Canada

Social distancing measures have been the main non-pharmaceutical intervention (NPI) against the COVID-19 pandemic. Numerous large-scale analyses have studied how these measures have affected human movement, finding sizeable drops in average mobility. Yet comparatively little attention has been paid to higher-order effects such as “superspreading events” which are known to be outsized drivers of pandemics. Networks with heterogeneous (high variance) distributions of contacts can dramatically accelerate spreading processes, even if the average number of contacts is low. This stresses the need to quantify higher-order effects, and the (in)ability of existing NPI to reduce them.

Here we assess this by applying tools of statistical physics to approximately 12 billion anonymized mobile phone traces from 2.33 million devices in the Chicago metropolitan area, from Jan.1 to Jun.30, 2020, covering the first wave of state- and city-level social distancing measures in the pandemic. To identify potential super-spreading events, we grid these data at a fine spatial and temporal resolution, revealing large, transient co-localizations of people which we term hotspot events. We then ask about the spatiotemporal distribution of these events and the mobility statistics of the people participating in them--both before and after the implementation of social distancing measures. 
Encouragingly, we find that distancing policies heralded a dramatic rarefaction of people, reflected in an increase in the entropy of their spatial distribution. As a result, we observe a concomitant drop in hotspot event frequency, with the largest reduction occurring in the urban core. This, however, belies a more worrisome trend: though we observe a large average reduction in the amount people travel (as measured by individual radius of gyration), this fails to be true for the subset of users participating in hotspot events. These users display higher-than-average baseline mobility, which persists (and even increases) during the post-lockdown period.

Our findings indicate that though social distancing policies may succeed in reducing average mobility, their effectiveness in reducing the key driver of spreading processes on networks (the second moment of the degree distribution) may be more limited. This in turn suggests the need for additional NPI specifically targeted at “super-spreading events”.

CAP Virtual Congress 2021

Quantifying Density Hotspots and Potential Superspreading Events During COVID-19

human mobility

superspeading

covid-19

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technical paper

Liquid Argon (LAr) is used as a target material by many WIMP dark matter search experiments for its high light-yield and excellent background rejection capability. LAr produces scintillation light at 128nm, which conventionally requires a wavelength shifting (WLS) material to be detected by photomultiplier tubes. Tetraphenyl-butadiene (TPB) is the WLS material of choice for most LAr detectors, including DEAP-3600 at SNOLAB, and is used to shift the primary scintillation light. Our work aims to characterize candidate thin films containing pyrene developed at Carleton University. Their long fluorescence time constant can be used for efficient rejection of pathological detector backgrounds. Fluorescence yield and mechanical stability in cryogenic conditions are also investigated. We describe the Queen's University cryogenic test facility which boasts good optical efficiency and a base temperature of 4K. We present results from pyrene thin films with various concentrations and purity as well as TPB as a reference.

Characterization of wavelength shifters for background rejection in liquid argon dark matter experiments

We discuss interaction of gravitational waves with matter including plasma and its implications for cosmology.

Gravitational Waves interacting with matter in cosmology

Trapped-ion quantum states are well-known to be good candidates for qubits in quantum computing. I will study the application of an adiabatic method known as STIRAP to achieve qubit switching. Stimulated Raman Adiabatic Passage (STIRAP) is a method of quantum control that utilizes a specific atomic structure known as a 3-Level Lambda System (3LLS). The system consists of two ground states that are coupled to an intermediate excited state via two counter-intuitively ordered pulses known as the Stokes and pump pulses. STIRAP is a notable method of population transfer because not only is it robust against small experimental variations, but it also has the unique property to allow the complete transfer of population between the two ground states without ever suffering population loss due to spontaneous emission from the excited state. STIRAP can be extended to other chain-wise connected multi-level systems such as those that are present in the trapped-ion. In this study, I will numerically determine the optimal pump and Stokes pulses that will maximize qubit switching in 3, 5, and 7-quantum states of the trapped-ion, and discuss the potential applicability of this method in quantum computing.

