This section is dedicated to the experiments currently searching for interactions between dark matter particles and an experimental target. Originally, these ex-periments were designed to detect WIMPs, but their physics reach can easily be extended to light dark matter and other dark matter candidates.
Figure 2.6: A list of existing and proposed underground laboratories divided by geographic areas with the relative size (circles) and depth in meters water equiva-lent [105]. Currently, the deepest operational laboratory is CJPL located in China, while the largest is LNGS located in Italy. Figure from [105].
The shared feature of these experiments is that they are located in deep-underground laboratories, in order to suppress the background induced by cosmic rays, see Fig-ure 2.6. Otherwise, a wide variety of experimental techniques is employed in the field of direct detection, so it is useful to divide them in some categories: noble liquid detectors, bubble chambers, proportional counters, and cryogenic detectors.
2.2.1 Noble liquid detectors
Experiments using liquefied noble gases have been arguably the most successful in the direct dark matter search for WIMPs so far. These experiments employ either argon (A'40) or xenon (A'131) which are both excellent scintillators, but other noble gases could also be suitable targets [106]. In standard conditions, these ele-ments assume a gaseous state which would prevent the design of a large-exposure experiment, but they can be conveniently liquefied to very dense targets. Argon becomes liquid at 87.2 K and xenon at 162.5 K [90], so both can be easily liquefied only using liquid nitrogen.
A particle interacting with a noble liquid produces heat, scintillation, and ionization.
Current experiments are not able to measure heat, but focus instead on detecting scintillation and ionization signals. Liquid noble gases experiments can be divided in two main categories based on which detector they use to measure these signals:
single phase detectors or dual-phase Time Projection Chambers (TPCs), see Fig-ure 2.7.
Single phase detectors only measure the primary scintillation (S1) caused by the
Figure 2.7: Left: sketch of an interaction between a dark matter particle and a liquid noble gas inside a single-phase detector. This detector measures the primary scintillation induced by the interaction via an array of photosensors surrounding the experimental volume. Right: sketch of an interaction between a dark matter particle and a liquid noble gas inside a double-phase TPC. In this case, apart from measuring the primary scintillation with the photosensors, the electrons produced in the ionization of the medium are drifted via a strong electric field towards a volume filled with gas. There, the electrons can cause a secondary scintillation signal that is measured by the surrounding photosensors. Figure from [90].
scattering of a particle inside the experimental medium, while experiments employ-ing dual-phase TPCs can also measure the ionization signal (S2) by extracting the electrons produced in the interaction via a strong electric field. Both the primary scintillation and ionization signals are generally measured with photomultiplier tubes (PMTs), but some experiments are planning to shift in the future to silicon photo-multipliers (SIPMs) [107, 108].
Example of experiments using single-phase detectors are XMASS [109] for xenon and DEAP-3600 [110] for argon. Experiments employing dual-phase TPCs with a xenon target are PANDAX [111], XENON1T [112], and LUX/LZ [113], while DarkSide [114] is based on argon.
2.2.2 Bubble chambers
Bubble chambers for dark matter search employ superheated fluids kept above the boiling point in a metastable state. Examples of suitable targets are CF3I, C3F8, C4F10, C2ClF5, and C3ClF8 [90]. When a particle interacts with the fluid, it can deposit enough energy to cause a local phase transition which will result in the formation of a bubble, see Figure 2.8. This method of detection is extremely con-venient, since the bubble formation can be tuned to take place only in the presence of a nuclear recoil induced by α particles, neutrons, or dark matter particles. The bubbles are detected through cameras and the images can be used to determine the spatial coordinates of the events. Acoustic sensors can also be employed to record sound emissions caused by bubble nucleation: this technique is very effective in sup-pressing background events, since nuclear recoils induced by αparticles have a very distinctive signature [115].
The drawback of this kind of detectors is the inability of detecting the recoil en-ergy, which means that they can only be used as counters of events above a certain
Figure 2.8: Sketch of a dark matter interaction taking place with a superheated fluid inside a bubble chamber. The nuclear recoil induced by a dark matter parti-cles causes the formation of a bubble if the recoil energy is above a certain energy threshold. Bubbles are detected using cameras, while acoustic sensors are used to suppress the background induced by α particles. Figure from [90].
energy threshold. Furthermore, after each event the bubble chamber has to be com-pressed and subsequently decomcom-pressed, causing a substantial dead time. Despite these challenges, the PICO-60 experiment has achieved a world-leading sensitivity for dark matter-proton spin-dependent interactions with19F [116].
2.2.3 Spherical Proportional Counters
Spherical Proportional Counters (SPCs) are constituted by a spherical vessel filled with pressurized gas. The vessel is grounded and constitutes the cathode, while at the centre of the sphere there is a small resistive body acting as anode. The anode is supported by a metallic rod through which a high voltage is injected [117]: in this way the electric field inside the detector varies as 1/r2, where r is the distance from the center. This sharp change in the electric field divides the detector volumes into two regions: the amplification region and the drift region.
When a particle interacts inside the volume, it causes the ionization of the gas followed by the emission of primary electrons. These electrons drift towards the amplification region where they acquire enough energy to produce a ions-electrons avalanche. The ions resulting from the avalanche drift towards the cathode and induce the signal, which is read via the high voltage wire, see Figure 2.9. Any parti-cle interaction detected in a SPC has two distinctive observables: the amplitude of the signal, correlated to the energy deposition, and its rise time, correlated to the position of the interaction [117].
Advantages of this kind of detectors are the high radiopurity of the materials em-ployed for its construction, the flexibility in the choice of the target gas (Ne, He, H, CH4), and the possibility to achieve energy thresholds for nuclear recoils <1 keV.
While these detectors can be used for a variety of applications, the main experiment
Figure 2.9: Sketch of an interaction between a dark matter particle and the gas inside a Spherical Proportional Counter. After the scattering there is the production of a primary electron (1), which drifts towards the center (2). When the electron reaches the amplification region it produces a ion-electron avalanche (3). The positive ions drift then towards the outer vessel (4) inducing the signal read through the high voltage wire (5). Figure from [119].
of this kind involved in direct dark matter search is NEWS-G [118].
2.2.4 Cryogenic detectors
Cryogenic detectors have been among the first detectors to be employed in direct dark matter search. These detectors are based on crystalline targets cooled down to temperatures below 50 mK and coupled with mainly two different kind of sen-sors: Transition Edge Sensors (TES) and Neutron Transmutation Doped (NTD) germanium thermistors [90]. These sensors detect the heat signal induced by par-ticle interactions inside the crystals, see Figure 2.10, and are able to detect tiny energy depositions with high energy resolutions. To ensure an effective background rejection, experiments of this kind also detect a second signal coming from particle interactions inside the target. Depending on the experiment, in fact, the ioniza-tion or the scintillaioniza-tion produced by particle interacioniza-tions within the crystals are measured in coincidence with the heat signal: from the ratio of these signals it is possible, in most cases, to distinguish electronic recoils events from nuclear recoil events which are generally associated with dark matter interactions in the target.
The most prominent experiments of this kind are currently SuperCDMS [120], EDELWEISS-II [121], and CRESST-III [122].
Since this thesis focuses on novel physics results obtained with cryogenic detectors, the underlying physical aspects of this particular class of detectors will be explained in more detail in Chapter 3.
Figure 2.10: Sketch of a particle interaction taking place inside a crystal thermally coupled to a thermal bath in the∼10 mK range. The particle interaction produces a heat increase which is measured by a sensor placed on the crystal’s surface. The particle interaction can also cause ionization or scintillation that can be used for particle discrimination and background rejection. Figure from [90].