Percorrer por autor "Brás, P."
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- 3D Modeling of Electric Fields in the LUX DetectorPublication . LUX collaboration (93 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.This work details the development of a three-dimensional (3D) electric field model for the LUX detector. The detector took data to search for weakly interacting massive particles (WIMPs) during two periods. 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 generated 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 μC/m2. From our studies, we deduce that the electric field magnitude varied locally while the mean value of the field of ~200 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.
- $^{83\textrm{m}}$Kr calibration of the 2013 LUX dark matter searchPublication . LUX collaboration (93 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.LUX was the first dark matter experiment to use a Kr83m calibration source. In this paper, we describe the source preparation and injection. We also present several Kr83m 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.
- Calibration, event reconstruction, data analysis, and limit calculation for the LUX dark matter experimentPublication . LUX collaboration (98 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.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.
- First Searches for Axions and Axionlike Particles with the LUX ExperimentPublication . LUX collaboration (101 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.The first searches for axions and axionlike particles with the Large Underground Xenon experiment are presented. Under the assumption of an axioelectric interaction in xenon, the coupling constant between axions and electrons gAe is tested using data collected in 2013 with an exposure totaling 95 live days ×118 kg. A double-sided, profile likelihood ratio statistic test excludes gAe larger than 3.5×10-12 (90% C.L.) for solar axions. Assuming the Dine-Fischler-Srednicki-Zhitnitsky theoretical description, the upper limit in coupling corresponds to an upper limit on axion mass of 0.12 eV/c2, while for the Kim-Shifman-Vainshtein-Zhakharov description masses above 36.6 eV/c2 are excluded. For galactic axionlike particles, values of gAe larger than 4.2×10-13 are excluded for particle masses in the range 1–16 keV/c2. These are the most stringent constraints to date for these interactions.
- Identification of Radiopure Titanium for the LZ Dark Matter Experiment and Future Rare Event SearchesPublication . LZ collaboration (205 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a detector containing a total of 10 tonnes of liquid xenon within a double-vessel cryostat. The large mass and proximity of the cryostat to the active detector volume demand the use of material with extremely low intrinsic radioactivity. We report on the radioassay campaign conducted to identify suitable metals, the determination of factors limiting radiopure production, and the selection of titanium for construction of the LZ cryostat and other detector components. This titanium has been measured with activities of $^{238}$U$_{e}$~$<$1.6~mBq/kg, $^{238}$U$_{l}$~$<$0.09~mBq/kg, $^{232}$Th$_{e}$~$=0.28\pm 0.03$~mBq/kg, $^{232}$Th$_{l}$~$=0.25\pm 0.02$~mBq/kg, $^{40}$K~$<$0.54~mBq/kg, and $^{60}$Co~$<$0.02~mBq/kg (68\% CL). Such low intrinsic activities, which are some of the lowest ever reported for titanium, enable its use for future dark matter and other rare event searches. Monte Carlo simulations have been performed to assess the expected background contribution from the LZ cryostat with this radioactivity. In 1,000 days of WIMP search exposure of a 5.6-tonne fiducial mass, the cryostat will contribute only a mean background of $0.160\pm0.001$(stat)$\pm0.030$(sys) counts.
- Limits on spin-dependent WIMP-nucleon cross section obtained from the complete LUX exposurePublication . LUX collaboration (99 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.We present experimental constraints on the spin-dependent WIMP-nucleon elastic cross sections from the total 129.5 kg yr 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% C.L. upper limits to be set on the WIMP-neutron (WIMP-proton) cross section of σn=1.6×10-41 cm2 (σp=5×10-40 cm2) 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.
- Liquid xenon scintillation measurements and pulse shape discrimination in the LUX dark matter detectorPublication . LUX collaboration (92 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.Weakly interacting massive particles (WIMPs) are a leading candidate for dark matter and are expected to produce nuclear recoil (NR) events within liquid xenon time-projection chambers. We present a measurement of the scintillation timing characteristics of liquid xenon in the LUX dark matter detector and develop a pulse shape discriminant to be used for particle identification. To accurately measure the timing characteristics, we develop a template-fitting method to reconstruct the detection times of photons. Analyzing calibration data collected during the 2013–2016 LUX WIMP search, we provide a new measurement of the singlet-to-triplet scintillation ratio for electron recoils (ER) below 46 keV, and we make, to our knowledge, a first-ever measurement of the NR singlet-to-triplet ratio at recoil energies below 74 keV. We exploit the difference of the photon time spectra for NR and ER events by using a prompt fraction discrimination parameter, which is optimized using calibration data to have the least number of ER events that occur in a 50% NR acceptance region. We then demonstrate how this discriminant can be used in conjunction with the charge-to-light discrimination to possibly improve the signal-to-noise ratio for nuclear recoils.
- LUX trigger efficiencyPublication . Akerib, D.S. et al. (93 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.The Large Underground Xenon experiment (LUX) searches for dark matter using a dual-phase xenon detector. LUX uses a custom-developed trigger system for event selection. In this paper, the trigger efficiency, which is defined as the probability that an event of interest is selected for offline analysis, is studied using raw data obtained from both electron recoil (ER) and nuclear recoil (NR) calibrations. The measured efficiency exceeds 98\% at a pulse area of 90 detected photons, which is well below the WIMP analysis threshold on the S2 pulse area. The efficiency also exceeds 98\% at recoil energies of 1.3 keV and above, for both ER and NR. The measured trigger efficiency varies between 99\% and 100\% over the fiducial volume of the detector.
- Position Reconstruction in LUXPublication . LUX collaboration (93 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.The (x, y) position reconstruction method used in the analysis of the complete exposure of the Large Underground Xenon (LUX) experiment is presented. The algorithm is based on a statistical test that makes use of an iterative method to recover the photomultiplier tube (PMT) light response directly from the calibration data. The light response functions make use of a two dimensional functional form to account for the photons reflected on the inner walls of the detector. To increase the resolution for small pulses, a photon counting technique was employed to describe the response of the PMTs. The reconstruction was assessed with calibration data including 83mKr (releasing a total energy of 41.5 keV) and 3H (β− with Q = 18.6 keV) decays, and a deuterium-deuterium (D-D) neutron beam (2.45 MeV) . Within the detector's fiducial volume, the reconstruction has achieved an (x, y) position uncertainty of σ = 0.82 cm and σ = 0.17 cm for events of only 200 and 4,000 detected electroluminescence photons respectively. Such signals are associated with electron recoils of energies ~0.25 keV and ~10 keV, respectively. The reconstructed position of the smallest events with a single electron emitted from the liquid surface (22 detected photons) has a horizontal (x, y) uncertainty of 2.13 cm.
- Results from a search for dark matter in the complete LUX exposurePublication . LUX collaboration (101 authors); Brás, P.; Lindote, A.; Lopes, M.I.; Neves, F.; Silva, C.; Solovov, V.N.We report constraints on spin-independent weakly interacting massive particle (WIMP)-nucleon scattering using a 3.35×104 kg day exposure of the Large Underground Xenon (LUX) experiment. A dual-phase xenon time projection chamber with 250 kg of active mass is operated at the Sanford Underground Research Facility under Lead, South Dakota (USA). With roughly fourfold improvement in sensitivity for high WIMP masses relative to our previous results, this search yields no evidence of WIMP nuclear recoils. At a WIMP mass of 50 GeV c-2, WIMP-nucleon spin-independent cross sections above 2.2×10-46 cm2 are excluded at the 90% confidence level. When combined with the previously reported LUX exposure, this exclusion strengthens to 1.1×10-46 cm2 at 50 GeV c-2.