Quantum Control of Trapped Ions using Stimulated Raman Adiabatic Passage

**Introduction:** MRI’s low sensitivity, caused by the use of nuclei with low-gyromagnetic ratios or low magnetic field strength, can presently be improved with expensive high-field MRI-hardware and/or expensive enriched-isotopes. We propose a new method that does not require any extra signal-averaging or hardware to improve the quality of MRI images. We will use a significant k-space under-sampling acquisition method where only a certain percentage of the k-space points will be acquired per image, corresponding to the acceleration-factor (AF); it follows that one can acquire ten under-sampled images in the same time as one fully-sampled image. Averaging each possible combination of images of the under-sampled set, a density decay curve can then be fitted and reconstructed using the Stretched-Exponential-Model (SEM) combined with Compressed Sensing (CS).1
**Method:** 1H MR was performed on a resolution-phantom at the low-field (0.074T) MRI scanner using a home-built RF coil. Nine 2D fully-sampled k-spaces were acquired. Combinations of 2, 3, and 4 averages were carried out for each possible permutation, resulting in 14 k-spaces total (2 combinations for 4 averages, etc.); these were retroactively under-sampled for three AF’s (7, 10, 14). 3 Cartesian sampling schemes (FGRE, x-Centric2, & 8-sector FE Sectoral3) were used. The SNR attenuation is assumed to represent a decrease of the resonant isotope density in phantom after diluting it with the non-resonant isotope. The resulting signal decay (density) curve was fitted using the Abascal method.1
**Results:** The SNR of the 9 k-space averaged image and the original image was 16 and 5, respectively. The SNR of the three sampling schemes is 15 for FGRE, 19 for x-Centric, and 17 for FE Sectoral.
**Conclusion:** The improved SNR of the generated images for all sampling schemes demonstrate that the SEM equation can be adapted for fitting the SNR decay dependence of the MR signal. Since this technique does not require extra hardware, the proposed method could be implemented in current MRI-systems and yield improved images. Due to the CS-based reconstruction, the higher AF leads to more visible artefacting; this could be reduced by a Deep Learning-based correction after the fact.4
**References:**
**1** Abascal et al. IEEE Trans Med Imaging (2018); **2** Ouriadov et al. MRM (2017); **3** Khrapitchev et al. JMR (2006); **4** Duan et al. MRM (2019)

The Use of a Novel Sampling/Reconstruction Method for Non-Proton and Low Field MRI

**Introduction:** A non-invasive imaging method: inhaled hyperpolarized (HP) 129Xe magnetic resonance imaging (MRI) is currently used to measure lung structure and function.***1*** Simultaneous ventilation/perfusion (V/P) lung measurements of functional gas exchange within the lungs can be obtained using this MRI approach because of the high solubility of xenon in lung tissue as compared to other imaging gases. This measurement is possible due to the distinct and large range of chemical shifts (~200ppm) of 129Xe when residing within barrier and RBC (e.g., barrier and RBC phase xenon) compared to the gas phase. Therefore, 129Xe is a unique probe for exploring xenon within and beyond the lung, such as lung parenchyma (barrier), RBC, and even other organs such as the **brain**, heart and kidney.
[15O]water positron emission tomography (PET) is the gold standard imaging method for determining cerebral perfusion.***2,3*** In this study, simultaneous 129Xe-based MRI and [15O]water PET images were collected and compared. 

**Methods:** A 60mL plastic syringe was used in which 30mL of the hyperpolarized 129Xe gas was dissolved in [15O]-water solution (30mL) After dissolving, all leftover xenon gas was removed from the syringe. A turn-key, spin-exchange polarizer system (Polarean 9820 129Xe polarizer) was used for obtaining Hyperpolarized 129Xe gas. 129Xe dissolved phase images were acquired in a 3T PET/MRI (Siemens Biograph mMR) scanner. [15O]water PET data were acquired simultaneously with 129Xe MRI using the integrated PET system in the 3T PET/MRI. 

**Results:** Two consecutive 2D axial 129Xe MRI images and two (2D and 3D) [15O]water PET images were acquired simultaneously. 129Xe/PET images indicate that the diameter of the phantom from both PET and MRI images are similar. Both 129Xe images demonstrate a sufficient SNR level (80 and 10 respectively) suggesting that 3D 129Xe imaging is possible. 

