Felix Wagner
PostDoc, Superconducting Quantum Bits and Sensors
Analysis and characterization of helium targets with transition edge sensors (ACE of HEARTS) is a proposed project to be done as an Erwin Schrödinger Fellowship cooperatively between the University of California Berkeley (UCB) and the Institute of High Energy Physics of the Austrian Academy of Sciences. UCB is involved in the HeRALD experiment, a dark matter direct detection experiment using superfluid helium targets. ACE of HEARTS will be performed as part of the HeRALD collaboration, focusing on the development of suitable methods to analyze the experimental data. The experiment is expected to produce rich information through non-trivial pulse shapes and coincidences in sensors. Therefore the methods applied in the analysis have a significant impact on the physics reach of the experiment.
I am using this page to post updates on the status of the project. Read more on the topic below!
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The HeRALD experiment
The HeRALD concept paper (PhysRevD.100.092007) introduces the experiment and most of the plots on this page are currently sources from that paper. It is part of the larger TESSERACT collaboration that also includes the SPICE effort, summarized in a SNOWMASS letter of interest.
The HeRALD experiment aims to detect sub-GeV dark matter particles using a superfluid helium target in microcalorimeters, which will be sensitive to the scattering of dark matter particles off helium nuclei. The experiment utilizes the advantageous properties of superfluid helium, including a light nuclear mass for better kinematic matching with light dark matter particles, copious production of scintillation light, extreme intrinsic radiopurity, high impedance to external vibration noise, and a unique "quantum evaporation" signal channel enabling the detection of phonon-like modes via liberation of 4He atoms into a vacuum. The experiment detects both scintillation photons and triplet excimers using calorimeters, including calorimeters immersed in the superfluid. Kinetic excitations of the superfluid medium (rotons and phonons) are detected using quantum evaporation and subsequent atomic adsorption onto a calorimeter suspended in vacuum above the target helium.
The figure on the right is from PhysRevD.100.092007 and a schematic side view of the detector. The light blue medium is the 4He target, the red strips are carriers for TES sensors.
A pulse shape model for helium targets with transition edge sensors
A particle recoil in the 4He produces a temperature pulse in the TES with three components. The first component is from prompt scintillation light, the second from phonon-induces evaporation of helium atoms and the long tail is from scintillation light of triplet excimers. These pulse shapes are rich with information and not only the recoil energy of the particle can be reconstructed from the differences, but also the type and position of the interaction with the detector. One of the objectives of the ACE of HEARTS project is to understand these pulse shapes fundamentally and build practically useful (e.g. parametric) models to make simulations of the detector response and noise possible. These can then be used to improve the detector design, and simulate cut efficiencies and use analysis techniques based on supervised machine learning.
Mitigation of instrumental backgrounds through coincidence analysis
Instrumental backgrounds are currently a limiting factor for experiments using TES (see EXCESS). These backgrounds are likely stress-induced (see e.g. arXiv:2208.02790).
A method to veto these backgrounds can be to use multiple sensors simultaneously, and analyze the coincidences between them. For standard solid-state calorimeters the individual sensors would add non-negligible heat capacity to the system, making the sensor less sensitive. For the HeRALD detector concept this is not the case, due to the vacuum gap between the helium target and the sensors.
The figure left is adapted from D. N. McKinsey's talk at the IDM22.
Scalable light dark matter search with light nuclei and lowest backgrounds
The experiment is able to detect dark matter-nucleon elastic scattering, with projected sensitivities that demonstrate that even very small (sub-kg) target masses can probe wide regions of as-yet untested dark matter parameter space.
The figure right shows projected limits in the dark matter parameter space. The green region is already tested by existing experiments, the red lines surround the region that can be tested with HeRALD: 1 kg-day with 40 eV (curve 1, solid red), 1 kg-yr with 10 eV (curve 2, dashed red), 10 kg-yr with 0.1 eV (curve 3, dotted red), and 100 kg-yr with 1 meV (curve 4, dash-dotted red). The region below the black line is dominated by backgrounds from the "neutrino fog".
