LIST OF TABLES
2.3 SNOLAB Backgrounds and Projected Sensitivity
2.3.2 Projected Sensitivity
Sensitivity limits were calculated separately for several dark-matter models. The models include nuclear-coupled, electron-coupled (through a light mediator), electron- coupled (through a heavy mediator), dark-photon, and axion-like-particle dark mat- ter. The various sensitivity limits were calculated assuming an exposure equivalent to 4 years of data taking with 80% live time (the fraction of data-taking time actually recorded to data). The resultant sensitivity plots are shown in Figures 2.8, 2.11, 2.13, 2.15, and 2.16. The pre- and post-cut background spectra used to calculate these sensitivities are shown in Figures 2.9, 2.10, 2.12, 2.14, and 2.17.
Sensitivity to spin-independent nucleon-coupled dark matter is shown in Figure 2.8.
Si detectors are sensitive to lower masses than their Ge counterparts because the Si nucleus has a lower mass. HV detectors are sensitive to lower masses, because their higher Δ𝑉 creates a lower recoil-energy threshold. For higher masses, the iZIPs’
ability to remove backgrounds gives them better sensitivity. Although the Si iZIPs do not have the best sensitivity for any masses, they will provide useful information on the Si-HV backgrounds.
For nuclear-recoil dark matter, sensitivity was calculated using both the optimum interval method and using a profile-likelihood ratio. The latter utilizes information about known background spectra and is therefore capable of improving sensitivity.
For the subsequent sensitivity calculations, only the profile-likelihood-ratio result is presented.
The post-cut backgrounds used to produce Figure 2.8 are shown in Figures 2.9 and 2.10. We can see the effect of the iZIP detectors’ ER rejection. Both surface- and bulk-ER rates drop at high phonon energy for iZIP detectors. For lower energies, noise in the ionization measurement prevents accurate calculation of ionization
1 10 10-46
10-45 10-44 10-43
10-9 10-8 10-7
Dark Matter Mass[GeV/c2]
DarkMatter-nucleonσ DarkMatter-nucleon
Created Dec 15 2022
Ge HV Ge iZIP
Si iZIP
Figure 2.8: Projected SNOLAB sensitivity to spin-independent nucleon-coupled dark matter in the form of 90%-confidence exclusion limits. Produced by [33]. The pre- and post-cut backgrounds used to calculate the iZIP (HV) limits are shown in Figure 2.9 (2.10). Solid-line limits are calculated using a profile-likelihood ratio that utilizes information about background spectra. Dashed-line limits are calculated using the optimum interval method. The short-dashed pink line represents the single- neutrino sensitivity. An experiment with that sensitivity would see an average of one neutrino event over the course of taking data [35]. The long-dashed purple line and purple shaded region indicate the neutrino fog. In this mass range, the neutrino fog is dominated by solar fusion of7Be and 𝛽-decay of8B. The currently excluded parameter space is shaded in light grey.
yield. Surface rejection lowers the rate of surface NRs in both iZIP and HV detectors. Bulk NRs from neutron and neutrino interactions become significant for very low energies, but such energies are below the detector thresholds and do not impact the sensitivity.
Ge iZIP Si iZIP
ra w w/ cu ts
Figure 2.9: The expected backgrounds (in total phonon energy) used to calculate the iZIP nuclear-recoil sensitivities in Figure 2.8. (Top) The pre-cut spectra. (Bot- tom) The post-cut spectra. Produced by [33]. The vertical dashed lines indicate the estimated analysis thresholds (estimated to be seven times the phonon-energy reso- lution). Red backgrounds come from bulk ER events including Compton scattering and3H/32Si 𝛽-decay. Green backgrounds come from surface ERs. Mustard back- grounds come from surface NR events including 222Rn-descendent206Pb recoils.
Blue backgrounds originate from neutron-induced bulk NRs. Cyan backgrounds originate from CE𝜈NS. Grey backgrounds in Ge are spectral peaks due to cos- mogenic activation. Example candidate signal models are shown in pink. Each example model is set to have equivalent cross section to the projected sensitivity.
iZIP example candidates have masses of 1.6, 5, and 16 GeV. Higher-mass candidates produce higher-maximum energy.
Ge HV Si HV
ra w w/ cu ts
Figure 2.10: The expected backgrounds (in total phonon energy) used to calculate the HV nuclear-recoil sensitivities in Figure 2.8. The HV-style detectors use Δ𝑉 large enough to observe the quantization of the second term in Eq. 2.1. This quantization is the origin of the spectral peaks particularly noticeable in the ER- recoil spectra. (Top) The pre-cut spectra. (Bottom) The post-cut spectra. Produced by [33]. The vertical dot-dashed lines indicate the estimated degradation of HV- detector thresholds due to voltage-induced leakage events. Otherwise, uses the same legend as Figure 2.9. Example candidates have masses of 0.5, 1.6, and 5 GeV with higher-mass candidates producing higher-maximum energy.
The projected sensitivities to electron-coupled dark matter are shown in Figures 2.11 and 2.13 for heavy and light mediators, respectively. Projected limits are only shown using HV detectors because their lower threshold makes them superior in this mass region. The iZIP detectors’ ability to reject ERs also serves no purpose for ER-search events.
The pre- and post-cut backgrounds used to produce Figures 2.11 and 2.13 are shown in Figures 2.12 and 2.14, respectively.