**Conclusions:** The results of this proof-of-concept study clearly indicate the feasibility of the simultaneous hyperpolarized 129Xe MRI and [15O]water PET measurements. This demonstration enables the next step, namely, in-vivo double tracer brain perfusion imaging which we plan to perform using a small animal model.

***References:***
***1***.Kaushik, S. S. et al. MRM (2016); ***2***. Fan, A., et. al. JCBFM (2016); ***3.*** Ssali. T., et. al. JNM (2018).

Simultaneous Hyperpolarized 129Xe MRI and [15O]water PET Multi-Modal Imaging: A Proof of Concept Study

We are developing a time-reversal breaking test in radiative $\beta$ decay, using just the momenta of three outgoing particles. This type of time reversal is independent of nuclear spin, so explores time reversal-breaking physics unrelated to electric dipole moments (though there are model-dependent constraints at 1-loop order from null measurements of the neutron EDM). The scalar triple product of three momenta $\vec{p_1}\cdot \vec{p_2} \times \vec{p_3}$ provides a unique time-reversal odd observable, but trivially vanishes in ordinary $\beta$ decay when the three momenta sum to zero. So we need the fourth outgoing particle in radiative $\beta$ decay, considering the correlation between $\beta$, $\nu$, and $\gamma$. We add $\gamma$-ray detectors (GAGG scintillator with SiPM readout) to TRIUMF's magneto-optical trap for beta decay (TRINAT), which includes a uniform electrostatic field for efficient recoil ion detection. Explicit models produce this observable with an antisymmetric Chern-Simons term from QCD-like new interactions, interfering with the standard model vector electroweak interaction within the nucleon [S. Gardner and D. He, Phys. Rev. D 87 116012 (2013)], and among the predicted features are a quite different gamma-ray spectrum than normal bremsstrahlung. We will show initial data from the decay of 92Rb, a case without vector interactions not yet testing the explicit models.

Time-reversal test in radiative beta decay: progress

Much of what we know of the early universe comes from observations of the cosmic microwave background (CMB): a 13 billion-year-old field of microwave radiation that permeates the entire universe. Recent technological advances have made real the possibility of combining CMB measurements with other large data sets to extract hitherto inaccessible cosmological information. One such example is the novel technique of kinetic Sunyaev Zel’dovich (kSZ) tomography in which one combines a CMB temperature map with the positions and distances of galaxies across the sky, using statistical correlations between high-fidelity maps to reconstruct the velocity of those galaxies at the largest angular scales and over an appreciable fraction of the volume of the universe. At these scales and distances the motion of large-scale structure owes its statistical properties to the conditions of the early universe; measuring this velocity map would therefore provide an independent probe into the physics of that era.

The primary challenge in extracting a velocity map using kSZ tomography is characterizing the dominant sources of uncertainty introduced by non-idealities such as redshift measurement error, incomplete sky coverage, galactic and extragalactic contaminants, and confusion with other physical effects in the CMB. To this end we have designed a pipeline to perform the reconstruction using next-generation CMB and galaxy survey data that incorporates these contaminants into its design. We account for redshift errors and demonstrate that masking and other cut sky effects do not influence the reconstruction fidelity. We show that galactic processes do not contribute to the reconstruction noise, and estimate the impact of extragalactic sources that mimic the kSZ signal. We demonstrate that reconstruction is possible with data from next generation surveys and forecast how well the pipeline will perform with realistic contaminants. We estimate a strong signal-to-noise of the velocity map on large scales, making it a new data product on the cutting edge of cosmological research, as the field turns its attention to upcoming high-resolution datasets.

kSZ Tomography with Foregrounds and Systematics

Vertically stacked heterostructures of two-dimensional materials provide a platform for realizing novel electronic states due to proximity effects. In particular, moiré patterns in two-dimensional material heterostructures have been shown to create flat bands that favor the occurrence of correlated electronic states. In this work, we use scanning tunneling microscopy and spectroscopy to study moiré patterns in mechanically assembled heterostructures of graphene and transition metal dichalcogenides. By comparing to a theoretical model, in the case of graphene - ReS2 heterostructures we find the presence of stripped moiré patterns, reflecting the different crystal symmetry of the two lattices.

The experimental work was supported by: National Sciences and Engineering Research Council (NSERC) No. RGPIN-2016-06717, No. 521420 and NSERC Scholarship program. Theory work was supported by Singapore Ministry of Education AcRF Tier 2 (No. MOE2017-T2-2-140).

