Annotation of the results obtained in 2020
One of the perspective ways to search for weakly interacting massive particles of dark matter (Weakly Interacting Massive Particles, WIMP) is the direct detection of their scattering on target nuclei in low-background installations located in underground laboratories. Direct experiments on the registration of dark matter particles significantly expand and supplement the investigated range of WIMP masses which is possible for observation. Currently, the most sensitive instrument for direct search for WIMPs are detectors based on liquid noble gases (Ar and Xe). This year, the RSF project carried out searches for Dark Matter particles in international experiments on the DarkSide and DEAP-3600 facilities with liquid argon targets. This year, the DarkSide-50 experiment was completed in the underground low-background laboratory of Gran Sasso (Italy) with a mass of an argon target of 47 kg and an analysis of the collected data was carried out. As a result, a world record sensitivity to the cross section of spin-independent interaction of WIMP with nucleons was achieved for WIMP particles with masses <10 GeV/c^2 (cross section ~ 10^-42 cm^2 for masses ~ 3 GeV/c^2). The data analysis team applied machine learning methods and created a convolutional neural network model to improve the accuracy of spatial reconstruction of events and signal selection by the time-shape of the scintillation pulse in the low-energy hidden mass search area. Taking into account the achieved unique sensitivity of the DarkSide-50 detector when registering record low energy releases in the detector target, we investigated the possibility of detecting neutrinos from a Supernova flash in the DarkSide-20k detector with a sensitive mass of an argon target of 20 tons in the reaction of elastic coherent neutrino scattering on argon nuclei. The calculations used the physical model of the detector, created on the basis of the Monte Carlo package, taking into account the specific characteristics of the DarkSide-50 detector achieved. Based on many years of experience in operating the DarkSide-50 facility, a design of a low-background liquid-argon DArT detector has been developed to determine the content of the radioactive isotope 39Ar in radiation-pure argon. Works on the assembly and testing of two pilot two-phase time-projection cameras (TPC) with argon target masses of 10 kg (DarkSide-Proto) and 1 ton (DarkSide-LowMass) were performed in order to prepare for the launch of a large-scale low-background DarkSide-20k experiment. The main task of DarkSide-Proto is to test the operation of photo-detector elements (PDE), consisting of NUV-HD-Cryo silicon photomultiplier (SiPM) arrays and their electronic paths. These SiPMs are specially designed by FBK for the DarkSide experiment and are designed to operate at cryogenic temperatures. SiPMs are combined (6∙4=24 pcs.) into a single PDE with area 5∙5 cm^2. In the experimental chamber, 25 modules of photodetector elements were installed to register scintillation and electroluminescent photons, and the electronic path was debugged to collect information from the TPC. For the first time, the possibility of using matrices of silicon photomultipliers with a large sensitive surface area instead of traditional photomultiplier tubes for registering photons was proved in the physical experiment. Work continued on assembling a pilot two-phase liquid-argon time-projection camera with a target mass of 1 ton DarkSide-LowMass, optimized for the search for WIMP particles with low masses (below 10 GeV/s^2) based on electroluminescence signals from electrons extracted from the track from liquid to gas phase of TPC. The participants of the RSF project developed and tested data analysis packages (including the package for reconstruction of event coordinates in TPC using machine learning algorithms), analyzed the results of calibrations of the DarkSide-Proto detector and the data of the DarkSide-LowMass detector simulations in the Monte Carlo package. The characteristics of silicon photomultiplier matrices (SiPM) for detection of Dark Matter particles in liquid argon detectors were investigated. At the experimental stand of the National Research Center "Kurchatov Institute", a study and comparison of the technical characteristics of SiPMs from different manufacturers at liquid nitrogen temperatures was carried out for their use as photodetector elements in tagged sources of gamma-ray and neutrons in the DarkSide experiment. The RNF project participants were the first to calculate radiogenic backgrounds in the DarkSide-20k detector arising in a previously unaccounted reaction channel (alpha, n-gamma) for the main structural materials of the DarkSide-20k detector. As a result of calculations, it was shown that one of the sources of the neutron background in the detector is the material of its external cryostat, developed at CERN on the basis of the ProtoDune cryostat prototype. Recommendations have been developed for replacing a part of the structural materials of the external cryostat without changing its design. A possible way to reduce background events in TPC from (alpha, n) reactions on argon nuclei was also discovered, which consists in applying a thin protective coating on the detector elements (in particular, elements