Graduate Thesis Defense - 2020 Fall

Speaker: Atanu Pathak, University of Louisville (PhD Thesis Defense)

Abstract: Charged lepton flavor violation is a clear signal of new physics. Such decays are not allowed in the Standard Model but highly anticipated in a large class of new physics models. A direct search for lepton flavor violation in decays of the Higgs boson with the ATLAS detector at the LHC is presented here. The analysis is performed in the H → lτ channel, where the leading lepton (l) can be either an electron or a muon, and the τ lepton decays into an opposite flavored lighter lepton or via the hadronic decay channel.  Published results of this search are presented in this thesis based on a data sample of proton-proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 36 inverse-femtobarns (fb−1) at center-of-mass energy (√s) of 13 TeV during the 2015-2016 data-taking period. The analysis is found to be three times more sensitive than the previous analysis performed with 20 fb−1 of data collected at √s = 8 TeV during the 2012 data-taking period, and comparable to the one obtained by the CMS experiment with a similar-sized dataset at √s = 13 TeV during the 2015-2016 data-taking period. The direct search for lepton flavor violating decays of the Higgs boson obtained with the present analysis at the Large Hadron Collider is about twenty-five times lower than the indirect prediction. To complement my research goal of searching for new physics with lepton flavor violating signatures in final states containing the τ lepton,  the generator level modeling of decays of the τ lepton decaying into Standard Model processes at the Belle II experiment at the world’s highest luminosity collider is also presented.

Speaker: Ashan Vitharana, University of Louisville (PhD Thesis Defense)

Abstract: Our ability to understand and predict space weather has become vital due to its significant societal impacts, for example, communication, transportation, and national defense. One of the most exciting discoveries in the last decade has been the realization that tropospheric weather can strongly influence the space weather. It is now recognized that the atmospheric waves (gravity waves, atmospheric tides, and planetary waves) play a key role in coupling the lower and upper atmosphere. In this dissertation, we focused our study on atmospheric tides. While climatology of tides has been extensively studied, little is known about the tidal weather (tidal variability < 30- days). This dissertation constitutes a study to make the step from “seasonal/climatological tides” to “tidal weather/short-term variability” using the data from the extended Canadian Middle Atmosphere Model (eCMAM), and temperature observations from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the TIMED satellite. Particular attention is given to the short-term variability of the migrating diurnal tide (DW1). A hierarchy of statistical models, e.g. autoregressive (AR), vector AR (VAR), and parsimonious AR (PVAR), are developed to predict tidal weather based on the statistical properties such as the quasi 23‐day oscillation and the spatial correlations found in the tidal weather. We can predict the next day’s tidal weather at near R2 = 90% accuracy (correlation coefficient of 0.95). The total tidal variability is modulated on various temporal scales, hence a multi-linear regression model is fitted to the DW1 temperature amplitude using deterministic variables (solar cycle, ENSO, QBO, and the seasonal harmonics) and the fitted coefficients/amplitudes are examined. The absolute and relative variability of the short-term tidal time series shows significant 6-month variation. Short-term tidal variability contributes ~50-75% of the total tidal variability between 7 to 30-day window. Physical mechanisms for the short-term DW1 variability are also investigated using the eCMAM thermodynamic budget. Wave-mean flow interaction is mostly responsible for the tidal weather in DW1 in the mesosphere. Above 140 km, short-wave solar heating becomes the governing force for the short-term DW1 variability.

Abstract: Shah Riyadh, University of Louisville (PhD Candidacy)

Abstract: Quantum-state-specific investigation of NO₂ and NO₃ is of paramount importance in atmospheric chemistry and fundamental molecular physics. High-resolution laser spectroscopy investigations of these two free radicals can unravel the interplay mechanism between vibronic (vibrational-electronic) interactions, including the Renner-Teller, Jahn-Teller, and pseudo-Jahn-Teller effects. Also, vibronic interactions are often coupled to other intramolecular interactions such as the spin-orbit interaction and (pre-)dissociation. Therefore, a comprehensive investigation of the molecular energy level structure is desired for a quantitative understanding of intermolecular dynamics. However, selection rules dictate the ro-vibronic transitions so that there exist “dark states” that cannot be accessed in one-photon spectroscopy measurements of ground-state molecules. Two-photon cavity-enhanced spectroscopy can access and resolve these dark states and provide the much-desired information for understanding vibronic interactions. In our lab, two cavity-enhanced spectroscopy techniques are being developed: the double-resonance spectroscopy and the stimulated-emission pumping. Both techniques are based on the highly sensitive cavity ring-down technique and have the advantages of being two-photon spectroscopy techniques, including high quantum-state-selectivity, narrow linewidth, and simplified spectra. Doppler-free spectroscopy is a pre-requisite for the two-photon spectroscopy measurement. A Doppler-free saturation absorption spectra of CH_$ will be presented. We will also discuss the energy level structure and vibronic interaction mechanisms of NO₂ and NO₃, and the planned two-photon excitation scheme to access their dark states.