Study of the response of a large volume TPC prototype for the CYGNO experiment at LNGS

Astrophysical and cosmological observations have provided strong evidence that a significant portion of the Universe is composed of non-luminous matter, known as Dark Matter (DM). The Standard Model (SM) of particle physics successfully describes the fundamental particles and their interactions but does not account for any Dark Matter particle candidate. One of the most supported hypotheses suggests that DM is composed of particles different from those in the SM. Among the most promising candidates are Weakly Interactive Massive Particles (WIMPs), which are non-relativistic particles that interact weakly with SM particles. A stable, weakly interacting particle in thermal equilibrium in the early Universe would be capable of explaining the observed relic DM density. The motion of the Sun and the Earth with respect to the rest frame of the Galaxy produces an apparent "wind" of WIMPs coming from the direction of the Cygnus constellation. Assuming DM can interact with SM particles, it is possible to exploit the weak interaction with regular matter measuring the recoils induced by DM interactions. This is known as direct detection. The fundamental strategy is to expose a large amount of instrumented mass and wait for DM to produce recoils in it. Direct detection experiments look for nuclear recoils of low energy, 1-100 keV, with an expected rate below 1 event/kg/year. The low expected event rate implies detectors with extremely challenging requirements on the background reduction techniques, such as operating detectors underground to suppress cosmic rays, using radiopure materials, and implementing active or passive shielding. The CYGNO project aims to build a large O(30m 3 ) directional detector for rare event searches, including Dark Matter. The detector uses a gaseous Time Projection Chamber (TPC), which is sensitive to recoil topology and allows direction measurement. The TPC is filled with a gas mixture rich in helium and fluorine at atmospheric pressure, making it sensitive to both Spin-Independent and Spin-Dependent interactions. A triple Gas Electron Multiplier (GEM) stack provides ionization signal amplification, and the signals are optically read out using photomultiplier tubes (PMTs) and scientific CMOS cameras, enabling 3D track reconstruction. Several CYGNO prototypes have been built and tested in both overground and underground environments to assess their performance. Currently, the CYGNO project is at the end of the R&D phase with the Long Imaging ModulE (LIME), the largest prototype built, comprising a 50 liters active volume. After commissioning overground at Laboratori Nazionali di Frascati (LNF), LIME was installed underground at Laboratori Nazionali del Gran Sasso (LNGS) in February 2022. This underground operation marks a significant milestone in the CYGNO roadmap towards constructing a large-scale TPC for directional Dark Matter searches.