Low-mass WIMPs Research Articles

Searching for WIMPs with TREX-DM: achievements and challenges

The TREX-DM detector, a low background chamber with microbulk Micromegas readout, was commissioned in the Canfranc Underground Laboratory (LSC) in 2018. Since then, data taking campaigns have been carried out with argon and neon mixtures, at different pressures from 1 to 4 bar. By achieving a low energy threshold of 1 keV ee and a background level of 80 counts keV-1 kg-1 day-1 in the region from 1 to 7 keV ee , the experiment demonstrates its potential to search for low-mass WIMPs. Two of the most important challenges currently faced are the reduction of both, background level and energy threshold. With respect to the energy threshold, recently a new readout plane is being developed, based on the combination of Micromegas and GEM technologies, aiming to have a pre-amplification stage that would permit very low energy thresholds, close to the single-electron ionization energy. With respect to the background reduction, apart from studies to identify and minimize contamination population, a high sensitivity alpha detector is being developed in order to allow a proper material selection for the TREX-DM detector components. Both challenges, together with the optimization of the gas mixture used as target for the WIMP detection, will take TREX-DM to explore regions of WIMP's mass below 1 GeV c -2.

Study of low-energy nuclear recoils in liquid argon with the ReD experiment

Liquid Argon (LAr) Time Projection Chambers (TPC) operating in double-phase can detect the nuclear recoils (NR) possibly caused by the elastic scattering of WIMP dark matter particles via light signals from both scintillation and ionization processes. In the scenario of a low-mass WIMP (< 2 GeV/c2), the energy range for the NRs would be below 20 keV, thus making it crucial to characterize the ionization response in LAr TPCs as the lone available detection channel at such low energy. The Recoil Directionality (ReD) project, within the Global Argon Dark Matter Collaboration, aims to measure the ionization yield of a LAr TPC in the recoil energy range of 2–5 keV. The measurement was performed in winter 2023 at the INFN Sezione of Catania and the analysis is ongoing.

Electron transfer efficiency in liquid xenon across THGEM holes

Dual-phase liquid-xenon time projection chambers (LXe TPCs) deploying a few tonnes of liquid are presently leading the search for WIMP dark matter. Scaling these detectors to 10-fold larger fiducial masses, while improving their sensitivity to low-mass WIMPs presents difficult challenges in detector design. Several groups are considering a departure from current schemes, towards either single-phase liquid-only TPCs, or dual-phase detectors where the electroluminescence region consists of patterned electrodes. Here, we discuss the possible use of Thick Gaseous Electron Multipliers (THGEMs) coated with a VUV photocathode and immersed in LXe as a building block in such designs. We focus on the transfer efficiencies of ionization electrons and photoelectrons emitted from the photocathode through the electrode holes and show experimentally that efficiencies approaching 100% can be achieved with realistic voltage settings. The observed voltage dependence of the transfer efficiencies is consistent with electron transport simulations once diffusion and charging-up effects are included.

Search for low mass WIMP dark matter with DarkSide-50

DarkSide-50 is a direct dark matter detection experiment at Laboratori Nazionali del Gran Sasso that uses argon as the target material. Exploiting the ionization signal from a dual-phase time projection chamber filled with low-radioactivity argon, it has set the most stringent exclusion limit on WIMPs with a mass of few \mathrm{GeV}/c^2GeV/c2. A new analysis has recently been carried out with a larger exposure, profiting from an improved understanding of the detector response and background model. An improvement of about a factor 10 in sensitivity is expected for a WIMP mass of 2 GeV/c^22.

Coating μm TPB on a cylindrical detector and studying the sample films being cooled to LN and LHe temperatures

ALETHEIA is a newly established dark matter direct detection project that aims at hunting for low-mass WIMPs. TPB is widely implemented in liquid helium and argon experiments to shift VUV photons to visible light. We first report that we have successfully coated ∼ 3 μm TPB on the inner walls of a 10-cm cylindrical PTFE detector; we split the coating process into two steps to have all of the surfaces being coated with the same thickness; three independent methods were applied to figure out the thickness of the TPB coating layers, and consistent results were obtained. Second, with an SEM machine, we scanned the surface of TPB coating sample films exposed to different cryogenic temperatures. The first group of sample layers were immersed into a liquid nitrogen dewar for forty hours, the second group samples were cooled to 4.5 K for three hours, and the third group stayed at room temperature after coating. The SEM-scanned images of the sample films barely show any noticeable difference.

