Christopher Hearty


Research Classification

Research Interests

Experimental Particle Physics
e+e- collider
Physics beyond the Standard Model
Dark sector
dark matter

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Research Methodology

SuperKEKB e+e- collider at the KEK laboratory in Japan


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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Search for axion like particles with the BaBar detector and photon hadron separation using Zernike moments at Belle II (2021)

Even though the Standard Model of particle physics is a very successful model, we know that it is incomplete. For example, it does not explain why the world we see around us is made of matter rather than anti-matter and it does not incorporate the force of gravity. One extension of the Standard Model is the introduction of Axion Like Particles, ALPs. ALPs appear in string theories and supersymmetry and they might explain some astrophysical anomalies. ALPs can be produced in electron-positron colliders and detected in the specialized detectors built around their interaction point, like the PEP-II collider and the BaBar detector at the Stanford Linear Accelerator Center. This work presents an un-blinded search for an ALP that couples exclusively to photons in 5% of the BaBar data. We search for an excess in the invariant mass distribution of ALP candidates over a smooth background. The results are consistent with the data being composed only of Standard Model background. 90% credible interval upper limits are set on the ALP production cross section and coupling constant. These limits exclude previously unexplored regions of the phase space in the mass range 0.29 GeV/c² to 5 GeV/c². In searches involving photons, it is important to be able to efficiently detect them while rejecting other types of particles. Many high energy particle detectors detect photons in electromagnetic calorimeters that are made up of many cells. A photon interacting with the calorimeter typically leaves a different energy distribution in the cells than some other particle types, hadrons, for example. Discriminating variables for photons, based on Zernike moments, are developed in order to improve the photon identification at Belle II. One of the new variables is found to be the best at identifying photons among all other such variables used at Belle II for photons with energies in the energy range most relevant to e+e− → BB events.

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Cluster Counting in Drift Chambers for Particle Identification and Tracking (2016)

Drift chambers are a type of gaseous ionization detector used in high-energy physics experiments. They can identify charged particles and measure their momentum. When a high-energy charged particle crosses the drift chamber, it ionizes the gas. The liberated electrons drift towards positive-high-voltage wires where an ionization avalanche amplifies the signal. Traditional drift chambers use only the arrival time of the cluster of charge from the closest ionization for tracking, and use only the integral of the whole signal for particle identification. We constructed prototype drift chambers with the ability to resolve the charge cluster signals from individual ionization events. Different algorithms were studied and optimized to best detect the clusters. The improvementsto particle identification were studied using a single-cell prototype detector, while the improvements to particle tracking were studied using a multiple-layer prototype. The prototypes were built in the context of initial work for the now-cancelled SuperB project, but the results apply to any drift chambers used in flavour-factory experiments. The results show that the choice of algorithm is not as critical as properly optimizing the algorithm parameters for the dataset. We find that a smoothing time of a few nanoseconds is optimal. This corresponds to bandwidth of a few hundred megahertz, indicating that gigahertz-bandwidth electronics are not required to make use of this technique. Particle identification performance is quantified by the fraction of real pions correctly identified as pions with at most 10% of real pions mis-identified as muons. In our single-cell prototype, the performance increases from 50% to 60% of pions correctly identified when cluster counting is combined with a traditional truncated-mean charge measurement, compared to the charge measurement alone. Tracking performance is quantified by the single-cell resolution: the uncertainty in measuring the distance of charged particle tracks from a given sense wire. In our multiple-layer prototype, the single-cell tracking resolution using traditional methods is measured to be ~150μm. With cluster counting implemented, the resolution is unchanged, indicating that the additional cluster information is not useful.

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Search for a light Higgs boson with the BABAR detector : with proposals to improve drift chamber aging studies and particle identification (2015)

In the Standard Model of particle physics, the Higgs boson is an elementary particle required to explain the origin of mass. One extension, the next-to-minimal supersymmetric Standard Model, predicts the existence of a GeV/c² scale Higgs boson, denoted A^0. This can be produced at B meson factories such as the BABAR experiment at the Stanford Linear Accelerator Center. We study e⁺e- → Υ(2S)→ π⁺π-Y(1S), Y(1S)→ γA^0, A^0 → gg, ss-bar, cc-bar decays using data collected by the BABAR detector at a centre-of-mass energy of 10.02 GeV/c².The search for A^0 → gg or ss-bar is performed by fully reconstructing the entire decay chain. We search for an excess of events relative to expected backgrounds in the reconstructed A^0 mass spectra. No significant excess is found. We set upper limits on product branching fraction B(Y(1S) → γA^0)xB(A^0 → gg) from 1x10-⁶ to 2x10-² for A^0 masses from 0.50 to 9.00 GeV/c², and B(Y(1S) → γA^0)x B(A^0 → ss-bar) from 5x10-⁶ to 1x10-³ for A^0 masses from 1.50 to 9.00 GeV/c².The search for A^0 → cc-bar is done by reconstructing the dipion transition, the radiative photon, and a D meson as a tag for cc-bar decays. We infer the mass of an A^0 candidate from the recoil of the dipion transition and the radiative photon. No significant signal is seen in the data and we set upper limits on product branching fraction B(Y(1S) → γA^0)xB(A^0 → cc-bar) from 7x10-⁵ to 2x10-³ for A^0 masses from 4.00 to 8.95 GeV/c² and 9.10 to 9.25 GeV/c².These are the first measurements to date and set constraints on the next-to-minimal supersymmetric Standard Model.Proposals to improve drift chamber aging studies and particle identification are included as part of the thesis. We propose to improve aging studies with prototypes that better resemble a full-scale drift chamber. Using TRIUMF beam data, we establish that at a particle identification efficiency of 95%, misidentification rates may be reduced from 2.2% to 1.9% by including likelihood tests and Kolmogorov-Smirnov tests and to 0.5% by including cluster counting.

