There is evidence that most of the matter in the Universe is made yet unknown form of dark matter that builds large structures in the Universe but does not interact with light. The existence of dark matter, that might interact very weakly with normal matter, is one of the strongest indications that there must be physics beyond the standard model of particle physics, as no known particle can be associated with dark matter. The “dark matter problem” is one of the most important open issues in modern physics as it suggests that mankind actually only properly understands a tiny fraction (in terms of mass) of the Universe.
One strategy to detect dark matter is to search for their rare interactions with atomic nuclei. The huge background from cosmic rays and natural radioactivity requires that the experiments are well shielded and that the materials used to construct the detector, as well as the dark matter target itself, do not contain radioactive contaminants. The ultimate background for the dark matter search stems from neutrinos which produce a signal that is identical to that from a dark matter interaction. The most sensitive dark matter detectors so far are dual-phase time projection chambers (TPCs) filled with cryogenic xenon gas in liquid form (LXe).
ULTIMATE’s goal was to explore how one can build the ultimate LXe-based dark matter detector that is capable to explore the entire accessible parameter space, down to the limit from neutrinos. The background of such detector needs to be dominated by neutrino-induced interactions, i.e. all other background sources need to be suppressed well below this level. To reach the design sensitivity and background level, a cylindrical TPC of about 2.6m height and diameter, containing 40t of LXe, is required. We focused on several crucial aspects related to the background and how such large detector can be constructed mechanically. In addition, we studied several science channels beyond dark matter which can be addressed by a 40t LXe TPC as well.
The ULTIMATE objectives were
1. To experimentally demonstrate whether background from radon-222, dominating the background of current detectors, can be mitigated by a novel hermetic TPC design which separates the active from the passive LXe.
2. To investigate the intrinsic radiopurity of PTFE, an important structural and optical material in LXe TPCs. The level of purity will directly affect the detector’s background level.
3. Since a low-background LXe TPC of 2.6m diameter has never been built before, a test-platform was developed to allow testing full-scale detector components in a cryogenic LXe environment.
4. To investigate whether the charge signal from a particle interaction can be amplified in a novel way in liquid xenon (instead of in xenon gas) as this would simplify detector design and operation.
5. To investigate science channels beyond dark matter which could be explored with the ultimate dark matter detector.