Preliminary studies on TPB coating on inner walls of ten-cm-scale cylindrical PTFE detector

&lt;sec&gt;ALETHEIA (a liquid hElium time projection cHambEr In dark matter) project is an originally creative dark matter experiment aiming to search for low-mass (100 MeV/c&lt;sup&gt;2&lt;/sup&gt;–10 GeV/c&lt;sup&gt;2&lt;/sup&gt;) WIMPs. While there have existed more than ten experiments doing research on low-mass WIMPs, the ALETHEIA is supposed to grow up to be a leading project worldwide due to many unique advantages, including but are not limited to extremely low intrinsic backgrounds, easy purification , and strong potential capability of signal/background discrimination. Owing to the project’s original creativity, there has existed no direct experience of building such a detector yet; consequently, we have to launch a set of R&amp;D programs from scratch, including the TPB coating process conveyed in this paper.&lt;/sec&gt;&lt;sec&gt;An incident particle that hits a liquid helium detector would generate 80-nm-long scintillation. There are currently no commercially available photon detectors capable of efficiently detecting the scintillation light and a wavelength converter must be used to convert the 80-nm-long scintillator into visible light. Silicon photomultipliers (SiPMs) can then be implemented to detect the 450-nm-wavelength light. The TPB (Tetraphenyl Butadiene, 1, 1,4, 4-tetraphenyl-1, 3-butadiene) is widely used for realizing the conversion. Although in thedark matter experiment using argon pulse-shape discrimination (DEAP) , 2.3-μm-thick TPB is successfully coated on the inner wall of the sphere with a radius of 85 cm, we cannot mimic the whole process in our experiment directly out of the two following reasons: (a) our detector shape is cylindrical, not spherical, and (b) the diameter of the current detector prototype is only 10 cm, while the one of the DEAP detectors is as large as 1.7-meter. Consequently, we must design and build an appropriate coating apparatus suitable for our detector. Owing to the existence of necessary auxiliary parts (such as cables for heating and temperature sensors), on which some vapored TPB molecules would be deposited when the coating is in progress. As a result, a blind spot on the inner wall always exists that cannot be fully coated; the blind spot area will affect the visible light yield of 80-nm-long scintillation. To solve the problem, we split the coating process into two steps: coating the curved surface and one base together in the first step and coating another base in the second step. In this way, the cylindrical detector's whole inner wall (the curved surface and the two bases) will be coated. Another key technology is to design an appropriate source sphere containing TPB powder. There are 20 holes evenly distributed on the surface of the sphere. After the TPB powder is heated andevaporated into the gas, the TPB molecules should move slowly enough to ensure that they scatter from each other long enough within the source before randomly finding a hole to escape. As a result, the TPB molecules come out of the source in an isotropic way then adhere to the inner surfaces of a cylindrical detector (diameter and height are both 10 cm) with nearly the same thickness. The TPB coating thickness on the inner wall is in a range between 1.50 and 3.02 μm, which corresponds to the thinnest and thickest TPB plate, respectively. The variation mainly comes from the different distances from the coating place to the source, which lies at the center of the PTFE cylinder. The thickness difference will not bother us because the conversion efficiency for 80-nm-long scintillation is almost the same as that for the TPB thickness in a range from 0.7 to 3.7 μm.&lt;/sec&gt;&lt;sec&gt;In addition to introducing the ALETHEIA project briefly at the beginning, we mainly address several aspects of TPB coating: coating principle, source design, coating process, coating thickness monitoring, and the comparison of thickness among coating plates from three independent methods. The whole process addressed in this paper is expected to provide a valuable reference for other experiments with similar requirements.&lt;/sec&gt;

