Search for low-mass dark matter with CDMSlite using a likely-fit profile.” In: Physical review D99.6 (March 2019). First Dark Matter Constraints of a SuperCDMS Single-Charge Sensitive Detector.” In: Physical Review Letters121.5 (August 2018).
LIST OF TABLES
Astronomical Evidence
- Galactic Rotation Curves
- Galaxy Clusters
- Gravitational Lensing
When performed for many galaxies, this calculation consistently implied greater mass than that explained by light matter alone. This particular effect can be explained by modifying the laws of gravity or by assuming the presence of non-luminous matter that drives the velocity at a high radius (dark matter).
Cosmological Evidence
- CMB Power Spectrum
- Structure Formation
After recombination, the baryonic matter was gravitationally pulled into the dark matter structure, making the structure visible to modern astronomers. A requirement of this model is that most of the dark matter forming the structure was non-relativistic at the time of matter-radiation parity.
Candidates
- QCD Axion
- Dark Photons
- Axion-Like Particles
- Hidden-Sector WIMPs
Dark matter WIMP candidates include the lightest supersymmetric particle (LSP) predicted by supersymmetry (SUSY), the lightest Kaluza-Klein particle (LKP) predicted by compactified universal extra dimensions (UED), and the lightest odd-T parity particle ( as defined in little Higgs theories) [12]. Hidden-sector dark matter is a general term for candidates that rely on a "dark portal" for interaction with the Standard Model.
Direct-Detection Methodology and World-Leading Limits
- Exclusion Plot Basics
- Neutrino Fog
- Electron-Scattering Experiments
- Electron-Absorption Experiments
Energy deposition from such neutrinos (by coherent elastic neutrino scattering, CE𝜈NS) will look similar to dark matter in conventional direct detection experiments. The lower mass of the electron makes it sensitive to the scattering of dark matter with lower mass.
Detector Design
- HV Detectors
- iZIP Detectors
The reconstruction will be used to remove events that are close to the radial edge of the detectors. QETs will collect 1 mm of surface instead of accelerating it to the opposite surface.
SNOLAB Experimental Setup
- SNOLAB Site
- Cryogenic and Shielding Assembly
These stages are thermally isolated and will be maintained at 250mK, 1K and 1K using the same DR. Inside the outer neutron shield will be an aluminum structure filled with vaporized nitrogen.
SNOLAB Backgrounds and Projected Sensitivity
- Background Sources
- Projected Sensitivity
- Basic Charge Amplifier Design
- Charge Dissipation via Feedback Resistor
- Amplifier Noise and Mitigation
- Current Mirror and HEMT Biasing
- Amplification and HEMT Parameter Definitions
- Thermal Considerations
- Vibration Considerations
- HEMT Characterization Setup
- Transconductance and IV Curves
- Cutoff Voltage
- Ungrounded HEMT Characterization
The backgrounds before and after the cutout used to calculate the constraints are shown in Figure 2.17. With the corresponding𝑉𝑔 𝑠 known (from the transconductance measurement) for each HEMT and bias point, I measured the IV curves by setting 𝑉𝑔 𝑠, sweeping across𝑉𝑑𝑠 and recording 𝐼𝑑𝑠. The resulting graph for one HEMT is shown in Figure 2.26.
Properties of SuperCDMS SNOLAB HEMTs
- Gate-Voltage Biasing
- Transconductance and Amplifier Gain
- Drain Conductance
The second HEMT displayed an unusual property that I believe is a reading effect rather than a reflection of the HEMT. The difference is likely due to improvements in the HEMT design between [37] and the production of the SNOLAB HEMT in 2015. I have also included a scatterplot of the transconductance of the HEMTs drawn in the order they were tested (Figure 2.33).
Histograms of the snolab HEMT drain conductances for each bias setting are shown in Figure 2.34.
Ongoing Testing of the Ionization Readout
There is another amplifier between the output of the charge amplifier and the ADC input, so this does not correspond exactly to the output of the charge amplifier. Some of these are likely the result of the noisy laboratory environment and will be reduced in the carefully shielded SNOBOX. After debugging, the ionization noise is expected to look like that in Figure 2.38.
