Abstract
We investigate the diurnal modulation of the event rate for dark matter scattering on solid targets arising from the directionally dependent defect creation threshold energy. In particular, we quantify how this effect would help in separating dark matter signal from the neutrino background. We perform a benchmark analysis for a germanium detector and compute how the reach of the experiment is affected by including the timing information of the scattering events. We observe that for light dark matter just above the detection threshold the magnitude of the annual modulation is enhanced. In this mass range using either the annual or diurnal modulation information provides a similar gain in the reach of the experiment, while the additional reach from using both effects remains modest. Furthermore, we demonstrate that if the background contains a feature exhibiting an annual modulation similar to the one observed by DAMA experiment, the diurnal modulation provides for an additional handle to separate dark matter signal from the background.
Highlights
As the exclusion limits for direct detection of dark matter (DM) particles in the traditional WIMP mass range are approaching the neutrino floor, more focus is being shifted toward low-mass DM with mDM ≪ 10 GeV, where the recoil kinematics forbid efficient detection in, e.g., a liquid xenon target
It is plausible to take the DAMA result as a signal of an unknown background feature whose modulation coincides with that of the DM background. In such case we demonstrate that the diurnal modulation offers an additional quantitatively important criterion to separate the DM signal from the background
We plan to study the utility of the diurnal modulation feature in the NaI target in future work, and report here the potential gain in sensitivity of a germanium detector from using the intraday time information of the event rate, in the case that the background perfectly imitates the annual modulation of DM
Summary
As the exclusion limits for direct detection of dark matter (DM) particles in the traditional WIMP mass range are approaching the neutrino floor, more focus is being shifted toward low-mass DM with mDM ≪ 10 GeV, where the recoil kinematics forbid efficient detection in, e.g., a liquid xenon target. For the particular case study of a germanium target, since both modulation amplitudes are of the same order, we show that the improvement of the reach of the experiment below the neutrino floor is saturated by using either of the modulation signals, and additional gain from using both is only obtained for a prohibitively large exposure. This is because the energy scale for the threshold anisotropy (diurnal modulation) is of the same order as the DM energy modulation due to the seasonal variation of the velocity of Earth with respect to the DM halo (annual modulation).
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