Dark Matter

In 1933 Fritz Zwicky, a Swiss astronomer who worked at the California Institute of Technology, measured the velocity dispersion of galaxies in the Coma cluster finding a value larger than expected from the estimated total mass of the cluster. Zwicky defined such unseen mass as “dark matter”. These observations were confirmed later in the 1970s, when the data collected on the galactic rotational curves of spiral galaxies and definitively proved the presence of an amount of mass much larger than the optical size of galactic disks. The nature of dark matter is one of the most engaging challenges in modern physics. Physicists can look for its effects by measuring the movements of galaxies, but what it is made of is still a mystery. This discovery would have a significant impact on the understanding of the formation of the large scale structure in the Universe and, it could lead to the empirical evidence of new unknown particles. Some ideas for dark matter particles could also solve other problems in particle physics. Such particles should interact very weakly with matter and have about 1 to 1000 times the mass of a proton. Physicists called them WIMP, which is the acronym of Weakly Interacting Massive Particles. One promising candidate is the neutralino, a particle predicted by Supersymmetry. Supersymmetry supposes that every known particle has a partner, explaining some of the unknowns in Standard Model particles. The inevitable question which arises at this point is: how can we catch dark matter particles? Many experiments, in laboratories all around the word, are seeking dark matter particles using many different approaches. The current experiments can be divided in three categories: indirect search, direct search and accelerator based search experiments. The first category looks for Standard Model particles produced in dark matter particles annihilation and direct search experiments try to detect the scattering of such particles off an atomic nucleus inside their detection volume. In accelerator based experiments, dark matter particles could be created in the collision of highly energetic particle beams.

Experiments exploiting an indirect search approach probe regions of the Galaxy where a high density of dark matter particles is expected. In those regions, the random chance of dark matter particles annihilation is enhanced. When annihilations occur, gamma-rays or particle-antiparticle pairs are produced. This additional particle flux then could be observed by satellites or telescopes on Earth. This approach, however, does not yield a unique answer, as gamma-ray and particle backgrounds are often not understood to the level of precision needed. Examples of such experiments are AMS (Alpha Magnetic Spectrometer) and ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch). AMS is an experiment installed on the International Space Station. It includes several detectors which help in particle identification. AMS measures electrons, positrons, antiprotons and nuclei to the TeV energy range in order to study the cosmic ray spectrum. Dark matter particles are detected indirectly looking for an excess of positrons in the measured spectrum. Hamamatsu R5900 PMTs have been used to measure the signals generated when the particles pass through the AMS detectors. ANTARES is a neutrino telescope located 2475 meters under the Mediterranean Sea, 40 km off the coast of Toulon in France. It is very sensitive to WIMPs accumulated in the center of our galaxy, since it is visible from the ANTARES location, and in the Sun. This experiment uses an array of Hamamatsu photomultipliers, type R7081-20, in vertical strings, spread over an area of around 0.1 square kilometers.

Figure 1. Alpha Magnetic Spectrometer.
Figure 1. Alpha Magnetic Spectrometer.
Figure 2. Schematic drawing of the principle of detection used by the XENON100 experiment.
Figure 2. Schematic drawing of the principle of detection used by the XENON100 experiment.

Direct search experiments aim for the detection of dark matter particles looking for nuclear recoils produced by such particles scattered off target nuclei. This would produce a recoiling target nucleus which can be detected if sufficient energy is transferred by the collision. The main difficulty is the discrimination of events induced by external radiation that can mimic the scattering of a dark matter particle. Therefore, such experiments are usually located in deep underground laboratories and use pure material as a target and detectors. Further backgrounds can be reduced only by selecting detector materials which contain the tiniest traces of radioactive impurities. Most of today’s direct search experiments use three techniques to detect the nuclear recoil: induced heat, ionization signal and/or scintillation light. XENON100 (with the future upgrades XENON1T and XENONnT) as well as DarkSide are examples of direct search experiments. The XENON100 Dark Matter Search is an experiment located underground in the INFN Gran Sasso Laboratories, which use a double phase Time Projection Chamber (TPC) filled with liquid xenon and a gaseous xenon layer on top at a temperature of about -95 °C. A particle interacting in liquid xenon produces a prompt scintillation signal through excitation and recombination of ionization electrons. These electrons are drifted towards the liquid-gas interface where they produce a secondary scintillation signal (see Fig. 2). Two arrays of Hamamatsu R8520-06-AL 1 inch square PMTs, one on top of the TPC and the other on the bottom, detect the light in the TPC. With a combination of large mass and ultra-low background detectors, the XENON100 project is leading this field with some of the most stringent limits on the interaction between WIMPs and normal matter. The DarkSide program plans to develop a series of novel dual phase liquid argon TPCs. The detectors will use several innovative techniques to identify dark matter signals and to understand and suppress backgrounds. After a successful prototype (DarkSide-10), the first physics detector in the DarkSide program is DarkSide-50 (DS-50), currently situated in the INFN Gran Sasso National Laboratory. The detector has an active mass of 50 kg and has enough sensitivity to search for WIMPs with a mass 100 times greater than the mass of a proton. The DS-50 two-phase liquid argon TPC - shown in Fig. 3 - uses a detection technique similar to XENON100. The scintillation light is recorded by thirty-eight 3 inch Hamamatsu low-background R11065 PMTs, 19 each on the top and bottom to view the active argon. The construction of DarkSide-50 was completed in late 2012, and the physics data collection began in 2013.

Figure 3. Design of DS-50 liquid argon dewar containing the two-phase TPC.
Figure 3. Design of DS-50 liquid argon dewar containing the two-phase TPC.

Physicists using the Large Hadron Collider (LHC) at CERN are looking for dark matter particles produced in the proton-proton collision. The energies achieved by such an accelerator are high enough to create yet unknown particles that may have the properties required to account for dark matter. If dark matter particles are produced in proton-proton collisions at the LHC, experiments such as ATLAS and CMS should measure less energy than predicted. Indeed, dark matter must be stable over long timeframes and interact only very rarely. Therefore, if produced, it must escape the detector without leaving any further trace.

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