Topics covered:
Large scale structure formation, CMB, DM in the Milky Way, SUSY, Direct Searches, Indirect Searches, Axions, Neutrinos, Non-SUSY, statistical methods, DM@ LHC, Dark Energy
The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a liquid xenon TPC in the Davis cavern at the Sanford Underground Research Facility, Lead, South Dakota, 4850 feet below the surface. The underground environment reduces the cosmic ray flux by a factor of $10^6$, but there remains a potential background from γ-rays emitted from the decays of $^{40}$K, $^{238}$U and $^{232}$Th naturally occurring in the rock surrounding the laboratory. In-situ γ-ray measurements were taken with a sodium iodide detector in several locations within the cavern, yielding average radioactivity levels. We will present the radioactivity levels determined with these measurements, and a first attempt at mapping suspected non-uniformities in the γ-flux due to differences in rock composition.
LIGO's detection of the black hole quasinormal modes has proved the existence of the light rings outside the event horizon. Other sub-dominant long-lived modes depend on the existence and properties of the horizon, as well as the black hole hairs outside. These modes may reveal themselves in future gravitational-wave detectors as echoes. In this work, we study the ringdown of a de Sitter-Schwarzschild black hole in the dGRT theory of massive gravity. We find interesting signatures of ringdown echoes as we turn on the coupling between spacetime perturbations and the background vector field outside the black hole.
Many dark matter search experiments active today, and to be activated in the future, utilize xenon as a target mass in dual phase time projection chambers (TPC). Although xenon has seen widespread use in liquid noble gas detectors, and is currently employed by the XENON1T, LUX, and PandaX-II detectors; the process of energy deposition for nuclear recoils in liquid xenon is not precisely understood. This process has been described from first principles and measured precisely in the cases of liquid helium and argon, but this remains prohibitively difficult in the case of liquid xenon. The XENON1T detector possesses an external mono-energetic neutron source via deuterium-deuterium plasma fusion, producing neutrons with peaks at 2.2 Mev and 2.7 Mev. This research aims to reconstruct double scatter events using these mono-energetic neutrons, thus providing valuable information to constrain present understanding of energy deposition for nuclear recoils in liquid xenon.
Very high energy gamma rays up to 200 GeV have been observed from the quiescent Sun. While this emission is thought to originate from hadronic cosmic rays interacting in the Sun’s atmosphere, the mechanism is not well understood. Another potential source of gamma rays from the Sun is gravitationally captured dark matter. Weakly interacting massive particles (WIMPs) accumulating in the solar core can annihilate to long-lived mediators, which then decay to Standard Model particles. Gamma-ray emission from this process depends on the mass of the WIMPs, but is expected to become important above 1 TeV. Thus measurements of the Sun at TeV energies are crucial to constrain the properties of the WIMPs. The High Altitude Water Cherenkov (HAWC) Observatory is a wide field-of-view air shower array that is currently the only detector capable of measuring gamma rays from the Sun at multi-TeV energies. We present results from a search for excess gamma rays from the Sun using three years of data from HAWC. Our measurements provide the strongest upper limits on TeV gamma rays from the solar disk. We discuss the implications of these measurements on the origin of the solar gamma rays and present constraints for the spin-dependent WIMP-proton scattering cross section for masses above 1 TeV.
The LUX-ZEPLIN (LZ) experiment aims to directly detect WIMP particles interacting with xenon nuclei. These interactions produce scintillation and ionization signals within the active region of the detector which identify their presence. It is the goal to trigger correctly on ionization pulses that come from WIMPs and to avoid triggering on dark current from PMTs. This presentation demonstrates the process of identifying the criteria for a trigger decision.
John Armstrong
University of Maryland
DMSS Abstract
Modern dark matter direct detection experiments such as LUX-Zeplin require low backgrounds to reach sensitivities necessary to claim detection. When deep underground and isolated from cosmic rays, the dominant backgrounds come from radioactive impurities. While a getter can effectively remove electronegative impurities, noble gas impurities must be removed from the xenon volume using other methods or screened. The radioisotope $^{222}Rn$ is particularly difficult to eliminate because it emanates from trace amounts of $^{226}Ra$ contained in detector components. It is necessary to screen detector components for $^{222}Rn$ before assembly to reduce and quantify the radon burden. We make use of an electrostatic chamber with a PIN diode target to collect and count radon daughters of detector components. I will discuss some of the more relevant analysis techniques and results in this talk.
