Session Y20: Innovative Ideas and Techniques for Dark Matter Detection

Live

In recent years, much effort has been made to better understand the low energy nuclear recoil (NR) response in liquid xenon (LXe), allowing experiments to be increasingly sensitive to light dark matter. We propose a technique for ultra-low energy NR calibration using the recoils imparted to xenon nuclei during the de-excitation process following thermal neutron capture, where the instantaneous gamma cascade leaves the nuclei with less than 0.3 keV of recoil energy. A successful measurement of the quanta yields below this point will contribute to a greater sensitivity for LXe experiments that will benefit from a lower energy threshold, mainly those searching for light dark matter and coherent neutrino-nucleus scattering. We describe the calibration technique, including simulation and signal optimization, and its feasibility for a small LXe detector using a pulsed neutron source.   

Live

The Scintillating Bubble Chamber (SBC) is a rapidly developing new technology for 0.7 - 7 GeV nuclear recoil detection. Demonstrations in liquid xenon at the few-gram scale have confirmed that this technique combines the event-by-event energy resolution of a liquid-noble scintillation detector with the world-leading electron-recoil discrimination capability of the bubble chamber, and in fact maintains that discrimination capability at much lower thresholds than traditional Freon-based bubble chambers. The promise of unambiguous identification of sub-keV nuclear recoils in a scalable detector makes this an ideal technology for both GeV-mass WIMP searches and CEvNS detection at reactor sites. We will present progress from the SBC Collaboration towards the construction of a pair of 10-kg argon bubble chambers at Fermilab and SNOLAB to test the low-threshold performance of this technique in a physics-scale device and search for dark matter, respectively.   

Live

For low-mass dark matter search, an experiment with a cylindrical 1kg undoped CsI crystal coupled directly to two photomultiplier tubes at about 77K was conducted and a light yield of 26.0\textpm 0.4 photoelectrons per keV electron-equivalent was achieved. The presumed prototype consisted of 10kg undoped CsI or NaI scintillation crystals directly coupled with SiPM arrays operated at 77K was assumed to have a much higher light yield compared to the COHERENT CsI(Na) detector. This eliminated the concern of self light absorption in large crystals raised in some of the early studies. For neutrino detection, the light yield of an undoped CsI crystal at about 77 Kelvin was measured to be 33.5\textpm 0.7 PE per keVee in the energy range of [13, 60] keVee using X and $\gamma $-rays from an $^{\mathrm{241}}$Am radioactive source. Based on this experimental result, the performance of 10kg cryogenic inorganic scintillating crystals coupled to SiPM arrays to probe non-standard neutrino interactions (NSIs) through the detection of coherent elastic neutrino-nucleus scatterings (CEvNS) at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory (ORNL), was examined in detail.   

Not Participating

Dark Matter with an attractive Yukawa coupling to nucleons, at a level allowed by Direct Detection and CMB experiments, can bind to nuclei in Earth. Sexaquark DM (SDM), is an example of this type of scenario. With an expected mass ~ 1.8-2 GeV, binding would lead to formation of exotic isotopes with masses having the very challenging mass offset of O(2 amu) from standard isotopes. Predictions for abundances vary dramatically with the Yukawa coupling, due to possible resonant (and anti-resonant) DM-nuclei interactions, moreover geochemistry of residence times is in not all cases well-known, making a theoretical interpretation challenging as well. In this talk, we report results of a recent dedicated laboratory geochemical and accelerator-mass-spectrometer search for exotic isotopes of noble gases and Oxygen, having unprecedented sensitivity in this only-now-explored mass range. The implications for the SDM scenario will be discussed.   

Live

Nuclear recoil (NR) calibrations are vital for understanding detector responses to dark matter candidates and neutrino-nucleus signals in direct detection experiments. Low-mass ($<$5 GeV) dark matter candidates and $^8$B neutrinos drive the need for high-statistics/low-systematic calibrations at even lower NR energies. We report the results of measurements made at Brown University demonstrating the effectiveness of an Adelphi Technologies Inc. DD neutron generator and deuterated scintillator in a carefully shielded geometry to shift the neutron beam energy from 2.45 MeV (94 keV FWHM) to 350 keV (85 keV FWHM). This low energy, monoenergetic source is fully portable and usable in situ to measure NR events in a range of detector technologies. The lower neutron speed allows the tagging of distinct S1 signals for multiple scatters within tonne-scale liquid noble time projection chambers (TPCs) and permits direct in situ measurement of light yield (L$_y$) independent of charge yield (Q$_y$). The scintillator reflector allows per-neutron energy determination via time-of-flight (ToF) and pulse size measurements, providing a powerful calibration source with few systematic uncertainties. A ToF-based hydrogen reflector source with a tunable neutron energy from 10-100 keV is also discussed.   