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A Study of B --> CCBAR Gamma K in the BABAR Experiment (2009)

No abstract available.

Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Signal proxy and radiative Bhabha background studies : contributions to the search for invisibly decaying dark photons at Belle II (2023)

The nature of dark matter is one of the greatest extant mysteries in physics. Many hypotheses have been proposed to explain dark matter, but none have ever been substantiated. Some of these hypotheses propose dark sectors, i.e., entire families of dark matter particles. One such hypothesis is that of the dark photon, a massive, gauge boson which would be the mediator of the dark electromagnetic force. The dark photon would couple feebly to the photon, and thereby be producible through e⁺e⁻ annihilation at a particle collider. Described here are contributions toward a search for the dark photon at Belle II, an electron-positron collider experiment in Japan. This search focuses on invisible decays of the dark photon, with a single initial-state radiation photon serving as the only evidence of the production and decay. Due to the delicate nature of this event signature, the dark photon search requires a carefully constructed set of event requirements and vetoes that remove as much background as possible without removing a large amount of the signal. To further this goal, two control samples were designed. One control sample is of a clean radiative dimuon sample, used as a signal proxy, while the other is of the radiative Bhabha background. These control samples are then used to design vetoes and selection criteria for the search for the dark photon at Belle II, termed the single-photon analysis.

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Search for invisible decays of dark photon at Belle II (2021)

The Standard Model of particle physics displays a good description of the nature of fundamental particles. However, there are some inconsistencies between theory and experimental evidence. One of the observations made through astronomy and cosmology but is not explained by the Standard Model is dark matter. Many theories have been made to explain dark matter, which includes the dark photon particle. The dark photon is a hypothetical particle thought to exist as a mediator between the Standard Model photon and dark matter particles. The dark photons can be produced at an electron-positron collider like the Belle II experiment, located at the KEK particle physics facility. The work presented shows the first attempts of the search for dark photons with invisible decays, where studies on background were conducted for low dark photon masses. This involved understanding the different backgrounds for dark photon events, the geometry and efficiency of the main sub-detectors, and making the final background estimates. In addition, discrepancies between data and Monte Carlo events for the background control sample were found due to detector imperfections, which are also summarised here.

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A Low Gain Fine Mesh Photomultiplier Tube for Pure CsI (2015)

The increased luminosity of the upgraded SuperKEKB accelerator in turnmandates an upgrade to the Belle detector. One proposed upgrade is toexchange the existing thallium doped cesium iodide scintillation crystals(CsI(Tl)) in the endcap calorimeter with pure cesium iodide (CsI). Oneadvantage of pure CsI is its shorter decay time constant. This would reducethe amount of time taken to process each event, which in turn reduces thechance of simultaneously measuring the energy of different two particles(pileup). Hamamatsu Photonics has produced the R11283 photomultipliertube with a nominal average gain of 255 ± 11, ideal for measuring the lightproduced by scintillation in pure CsI while in a magnetic field. A prototypearray of 16 photomultiplier tubes was built and tested at TRIUMF. Thiswork documents the characterization of the photomultiplier tube as wellas University of Montreal’s pre-amplification and shaper electronics. Theprimary results can be split into four distinct measurements: the electronicnoise, the short term stability, the excess noise factor, and the lifetime. Theelectronic noise was initially measured with cosmic rays and was found to be(77 ± 2) keV using a Belle II pure CsI crystal. The short term stability wasmeasured with a set of calibration sources, and the variation over a weekwas (0.28 ± 0.03)% after temperature corrections. The excess noise factorwas found to be (1.9 ± 0.1 ± 0.4) using a pulsed UV laser. This result wasaccompanied by an additional electronic noise measurement of 1730 ± 33electrons at the anode. The lifetime was found using a UV LED array anda ²⁰⁷Bi source, with the gain × quantum efficiency reduced to (93 ± 3)%after about 48 days of aging in real time. This was equivalent to 70 yearsof standard Belle II operation with 7 C having passed through the anode.There were several sets of aging behaviours observed, with some evidencethat the anode charge is not the sole factor in aging.

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