An important topic in dark matter research is the search for sub-GeV dark matter. Direct detection searches for sub-GeV dark matter can be conducted through scattering on Silicon nuclei utilizing a detector with energy threshold on the order of 10 eV. This low threshold can be achieved by thermalizing a Transition-Edge Sensor (TES) based detector with gram scale silicon absorbers. This talk describes the design of a 10 eV threshold TES based detector as well as recent fabrication and testing progress.
In noble liquid detectors searching for rare interactions, radon, a decay product of trace amounts of natural uranium present in detector components, leads to the dominant background of the detector. The results of the R&D studies of radon reduction system for noble liquid detectors preformed at The University of Michigan will be presented. The adsorption characteristics of radon in nitrogen, argon, and xenon carrier gases on various types of charcoals with different adsorbing properties and intrinsic radioactive purities have been studied in the temperature range of 190-295 K at flow rates of 0.5 and 2 standard liters-per-minute. Performances of different charcoals have been quantified using parameters such as average breakthrough time, dynamic adsorption coefficient, and number of theoretical stages.
The latest release of the Noble Element Scintillation Technique (NEST) is presented here. Noble element target media have become common in rare event searches, and an accurate comparison model is critical for understanding and predicting signals and unwanted backgrounds. Like its predecessors, NESTv2.0 is a simulation tool written in C++ and is based heavily on experimental data, taking into account most of the existing ionization and scintillation data for solid, liquid, and gaseous xenon. Due to the large amount of precise data for liquid xenon, most theoretical models in NEST have been replaced with simple,well-behaved, empirical formulas, such as sigmoids and power laws. NESTv2.0 also uses an empirical, non-binomial, recombination fluctuations model. In addition, NESTv2.0 simulates S1 and S2 scintillation signals with correct energy resolutions in dual-phase xenon time-projection chambers, and this is done without using an external package. While NEST can be used with GEANT, NESTv2.0 is fully capable of operating as a stand-alone command-line tool.
Dual-phase time projection chambers (TPCs) filled with liquid xenon (LXe) are a
widely-used technique for the direct search for dark matter in the form of weakly
interacting massive particles (WIMPs). One of the upcoming experiments exploiting
this technique is XENONnT, which aims at Dark Matter detection through the
scattering of WIMPs off the xenon nuclei. XENONnT will be filled with 8 t of ultra-
pure liquid Xenon, out of which will be 5.9 t in the active TPC. XENONnT will be
located at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, at an average
depth of 3600 m water equivalent. After an overview on the XENONnT project, this
talk will present the working principles of the XENONnT TPC as well as the lessons
learned from the operating XENON1T.
This work is supported by the Federal Ministry of Education and Research of
Germany (BMBF).
Self-shielding in ton-scale liquid xenon (LXe) time projection chambers (TPCs) presents a unique challenge for calibrating detector response to interactions in the TPCs innermost LXe volume. Calibration isotopes must be injected directly into the LXe to reach the central volume of the TPC, where they must either decay away with a short half life, or be purified out. I present a summary of the hardware effort at UMass Amherst, which is designed to refine techniques for the injection and removal of precise activities of various calibration isotopes that are useful in LXe TPC experiments such as LUX-ZEPLIN.
XENON1T is a liquid xenon time projection chamber (TPC) designed to look for elastic recoils of dark matter particles with xenon nuclei. To increase the sensitivity of the experiment, we need to decrease the background in the detector. One background that is specific to low-energy recoils comes from so called single electrons trains that follow interactions in the TPC over time scales of hundreds of milliseconds. To mitigate this background, we first need to understand where it comes from. This talk will discuss different theories for these electron trains and how I test these ideas through analysis of XENON1T data.
Many leading WIMP direct detection experiments use liquid xenon as a target to observe scattering dark matter. Currently, these detectors search for WIMPS in the GeV-TeV mass energy range, and require larger and larger detectors to gain sensitivity. An alternate analysis channel investigates the energy spectrum of individual extracted electrons. Through this channel, xenon TPCs, regardless of size, may be sensitive to $MeV/c^2$ dark matter. However, backgrounds from so-called electron trains make this analysis challenging. The focus of the new LBECA collaboration is to design a small xenon TPC with minimal single-electron backgrounds to search for dark matter through this analysis channel. This talk presents the progress of the LBECA Collaboration in identifying leading sources of these backgrounds and in the work to reduce them, with the goal of rapidly realizing a dedicated low-mass dark matter search experiment.