This figure shows the load amplifier noise that was measured during the design phase of the SNOLAB experiment using a similar HEMT and a functionally similar (but not identical) amplifier design.
HVEV RUN 3 AT NEXUS
HVeV Detectors
Each pair produced is accelerated by the voltage bias to generate 150 eV of phonon energy observed in the detector. The sensitivity of the HVeV detectors to NTL phonon energy is a powerful tool for measuring ionization events. Therefore, the sensitivity of the HVeV detector for single 𝑒−ℎ+ pairs corresponds to an electron recoil sensitivity of only ∼1.2 eV.
Quasiparticles transitioning to the lower-gap material release excess energy as phonons that head the TES (grey).
Previous HVeV Science
- Run 1 at Stanford
- Run 2 at Northwestern
- Low-Energy Excess
The electron scattering and dark photon absorption exclusion limits produced by Run 2 analysis can be seen in Figures 3.8 and 3.9, respectively. For most of the mass range, the limits of Run 2 were slightly worse than those of Run 1 (despite Run 2 having improved energy resolution and ≥ 2× exposure). The weaker limits were actually the result of the higher CT rate of the Run 2 detector and the use of a more conservative limit setting method.
The excess of Run 2 (compared to the dark matter model, which fits well in the ≥1𝑒−ℎ+ region) can be seen in Fig. 3.11.
Setup at NEXUS
- NEXUS facility
- Cryogenics and Shielding
- Payload Arrangement
- Detector Designs
During Run 3, cosmogenics was also considered a potential origin for burst events, so the underground site was part of the broader plan to reduce the low-energy remnant. With this arrangement, each detector acts as a veto detector for each of the others (especially effective for those sharing a bracket). Three vertical bars thermally connect the bottom plate to the mixing chamber stage of the dilution refrigerator (above the inner lead shield).
Upon inspection of the NF-F design, it became clear that the inner channel of QETs had an unintentional short circuit between input and output (see Figure 3.15).
Data Taking
While laser data confirmed that events were generated in the detectors, it also showed a larger fraction of non-quantized events than seen in previous runs. An inadvertent short circuit between the input and output wire connector blocks of the inner channel is indicated by red arrows. On January 11, the data-taking team attempted to take laser data, but could not find signals that coincided with the TTL trigger (the transistor logic that indicates when the laser diode is pulsed).
Day 9 was the last day of science data recording, so the analysis had access to laser data from the start and end of science data collection.
Data Processing
- Triggering
- Optimal Filtering
For run 3, the expected signal template for each detector was calculated using laser-generated events from January 11. Although the expected signal template from each detector was constant throughout the run, the noise was re-estimated for each minute of data recording. For each minute, the noise was calculated in each detector using traces activated at random times.
By combining the templates and noise measurements, the OFs were calculated for each detector and each minute of data acquisition.
Relative channel weighting
- Resolution-Optimizing Method
- Partition-Dependence-Minimizing Method
An example of the iterative 1 𝑒−ℎ+ cut applied to NF-C data can be seen in Figure 3.20. The model used is given in Eq. 3.3) The square prefactor is responsible for normalizing the weighted sum to 100 eV. An example dataset's OF0 random trigger events in the NF-C detector can be seen in Figure 3.23.
I followed the same procedure to select the 1 𝑒−ℎ+ events from triggered science data (also in Figure 3.23).
Energy Calibration Using Laser Data
- Low- 𝜆 Laser Data
- Daily Responsivity Variation
- Peak Fitting and Background Estimation
- Calibration Fitting
- Residuals and Uncertainties
- Calibration Extrapolation
The NF-C and NF-H values were considered close enough to the results in Figure 3.22 that the old values were kept in use. We modeled the distribution as a sum of asymmetric Gaussian2 distributions with location and width determined by the amplitude and individual uncertainties of the data (see Figure 3.27). It was observed that the background between the peaks had a significant energy dependence (see the negative slopes evident in Figure 3.28).
The green uncertainty band shown in Figure 3.30 shows one standard deviation of the uncertainty from the fit result.
