The Physics Beyond Colliders initiative is an exploratory study aimed at exploiting the full scientific potential of the CERN's accelerator complex and scientific infrastructures through projects complementary to the LHC and other possible future colliders. These projects will target fundamental physics questions in modern particle physics.ii 7 Physics reach of PBC projects 66 8 Physics reach of PBC projects in the sub-eV mass range 66 8.1 Axion portal with photon dominance (BC9) 66 9 Physics reach of PBC projects in the MeV-GeV mass range 73 9.1 Vector Portal 78 9.1.1 Minimal Dark Photon model (BC1) 78 9.1.2 Dark Photon decaying to invisible final states (BC2) 83 9.1.3 Milli-charged particles (BC3) 90 9.2 Scalar Portal 93 9.2.1 Dark scalar mixing with the Higgs (BC4 and BC5) 93 9.3 Neutrino Portal 97 9.3.1 Neutrino portal with electron-flavor dominance (BC6) 98 9.3.2 Neutrino portal with muon-flavor dominance (BC7) 101 9.3.3 Neutrino portal with tau-flavor dominance (BC8) 103 9.4 Axion Portal 106 9.4.1 Axion portal with photon-coupling (BC9) 106 9.4.2 Axion portal with fermion-coupling (BC10) 110 9.4.3 Axion portal with gluon-coupling (BC11) 113 10 Physics reach of PBC projects in the multi-TeV mass range 115 10.1 Measurement of EDMs as probe of NP in the multi TeV scale 115 10.2 Experiments sensitive to Flavour Violation 116 10.3 B physics anomalies and BR(K → πνν) 120 11 Conclusions and Outlook 121 A ALPS: prescription for treating the FCNC processes 123 B ALPs: production via π 0 , η, η mixing 126 Executive SummaryThe main goal of this document follows very closely the mandate of the Physics Beyond Colliders (PBC) study group, and is "an exploratory study aimed at exploiting the full scientific potential of CERN's accelerator complex and its scientific infrastructure through projects complementary to the LHC, HL-LHC and other possible future colliders. These projects would target fundamental physics questions that are similar in spirit to those addressed by high-energy colliders, but that require different types of beams and experiments 1 ". Fundamental questions in modern particle physics as the origin of the neutrino masses and oscillations, the nature of Dark Matter and the explanation of the mechanism that drives the baryogenesis are still open today and do require an answer.So far an unambiguous signal of New Physics (NP) from direct searches at the Large Hadron Collider (LHC), indirect searches in flavour physics and direct detection Dark Matter experiments is absent. Moreover, theory provides no clear guidance on the NP scale. This imposes today, more than ever, a broadening of the experimental effort in the quest for NP. We need to explore different ranges of interaction strengths and masses with respect to what is already covered by existing or planned initiatives.Low-mass and very-weakly coupled particles represent an attractive possibility, theoretically and phenomenologically well motivated, but currently poorly explored: a systematic investigation should be pursued in the next decades both at acc...

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This Conceptual Design Report describes LUXE (Laser Und XFEL Experiment), an experimental campaign that aims to combine the high-quality and high-energy electron beam of the European XFEL with a powerful laser to explore the uncharted terrain of quantum electrodynamics characterised by both high energy and high intensity. We will reach this hitherto inaccessible regime of quantum physics by analysing high-energy electron-photon and photon-photon interactions in the extreme environment provided by an intense laser focus. The physics background and its relevance are presented in the science case which in turn leads to, and justifies, the ensuing plan for all aspects of the experiment: Our choice of experimental parameters allows (i) field strengths to be probed where the coupling to charges becomes non-perturbative and (ii) a precision to be achieved that permits a detailed comparison of the measured data with calculations. In addition, the high photon flux predicted will enable a sensitive search for new physics beyond the Standard Model. The initial phase of the experiment will employ an existing 40 TW laser, whereas the second phase will utilise an upgraded laser power of 350 TW. All expectations regarding the performance of the experimental set-up as well as the expected physics results are based on detailed numerical simulations throughout.

X-ray crystallography is one of the main methods to determine atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes consisting of hundred-thousands of atoms that constitute the macromolecular machinery of life. Life is not static, and unravelling the structure and dynamics of the most important reactions in chemistry and biology is essential to uncover their mechanism. Many of these reactions, including photosynthesis which drives our biosphere, are light induced and occur on ultrafast timescales. These have been studied with high time resolution primarily by optical spectroscopy, enabled by ultrafast laser technology, but they reduce the vast complexity of the process to a few reaction coordinates. In the AXSIS project at CFEL in Hamburg, funded by the European Research Council, we develop the new method of attosecond serial X-ray crystallography and spectroscopy, to give a full description of ultrafast processes atomically resolved in real space and on the electronic energy landscape, from co-measurement of X-ray and optical spectra, and X-ray diffraction. This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology like energy conversion processes. For that purpose, we develop a compact, fully coherent, THz-driven atto-second X-ray source based on coherent inverse Compton scattering off a free-electron crystal, to outrun radiation damage effects due to the necessary high X-ray irradiance required to acquire diffraction signals. This highly synergistic project starts from a completely clean slate rather than conforming to the specifications of a large free-electron laser (FEL) user facility, to optimize the entire instrumentation towards fundamental measurements of the mechanism of light absorption and excitation energy transfer. A multidisciplinary team formed by laser-, accelerator,- X-ray scientists as well as spectroscopists and biochemists optimizes X-ray pulse parameters, in tandem with sample delivery, crystal size, and advanced X-ray detectors. Ultimately, the new capability, attosecond serial X-ray crystallography and spectroscopy, will be applied to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.