An important step in the XENON1T analysis pipeline is distinguishing electronic recoil events (ERs) from nuclear recoil events (NRs) inside the detector. ERs come from background, while NRs are evidence of a possible signal. This is currently done by hand using a profile likelihood method. The goal is to maximize the acceptance of correctly classified NRs while rejecting as many ERs as possible; this step has a significant impact on the amount of background that can be discarded and therefore the sensitivity of the detector. This is a binary classification problem, a well-understood domain for machine learning and one of its major strengths. Various supervised learning algorithms were tested on calibration data. The best-performing algorithm, a support vector machine (SVM), approximately matched but did not exceed the performance of the profile likelihood method. Various ways that the quality of the dataset affects the performance of machine learning algorithms are discussed. It is shown that machine learning has a comparative advantage over manual statistical analysis only when dealing with large, high-dimensional, poorly-understood datasets. Although many different types of data are collected on each recoil event, most of it can be used to distinguish calibration events from run events, invalidating its use for machine learning. A more promising approach is to use exploratory machine learning techniques to augment human analysis.
A brief overview of experimental activities at the University of Montreal is given concerning the search for the Dark Matter (DM) in the PICO experiment (SNOLAB, Ontario, Canada) as well as at the accelerator laboratory in Montreal. Newly proposed methods of DM detection using the superheated liquid gas will be briefly discussed.
Weakly interacting massive particles (WIMPS), the leading candidate for dark matter, interacts differently than what we would expect with ordinary baryonic matter. These interactions require very sensitive detectors. Being much cheaper than xenon, when superheated, ultra-purified water can act as a sensor to detect these rare events. Alongside replicating other WIMP detectors the bubble chamber can also be used to validate through detection of neutrons, which should elastically scatter through the same mechanism as dark matter. The design of this sensor consists of four essential parts: an temp-adjustable hotplate, a thermal bath, a test tube that holds both the metastable liquid and oil that acts as a surface tension buffer, and a recondenser that is attached to the test tube. We’ve reached the rate of one major event per minute. The approximate average cosmic-ray background, dominated by GeV-scale muons near sea level, and partially verified that a statistically significant increase in rate occurs in the presence of neutron sources like AmBe or Cf-252, but not in gammas.
As higher mass particles are eliminated as possibilities in the search for dark matter, it is important to explore new types of detectors that are more specialized at looking for lower mass particles. For this purpose, super-cooled water has been explored as a target material for future detectors. This talk will go over the motivations for a detector of this type, the operational data that has been collected—including the first evidence of radiation induced nucleation of super cooled water, and additional applications beyond searching for dark matter.
In the Milky Way Galaxy, the rotation curve is related to the mass distribution of visible matter and dark matter. By comparing the measured rotation curve with the calculated rotation curve due to visible matter, we can determine the mass density of dark matter in our galaxy. The components of visible matter includes the black hole at the center of our galaxy, the two bulges, and the galactic disk. Due to their spherical symmetry (and the shell theorem) calculating the gravitational force due to the black hole and the bulges is relatively simple. Surprisingly, calculating the gravitational force due to the galactic disk is much harder. We break up the disk into numerous rings, calculate the gravitational force due to each ring, and sum the forces due to all rings. The total gravitational force, due to these four mass distributions, can be used to calculate the rotation curve due to visible matter. Using the measured rotation curve, we extract the rotation curve due to the dark matter in our galaxy. The rotation curve can be used to determine the mass distribution of the dark matter in our galaxy.
If dark matter interacts weakly then one possible experimental signal would be an annual modulation in the interaction rate caused by the combined motion of the sun and earth moving through the dark matter halo. The DAMA-LIBRA experiment has observed such a modulation in their detector in a way not described by known modulating backgrounds such as atmospheric muon rate (thickness/density of atmosphere), radon gas, or solar neutrino interactions (apogee/perigee effect). In this talk, we describe the prototype development of a device to collect aerosols on a daily basis to search for modulations of atmospheric concentrations of elements which have naturally occurring radioactive isotopes like potassium. Depending on the size of the effect, this background process could theoretically be misinterpreted as a dark matter signal.
The HAYSTAC experiment looks for axions, a type of cold dark matter (CDM) particle predicted in the context of the standard model of electroweak interactions. If they exist, axions fall very low on the mass scale, and their presence could be detected when they convert to radiofrequency photons in the presence of a strong magnetic field. HAYSTAC consists of a microwave cavity resonator that is tunable and builds up the axion signal to a point where it is detectable, and a quantum limited amplifier that exploits the Josephson effect. I will discuss is current status, and plans for the upcoming run.
