# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 4 MKIDs in Particle Physics
### MKIDs for Dark Matter Experiments  
WIMPs are a possible dark matter candidate, but unfortunately their interaction rate, usually intended as interaction with some nuclei, is extremely low, \(<\)0.1 event/kg/day and the expected energy deposited per interaction is in the order of 10 keV. State of the art experiments which are trying to detect WIMPs exploit phonon and photon detectors to measure the ionization of the interacting medium as well as the phonon excitation in the medium to try and distinguish electron recoils from nuclear recoils, which might be due to WIMPs. Further improvements are being pursued in order to increase the sensitivity of the detectors to WIMPs and to further reduce unwanted background signals. Improvements to the state-of-the-art detectors are intended to go in the direction of increasing the sensitive area, increased energy collection and fabrication reliability. In this framework MKIDs can be a means to achieve such desired improvements given their sensitivity, their multiplexability and their insensitivity to inhomogeneities in critical temperature which, in return, coincides with a much easier to fabricate detector. To this day, only few applications for MKIDs to be deployed as dark matter detectors have been proposed. [PERSON] pioneered the idea [135] in 2008, and prototype detectors have been developed by Caltech and JPL [136; 137] based on multiple MKIDs spread on a massive absorber. Phonons caused by particle absorption would trigger most of them and time delays and signal intensities allow to localize the interaction location. In 2020 the idea was followed up by the [PERSON] group in LNGS which published their white paper on BULKIDs, a R&D project to exploit MKIDs for the detection of rare, low-energy processes [129]. [PERSON] et al. imagined the interacting medium to be the silicon substrate on which the MKIDs are fabricated. The interaction would excite phonons which are then trapped in the superconducting films exciting quasi-particles. The resulting quasi-particles are eventually trapped in the MKIDs provided that they are fabricated with a superconductor whose band gap is smaller than that of the absorbing material. In particular, the proposed instrument would be produced with a Ta phonon absorber and Al MKIDs. Said instrument would be expected to have an energy resolution of 58 eV for a 20 keV event in the detector. The BULKID devices proposed by [PERSON] et al. in [129] instead use 108 individual \(5\times 5\times 5\) mm\({}^{3}\) Si voxels weighing 0.29 g each as the interacting medium. These voxels are fabricated on the backside of the Si wafer where the MKIDs will then be structured. The primary goal of BULKID is the high-precision measurement of coherent elastic scattering of neutrinos off nuclei and could also be employed as a dark matter detector [129]. Dark matter WIMPs are intended to interact with the silicon voxels producing phonons that the MKIDs on the other side of the wafer is expected to detect. In particular, the fact that the voxels are separated one from another allows for a rough, but immediate, determination of the position where the interaction might have occurred. The superconducting material intended to be used for these BULKIDs detectors is a multilayer stacking of Al/Ti/Al whose critical temperature can be tuned to preference. A \(T_{c}\) of 805 mK is expected for tri-layers whose respective thickness are 13, 33, and 30 nm. An optimization of these devices is projected to allow for an energy resolution as low as 20 eV. In addition to the more canonical approaches just described, two further applications are being investigated. The detection mechanism remains mostly unvaried: weakly interacting massive particles (WIMPs) need to recoil off other particles. This interaction, in return,produces excited particles whose energy can be measured.