Moiré patterns in graphene and transition metal dichalcogenide heterostructures

**THESIS:** The ontology of spacetime should not be modelled as a single universally-shared 4D spacetime manifold.

**THE CONSENSUS SPACETIME ONTOLOGY:** The accepted ontology of classical spacetime is of a 4D differentiable manifold of events. Although different observers may assign different coordinate labels to events, it is conventionally asserted that the underlying reality is that of a single spacetime manifold. 
This single classical manifold ontology prohibits nonlocality, because there is no possibility for any form of physical interaction, except those occurring along paths in the manifold. Consequently, Bell nonlocality (EPR) remains mysterious [1].

**DIAGNOSIS:** The existence of an invariant distance measure (the spacetime interval), and smooth coordinate manifolds has been taken to imply the existence of a universally-shared smooth spacetime manifold. However, this move involves an improper conflation of two different modes of description.
The causal structure possesses invariant properties, but lacks the smooth neighborhood properties required for a manifold, whereas exactly the converse is true of the coordinate manifolds. Manifolds are smooth, but not invariant. We cannot borrow invariance from the causal structure and glue it to coordinate manifolds to create an invariant manifold.

**GR:** The tensor calculus of GR is "coordinate-free" in the sense that it gives consistent results, irrespective of coordinate system. However, the existence of coordinate mappings between spacetime manifolds does not imply the physical existence of a universal shared manifold. By analogy, the existence of an atlas of the human brain (allowing anatomical mapping between individual brains) does not imply the physical existence of a shared `universal brain’. A mathematical mapping does not imply a physical process.

**CONCLUSION:** To move beyond classical GR, it is necessary to abandon the image of spacetime as a single deformable 4D entity, and to replace it with a structure acknowledging the reality of the multiple coordinate manifolds. See [2] for such a `many-spaces’ proposal, including an account of quantum nonlocality.

**REFERENCES**

[1] Nicolas Brunner et al. “Bell nonlocality”. In: Rev. Mod. Phys. 86 (2 Apr. 2014), p. 419.

[2] J. C. Sharp. One Universe, Many Spaces: A Non-Local, Relativistic Quantum Spacetime
https://doi.org/10.20944/preprints201805.0003.v1

The Collapse of the Manifold

Introduction: A great challenge in quantitative dynamic positron emission tomography (PET) imaging is to determine the exact volumes of interest (VOI) with which one wants to work. They have a tremendous impact on the time-activity curves that are used to extract the pharmacokinetic coefficients. Since PET images are functional and not anatomical, using a bijective relationship with a computed tomography (CT) co-image is neither the sole nor the best possibility. In recent years, many publications have come with ingenious methods to work directly with the PET images, ranging from machine learning algorithms to manual toil to define regions. These techniques have different uses, mainly in the hope of enabling easier and more efficient tumor delimitation. In the case of dynamic images, the temporal aspect of the imaging procedure changes the methods that can be implemented. Furthermore, the need to delimitate specific and precise functional sites render the whole operation computationally and physically challenging, especially in the absence of a common and well-established methodology. 
Methodology: In this project, a novel approach using a gradient-based segmentation was used on pre-clinical dynamic PET images on rats. Fourteen different animals were used under similar pre-clinical conditions. The developed segmentation technique uses properties of the image itself, relying on already known properties of the radio-drug used in order to segment automatically the kidney of the animal. The work was conducted using the mini-PET scanner at the Montreal Neurological Institute, according to the ethical guideline from the University of Montréal and the Canadian Tri-Council.
Results: From the preliminary data, the proposed method has a relevant rate of success on clinical images in delimitating the volumes of interest. The respective time-activity curves follow the general pattern of the manual delimitations done by experts, yet with non-negligible differences. So far, the proposed technique offers good result on 8 of the 14 rats, as compared to 12 rats when using a manual segmentation. The greatest strength of the algorithm is its ability to reproduce the same results notwithstanding the operator. The technique can also quantify movement in the organ of interest and work in spite of a great amount of Gaussian noise. 
Keywords: Nuclear Medicine, pre-clinical dynamic PET imaging, pharmacokinetic

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