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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