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Dark matter (DM) is one of the most outstanding problems in physics and is a promising hint for physics beyond the Standard Model. Many dark matter detection experiments have been built, with weakly-interacting massive particles (WIMP) as a popular DM candidate. There are also growing interest in light (keV-GeV) dark matter (LDM). The Super Cryogenic Dark Matter Search (SupreCDMS) experiment, which is based on transition-edge sensor (TES), is designed for low-mass WIMPs detection. In the meanwhile, SuperCDMS is using the same technology to build more sensitive detectors to probe some LDM models. In this thesis, I will introduce one of the SuperCDMS R\\&D programs called HVeV which develops high-voltage detectors with eV-scale resolution. The outstanding performance of HVeV detector enabled two topics that are important for DM research and related fields. First, ionization yield is an essential parameter to calibrate the detector response for low-mass WIMP but is not characterized in the target energy region. I used the HVeV detector to measure the ionization yield in silicon down to 100~eV. Second, I studied the backgrounds in HVeV detectors and identified one that dominates. The sensitivity of HVeV detectors can be increased by two orders of magnitude if this background source can be eliminated. I also show a DM exclusion limit with a half day of measurement from the HVeV detector.

Microcalorimeters that leverage microwave kinetic inductance detectors to read out phonon signals in the particle-absorbing target, referred to as kinetic inductance phonon-mediated (KIPM) detectors, offer an attractive detector architecture to probe dark matter (DM) down to the fermionic thermal relic mass limit. A prototype KIPM detector featuring a single aluminum resonator patterned onto a 1-gram silicon substrate was operated in the NEXUS low-background facility at Fermilab for characterization and evaluation of this detector architecture's efficacy for a dark matter search. An energy calibration was performed by exposing the bare substrate to a pulsed source of 470 nm photons, resulting in a baseline resolution on the energy absorbed by the phonon sensor of \\(2.1\\pm0.2\\) eV, a factor of two better than the current state-of-the-art, enabled by millisecond-scale quasiparticle lifetimes. However, due to the sub-percent phonon collection efficiency, the resolution on energy deposited in the substrate is limited to \\(\\sigma_E=318 \\pm 28\\) eV. We further model the signal pulse shape as a function of device temperature to extract quasiparticle lifetimes, as well as the observed noise spectra, both of which impact the baseline resolution of the sensor.

We demonstrate single electron-hole pair resolution in a single-sided, contact-free 1 cm\\(^2\\) by 1 mm thick Si crystal operated at 48 mK, with a baseline energy resolution of 3 eV. This crystal can be operated at voltages in excess of \\(\\pm50\\) V, resulting in a measured charge resolution of 0.06 electron-hole pairs. The high aluminum coverage (\\(\\sim\\)70%) of this device allows for the discrimination of surface events and separation of events occurring near the center of the detector from those near the edge. We use this discrimination ability to show that non-quantized dark events seen in previous detectors of a similar design are likely dominated by charge leakage along the side wall of the device.

We present a detailed characterization of a new generation of athermal-phonon single-charge sensitive Si HVeV detectors, the best of which achieved 612 meV \\(\\pm\\) 4 meV baseline resolution. Our sub-eV energy resolution enables precise measurements of single-photon events and reveal consistent energy losses of 0.81 eV \\(\\pm\\) 0.03 eV per charge excitation across two facilities. We demonstrate that the noise for these detectors is well described using a standard Transition Edge Sensor noise model. We also place upper bounds on the nominal phonon collection efficiency of 45\\%, establishing these detectors as the most efficient athermal phonon detectors to date, limited only by intrinsic limitations of quasiparticle generation.

We present the current status of the MATHUSLA (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles) long-lived particle (LLP) detector at the HL-LHC, covering the design, fabrication and installation at CERN Point 5. MATHUSLA40 is a 40 m-scale detector with an air-filled decay volume that is instrumented with scintillator tracking detectors, to be located near CMS. Its large size, close proximity to the CMS interaction point and about 100 m of rock shielding from LHC backgrounds allows it to detect LLP production rates and lifetimes that are one to two orders of magnitude beyond the ultimate reach of the LHC main detectors. This provides unique sensitivity to many LLP signals that are highly theoretically motivated, due to their connection to the hierarchy problem, the nature of dark matter, and baryogenesis. Data taking is projected to commence with the start of HL-LHC operations. We summarize the new 40m design for the detector that was recently presented in the MATHUSLA Conceptual Design Report, alongside new realistic background and signal simulations that demonstrate high efficiency for the main target LLP signals in a background-free HL-LHC search. We argue that MATHUSLA's uniquely robust expansion of the HL-LHC physics reach is a crucial ingredient in CERN's mission to search for new physics and characterize the Higgs boson with precision.

