Periodic Reporting for period 2 - LightAtLHC (Search for Axion-Like Particles at the LHC) Reporting period: 2021-09-01 to 2023-02-28 Summary of the context and overall objectives of the project Our current understanding of the universe is based on the theory of general relativity as well as the Standard Model of particle physics. Both theories are highly successful, however, several experimental aspects imply that our knowledge of the fundamental building blocks of matter is not yet complete. Most prominently, we observe a significant discrepancy between the prediction and the measurement of a property of the muon particle, known as the muonic (g-2) puzzle. One possible explanation is based on the existence of new particles, generally referred to as axions. The main goal of the Light@LHC project is the search for such axions at the large hadron collider (LHC), both at the ATLAS and the FASER Experiment, using highly innovative experimental techniques. In particular, we developed new algorithms to search for axion with long live times at ATLAS as well as new detector components for the FASER Experiment during the first phase of the project, which is now concluded. We will use those developed technologies to conduct new searches for axions during the second phase of the project, hoping to find the door to physics beyond the standard model and completing the knowledge of mankind about our universe. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far Three main achievements could be realized in the past years: The development of neuronal-network based classifiers for collinear and low energetic photon signatures allows us to search for axions, produced in Higgs boson decays as well as for axions produced in light-by-light scattering events. Furthermore, the Light@LHC team invented a possibility to assign data-driven uncertainties on the reconstruction of photon signatures with displaced vertices, allowing us to perform a reliable search for axions with higher masses. Most notably, we constructed the support frame of the FASER experiment and successfully installed it next to the LHC, as shown in the attached photo. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) By the end of the Light@LHC project, we will have performed the most stringent search for axion like particles with masses between few MeV to the TeV scale, probing a completely new parameter space. In particular, we will answer the question, if axions can explain the muonic (g-2) puzzle. Moreover, we will have developed new technologies to search for long-lived particles at the LHC, thus having a long lasting impact in the field of experimental high energy physics. Installation of the FASER Experiment at CERN
