Session H9: Dark Matter with Lux/LZ, DarkSide and Axions and ADMX

First results from a microwave cavity axion search at $25~\mu\mathrm{eV}$: Overview

The axion is a well-motivated cold dark matter candidate first postulated to explain the absence of CP violation in strong interactions. Dark matter axions may be detected via their resonant conversion into photons in a high-$Q$ microwave cavity permeated by a strong magnetic field. In this talk I will present an overview of a newly operational cavity detector at Yale, which is the first such detector to incorporate a dilution refrigerator and Josephson parametric amplifier and thereby approach quantum-limited noise performance. I will discuss the first results from this experiment, which has excluded axion models with two-photon coupling $g_{a\gamma\gamma} > 2\times10^{-14} \mathrm{GeV}^{-1}$, a factor of $\simeq2.3$ above the benchmark KSVZ model, over the mass range $23.55~\mu\mathrm{eV} < m_a < 24.0~\mu\mathrm{eV}$. These are the first limits within the axion model band in the $10~\mu\mathrm{eV}$ mass decade.   

First results from a microwave cavity axion search at $25~\mu\mathrm{eV}$: Analysis

ADMX-HF searches for dark matter axions via Primakoff conversion into microwave photons in the gigahertz domain. Since 2012, tremendous effort has been made to build an axion detector working in this frequency range. By operating the system in a cryogen-free dilution refrigerator ($T\,=\,127\,\mathrm{mK}$) and integrating a Josephson Parametric Amplifier (JPA), we obtain a sufficiently low system noise temperature to exclude axion models with $g_{a\gamma\gamma} > 2\times10^{-14}\, \mathrm{GeV}^{-1}$ over the mass range $23.55~\mu\mathrm{eV} < m_a < 24.0~\mu\mathrm{eV}$. In this talk, I will discuss the statistical nature of the data, simulations, and the detailed data analysis procedure we used to optimize the sensitivity to axion signals.   

The ADMX Microwave Cavity: Present and future

The Axion Dark Matter eXperiment (ADMX), a direct-detection axion search, uses a tunable resonant cavity to enhance axion to photon conversion rates to a detectable level when the cavity resonance matches the mass of the axion. It has successfully taken data in the 460 -- 890 MHz frequency range and is now probing a similar range with much higher sensitivity.~However the axion mass is unknown and may be at higher frequencies than the currently operating system. In anticipation of future runs with an increased mass range, ADMX is conducting extensive research and development of microwave cavities. These developments include photonic band-gap cavities, multi-vane cavities, partitioned cavities, in-phase coupled cavities, and superconducting hybrid cavities. Many of these projects are in different stages between simulations and testing of physical prototypes. The status and current objectives of these projects will be presented.   

Recent progress on the Axion Dark Matter eXperiment (ADMX)

The Axion Dark Matter eXperiment (ADMX) is one of the three “Generation-2” direct dark matter searches and the only one dedicated to finding the axion. It looks for axions that convert into photons through the Primakoff process in the presence of a strong magnetic field. The mass of the axion is unknown but expected to be ~few to tens of $\mu$eV, which corresponds to photons in the GHz range. The expected signal power is of the order $10^{-24}$ W, which puts stringent requirements on the system’s noise level. ADMX has recently started its Generation-2 data run with the recent upgrades of a dilution refrigerator, which cools the system to sub-K temperature suppressing the thermal background noise and tunable, near quantum noise-limited SQUID amplifiers. This talk will summarize the current status and operation of ADMX experiment as it searches for dark matter axions.   

Reflectance measurements of PTFE, Kapton, and PEEK for xenon scintillation light for the LZ detector.

LZ (LUX-Zeplin) is an international collaboration that will look for dark matter candidates, WIMPs (Weakly Interacting Massive Particles), through direct detection by dual-phase time projection chamber (TPC) using liquid xenon. The LZ detector will be located nearly a mile underground at SURF, South Dakota, shielded from cosmic background radiation. Seven tons active mass of liquid xenon will be used for detecting the weak interaction of WIMPs with ordinary matter. Over three years of operation it is expected to reach the ultimate sensitivity of 2x10$^{\mathrm{-48}}$ cm$^{\mathrm{2}}$ for a WIMP mass of 50 GeV. As for many other rare event searches, high light collection efficiency is essential for LZ detector. Moreover, in order to achieve greater active volume for detection as well as reduce potential backgrounds, thinner detector walls without significant loss in reflectance are desired. Reflectance measurements of polytetrafluoroethylene (PTFE), Kapton, and PEEK for xenon scintillation light (178 nm), conducted at the University of Michigan using the Michigan Xenon Detector (MiX) will be presented.   

An Improved Nuclear Recoil Calibration in the LUX Detector Using a Pulsed D-D Neutron Generator

The LUX dark matter search experiment is a 370 kg (250 kg active mass) two-\textunderscore phase liquid/gas xenon time projection chamber located at the 4850 ft level of the Sanford Underground Research Facility in Lead, SD. The first absolute charge (Qy) and light (Ly) measurement performed in situ in the LUX detector with a D-D calibration technique for nuclear recoil spanning 0.7 to 74 keV and 1.1 to 74 keV respectively have been reported in \underline {arXiv:1608.05381}. The D-D calibration has subsequently been further improved by incorporating pulsing technique, i.e. the D-D neutron production is concentrated within narrow pulses (20 us / 250 Hz) with the timing information recorded. This technique allows the suppression of accidental backgrounds in D-D neutron data and also provides increased sensitivity for the lower energy NR calibrations. I will report the improved NR absolute Qy and Ly measurements using the pulsed D-D calibration technique performed in situ in the LUX detector.   

The DarkSide-50 liquid argon dark matter search

The DarkSide-50 experiment uses three nested detectors to directly search for WIMP dark matter, with the innermost detector a time projection chamber filled with a target of liquid argon (LAr). The unique difference in pulse shape between electron recoils and nuclear recoils in LAr allows for exceptional discrimination of beta and gamma backgrounds. Event discrimination due to pulse shape coupled with the neutron discrimination power of the outer detectors is used to create a background-free environment for the DarkSide-50 WIMP search. Atmospheric argon, including the radioactive $^{39}$Ar isotope, was first used to search for WIMPs in a 50-day campaign, and later a search with 70.9 days of livetime was performed with argon extracted from underground wells, reducing the $^{39}$Ar isotope by a factor of $1.4 \times 10^3$. The status of the experiment will be discussed.   

Analysis of Alpha Backgrounds in DarkSide-50

DarkSide-50 is the current phase of the DarkSide direct dark matter search program, operating underground at the Laboratori Nazionali del Gran Sasso in Italy. The detector is a dual-phase argon Time Projection Chamber (TPC), designed for direct detection of Weakly Interacting Massive Particles, and housed within an active veto system of liquid scintillator and water Cherenkov detectors. Since switching to a target of low radioactivity argon extracted from underground sources in April, 2016, the background is no longer dominated by naturally occurring $^{39}$Ar. However, alpha backgrounds from radon and its daughters remain, both from the liquid argon bulk and internal detector surfaces. I will present details of the analysis used to understand and quantify alpha backgrounds, as well as to understand other types of radon contamination that may be present, and our sensitivity to them.