Operation update and Calibration plan for the Scintillating Bubble Chamber (SBC) Collaboration’s 10-kg LAr detector at Fermilab

The Scintillating Bubble Chamber (SBC) Collaboration is developing a novel detection technique aimed at detecting low-mass (0.7-7 GeV/c2) WIMP interactions and coherent elastic neutrino-nucleus scattering (CEυNS) of reactor neutrinos. Using a target volume composed of super-heated argon with a tiny amount of dissolved xenon, the nucleation signal from electron recoils (the limiting factor for low-threshold studies in bubble chambers) is suppressed, allowing for the exploration of new parameter space. Particle interactions with the target fluid can lead to the production of heat (bubbles) and scintillation light. By combining these observables, the SBC Collaboration is aiming to reach a threshold of 100 eV for nuclear recoil detection with discrimination, to be demonstrated for the first time in a 10-kg device at Fermilab.In this paper, the current activities at Fermilab towards the commissioning and operation of the detector will be presented as well as the calibration run plan, which aims to measure the response of this chamber to both electron- and nuclear-recoils at the target 100 eV threshold.

The search for Light Dark Matter with NEWS-G

The NEWS-G direct dark matter search experiment uses spherical proportional counters (SPC) with light noble gases to explore low WIMP masses. The first results obtained with an SPC prototype operated with Ne gas at the Laboratoire Souterrain de Modane (LSM) have already set competitive results for low-mass WIMPs. The forthcoming next phase of the experiment consists of a large 140 cm diameter SPC installed at SNOLAB with a new sensor design, with improved detector performance and data quality. Before its installation at SNOLAB, the detector was commissioned with pure methane gas at the LSM, with a temporary water shield, offering a hydrogen-rich target and reduced backgrounds. After giving an overview of the improvements of the detector, preliminary results of this campaign will be discussed, including UV laser and Ar-37 calibration data.

Studies on liquid argon S1 and S2 properties for low mass WIMP search experiments

Liquid argon is known as an excellent target material for WIMP dark matter direct search experiment. Relatively small atomic mass (A=40) gives high nuclear recoil energy for WIMP-Ar nuclear scattering, thus it potentially has high sensitivity for low mass WIMP (<10 GeV/c2) . There are two crucial R&D topics for the liquid argon detector to explore such low mass WIMP signal, namely, deep understanding the liquid argon scintillation and ionization process for low energy deposition (<10 keV), and increasing the signal light yield by improving the argon scintillation light collection efficiency. In this paper, we report recent results from Waseda university liquid argon test facility.

The TREX-DM experiment at the Canfranc Underground Laboratory

TREX-DM (TPC Rare Event eXperiment for Dark Matter) is intended to look for low mass WIMPs in the Canfranc Underground Laboratory (LSC) in Spain, using light elements (Ne, Ar) as target in a high pressure TPC equipped with Micromegas readouts. Here, a description of the detector, the first results from commissioning data and the expected sensitivity from the developed background model are briefly presented.

Status and prospect of the ANKOK project: Low mass WIMP dark matter search using double phase argon detector

The ANKOK project is a dark matter search experiment using an argon detector specialized for the low mass (∼ 10 GeV/c2) WIMP detection. We are currently proceeding with R&D efforts to understand the background sources, develop and apply new photo sensor that is directly sensitive to 128 nm argon scintillation light and improve electron recoil type background rejection. In the next few years, we are planning to construct such an argon detector in an underground laboratory with enough sensitivity to detect low mass WIMP.

Status of the TREX-DM experiment at the Canfranc Underground Laboratory

The TREX-DM experiment is conceived to look for low mass WIMPs by means of a gas time projection chamber equipped with novel micromegas readout planes at the Canfranc Underground Laboratory. The detector can hold 20 l of pressurized gas up to 10 bar, which corresponds to 0.30 kg of Ar, or alternatively, 0.16 kg of Ne. The micromegas will be read with a self-triggered acquisition, allowing for effective thresholds below 0.4 keV (electron equivalent). The preliminary background model, following a complete material screening program, points to levels of the order of 1-10 counts keV−1 kg−1 d−1 in the region of interest, making TREX-DM competitive. The status of the commissioning, description of the background model and the corresponding WIMP sensitivity will be presented here.

