisor
Mijovic, Liza
dc.contributor.author
Heath, Matthew Peter
dc.contributor.sponsor
Science and Technology Facilities Council (STFC)
en
dc.date.accessioned
2022-03-22T11:03:52Z
dc.date.available
2022-03-22T11:03:52Z
dc.date.issued
2022-03-21
dc.description.abstract
The Higgs boson to diphoton (H → γγ) branching ratio is only 0.227 %, but this
final state has yielded some of the most precise measurements of the particle. As
measurements of the Higgs boson become increasingly precise, greater import is
placed on the factors that constitute the uncertainty. Reducing the effects of these
uncertainties requires an understanding of their causes. The research presented
in this thesis aims to illuminate how uncertainties on simulation modelling are
determined and proffers novel techniques in deriving them.
The upgrade of the FastCaloSim tool is described, used for simulating events in
the ATLAS calorimeter at a rate far exceeding the nominal detector simulation,
Geant4. The integration of a method that allows the toolbox to emulate the
accordion geometry of the liquid argon calorimeters is detailed. This tool allows
for the production of larger samples while using significantly fewer computing
resources.
A measurement of the total Higgs boson production cross-section multiplied
by the diphoton branching ratio (σ × Bγγ) is presented, where this value was
determined to be (σ × Bγγ)obs = 127 ± 7 (stat.) ± 7 (syst.) fb, within agreement
with the Standard Model prediction. The signal and background shape modelling
is described, and the contribution of the background modelling uncertainty to the
total uncertainty ranges from 18–2.4 %, depending on the Higgs boson production
mechanism.
A method for estimating the number of events in a Monte Carlo background
sample required to model the shape is detailed. It was found that the size of
the nominal γγ background events sample required a multiplicative increase by
a factor of 3.60 to adequately model the background with a confidence level of
68 %, or a factor of 7.20 for a confidence level of 95 %. Based on this estimate,
0.5 billion additional simulated events were produced, substantially reducing the
background modelling uncertainty.
A technique is detailed for emulating the effects of Monte Carlo event generator
differences using multivariate reweighting. The technique is used to estimate the
event generator uncertainty on the signal modelling of tHqb events, improving the
reliability of estimating the tHqb production cross-section. Then this multivariate
reweighting technique is used to estimate the generator modelling uncertainties
on background V γγ samples for the first time. The estimated uncertainties were
found to be covered by the currently assumed background modelling uncertainty.
en
dc.identifier.uri
https://hdl.handle.net/1842/38781
dc.identifier.uri
http://dx.doi.org/10.7488/era/2035
dc.language.iso
en
en
dc.publisher
The University of Edinburgh
en
dc.subject
Large Hadron Collider
en
dc.subject
ATLAS
en
dc.subject
Higgs boson
en
dc.subject
simulation production
en
dc.title
Modelling uncertainties for measurements of the H → γγ Channel with the ATLAS Detector at the LHC
en
dc.type
Thesis or Dissertation
en
dc.type.qualificationlevel
Doctoral
en
dc.type.qualificationname
PhD Doctor of Philosophy
en
Files
Original bundle
1 - 1 of 1
- Name:
- Heath2022.pdf
- Size:
- 14.01 MB
- Format:
- Adobe Portable Document Format
- Description:
This item appears in the following Collection(s)