Live

The microwave cavity experiment based on the resonant conversion of axions to photons in a magnetic field proposed by Sikivie in 1983 has provided the strongest limits to date on the QCD axion. Current searches are extendable up to the range of 10 GHz (\textasciitilde 40 \textmu eV), but concepts for practical resonators for higher frequencies that possess both large volume and good form factor are lacking. A concept has recently been published however, based on a metamaterial comprised of a 2D array of thin wires, whereby in a magnetic field the axion couples directly to its plasmonic modes [1]. With wire arrays similar to those used in high energy physics detectors (tens of microns diameter, few millimeter spacing), the plasma frequency can be adjusted to be \textgreater 10 GHz and importantly, the volume can be arbitrarily large. We have carried out experimental studies of 2D wire array metamaterials built up of individual wire planes extending earlier measurements [2,3], and finding excellent agreement with theoretical models [2,4]. Furthermore, we have explored new configurations to find a practical mechanism for tuning the array over a wide dynamic range in frequency, with promising results.   

Live

The identity of dark matter remains one of the most urgent mysteries in fundamental physics. With some leading direct detection experiments now observing background events and WIMP-nucleon scattering limits approaching the neutrino floor, there is renewed interest in constructing an observatory capable of detecting and distinguishing WIMP and coherent elastic neutrino-nucleus scattering (CEvNS) via directionality. The CYGNUS proto-collaboration aims to deploy gas-target time projection chambers (TPCs) capable of event-by-event nuclear recoil imaging. Smaller, near-term detectors with this capability would enable new precision measurements, searches for beyond the Standard Model (BSM) physics, and measurements of solar neutrinos. A large detector could establish the galactic origin of a dark matter signal, and subsequently be used to map the local WIMP velocity distribution and explore the particle phenomenology of dark matter. Therefore, there exists an opportunity to develop a long-term, diverse, and cost-effective experimental program around directional detection of nuclear recoils in gas TPCs at different scales. I will discuss the projected dark matter sensitivity, compare the suitability of different technological approaches, and comment on the broader physics case.   

Live

The ALETHEIA (A Liquid hElium Time projection cHambEr In dArk matter) aims to hunt for low-mass WIMPs with a novel technology: filling liquid helium into a TPC (Time Projection Chamber). The liquid helium is at $\sim$ 4 K, not in a superfluid state. While the TPC has been implemented for LXe (Liquid Xenon) and LAr (Liquid Argon) DM (Dark Matter) experiments and achieved great success, no single LHe TPC has ever been demonstrated suitable for DM hunting yet. The ALETHEIA collaboration would pioneer the research of an LHe TPC for DM hunting. The project has been successfully reviewed by a panel composed of leading physicists in the community of DM and LHe in Oct 2019. We are currently at the stage of prototypes R\&D. We will present our preliminary results on testing some of the prototype detectors in the APS April meeting of 2021.   

Live

In 2016, the ATOMKI collaboration announced [PRL \mathbf{116}, 042501 (2016)] observing an unexpected enhancement of the $e^+$-$e^-$ pair production signal in one of the $^8$Be nuclear transitions induced by an incident proton beam on a $^7$Li target. Many beyond-standard-model physics explanations have subsequently been proposed. One popular theory is that the anomaly is caused by the creation of a protophobic vector boson ($X$) with a mass around 17 MeV [e.g., PRL \mathbf{117}, 071803 (2016)] in the nuclear transition. In this talk, I will discuss our recent study (arXiv: 2008.11288) which disproves this hypothesis. I will start with an isospin relation between photon and $X$ couplings to nucleons, and then explain how these relationships suggest that the $X$ production must be dominated by direct transitions without going through any nuclear resonance (i.e., Bremsstrahlung radiation) with a smooth energy dependence that occurs for all proton beam energies above threshold. This energy dependence contradicts the experimental observations and invalidates the protophobic vector boson explanation.