In the following figures the projections for the discussed improvements are compared to the projection for the performance of TUM40, which is depicted as light red band.
Although, the model depends on assumptions the simulations give an indication which modifications influence the sensitivity most in the different regions of the parameter space. In conclusion, the effects on the sensitivity should be considered rather qualita-tively than quantitaqualita-tively.
3.2 Impact of Relevant Parameters
In the following the impact of the relevant parameters on the sensitivity of a direct dark matter experiment is investigated. In order to disentangle the influence of exposure, background, as well as threshold, these modifications are studied separately.
3.2.1 Exposure
As long as no background is observed, the sensitivity of a direct dark matter experiment scales with the exposure, i.e. the product of detector mass and measurement time. The exposure can be increased either with longer measurements or with an increased target mass. As it is not feasible to increase the measurement time arbitrarily, the exposure usually is increased with an enlarged detector mass. One possibility to do this is to increase the mass of each single detector, which is difficult to achieve without reducing the detector performance. Otherwise, the detector mass can be increased by operating
Figure 3.3: The expected sensitivities (1σC.L.) for an exposure of 500 kg days (medium red) and 5 tonne days (dark red) with a TUM40 like module shown in the parameter space for elastic spin-independent dark matter nucleon scattering. A larger exposure results in an increased sen-sitivity in the high mass region while at low masses the exposure is not an important parameter for the sensitivity. For comparison the currently leading limits of different experiments (solid lines) and the neutrino floor for CaWO4(gray-shaded area) are shown [37, 45, 49, 67, 74].
a large number of detectors, which in the case of the CRESST experiment implies a large number of readout channels for which an upgrade of the setup would be necessary.
In figure 3.3 the sensitivity expected for an exposure of 500 kg days with the per-formance of the module TUM40 (medium red) demonstrates that increased statistics improve the limit for masses above ∼15 GeV/c2 compared to the projection with an exposure of 29 kg days (light red). At a mass of 100 GeV/c2 the limit can be increased by a factor of∼10, while for masses.13 GeV/c2 the improvement is less than∼20 %.
A projection for an exposure of 5 tonne days (dark red) illustrates that in the high mass region the sensitivity directly scales with exposure, as this projection with a ten times higher exposure improves the limit by a factor of ten at a mass of 100 GeV/c2. However, for masses.13 GeV/c2 there is hardly any improvement in the sensitivity compared to the sensitivity expected with an exposure of 500 kg days.
In conclusion, in the high mass region the sensitivity directly scales with exposure while at low masses exposure is not an important parameter.
3.2.2 Background
Another parameter that limits the sensitivity is the observed background. As explained before (see section 2.6), the limits derived from data of TUM40 and Lise both agree with the presence of e−/γ-background only, throughout the whole mass range. This indicates that the events in the acceptance region are probably produced by this background only.
Therefore, reducing the background means reducing the leakage of the e−/γ-band, which can be achieved byreducing the content of the band or/andreducing the overlap of the bands.
A reduction of the e−/γ-band content can be achieved byincreasing the radiopu-rity of the crystals (selection and/or cleaning of the raw materials, multiple grow-ing of the crystals [77, 83]) or by reducing the background from the surrounding of the detectors (e.g. selection and improved cleaning of materials, active veto).
The overlap of the bands can be reduced by areduction of the width of the bands, which is equivalent to anenhancement of the light detector resolution (e.g. increase of the amount of the detected light or an increase of the sensitivity of the light detector) or areduction of the non-proportionality of the light yield.
As the e−/γ-background is dominated by the intrinsic contamination of the crystals [83], a reduction of the overall background can be achieved with an improvement of the crystal radiopurity. An investigation of the crystal growth process and a development of a cleaning procedure of the raw materials is currently in progress [91]. Within the next years a reduction of the internal impurities of the crystals by a factor of 100 is in reach [84].
The expected sensitivities for a reduction of the total background by a factor of 10 (light magenta) and 100 (dark magenta) are shown in figure 3.4. As can be seen, a reduction of the background improves the sensitivity over the whole mass range.
For an improvement of the background by a factor of 10 a gain of sensitivity by a factor of 2–3 is expected for masses of 2–100 GeV/c2. In the mass range where the improvement is more pronounced (10–20 GeV/c2) the current sensitivity is more limited by backgrounds than in regions where a smaller improvement is expected. Only for masses below ∼ 2 GeV/c2 the gain in sensitivity is only ∼ 10 %. However, a further improvement of the background by a factor of 10, i.e. a factor of 100 in total, only results in a slight gain of the expected sensitivity. The maximum gain in sensitivity is ∼ 2 for a mass of ∼ 13 GeV/c2, while at masses . 4 GeV/c2 and & 40 GeV/c2 an improvement of.20 % is expected. This is due to the fact that the expected amount of background events in the assumed exposure is close to zero.
In conclusion, for the given exposure of 29 kg days a reduction of the background by a factor of 10 improves the sensitivity for almost the complete mass range shown here, while a further reduction has only little effect. However, a background reduction pays out more when the exposure is increased significantly.
3.2.3 Threshold
The performance of the phonon detector is defined by its signal-to-noise ratio. A larger signal-to-noise ratio improves the energy resolution and accordingly reduces the energy
Figure 3.4: The expected sensitivities (1σ C.L.) for a module with a background reduced by a factor of 10 (light magenta) and 100 (dark magenta) compared to TUM40 shown in the parameter space for elastic spin-independent dark matter nucleon scattering. All projections are simulated for an exposure of 29 kg days in order to compare them to the projection for TUM40.
Compared to the current sensitivity obtained with the given background a slight improvement of the sensitivity over the whole mass range is expected. However, improving the background by a factor of 100 only results in a marginal sensitivity increase due to the low number of expected background events in the exposure of 29 kg days. For comparison the currently leading limits of different experiments (solid lines) and the neutrino floor for CaWO4 (gray-shaded area) are shown [37, 45, 49, 67, 74].
threshold. The lowest energy threshold achieved with CRESST detectors in phase 2 was 307 eV [74]. A further reduction can be achieved by either increasing the signal or by reducing the noise of the detector. While the noise is dominated by the parameters of the TES and the SQUID (see section 4.3) and is difficult to reduce, an increase of the signal can be achieved by reducing the size of the target crystal or a further optimization of the TES.
From the detector physics (see chapter 4) it is expected that a reduction of the crystal volume by a factor of 10 combined with an optimization of the detector layout translates in a reduction of threshold from the 603 eV reached in TUM40 [67] to at least 100 eV (see section 5.1). First measurements indicate that with the optimized detector module even lower thresholds down to∼20 eV can be reached (see section 7.2.2).
Figure 3.5 illustrates the sensitivities that can be reached with a threshold of 20 eV (light yellow), 50 eV (medium yellow), and 100 eV (dark yellow) with all parameters concerning the light detector performance and backgrounds as observed in the TUM40 module. It can be seen that for masses below∼3 GeV/c2 the sensitivity increases with a lower threshold, whereas in the high-mass region almost no difference is visible. With thresholds of 100 eV and 20 eV the parameter space down to masses of ∼0.25 GeV/c2 and ∼ 0.13 GeV/c2 can be explored, respectively. For a dark matter particle mass of 1 GeV/c2 a threshold of 100 eV enhances the sensitivity by three orders of magnitude and for a threshold of 20 eV the sensitivity is improved by more than four orders of magnitude.
In conclusion, a reduction of threshold enhances the sensitivity in the low mass region drastically and allows to explore lower dark matter particle masses. For the high-mass region this parameter is not decisive.