of the SiPM electronic path with an increased content of natural radioactivity impurities). The project team actively participated in the search for Dark Matter particles in the DEAP-3600 experiment, using the accumulated experience of operating the DarkSide-50 detector, developed techniques and software for data processing and analysis. At present, the DEAP-3600 experiment has set the world's best limit on the spin-independent scattering cross section of dark matter particles on an argon target – 3.9∙10^-45 cm^2 for a WIMP mass of 100 GeV/c^2 at a confidence level of 90 %. The areas of responsibility of our group in the experiment are: analysis and processing of experimental data, development of methods for the background suppressing of the facility (including using machine learning algorithms), expanding the search area for dark matter particles using effective field theory and other possible models of the interaction of dark matter with matter, carrying out scheduled shifts at the facility. The design of the DEAP-3600 detector is such that one of the main sources of background is scintillation signals of alpha decays of the 210Po isotope on the inner surfaces of the detector neck. To suppress this background component, a software package for analyzing experimental data using machine learning algorithms was developed. The classification and rejection of background events in the detector was significantly improved. Based on the analysis of the data of the DEAP-3600 experiment and its calibrations and taking into account the specific characteristics of the detector, a physical model was developed to describe the shape of a scintillation pulse in liquid argon. The developed model includes: scintillations in liquid argon taking into account the so-called intermediate emission component, the temporal characteristics of the TPB spectrum shifter, including the delayed reemission of ultraviolet photons in a millisecond time interval, and the response of the detector photomultipliers. For the first time, it was found that about 10% of the intensity of the re-emitted light in TPB is accounted for by the long-lived component. Young participants in the project analyzed the data of the DEAP-3600 experiment taking into account the velocity distribution of Dark Matter in our Galaxy within the effective field theory models, which made it possible to limit the range of possible parameters of the existing models of the interaction of Dark Matter particles with matter. The tasks stated in the draft RSF 16-12-10369P for the reporting period were fully completed. Based on the material of the carried out work in the RSF project, 6 articles were published with gratitude to the Russian Science Foundation, of which 2 in the journals of the quartile Q1.

 

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Annotation of the results obtained in 2019
Astronomical observations of the last decades testify to the presence in the Universe of a non-baryon dark matter, which manifests itself only through gravitational interaction and is thus "hidden". Explanation of its nature, detection and subsequent study of its properties is a fundamental problem of modern science. The most substantiated and natural at present is the hypothesis that dark matter consists of massive, weakly interacting nonrelativistic WIMP (Weakly Interacting Massive Particles) particles. Research aimed at finding dark matter particles has progressed noticeably in recent years: the weight of the detector targets has increased, and the sensitivity of the experiments has improved. Intensive searches for hypothetical particles are carried out by elastic scattering processes on atomic nuclei by registering recoil nuclei. However, the cross section of the WIMP interaction with the nucleus is extremely small and the detection of recoil nuclei in the detector requires extremely sensitive detectors and low background conditions created in underground laboratories with protection against cosmic radiation. This project is implementing a set of activities in the development of dark matter particle detection technologies in large mass liquid noble gas detectors. This year, the project Global Argon Dark Matter Collaboration (GADMC) was initiated, which included participants in four international experiments: DarkSide, DEAP-3600, ArDM and MiniCLEAN. The main objective of the GADMC project is to pool resources and use accumulated experience in the development and operation of liquid argon detectors to develop a next-generation Dark Matter search facility with a record argon target mass of ~ 300 tons. Our project team actively participates in the DarkSide and DEAP-3600 experiments that are part of the GADMC collaboration. This year, the DarkSide-50 detector continued to collect and analyze data from the experiment to search for dark matter particles. The statistics of the events collected for the analysis of events (compared to the data in the previous publications of the DarkSide-50 experiment results) has been doubled. Special attention was paid to the analysis of low energies of the registered recoil nuclei corresponding to the observed electroluminescent signal in the range of 1-10 electrons pulled from the ionization track into the gas phase of the detector. This energy range allows the search for dark matter