Searching for Light Dark Matter with the NA64-POKER Experiment

The determination of the Dark Matter's microscopic nature represents one of the fundamental motivations for the search for new physics beyond the Standard Model. While multiple astrophysical and cosmological measurements robustly probe the existence of Dark Matter, these observations do not provide a direct indication of its particle-physics properties. Over the past decades, experimental efforts have mainly focused on the hypothesis that Dark Matter is composed of Weakly Interacting Massive Particles (WIMPs), having mass at the electroweak scale (1~GeV-1~TeV) and interacting with Standard Model particles through the weak force. Despite a broad experimental effort, no definitive confirmation of the existence of WIMPs has been found. As a result, the search for Dark Matter extended from the minimal WIMP model toward new alternative theories. Among these, the Light Dark Matter (LDM) models, in which Dark Matter has a mass in the 1~MeV—1~GeV range, are theoretically well motivated if a new mediator is introduced to allow the interaction between LDM and Standard Model particles. A simple, representative Light Dark Matter model introduces a new massive boson, called the Dark Photon ($A^\prime$), which interacts with both the Standard Model photon and LDM particles ($\chi$). In this scenario, the Dark Photon can be generated by the interaction of electrically charged particles with ordinary matter. The Light Dark Matter models can be effectively tested at particle accelerators with experiments searching for the production of Dark Matter in high-energy scattering processes. This thesis focuses on experiments employing particle beams impinging on a fixed, active target, adopting the missing energy signature to investigate Light Dark Matter particle production. Specifically, if an $A^\prime$ is generated within the target, it decays into Light Dark Matter particles that escape undetected, carrying away a fraction of the beam energy. In this context, experiments making use of high-energy positron beams maximise the production of LDM through the resonant annihilation of positrons with target atomic electrons ($e^+e^-\rightarrow A^\prime\rightarrow \chi \bar{\chi}$). Due to the kinematics of the resonant process, this production channel presents a unique experimental signature: the produced Dark Photon carries a kinetic energy that depends solely on its mass. My PhD thesis develops within the POKER (POsitron resonant annihilation into dark mattER) project, searching for Light Dark Matter through missing-energy measurements with positron beams impinging a thick active target. This effort is performed in the context of the NA64 experimental program, operating at the CERN Super Proton Synchrotron (SPS) and originally designed for LDM search via a 100~GeV electron beam. The main goal of my thesis was to perform a pilot missing-energy measurement with a $\sim100$~GeV positron beam exploiting the NA64 experimental setup to prove the feasibility of this experimental approach to search for Light Dark Matter production at accelerators. The thesis consists of six chapters. In Chapter~\ref{chapDM}, I introduce the Dark Matter phenomenology, describing the evidence for its existence, the open questions regarding its nature and the constraints imposed by cosmological and astronomical observations. I then present the Light Dark Matter theories and the hypotheses they are grounded on, discussing how experiments at accelerators represent an effective solution for testing this scenario. Moreover, I describe the adopted experimental technique based on a missing-energy measurement performed with an active thick target, focusing on the advantages of using a positron beam. Chapter~\ref{chapNA64} describes the NA64-e experimental setup and its latest results in the Dark Photon invisible decay search with the SPS-H4 100~GeV electron beam. In this context, I studied the contribution of secondary positrons generated in the target, possibly producing Dark Photons through the resonant annihilation channel, testing the effectiveness of the POKER experimental technique for the first time. Considering secondary positrons improved the NA64-e experimental sensitivity, extended the exclusion limits in the LDM parameter space and motivated a dedicated positron-beam effort within the collaboration. The first step towards this program was the H4 positron-beam characterisation, with special attention to beam purity, for which no precise measurements were available. Chapter~\ref{chapContam} illustrates how the H4 secondary positron beam is extracted from the SPS 400~GeV proton beam and what are the main mechanisms responsible for the contamination. I describe the dedicated analysis I was responsible for, which precisely measured, for the first time, the fraction of hadronic contaminants in the 100~GeV electron and positron beam at the SPS-H4 beamline. The results confirmed the feasibility of a dedicated positron-beam measurement searching for Light Dark Matter. During the summer of 2022, for the first time, the NA64-e experiment collected data to perform a Dark Photon search with the 100~GeV positron beam. Chapter~\ref{chapAna}, the core of this thesis, illustrates the relative analysis and the obtained results. The adopted analysis strategy is presented, highlighting the crucial elements: the single event selection, the background estimate, and the evaluation of signal efficiency. Thanks to the yield enhancement induced by the positron resonant annihilation, the obtained limits in the LDM parameter space touched the NA64-e results, corresponding to about two orders of magnitude larger accumulated statistics. This result proved the outstanding potential of NA64-POKER to probe Dark Sectors with positron beams, paving the way for a multi-energy measurement program to explore a large portion of the LDM parameter space. The crucial element of this experimental effort is a new high-resolution electromagnetic calorimeter to improve the NA64 discovery potential, allowing the signal line shape measurement in the missing energy spectrum. Chapter~\ref{chapPOKER} introduces the PKR-CAL detector, composed of lead tungstate crystals with a silicon photomultiplier-based readout system, which will be integrated within the NA64 setup, replacing the existing active thick target. Moreover, the measurements I performed to verify the effectiveness of these construction choices are summarised. POKERINO, a small-scale prototype of the PKR-CAL detector, was constructed to validate the PKR-CAL technical choices and the overall detector design. It adopts all the main elements of the PKR-CAL, including the single-crystal mechanical assembly, the PKR-CAL-SiPM board, the readout electronics, and the electrical connections scheme. The POKERINO performance was assessed with cosmic rays in Genova, and its response to high-energy beams was measured at the CERN H8 beamline. Chapter~\ref{chapPOKERINO} describes the POKERINO test measurements and the corresponding data analysis for which I was responsible, highlighting how the results influenced the design of the PKR-CAL calorimeter.