We present the Conceptual Design Report (CDR) for the MATHUSLA (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles) long-lived particle detector at the HL-LHC, covering the design, fabrication and installation at CERN Point 5. MATHUSLA is a 40 m-scale detector with an air-filled decay volume that is instrumented with scintillator tracking detectors, to be located near CMS. Its large size, close proximity to the CMS interaction point and about 100 m of rock shielding from HL-LHC backgrounds allows it to detect LLP production rates and lifetimes that are one to two orders of magnitude beyond the ultimate sensitivity of the HL-LHC main detectors for many highly motivated LLP signals. Data taking is projected to commence with the start of HL-LHC operations. We present a new 40m design for the detector: its individual scintillator bars and wavelength-shifting fibers, their organization into tracking layers, tracking modules, tower modules and the veto detector; define a high-level design for the supporting electronics, DAQ and trigger system, including supplying a hardware trigger signal to CMS to record the LLP production event; outline computing systems, civil engineering and safety considerations; and present preliminary cost estimates and timelines for the project. We also conduct detailed simulation studies of the important cosmic ray and HL-LHC muon backgrounds, implementing full track/vertex reconstruction and background rejection, to ultimately demonstrate high signal efficiency and \\(\\ll 1\\) background event in realistic LLP searches for the main physics targets at MATHUSLA. This sensitivity is robust with respect to detector design or background simulation details. Appendices provide various supplemental information.

The simple ABO 3 and A-site-ordered AA′ 3 B 4 O 12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg 3 Ti 4 O 12 , the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg 2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg 3 Ti 4 O 12 . Thus PbHg 3 Ti 4 O 12 appears to be a combinatory link from simple ABO 3 perovskites to A-site-ordered AA′ 3 Ti 4 O 12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′ 3 B 4 O 12 -type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials. There are few reports of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here, the authors find one with phase transition from a high-temperature centrosymmetric paraelectric phase to a low-temperature non-centrosymmetric ferroelectric phase in a high pressure synthesized compound.

The water-splitting reaction using photocatalyst particles is a promising route for solar fuel production 1 – 4 . Photo-induced charge transfer from a photocatalyst to catalytic surface sites is key in ensuring photocatalytic efficiency 5 ; however, it is challenging to understand this process, which spans a wide spatiotemporal range from nanometres to micrometres and from femtoseconds to seconds 6 – 8 . Although the steady-state charge distribution on single photocatalyst particles has been mapped by microscopic techniques 9 – 11 , and the charge transfer dynamics in photocatalyst aggregations have been revealed by time-resolved spectroscopy 12 , 13 , spatiotemporally evolving charge transfer processes in single photocatalyst particles cannot be tracked, and their exact mechanism is unknown. Here we perform spatiotemporally resolved surface photovoltage measurements on cuprous oxide photocatalyst particles to map holistic charge transfer processes on the femtosecond to second timescale at the single-particle level. We find that photogenerated electrons are transferred to the catalytic surface quasi-ballistically through inter-facet hot electron transfer on a subpicosecond timescale, whereas photogenerated holes are transferred to a spatially separated surface and stabilized through selective trapping on a microsecond timescale. We demonstrate that these ultrafast-hot-electron-transfer and anisotropic-trapping regimes, which challenge the classical perception of a drift–diffusion model, contribute to the efficient charge separation in photocatalysis and improve photocatalytic performance. We anticipate that our findings will be used to illustrate the universality of other photoelectronic devices and facilitate the rational design of photocatalysts. Photovoltage measurements on cuprous oxide photocatalyst particles are used to spatiotemporally track the charge transfer processes on the femtosecond to second timescale at the single-particle level.

In the behavioral analysis of children with Autism Spectrum Disorder (ASD), virtual reality (VR)-based eye-tracking technology offers a precise method for assessing social and cognitive characteristics. It overcomes the limitations of traditional diagnostic methods, such as clinician subjectivity and experience bias. VR also addresses ASD-related challenges like attention instability and emotional variability during social interactions. This paper combines eye-tracking with VR environments to analyze gaze patterns in children with ASD. It proposes a new diagnostic framework to improve objectivity and accuracy.The gaze estimation model integrates head and eye movement data to predict gaze direction. It enhances precision using binocular fusion and employs multi-scale convolutional kernels to extract hierarchical eye movement features. The model simplifies network connections to retain essential information. A lightweight Transformer architecture models long-range temporal dependencies in eye movements. A Bayesian decision model is used to classify fixations, saccades, and smooth pursuit.To test the model, an emotion recognition task was designed in a WebVR environment. Gaze data from children with ASD were collected, key features were extracted, and abnormal patterns were identified for diagnostic support. The experimental results showed an 85.88% accuracy rate. This confirms the effectiveness of combining VR and eye-tracking technology in ASD diagnosis, advancing intelligent medical tools, and reducing reliance on subjective clinical judgment.

Mesenchymal stem cells (MSCs) are considered the most promising seed cells for regenerative medicine because of their considerable therapeutic properties and accessibility. Fine-tuning of cell biological processes, including differentiation and senescence, is essential for achievement of the expected regenerative efficacy. Researchers have recently made great advances in understanding the spatiotemporal gene expression dynamics that occur during osteogenic, adipogenic and chondrogenic differentiation of MSCs and the intrinsic and environmental factors that affect these processes. In this context, histone modifications have been intensively studied in recent years and have already been indicated to play significant and universal roles in MSC fate determination and differentiation. In this review, we summarize recent discoveries regarding the effects of histone modifications on MSC biology. Moreover, we also provide our insights and perspectives for future applications.