Electron-interacting dark matter: Implications from DAMA/LIBRA-phase2 and prospects for liquid xenon detectors and NaI detectors

We investigate the possibility for the direct detection of low mass (GeV scale) WIMP dark matter in scintillation experiments. Such WIMPs are typically too light to leave appreciable nuclear recoils, but may be detected via their scattering off atomic electrons. In particular, the DAMA Collaboration [R. Bernabei et al., Nucl. Phys. At. Energy 19, 307 (2018)] has recently presented strong evidence of an annual modulation in the scintillation rate observed at energies as low as 1 keV. Despite a strong enhancement in the calculated event rate at low energies, we find that an interpretation in terms of electron-interacting WIMPs cannot be consistent with existing constraints. We also demonstrate the importance of correct treatment of the atomic wavefunctions, and show the resulting event rate is very sensitive to the low-energy performance of the detectors, meaning it is crucial that the detector uncertainties be taken into account. Finally, we demonstrate that the potential scintillation event rate can be much larger than may otherwise be expected, meaning that competitive searches can be performed for GeV scale WIMPs using the conventional prompt S1 scintillation signals. This is important given the recent and upcoming very large liquid xenon detectors.

Status of low mass WIMP detector TREX-DM

TREX-DM (TPC Rare Event eXperiment for Dark Matter) is a high-pressure Micromegas-based TPC designed to host a few hundreds of grams of light nuclei (argon or neon) with energy thresholds potentially at the level of 0.4 keVee or below. Preliminary background studies suggest that the levels expected are of the order of 1–10 counts keV−1 kg−1 d−1, making TREX-DM competitive in the search for low mass WIMPs (<10 GeV). The experiment has been approved by the LSC (Laboratorio Subterrneo de Canfranc) and the last months have been devoted to its installation underground. Here we will report on the first commissioning phase.

A HEMT-based cryogenic charge amplifier with sub-100 eVee ionization resolution for massive semiconductor dark matter detectors

We present the measured baseline ionization resolution of a HEMT-based cryogenic charge amplifier coupled to a CDMS-II detector. The amplifier has been developed to allow massive semiconductor dark matter detectors to retain background discrimination at the low recoil energies produced by low-mass WIMPs. We find a calibrated baseline ionization resolution of $\sigma_E = 91\,\text{eV}_{ee}$. To our knowledge, this is the best direct ionization resolution achieved with such massive ($\approx$150 pF capacitance) radiation detectors.

Novel approach to assess the impact of the Fano factor on the sensitivity of low-mass dark matter experiments

As first suggested by U. Fano in the 1940s, the statistical fluctuation of the number of pairs produced in an ionizing interaction is known to be sub-Poissonian. The dispersion is reduced by the so-called "Fano factor", which empirically encapsulates the correlations in the process of ionization. In modelling the energy response of an ionization measurement device, the effect of the Fano factor is commonly folded into the overall energy resolution. While such an approximate treatment is appropriate when a significant number of ionization pairs are expected to be produced, the Fano factor needs to be accounted for directly at the level of pair creation when only a few are expected. To do so, one needs a discrete probability distribution of the number of pairs created $N$ with independent control of both the expectation $\mu$ and Fano factor $F$. Although no distribution $P(N|\mu,F)$ with this convenient form exists, we propose the use of the COM-Poisson distribution together with strategies for utilizing it to effectively fulfill this need. We then use this distribution to assess the impact that the Fano factor may have on the sensitivity of low-mass WIMP search experiments.