Live-Time Selection
- Time-Stream Overlap Cut
- Mean-Baseline cut
- Temperature Cut
- Coincidence and Δt Cut
- Live-Time Cut Summary
- Combined Quality-Cut Effect
No correlation was apparent between the NF-H detector and any of the others (in these plots). The 𝜒2 cut was produced using OFL data from the first half of the nighttime laser data set. This was confirmed by applying the cut to the second half of the nighttime laser data.
The effect of the quality reductions on the scientific NF-C data can be seen in Figure 3.40.
Detector Energy Resolution
The maximum calculated energy resolution (from the laser peak 4-𝑒−ℎ+) was used as an upper limit. The lowest calculated energy resolution (from the science data baseline) was used as a lower limit. The nominal value was the average of the upper and lower limits.
To ensure that the energy resolution was stable during the run, the 1-𝑒−ℎ+ peak resolution was calculated for each science data set individually.
Charge Trapping and Impact Ionization Model
Poisson Limit Setting
- Energy Regions of Interest
- Signal Models: Dark-Matter Rates
- Signal Models: Signal Ionization
- Signal Models: Combined
- Limit Setting
- Combining Limits
- Limit Sensitivity to Systematic Uncertainties
𝐹𝐷 𝑀 is the momentum transfer shape factor, which determines the dependence of the distribution on the exchanged momentum (𝑞). The Run 3 mean Earth velocity (w.r.t. DM halo) was calculated using the mean DM velocity (w.r.t. Galactic frame), the specific solar velocity and the Earth velocity (w.r.t. the Sun) on 9 March. Dark matter models return the event rate as a function of electron recoil energy (𝐸𝑒𝑟), but the total phonon energy observed by the detector (𝐸𝑒𝑡) will be dominated by energy from ionization-dependent NTL phonons.
One of the following ratios (derived from the models in Section 3.12.2) is then used to extract the bound on the interaction parameter.
HVeV Run 3 Results
- Electron-Scattering Limits
SENSEI_2020 Xe_22td
SENSEI_2020 damic_2019
HVeV Run 4
A continuation of Series 3 HVeV was planned and data collection was performed during the Series 3 analysis. Experiment Run 4 also used four HVeV detectors in the same underground dilution cooler at NEXUS. Separate cooling was performed with an LED replacing the light-tight cover to obtain Run 4 calibration data.
We scaled the Run 2 limit by the square root of the refractive index of silicon to account for the difference in model used to set the limit (see section 3.12.2).
KIPM-DETECTOR DEVELOPMENT
Overview of MKIDs
- Kinetic Inductance in Superconductors
- MKID Readout Basics
- MKID Responsivity
The MKID then creates an effective notch filter in the transmission feedline, centered on its resonant frequency. The quality factor (𝑄𝑟) of the resonance and its coupling to the feed line is reflected in the full width at half maximum (FWHM) of the notch filter. It is a function of the normal state conductivity (𝜎𝑛), superconducting energy gap (Δ), temperature (𝑇), readout frequency (𝜔) and effective chemical potential of quasiparticles (𝜇∗).
It has a characteristic decay length equivalent to the minimum of the coherence length (𝜉0) and the mean free path of the electron (ℓ𝑒).
MKIDs for Dark Matter
- MKID Energy Resolution
- KIPM-Detector Energy Resolution
- MKID and KIPM Designs
Next, we would like to estimate the resolution of the phonon energy measurements in the detector substrate. We must also consider the fact that the active volume of the MKID will not absorb all the phonon energy in the substrate. If only 10% of the phonon energy is absorbed in the MKID quasiparticle system, the resolution will be 10× worse than the theoretical best value.
Due to the difference in Δ, the phonon energy is more likely to be absorbed in the Al MKID inductors instead of the Nb feed line, which improves𝜂𝑝 ℎ.
Early Device Tests
- Feedline-only Tests
- Resonance Identification and Characterization
The main purposes of the box are thermal sinking, connection of coaxial cables to the feed line and minimization of optical load. We suspect that the uneven transmission is (at least in part) due to the coupling of the main line to the resonant modes of the cavity formed by the device case. The green transmission is mounted in the case with both covers removed and raised from the surface of the cold plate.
The blue transmission is mounted in the box with both covers attached and with Eccosorb microwave absorbing foam placed between the device and the cover run by the supply line.
