Light particles in string and holographic frameworks

The Standard Model (SM) is the most accurate theory ever made, and after the discovery of the celebrated Higgs particle in 2012, it is also complete. However, several theoretical issues like the fact that neutrinos are massive, the existence of Dark Matter (DM) being five times more abundant than ordinary matter, and the absence of a consistent theory of quantum gravity lead physicists to believe that SM is only an effective manifestation of a more Fundamental Theory. The current project aims to explore an unknown possibility considering an extension of the SM by a large, invisible sector/world which interacts very weakly with the SM. In this model, new particles emerge from the hidden sector, and this project aims to analyse their behaviour and study their phenomenological implications. We will mainly focus on Anomalous Z`: a special type of massive vector particle that interacts with the SM particles (they behave like the Z particle of the SM). Z` is one of the most popular scenarios for physics beyond the SM, with extended searches at LHC at CERN. Such particles can explain differences between theoretical predictions and experimental observations, like the discrepancy in the muon`s magnetic moment. Therefore, the phenomenological implications of their presence are very interesting. Emergent axions: new particles, weakly coupled to the SM (therefore almost invisible) with a possible contribution to the evolution of the Universe and also potential Dark Matter candidates. These particles are also extensively studied; however, our emergent versions have different behaviours from the axions studied in the past. Emergent neutrinos: a particular version of neutrinos coming from the additional/invisible sector is an unexplored possibility. It allows for a different approach to neutrino physics, giving rise to new approaches to Leptogenesis and Baryogenesis in the early stages of the Universe. All these emergent/new fields differ qualitatively from what has been considered so far. Our results are expected to be compared with experimental data from the LHC and other experiments in the future.

Our current understanding of nature is described by the Standard Model of particle physics. It is the most precise scientific theory ever built and became "complete" after the discovery of the Higgs particle in 2012. Yet, several open questions remain: we know that neutrinos have mass, that most of the matter in the Universe is invisible "dark matter", and that gravity still cannot be combined consistently with the quantum world. These puzzles suggest that the Standard Model is only a visible part of a deeper and more fundamental picture. This project explored an original possibility - the existence of a hidden world of particles that interact only very weakly with ordinary matter. Although invisible to our current detectors, this hidden world could leave subtle traces in high-precision experiments or in the history of the Universe. The research focused on three key ideas: Anomalous Z particles - new, heavy cousins of the known Z boson that could explain tiny differences between theory and experiment, such as the long-standing anomaly in the magnetic moment of the muon. Emergent axions - extremely light, weakly interacting particles that might form part of the dark matter filling the Universe and influence its evolution after the Big Bang. Emergent neutrinos - new types of neutrinos coming from the hidden sector that could help explain why matter dominates over antimatter in the cosmos. During one year of research, theoretical models were developed that show how such particles might arise naturally and how they could be detected in experiments like those at CERN's Large Hadron Collider. The results, published in international scientific journals, provide new ways of connecting fundamental theories such as string theory with real experimental searches. Beyond the scientific insight, this project strengthens the link between abstract mathematical physics and experimental observation. It opens new routes to understand dark matter, the evolution of the early Universe, and possibly future technologies based on quantum fields and fundamental interactions. The project was completed in 2024 when the principal investigator took up a position as Scientific Officer at the European Research Council Executive Agency (ERCEA), continuing to support frontier research in Europe.