Paving the path to Dark Matter detection

Ciencias exactas e ingenierías   Paving the path to Dark Matter detection   Adoquinando el camino para la detección de materia oscura   Marco A. Reyes*, Juan Estrada**   *Departamento de Física, División de Ciencias e Ingeniería, Campus León, Universidad de Guanajuato. Loma del Bosque núm. 103, Col. Lomas del Campestre, apartado postal E-143, León, Guanajuato, México, C.P. 37150. Tel.: (477) 788 51 00. ** Fermi National Accelerator Laboratory. Pine St. and Kirk Rd., Batavia, IL, USA, C.P. 60510.   Recibido: 24 de febrero de 2015 Aceptado: 18 de mayo de 2015   Abstract In order to detect Dark Matter (DM), scientists in the Dark Matter In CCDs (DAMIC) Collaboration have set an experimental array of Charge-Coupled Devices (CCDs) in a nickel mine underground, and have developed all analysis tools to discern any known trace of conventional matter from what they expect to find in case a DM particle crosses the CCDs. In order to calibrate the signals from the CCDs, they have also designed experiments to quantify neutron-silicon interactions, assuming that neutrons can mimic DM interactions in the CCDs. Here we present preliminary results from the analysis of data obtained in these experiments. Keywords: Dark Matter; CCDs; quenching factor.   Resumen Para poder detectar materia oscura (DM, por sus siglas en inglés), científicos de la Colaboración DAMIC (Dark Matter In CCDs) han puesto un arreglo experimental de dispositivos de carga acoplada (CCDs, por sus siglas en inglés) en una mina subterránea de niquel, y han desarrollado todas las herramientas necesarias para discernir entre las trazas que pudieran dejar en ellos partículas de materia convencional, para comparar con las que podrían ser encontradas si una partícula de DM atravesara al arreglo de CCDs. Con el objeto de calibrar las señales de los CCDs, también han desarrollado experimentos para cuantificar las interacciones neutrón-silicio, suponiendo que los neutrones pueden imitar las interacciones de DM en los CCDs. Aquí presentamos resultados preliminares del análisis efectuado. Palabras clave: Materia oscura; CCDs; quenching factor.   The DAMIC experiment At two kilometers underground, the Vale InCo's Creighton nickel mine in Ontario, Canada, hosts the Sudbury Neutrino Observatory Laboratory (SNOLAB). There, after riding an elevator at 50 km/h, walking another 1.5 km, and bathing and changing clothes to prevent dust to come into the laboratory, physicists, astronomers and engineers who have already been certified as miners, can visit the charged coupled devices (CCDs) array constructed by the Dark Matter In CCDs (DAMIC) Collaboration to study Dark Matter (DM). In fact, DAMIC is the acronym for Dark Matter in CCDs, (Tiffenberg, 2013) and it is one of the international collaborations which has deployed experiments in underground mines to conduct these studies. Another example is the Large Underground Xenon Collaboration (LUX) (Akerib et al., 2013), which uses a 1.4 km underground gold mine in South Dakota, United States, to also study DM. According to scientists, in order to understand galaxies' rotation, large structure formation, and gravitational lensing, there must exist about 5.5 times more DM than ordinary baryonic matter. Furthermore, models predict that the DM local density should be around 0.3 GeV/cm3, with velocities in the Earth's reference frame of hundreds of km/s. There exist different candidates to DM particles, and one of these is the so called weakly interacting massive particle (WIMP). DAMIC is an experiment dedicated to study WIMP-nucleon spin independent interactions, where the nucleons nucleons are those of silicon conforming a CCD, which are low threshold and low background particle detectors. The scientific grade CCDs used by DAMIC are three 3 cm ´ 6 cm silicon wafers, two of them 500 microns thick and the other one 675 microns (2.2 g and 2.5 g, respectively), similar to those that were originally designed to construct the Dark Energy Survey experiment (DES) (Salles, 2013). DES studies the origin of the universe since 2012. In the near future, DAMIC will be upgraded to DAMIC 100, which will consist of 24 CCDs, 16 Mpixel, 675 μm CCDs, each 5.5 g of weight. The experimental setup consists of a copper box where the CCDs are assembled, which is immersed in a vacuum vessel (VV) where the temperature is kept at about -131 °C (figure 1). Since even dust includes radioactive particles, this array has to be very well blinded from ambient radiation. Therefore, the VV is protected by a 22 cm thick lead shield to protect from gamma rays, and a 46 cm thick poliethylene shield to protect from neutrons from radioactive decays. The way DAMIC CCDs are used to detect particles is the following. When a particle hits the Si nucleus, part of the recoil nucleus' energy, ER, is used to produce ionization. This ionization energy, EI, produces a charge Q deposit in the Si net, which is then collected and read bin by bin in a timed sequence. A schematic of the CCD operation is given in figure 2. In order to pick out DM signals from those of conventional matter, DAMIC has undergone detailed background studies from all possible sources, including for example, that coming from the epoxy used in the CCD package assembly. Also, this Collaboration has dedicated great efforts to understand the EI to Q conversion, and the ER to EI ionization efficiency, sometimes referred to as quenching factor. Leonel A. Villanueva and Marco A. Reyes, a student and a professor of the Department of Physics of the Sciences and Engineering Division of the University of Guanajuato, have participated in these efforts, which we shall describe below.   Charge and time characterization of phototube EMI-9954KB with a 0.2 photoelectron threshold Due to the searches of very low mass WIMPs as DM candidates, conducted by DAMIC or other experiments, measurement of nuclear recoil quenching factors for energies of about 1 KeVs - 10 KeVs has become very important in the past few years. DAMIC Collaboration is setting up a neutron scattering experiment on a silicon target to measure nuclear recoil quenching factor. Scintillator bars and phototubes (PMTs) are used to measure angular distribution of the scattered neutrons. Description of this experiment is given in the following section. In such experiment, to increase the neutron detection efficiency, the phototubes are operated with a very low threshold, of the order of 0.2 photoelectrons (p.e.'s). A group of scientists from DAMIC carried out an experiment at the Fermilab Test Beam Facility (FTBF) to characterize the charge and time resolution behavior of the PMTs that would be used in the neutron scattering experiment. For this PMT characterization experiment, the experimental setup consisted of two crossed Eljen EJ-200 scintillator bars, with two PMTs attached at the ends of each bar (figure 3). One bar had two FEU-115M PMTs, and the (...truncated)