1 10 100
10-43 10-42 10-41 10-40 10-39 10-38 10-37 10-36 10-35 10-34 10-33
10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103
Dark Matter Mass[MeV/c2] DarkMatter-electronσe[cm2 ]
LDM F=1 Sensitivity w/HV(Ge)
DarkMatter-electronσe[pb]
Created Dec 15 2022
DarkS ide
-50
Asymmet ric Fermion ELDE
R
Majorana ER Majorana
NR Freeze
-Out
Ge HV Si HV
Figure 2.11: Projected SNOLAB sensitivity to electron-scattering dark matter with a heavy mediator (momentum-transfer form factor = 1). Produced by [33]. The pre- and post-cut backgrounds used to calculate the limits are shown in Figure 2.12.
The currently excluded parameter space is shaded in light grey. Parameter space for some candidate models are shown in magenta.
Ge HV Si HV
ra w w/ cu ts
Figure 2.12: The expected backgrounds (in total phonon energy) used to calculate the electron-recoil sensitivity in Figure 2.11. (Top) The pre-cut spectra. (Bottom) The post-cut spectra. Produced by [33]. Uses the same legend as Figure 2.10. Example candidates have masses of 1, 3, 10, and 30 MeV with higher-mass candidates producing higher-maximum energy.
1 10 100 10-43
10-42 10-41 10-40 10-39 10-38 10-37 10-36 10-35 10-34 10-33
10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103
Dark Matter Mass[MeV/c2] DarkMatter-electronσe[cm2 ]
LDM F=1/q2Sensitivity w/HV(Ge)
DarkMatter-electronσe[pb]
Created Dec 15 2022
XENO N10
Freeze-In
Ge HV Si HV
Figure 2.13: Projected SNOLAB sensitivity to electron-scattering dark matter with a light mediator (momentum-transfer form factor = (𝛼𝑚𝑒/𝑞)2). Produced by [33].
The pre- and post-cut backgrounds used to calculate the limits are shown in Figure 2.14. The currently excluded parameter space is shaded in light grey. Parameter space for freeze-in dark matter is shown in magenta.
Ge HV Si HV
ra w w/ cu ts
Figure 2.14: The expected backgrounds (in total phonon energy) used to calculate the electron-recoil sensitivity in Figure 2.13. (Top) The pre-cut spectra. (Bottom) The post-cut spectra. Produced by [33]. Uses the same legend as Figure 2.10. Example candidates have masses of 1, 3, 10, and 30 MeV with higher-mass candidates producing higher-maximum energy.
The projected sensitivities to dark-photon and axion-like-particle absorption are shown in Figures 2.15 and 2.16, respectively. The entire rest energy of either particle would be deposited as electron-recoil energy. Therefore, the projected limits extend down to the energy threshold of each detector. The HV detectors are superior in this range. Both absorption processes share the same backgrounds (shown in Figure 2.17).
1 10 100
10-18 10-17 10-16 10-15 10-14 10-13 10-12 10-11
10-17 10-16 10-15 10-14 10-13 10-12 10-11
Dark Photon Mass[eV/c2]
KineticMixingAngleε
DPDM Sensitivity w/HV
KineticMixingAngleε
Created Dec 15 2022
Ge HV Si HV
Figure 2.15: Projected SNOLAB sensitivity to dark-photon absorption. Produced by [33]. The pre- and post-cut backgrounds used to calculate the limits are shown in Figure 2.17. The currently excluded parameter space is shaded in light grey.
1 10 100 10-14
10-13 10-12 10-11 10-10 10-9 10-8
10-13 10-12 10-11 10-10 10-9 10-8
Axion Mass[eV/c2] Axioncouplinggae
ALPDM Sensitivity w/HV
Axioncouplinggae
Created Dec 15 2022
Ge HV Si HV
Figure 2.16: Projected SNOLAB sensitivity to axion-like-particle (ALP) absorption.
Produced by [33]. The pre- and post-cut backgrounds used to calculate the limits are shown in Figure 2.17. The parameter space excluded by direct-detection experiments is shaded in dark grey. The parameter space excluded by stellar-cooling observations is shaded in light grey. The bottom of the latter region is marked in yellow and represents the best estimate based on stellar-cooling observations [36]. If ALPs do exist at the best-estimate level, the SNOLAB HV detectors should be able to detect them.
Ge HV Si HV
ra w w/ cu ts
Figure 2.17: The expected backgrounds (in total phonon energy) used to calculate the electron-recoil (via particle absorption) sensitivities in Figures 2.15 and 2.16.
(Top) The pre-cut spectra. (Bottom) The post-cut spectra. Produced by [33]. Uses the same legend as Figure 2.10. Example candidates have masses of 6, 20, and 60 eV with higher-mass candidates producing higher-maximum energy.
appears on the equivalent capacitor and is measured and amplified using the iZIP ionization readout. Each iZIP requires four independent ionization readouts (one for each of the four ionization channels).
The SNOLAB experiment uses charge (current-integrating) amplifiers as the first- stage amplifier of each ionization readout. Each charge amplifier is split into a cryogenic input stage (mounted on the detector tower) and a room-temperature stage with the remaining components. We will discuss the purpose for the division in the following section. Before doing so, it will be useful to discuss the operation of a basic charge amplifier.