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Particle physics experiments at CERN, Geneva.
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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Nov 2019)
The PIENU experiment at TRIUMF aims to measure the pion decay branching ratio, defined as the relative rate of decay of pions into electrons over muons including associated neutrinos and radiative components (denoted R_π) to a precision level of O(0.1%). This Standard Model (SM) observable provides a sensitive test of lepton universality, where weak coupling strengths are assumed to be equal for all leptons (g=g_e=g_μ= g_τ). Comparing the measured experimental (R_π^exp) and calculated SM (R_π^SM) ratios, the ratio of the coupling constants can be extracted and compared with the SM expectation g_e/g_μ=1 as follows g_e /g_μ = (R_π^exp/R^π^SM )¹/².The current theoretical calculation of the SM prediction R_π^SM=1.2352±0.0002)×10-⁴ with a precision of 0.016% is more precise than the measurements of previous generation experiments by a factor of 30; thus, there is scope for significant improvement. If the measurement is consistent with the SM, new constraints could be set on new physics scenarios for SM extensions, such as R-parity-violating super-symmetry, leptoquarks, and heavy neutrinos lighter than the pion. Most remarkably, a deviation from the SM could result from a new pseudo-scalar interaction with an energy scale of up to O(1000TeV) which would enhance the branching ratio by O(0.1%). In some instances, these constraints can far exceed the reach of direct searches at colliders.Between 2009 and 2012 around 6.5 million π+→e+ν_e events were gathered. The analysis of a subset of the 2010 data with 0.4 million events was published in 2015, giving R_π^exp=(1.2344±0.0023(stat.)±0.0019(syst.))×10-⁴, with a precision of 0.24%. This is in agreement with the SM, representing a 0.12% measurement of lepton universality at g_e/g_μ=0.9996±0.0012. The analysis presented in this thesis is blinded but includes the highest quality data portion available, around 3 million π⁺→e⁺ν_e events. For this work, major experimental systematic problems have been solved allowing for increased precision up to 0.12% for R_π^exp and up to 0.06% for lepton universality.
The pion decay branching ratio is an importantobservable in the Standard Model of particle physics. The value of thebranching ratio has been calculated within the Standard Model tobe (1.2352 ± 0.0002) ×10^−4. The PIENU experiment at TRIUMFaims to measure this quantity to a precision of
The pion branching ratio (R^π = [formula omitted] ) is an auspicious observable for a test of the standard model of particle physics (SM). Rπ has been calculated within this framework with high precision because the strong interaction dynamics cancel out in the ratio and the structure dependence only appears through electroweak corrections. Since the discovery of the electronic pion decay in 1958, Rπ was measured with increasing precision and confirmed the SM value of RπSM = 1.2352(2) x 10⁻⁴. However, the current experimental precision is 20 times worse than the theoretical one leaving a large window for potential new physics at “high-mass" scales (up to ∽1000 TeV).The PIENU experiment aims at measuring Rπ with an improved precision by a factor larger than 5 over the previous experiment at TRIUMF (Rπexp = (1.2265 ± 0:0056) x 10⁻⁴) in order to confront the theoretical prediction at the 0.1% level. The result presented in this thesis focuses on a fraction of the data taken since the beginning of physics data taking in 2009. A blind analysis has been implemented in order to avoid a human bias. With this set of data, the procedure is established for the final analysis. An improvement by a factor 1.17, dominated by statistical uncertainty, has been reached in the branching ratio precision. If added to the current Particle Data Group value, the result of this analysis reduces the uncertainty on the branching ratio by ∽25%.
No abstract available.
Master's Student Supervision (2010 - 2018)
The branching ratio of pions decaying to positrons and muons R = (π→eυ + π→eυγ)/(π→μυ + π→μυγ) has been calculated with very high precision in the Standard Model of particle physics. So far, the theoretical value of R = 1.2352(1) x 10-⁴ is 40 times more precise than the current experimental value of R = 1.230(4) x 10-⁴. To test this variable with respect to deviations from the Standard Model, the experimental precision needs to be improved, which is why the PIENU experiment aims at a precision of less than 10-³, i.e. an improvement of an order of magnitude over the current precision. At this level, mass scales ∼ TeV/c² can be probed for evidence of new pseudo-scalar interactions. The data collected with the experimental setup also allows for a search of sterile neutrinos. When determining the branching ratio, various systematic corrections are applied. The largest among these is due to electro-magnetic shower leakage out of the calorimeters and radiative decays. It was calculated to be (2.25 ± 0.06) % in this thesis.In the second part of the thesis, an experiment on the direct radiative capture of muons in zirconium is described. One theoretical extension to the Standard Model involves a new light and weaklyinteracting particle in the muon sector which does not conserve parity. This can be studied experimentally with polarized muons that undergo the direct radiative capture into the 2S state of a medium mass target nucleus. During this capture, longitudinal muon polarization is conserved and the muons instantly undergo the 2S-1S transitionemitting a second photon. Studying the angular distribution of this second photon indicates whether or not the process is parity violating, which would manifest physics beyond the Standard Model. The direct radiative capture of a muon into an atom in the 1S or 2S state has not been observed yet. Therefore, data was taken in 2012 to studythe radiative capture of muons in zirconium. The analysis method of this data set is described with a blind analysis technique.
Electric field calculations and ionization signal simulations in a liquid xenon detector for Positron Emission Tomography have been performed. The electric field was calculated using Opera 3D, a Finite Element Method application software. The uniformity of the electric field inside of the detector was evaluated by calculating the deviation of drifting electrons under the applied electric field. The ionization signals of the detector have been simulated. The comparison between simulation results and measurement signals was made.
The capabilities and system performance of a high-resolution micro-PET system based on liquid xenon have been studied. Monte-Carlo simulations of scintillation events within a single sector of a twelve-sector prototype have been performed, and the results have been analyzed using Neural Network algorithms. The ability of the system to distinguish interaction points using scintillation information has been analyzed and presented. Monte-Carlo simulations of a full scale prototype have also been performed using NEMA standard mouse and rat phantoms. A novel scatter suppression scheme based on weighted Line of Response data is presented, and the effects on scatter fraction and background noise are analyzed.