Defect Creation in Crystals: A Portal to Directional Dark Matter Searches

A large body of astrophysical observations indicate that around 85% of the matter in the universe is not made of recognized standard model particles. Understanding the nature of this so-called dark matter is of fundamental importance to cosmology, astrophysics, and high-energy particle physics. We examine the response of commonly used semiconductor materials to low-mass WIMP interactions using numerical simulations based on classical interatomic potentials in these materials. These simulations, backed up by more precise density functional theory simulations and experiments, predict a nonlinear energy loss that never produces phonons due to the nonzero energy required to form crystallographic defects. We argue that such nonlinear effects related to defect formation in electron-volt-scale resolution semiconductor detectors allows for very effective directional sensitivity and possible statistical nuclear recoil discrimination to dark matter signals for masses below 1 $$\hbox {GeV}/c^2$$ .

Micromegas for dark matter searches: CAST/IAXO &amp; TREX-DM experiments

The most compelling candidates for Dark Matter to day are WIMPs and axions. The applicability of gasesous Time Projection Chambers (TPCs) with Micromesh Gas Structures (Micromegas) to the search of these particles is explored within this work. Both particles would produce an extremely low rate at very low energies in particle detectors. Micromegas detectors can provide both low background rates and low energy threshold, due to the high granularity, radiopurity and uniformity of the readout. Small (few cm wide) Micromegas detectors are used to image the axion-induced x-ray signal expected in the CERN Axion Solar Telescope (CAST) experiment. We show the background levels obtained in CAST and the prospects to further reduce them to the values required by the Internation Axion Observatory (IAXO). We also present TREX-DM, a scaled-up version of the Micromegas used in axion research, but this time dedicated to the low-mass WIMP detection. TREX-DM is a high-pressure Micromegas-based TPC designed to host a few hundreds of grams of light nuclei (argon or neon) with energy thresholds potentially at the level of 100 eV. The detector is described in detail, as well as the results of the commissioning and characterization phase on surface. Besides, the background model of TREX-DM is presented, along with the anticipated sensitivity of this search, which could go beyond current experimental limits.

Supernovae and Weinberg\u2019s Higgs portal dark radiation and dark matter

The observed burst duration and energies of the neutrinos from Supernova 1987A strongly limit the possibility of any weakly-interacting light particle species being produced in the proto-neutron star (PNS) core and leading to efficient energy loss. We reexamine this constraint on Weinberg’s Higgs portal model, in which the dark radiation particles (the Goldstone bosons) and the dark matter candidate (a Majorana fermion) interact with Standard Model (SM) fields solely through the mixing of the SM Higgs boson and a light Higgs boson. In order for the Goldstone bosons to freely stream out of the PNS core region, the Higgs portal coupling has to be about a factor of 4-9 smaller than the current collider bound inferred from the SM Higgs invisible decay width. We find that in the energy loss rate calculations, results obtained by using the one-pion exchange (OPE) approximation and the SP07 global fits for the nucleon-nucleon total elastic cross section differ only by a factor ≲ 3. The SN 1987A constraints surpass those set by laboratory experiments or by the energy loss arguments in other astrophysical objects such as the gamma-ray bursts, even with other nuclear uncertainties taken into account. Furthermore, the SN 1987A constraints are comparable to bounds from the latest dark matter direct search for low-mass WIMPs (≲10 GeV.)

Proposed low-energy absolute calibration of nuclear recoils in a dual-phase noble element TPC using [formula omitted] neutron scattering kinematics

We propose a new technique for the calibration of nuclear recoils in large noble element dual-phase time projection chambers used to search for WIMP dark matter in the local galactic halo. This technique provides an insitu measurement of the low-energy nuclear recoil response of the target media using the measured scattering angle between multiple neutron interactions within the detector volume. The low-energy reach and reduced systematics of this calibration have particular significance for the low-mass WIMP sensitivity of several leading dark matter experiments. Multiple strategies for improving this calibration technique are discussed, including the creation of a new type of quasi-monoenergetic neutron source with a minimum possible peak energy of 272keV. We report results from a time-of-flight-based measurement of the neutron energy spectrum produced by an Adelphi Technology, Inc. DD108 neutron generator, confirming its suitability for the proposed nuclear recoil calibration.