particles in the small mass region (0.5 - 6 GeV /c^2), for which the detector DarkSide-50 has the best sensitivity in the world. In order to describe the processes of low energy release registration in two-phase liquid-argon detectors, a semi-empirical model of single-electron pulses generated at the gas-liquid boundary was developed and created. To take into account the Migdal effect of dark matter particle scattering in the Darkside-50 experiment, we developed a software module MIGDAL, which calculates the energy release in liquid argon during the transfer of a fixed pulse to the argon nucleus. The MIGDAL software package developed by us allows us to significantly improve the sensitivity of the installation to the WIMP interaction section on the argon nuclei for small masses of dark matter particles (0.5-6 GeV/c^2). A cycle of works on assembly, debugging and physical start-up of the DarkSide-Proto two-phase projection camera (TPC) with argon target was performed. TPC DarkSide-Proto camera has the size of the sensitive area of 25*25*12 cm^3 and its main task is to test the work of photodetector elements (PDE), consisting of silicon photomultiplier (SiPM) matrices, specially developed by FBK for the experiment DarkSide and designed to work at cryogenic temperatures. The laboratory facilities for testing and selecting silicon photomultiplier (SiPM) matrices have been completed and will be used as photodetectors for the DarkSide-20k unit. A stand was put into operation at the Kurchatov Institute to study the characteristics of silicon photomultipliers at liquid nitrogen temperatures. To measure the absolute efficiency of neutron registration in the Darkside detector (TPC-camera), a miniature recording device consisting of a ZnSe(Tl) scintillation crystal, a silicon photomultiplier and a spontaneous fission source of 252Cf of low activity (~20 fission/s) was developed. A fundamentally new scheme of calibration of a liquid-argon TPC-camera with Am-Be and Am-C13 neutron sources was proposed for implementation and calculated. To record the neutron scattered on the nuclei of the neutron argon, a fast neutron detector based on stylus crystals with dimensions (or a liquid scintillator based on pseudocolumol, similar to that used in the Borexino detector) was used with the separation of the signal pulses in the temporal form during the registration of recoil protons (detecting the scattered neutron) and gamma-quanta. The location of the fast neutron detector at different angles relative to the straight line between the center of the source and the center of the argon TPC chamber at coincidences with the TPC triggering allows to study the TPC response for low energy recoil nuclei. The event coordinate of the neutron scattering event on the argon nucleus during this calibration is recorded due to the possibility of three-dimensional (or two-dimensional at very low energies) spatial reconstruction of events in the two-phase argon TPC. Within the framework of cooperation with DarkSide collaboration, the project team is actively involved in the DEAP-3600 dark matter search experiment. The DEAP-3600 detector is a liquid argon scintillation detector operating in the underground low-background SNOLAB laboratory (Canada). The target of the detector is 3280 kg of liquid argon, which is a record value for argon detectors today. This year, the collection and analysis of experimental data for the search for dark matter particles continued. Participants regularly conducted 24-hour online maintenance at the DEAP-3600 facility during the year, during which work was carried out to monitor the cryogenic system, pre-assess the quality and stability of the data collected and calibrate the detector. Duty duties were carried out in the remote access mode to the DEAP-3600 control unit via the Internet. For this purpose, the Remote Experiment Control Center was equipped at the site of SIC "Kurchatov Institute", and software was developed for continuous communication with the electronic measuring system of the facility via the Internet channel. Information about the detector operation, current technical characteristics, signals, the status of the detector cryogenic system, etc. is displayed on large scale screens of online-monitors. The developed software allows remote control of the installation (including the parameters of the cryogenic installation and detector parameters) and operatively control the quality of the data collected. The work carried out provided closer contact with the experiment and training of young project participants. To effectively discriminate against background events in the DEAP-3600 detector, a software package of experiment data analysis using machine learning algorithms was developed. This package combines several different approaches to classify events (decision trees, neural network, etc.), which significantly improved the classification and rejection of background events in the detector. The number of events selected for analysis, which remained after subtraction of the background, was 48% (about 2 times more than the old selection criteria) with the rejection at the level of 99.9%. The use of machine learning algorithms provided more than a two-fold increase in experiment statistics.

 

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