Among the proposed explanations for the hierarchy problem, or the relative weakness of gravity compared to the other fundamental forces, is the existence of extra dimensions. To test this, we probe the diphoton spectrum for high-mass events using proton-proton collisions at $\sqrt{s}$ = 13 TeV. We present here the analysis results using 2016 data, corresponding to a 35.9 fb$^{-1}$ integrated luminosity, collected from the CMS detector of the Large Hadron Collider at CERN. Resonant and nonresonant signatures of extra dimensions are searched using different background estimation techniques. Limits were set for both the warped (resonant) and large extra dimensional (nonresonant) models. We also show preliminary work on improving sensitivity in this spectrum. In particular, we explore the angular distribution of the diphoton final states in preparation for the analysis of the 2017 data.
Standard searches for dark matter attempt to identify interactions with dark matter particles; a complementary search would involve looking for “dark forces”. As an example, if a dark force mediated by a “dark photon” exists, dark matter particles could pick up a tiny electric charge due to kinetic mixing with normal photons. Stable millicharged dark matter particles could become bound in matter, and potentially be detected by using an optical trap to measure the response to an applied electric field on a charge-neutral object that may contain such particles. This is a very well-isolated system, with an acceleration sensitivity on the order of 500 $\text{n}g/\sqrt{\text{Hz}}$ for a 20$\text{um}$ diameter SiO$^2$ levitated sphere. Within this experiment, the acceleration sensitivity can be translated to a fractional electron number sensitivity of $10^{-7} \text{electrons}/\sqrt{\text{Hz}}$ regardless of the mass of the millicharged particle, allowing exploration of uncharted regions of parameter space for such dark matter candidates.
In the last century, several astrophysical observations have provided strong evidences of the existence of dark matter in the Universe. This dark matter, non-luminous, and weakly interactive with ordinary matter, could be composed of massive particles called WIMPs.
XENON1T, the third direct detection experiment designed by the XENON collaboration, consists of a dual-phase (liquid-gas) time projection chamber (TPC), filled with xenon. XENON1T is the largest TPC ever built for dark matter search and conceived to detect an elastic scattering of a WIMP with the target nuclei. The energy transfer during such interaction induces a low energy nuclear recoil (below 100 keV).
In order to study the detector response to an energy deposit, the XENON collaboration carries out several calibration campaign with different sources. In particular, an internal source of Kr83m, which emits two radiations at low energies (32.2 keV and 9.4 keV), is used to calibrate XENON1T in the energy range where dark matter is expected.
Since data taking period dedicated to the search for dark matter lasts for several months, it is essential to monitor the detector stability and to study its response over time: regular Kr83m calibrations are carried out for this purpose.
Keywords: dark matter, direct detection, XENON1T, calibration
Krypton 85 is a long lived source of electron recoil backgrounds in LUX-ZEPLIN (LZ), a large scale 2-phase liquid xenon time projection chamber (TPC). Being a noble gas makes krypton difficult to remove using getters, and its long half life compared to the livetime of LZ (5 years), means that krypton impurities must be measured and tracked throughout xenon preparation, and use in the detector. The concentration of krypton must remain below the allowed concentration of 15 ppq. This talk outlines the motivation, equipment, and procedure to quantify ppq levels of krypton impurities.
LUX-ZEPLIN is dark matter direct detection experiment which will take data a mile underground in Lead, South Dakota starting in 2020. Krypton backgrounds must be minimized to decrease backgrounds to an acceptable level. Special cylinders fitted with purge volumes around the neck of the cylinders will be constructed to store the xenon. The cylinders have an upper limit of 15 parts-per-quadrillion $g_{Kr}/g_{Xe}$ when filled and a maximum ingress rate of 6 ppq/year $g_{Kr}/g_{Xe}$ while in storage. Characterization of the prototype 80-kg cylinder, Cylinder ID LZ-R3d-CYL-01, concluded that LZ will meet the maximum ingress rate using LN boil-off as a purge gas for the fitted N\textsubscript{2} purge volume. We show that we can achieve a rise in krypton concentration of $4.08\pm 0.20$ ppq/year while the volume around the neck of the prototype LZC is purged with LN boil-off.
COSINE-100 is a NaI(Tl) dark matter direct detection experiment, with the goal of testing DAMA's claim of dark matter detection by looking for an annual modulation signal. It consists of eight NaI(Tl) crystals, adding to a total of 106 kg, and 2000 liters of a liquid scintillator veto. Located at the Yangyang Underground Laboratory, South Korea, COSINE-100 has been running since September 2016. We observe a background rate of ~3 counts/kg/keV/day between 2 and 6 keV and have performed several analyses based on the current energy threshold of 2 keV. I will present on the detector performance and recent results.
