The Low-Energy Frontier of Particle Physics JOERG JAECKEL Institute for Particle Physics Phenomenology, Durham University, Durham DH1 3LE, United Kingdom ANDREAS RINGWALD Deutsches Elektronen-Synchrotron, Notkestraße 85, D-22607 Hamburg, Germany Key Words: Theoretical and experimental low energy particle physics, extensions of the Standard Model, axions, extra gauge bosons, hidden matter particles Abstract: Many embeddings of the Standard Model into a more unified theory predict the existence of a hidden sector of particles that interact very weakly with the visible sector. Some of these exotic particles, such as axions and hidden U(1) gauge bosons, may be very light and have very weak interactions with photons. These very weakly interacting sub-eV particles (WISPs) may lead to observable effects in experiments and astrophysical observations. The paper presents the physics case and a status report of this emerging low-energy frontier of fundamental physics. The paper discusses the physics case for WISPs, including axions and axion-like particles, ultralight hidden-sector particles, and their astrophysical and cosmological constraints. It also explores possible indirect hints for WISPs and discusses various experiments that search for WISPs using low-energy photons, such as photon regeneration experiments, laser polarization experiments, and experiments using strong electromagnetic fields. The paper concludes with a summary and outlook on the current situation and future prospects for WISP research.The Low-Energy Frontier of Particle Physics JOERG JAECKEL Institute for Particle Physics Phenomenology, Durham University, Durham DH1 3LE, United Kingdom ANDREAS RINGWALD Deutsches Elektronen-Synchrotron, Notkestraße 85, D-22607 Hamburg, Germany Key Words: Theoretical and experimental low energy particle physics, extensions of the Standard Model, axions, extra gauge bosons, hidden matter particles Abstract: Many embeddings of the Standard Model into a more unified theory predict the existence of a hidden sector of particles that interact very weakly with the visible sector. Some of these exotic particles, such as axions and hidden U(1) gauge bosons, may be very light and have very weak interactions with photons. These very weakly interacting sub-eV particles (WISPs) may lead to observable effects in experiments and astrophysical observations. The paper presents the physics case and a status report of this emerging low-energy frontier of fundamental physics. The paper discusses the physics case for WISPs, including axions and axion-like particles, ultralight hidden-sector particles, and their astrophysical and cosmological constraints. It also explores possible indirect hints for WISPs and discusses various experiments that search for WISPs using low-energy photons, such as photon regeneration experiments, laser polarization experiments, and experiments using strong electromagnetic fields. The paper concludes with a summary and outlook on the current situation and future prospects for WISP research.