November 9, 2020 feature
Study sets the first germanium-based constraints on dark matter
Cosmological observations and measurements collected in the past suggest that ordinary matter, which includes stars, galaxies, the human body and countless other objects/living organisms, only makes up 20% of the total mass of the universe. The remaining mass has been theorized to consist of so-called dark matter, a type of matter that does not absorb, reflect or emit light and can thus only be indirectly observed through gravitational effects on its surrounding environment.
While the exact nature of this elusive type of matter is still unknown, in recent decades, physicists have identified many particles that reach beyond the standard model (the theory describing some of the main physical forces in the universe) and that could be good dark matter candidates. They then tried to detect these particles using two main types of advanced particle detector: gram-scale semiconducting detectors (usually made of silicon and used to search for low-mass dark matter) and ton-scale gaseous detectors (which have higher energy detection thresholds and are better suited to perform high-mass dark matter searches).
The EDELWEISS Collaboration, a large group of researchers working at Université Lyon 1, Université Paris-Saclay and other institutes in Europe, recently carried out the first search for Sub-MeV dark matter using a germanium(Ge)-based detector. While the team was unable to detect dark matter, they set a number of constraints that could inform future investigations.
"EDELWEISS is a direct dark matter search experiment. As such, our primary goal is to detect dark matter to bring irrefutable proof of its existence," Quentin Arnaud, one of the researchers who carried out the study, told Phys.org. "Still, the absence of detection is an important result itself, because this allows us to test and set constraints on existing dark matter particle models."
There are two key reasons why dark matter particles have so far eluded detection. First, the probability that these particles will interact with ordinary matter, such as the one inside conventional particle detectors, is extremely small.
Second, the signal that researchers expect would arise from a dark matter particle impinging the detector is several orders of magnitude lower than the signals produced by natural radioactivity. Detecting these signals would thus require very long detector exposure times and the use of instruments that are made of radio-pure materials, but that are also adequately shielded and operated deep underground, as this prevents them from picking up ambient radioactivity and cosmic rays.
"Eventually (in spite all our efforts), there will always be some residual background that we need to be able to discriminate against," Arnaud explained. "Therefore, we develop detector technologies with the capability to determine whether the signals we detect are induced by a dark matter particle or are originating from the radioactive background."
Arnaud and his colleagues were the first to search for sub-MeV dark matter using a 33.4g germanium cryogenic detector instead of a silicon-based particle detector. They specifically searched for dark matter particles that would interact with electrons. The detector they used was operated underground at the Laboratoire Souterrain de Modane, in France.
"The energy deposited in our detector following a dark matter particle interaction is expected to be extremely small (<1 keV)," Arnaud said. "When searching for light dark matter particles (sub-MeV masses), it is even worse: The deposited energy can be as small as a few eV, energy deposits so small that only a few state-of-the art detector technologies can be sensitive to them."
The detector used by the EDELWEISS collaboration essentially consists of a germanium cylindrical crystal cooled down to cryogenic temperature (18 mK or -273,13 C° ), with aluminum electrodes on each side of the crystal, on which the team applied a high voltage difference. Collisions between particles and nucleus/atoms inside the crystal lead to the production of electron-hole pairs, which induce a small charge signal (i.e., current) as they drift towards collecting electrodes.
In addition, a particle's collision with the crystal lattice induces a small increase in temperature (i.e., under 1 micro-Kelvin). This change in temperature can be measured using a very sensitive thermal sensor known as a neutron transmutation doped (NTD) sensor. As the energy deposits that should theoretically arise from sub-MeV dark matter particles are incredibly small (i.e., in the eV-scale), however, the associated charge signal would be too small to be measurable and the increase in temperature too slight to be measured by an NTD sensor.
"To solve this issue, our detector exploits what's called the Neganov-Trofimov-Luke (NTL) effect (which to some extent is similar to the Joule effect): In cryogenic semiconductor detectors, the drift of N electron-hole pairs across a voltage difference produces additional heat whose energy adds up to the initial deposited," Arnaud said. "This Neganov-Trofimov-Luke (NTL) effect essentially turns a cryogenic calorimeter (operated at ΔV=0V) into a charge amplifier. A small energy deposit ends up giving rise to a high (measurable) temperature elevation and he higher the voltage, the higher the amplification gain."
Arnaud and his colleagues set new constraints on the kinetic mixing of dark photons. Overall, the findings they collected demonstrate the high relevance and value of cryogenic germanium detectors in the ongoing search for dark matter interactions that produce eV-scale electron signals.
The EDELWEISS collaboration is now developing a set of more powerful detectors called SELENDIS (Single ELEctron Nuclear recoil DIScrimination). The most important feature of these new detectors is an innovative discrimination technique that will allow the team to differentiate between nuclear and electronic recoils down to a single electron-hole pair with the sole measurement of heat signals rather than requiring the simultaneous measurement of two observables (e.g,, heat/ionization, ionization/scintillation or heat/scintillation), as is the case with previously proposed discrimination techniques.
"No currently existing detector technologies can combine single electron detection sensitivity and discrimination capabilities," Arnaud said. "Direct detection experiments optimized for high-mass dark matter searches are very good at discriminating the signal from the background but have relatively high energy detection thresholds. Low-mass dark matter search experiments—including EDELWEISS—have unprecedented low energy detection thresholds but cannot discriminate the signal from the background. With SELENDIS, our goal is to combine the two by developing the first detector combining single electron hole pair sensitivity and background discrimination capabilities."
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