Optimisation of amplification and gas mixture for directional Dark Matter searches with the CYGNO/INITIUM project

Astrophysical and cosmological observations performed within the last century provide extensive evidences that the comprehension of the Universe is not complete and that an ingredient known as dark matter (DM) is required to interpret the measurements. One of the most supported class of theories suggests that dark matter is composed of weakly interactive massive particles (WIMPs). As the Milky Way is expected to reside in a halo of these particles, a possible experimental methodology to observe them is to exploit the potential weak interaction between WIMPs and standard matter, which results in the recoil of the latter. Due to the strength of this interaction, direct detection experiments for WIMPs are searching for ultra rare events, by exposing large volume of instrumented mass to detect recoils of regular matter. The motion of the Sun and Earth with respect to the Galactic Rest Frame and the foreseen non relativistic nature of the WIMPs, induces an apparent wind of DM particles coming from the Cygnus constellation, which imprints a strong directional dependence in the recoil spectrum. Direct detection experiments capable of measuring the angular features of the recoils gain access to a wide range of advantages. These include the possibility to positively claim a discovery of DM, along with the ability to constrain DM properties. The CYGNO project sets into this context, with the aim to deploy a large directional detector for rare event searches as DM. It comprises a gaseous time projection chamber filled with a He:CF$_4$ gas mixture, an amplification stage based on a triple stack of gas electron multiplier (GEM) optically readout employing scientific CMOS cameras and photomultiplier tubes (PMTs). This setup allows to perform the imaging of the recoil tracks, measuring on an event by event basis the energy, the direction and the topology of the tracks. The advantage of the optical readout through sCMOS cameras is that it allows to image large areas with an high granularity but a reduced amount of sensors. The major drawback is that the resulting small solid angle coverage of the optical setup ends up limiting the number of photons that reach the light detectors, effectively affecting the energy threshold. In this thesis, it is presented the work carried out with small CYGNO prototypes in order to maximise the light yield without degrading spatial and energy resolution. Different number of GEMs of various thicknesses are tested together with the addition of an extra electrode to produce strong electric fields below the last GEM. The latter addition proved to be an innovative way to enhance the light production below the GEM holes for the He:CF$_4$ gas mixture, which allowed to improve the light yield by a factor close to 2 and reducing the intrinsic diffusion of the amplification structure by tens of micrometers. Minimising the electron cloud diffusion while drifting towards the amplification stage is very relevant to precisely measure the topological information of the recoil tracks. The addition of highly electronegative gases induces the electrons freed by the passage of an ionising particle to be captured by them in few micrometres. The negative ions produced are drifted in place of the electrons and their larger mass allows them to remain thermal with the gas, strongly reducing the diffusion. This technique is usually referred to as Negative Ion Drift (NID) operation. Using a CYGNO prototype, the addition of 1.6\% of SF$_6$ to the He:CF$_4$ gas mixture is demonstrated to produce negative ions drift operation reaching gas gains of the order of 10$^4$. A strong reduction in diffusion is measured, with diffusion coefficient as low as 45 $\mu$m/$\sqrt{\text{cm}}$, among the smallest ever measured in a gas detector. The potential performances of directional detectors in the context of a direct DM search are analysed with the use of rigorous statistical tools. The improvement in setting limits in the \W to nucleon and DM mass parameter space with a directional detector are evaluated employing the future expected performances of the CYGNO experiment. In addition, the improvement in the capability of discerning two different DM models exploiting a directional detector in comparison to a non directional one is analysed. It is found that, taking advantage of the angular feature, the two models considered can be distinguished with orders of magnitude less events than conventional non directional detectors.