The LUX experiment has performed searches for dark matter particles scattering elastically on xenon nuclei, leading to stringent upper limits on the nuclear scattering cross sections for dark matter. ...Here, for results derived from \({1.4}\times 10^{4}\;\mathrm{kg\,days}\) of target exposure in 2013, details of the calibration, event-reconstruction, modeling, and statistical tests that underlie the results are presented. Detector performance is characterized, including measured efficiencies, stability of response, position resolution, and discrimination between electron- and nuclear-recoil populations. Models are developed for the drift field, optical properties, background populations, the electron- and nuclear-recoil responses, and the absolute rate of low-energy background events. Innovations in the analysis include in situ measurement of the photomultipliers' response to xenon scintillation photons, verification of fiducial mass with a low-energy internal calibration source, and new empirical models for low-energy signal yield based on large-sample, in situ calibrations.
This work details the development of a three-dimensional (3D) electric field model for the LUX detector. The detector took data during two periods of searching for weakly interacting massive particle ...(WIMP) searches. After the first period completed, a time-varying non-uniform negative charge developed in the polytetrafluoroethylene (PTFE) panels that define the radial boundary of the detector's active volume. This caused electric field variations in the detector in time, depth and azimuth, generating an electrostatic radially-inward force on electrons on their way upward to the liquid surface. To map this behavior, 3D electric field maps of the detector's active volume were built on a monthly basis. This was done by fitting a model built in COMSOL Multiphysics to the uniformly distributed calibration data that were collected on a regular basis. The modeled average PTFE charge density increased over the course of the exposure from -3.6 to \(-5.5~\mu\)C/m\(^2\). From our studies, we deduce that the electric field magnitude varied while the mean value of the field of \(\sim200\)~V/cm remained constant throughout the exposure. As a result of this work the varying electric fields and their impact on event reconstruction and discrimination were successfully modeled.
The Large Underground Xenon (LUX) experiment operates at the Sanford Underground Research Facility to detect nuclear recoils from the hypothetical Weakly Interacting Massive Particles (WIMPs) on a ...liquid xenon target. Liquid xenon typically contains trace amounts of the noble radioactive isotopes \(^{85}\)Kr and \(^{39}\)Ar that are not removed by the in situ gas purification system. The decays of these isotopes at concentrations typical of research-grade xenon would be a dominant background for a WIMP search exmperiment. To remove these impurities from the liquid xenon, a chromatographic separation system based on adsorption on activated charcoal was built. 400 kg of xenon was processed, reducing the average concentration of krypton from 130 ppb to 3.5 ppt as measured by a cold-trap assisted mass spectroscopy system. A 50 kg batch spiked to 0.001 g/g of krypton was processed twice and reduced to an upper limit of 0.2 ppt.
We report an absolute calibration of the ionization yields(\(\textit{Q\)_y\(})\) and fluctuations for electronic recoil events in liquid xenon at discrete energies between 186 eV and 33.2 keV. The ...average electric field applied across the liquid xenon target is 180 V/cm. The data are obtained using low energy \(^{127}\)Xe electron capture decay events from the 95.0-day first run from LUX (WS2013) in search of Weakly Interacting Massive Particles (WIMPs). The sequence of gamma-ray and X-ray cascades associated with \(^{127}\)I de-excitations produces clearly identified 2-vertex events in the LUX detector. We observe the K- (binding energy, 33.2 keV), L- (5.2 keV), M- (1.1 keV), and N- (186 eV) shell cascade events and verify that the relative ratio of observed events for each shell agrees with calculations. The N-shell cascade analysis includes single extracted electron (SE) events and represents the lowest-energy electronic recoil \(\textit{in situ}\) measurements that have been explored in liquid xenon.
LUX was the first dark matter experiment to use a \(^{83\textrm{m}}\)Kr calibration source. In this paper we describe the source preparation and injection. We also present several ...\(^{83\textrm{m}}\)Kr calibration applications in the context of the 2013 LUX exposure, including the measurement of temporal and spatial variation in scintillation and charge signal amplitudes, and several methods to understand the electric field within the time projection chamber.
The first searches for axions and axion-like particles with the Large Underground Xenon (LUX) experiment are presented. Under the assumption of an axio-electric interaction in xenon, the coupling ...constant between axions and electrons, gAe is tested, using data collected in 2013 with an exposure totalling 95 live-days \(\times\) 118 kg. A double-sided, profile likelihood ratio statistic test excludes gAe larger than 3.5 \(\times\) 10\(^{-12}\) (90% C.L.) for solar axions. Assuming the DFSZ theoretical description, the upper limit in coupling corresponds to an upper limit on axion mass of 0.12 eV/c\(^{2}\), while for the KSVZ description masses above 36.6 eV/c\(^{2}\) are excluded. For galactic axion-like particles, values of gAe larger than 4.2 \(\times\) 10\(^{-13}\) are excluded for particle masses in the range 1-16 keV/c\(^{2}\). These are the most stringent constraints to date for these interactions.
We present experimental constraints on the spin-dependent WIMP-nucleon elastic cross sections from the total 129.5 kg-year exposure acquired by the Large Underground Xenon experiment (LUX), operating ...at the Sanford Underground Research Facility in Lead, South Dakota (USA). A profile likelihood ratio analysis allows 90% CL upper limits to be set on the WIMP-neutron (WIMP-proton) cross section of \(\sigma_n\) = 1.6\(\times 10^{-41}\) cm\(^{2}\) (\(\sigma_p\) = 5\(\times 10^{-40}\) cm\(^{2}\)) at 35 GeV\(c^{-2}\), almost a sixfold improvement over the previous LUX spin-dependent results. The spin-dependent WIMP-neutron limit is the most sensitive constraint to date.
Background/Aims
: Cytoskeletal reorganization plays an important role in the regulation of different cell functions, such as proliferation and migration. Since platelet-derived growth factor (PDGF) ...stimulates both proliferation and chemotaxis of hepatic stellate cells (HSC), we investigated the effects of this cytokine on cytoskeletal components of cultured rat HSC.
Methods/Results
: Exposure of HSC to PDGF induced the formation of stress fibres and of a ruffled configuration of the plasma membrane, evaluated by both fluorescence and electron microscopy. These modifications were also induced by exposure to the protein kinase C (PKC) activator phorbol-12-myristate-13-acetate (PMA) and abolished by pretreatment with the PKC inhibitor calphostin C, with the Rho inhibitor C3 exoenzyme and with the intracellular calcium chelator MAPTAM, but not with the PI-3 kinase inhibitor wortmannin or with the mitogen-activated protein kinase kinase inhibitor PD 98059. PDGF induced a translocation of Rho from the cytosol to the membrane which was inhibited by C3 exoenzyme and by calpostin C, and which was also induced by PMA. Moreover, PDGF induced a rearrangement of vinculin which was prevented by C3 exoenzyme and calphostin C.
Conclusions
: PDGF-induced cytoskeletal reorganization in HSC is dependent on PKC and Rho, thus suggesting that these two pathways may play an important role in the response of liver to injury.
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