Significant evidence, both astrophysical and cosmological, points to the existence of dark matter. The fundamental nature of dark matter, however, remains unknown. One appealing candidate for dark matter is the weakly interacting massive particle (WIMP). Direct detection experiments aim to observe the low energy scattering of WIMP particles off normal matter, and detectors based on noble liquid technology lead the way in WIMP sensitivity at high WIMP masses. To this day, no conclusive WIMP signals have been observed. The DarkSide experiment uses a liquid argon target in a dual-phase time projection chamber to search for WIMP dark matter. The DarkSide-50 (DS-50) experiment is located at Gran Sasso National Laboratory (LNGS) in Italy and its argon target comes from underground sources, shown to have much reduced 39Ar radioactivity compared to atmospheric argon. It nonetheless (and somewhat unexpectedly) contains measurable amounts of the radioactive contaminant 85Kr.
The work described in this talk covers the analysis of Monte Carlo simulated as well as real data from the DS-50 detector to search for a tell-tale delayed coincidence signature of 85Kr decays to the 85mRb state. The results of this search provide valuable information about the backgrounds inside of the detector as well as on its energy and spatial resolution, thus aiding in the search for WIMPs.
Liquid Xenon detectors are playing a pivotal role in the search for both dark matter and neutrinoless double beta decay. The principle of these detectors is based upon the measurement of scintillation light from the liquid xenon. The light response of these detectors to nuclear and electronic recoils must therefore be particularly well understood. While many such calibrations have been performed before, none have ever been done spanning a full energy range from sub-keV to MeV. As liquid xenon detectors are getting bigger and more multiphysics oriented, such a full calibration becomes not only important but necessary. To do this, a liquid xenon detector is being built at UAlbany. Once the detector's response to light is properly modelled for both nuclear and electronic recoils, this detector will be used to investigate means of increasing scintillation yield in liquid xenon.
nEXO (next-generation Enriched Xenon Observatory) is a proposed experiment to search for neutrinoless double beta (0νββ) decay of 136Xe with a projected half-life sensitivity of ~10^28 years using 5×10^3 kg of isotopically enriched liquid-xenon in a time projection chamber (TPC). Targeting this sensitivity requires addressing and reducing the backgrounds in the detector. Of particular interest is the 222Rn daughter 214Bi, whose decay includes a γ-ray line at 2448 keV, close to the 136Xe 0νββ decay Q-value of 2458.07 ± 0.31 keV. 214Bi decays are followed in close time-succession by the alpha decay of 214Po. These pairs of correlated decays are easy to tag if they occur in the bulk of detector. However, 214Bi that drift and decay at the cathode are tagged with much reduced efficiency and represent a non-negligible background for nEXO. Using data from EXO-200, ~80% of 214Bi decays in nEXO will occur at the cathode. Additionally, 214Bi decays occurring behind the TPC field cage can also contribute to nEXO background if they cannot be tagged using scintillation-only BiPo coincidences. We explore strategies to identify event-by-event and with high efficiency the 214Bi decays at the nEXO cathode and in the xenon outside the field cage. This study includes optimizing the light collection efficiency for alpha and beta decays occurring on detector surfaces, as well as careful design of the cathode electrode.
The search for neutrinoless double beta decay has spawned dozens of detectors looking for this rare decay and to determine whether neutrinos are Majorana or Dirac particles. Majorana neutrinos are required for most See-Saw Mechanisms for mass generation, as well as many models of sterile neutrinos, but is severely limited in what mechanisms it can rule out. Should a discovery be made, the next stage in 0vbb research requires the ability to resolve the angular/energy spectrum of 0vbb-decay. This spectrum is dominated by various nuclear processes as as well as different mass generation mechanisms and right/left handed current mixing for neutrinos. Making measurements of these spectra would then shed light on the nature of heavy right handed neutrinos and neutrino generation. Presented is preliminary work for a thin-film tracking calorimeter based of CdTe semiconductor research. This design allows for investigation of multiple 0vbb candidate isotopes simultaneously, and the tracking ability provides both a significant background reduction by means of vetoing external sources, and the ability to discern angular spectra. The design and simulation of this detector is done with Geant4 simulation toolkit, and convolutional neural net development has begun with simulated signals to resolve energy and and angle of the two emitted electrons.
DAMIC for Dark Matter in CCDs, will search for dark matter particle interaction with silicon CCDs. The experiment takes place at SNOLAB where 8 CCDs are used. We measure the energy spectrum of ionization events in the bulk silicon of charge-coupled devices down to a signal of 60 eV electron equivalent. Thus it’s necessary to measure and understand our background noise. I will give an introduction on the DAMIC experiment (pros and cons of the CCDs, Dark Matter Particles). Then I will give a presentation of the different works produced to evaluate the background noise ( Geant4 simulations, Analysis of the data).