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We calculate a new contribution to the axion mass that arises from gluons propagating in a 5th dimension at high energies. By uplifting the 4D instanton solution to five dimensions, the positive frequency modes of the Kaluza-Klein states generate a power-law term in the effective action that inversely grows with the instanton size. This causes 5D small instantons to enhance the axion mass in a way that does not spoil the axion solution to the strong CP problem. Moreover this enhancement can be much larger than the usual QCD contribution from large instantons, although it requires the 5D gauge theory to be near the non-perturbative limit. Thus our result suggests that the mass range of axions (or axion-like particles), which is important for ongoing experimental searches, can depend sensitively on the UV modification of QCD.

This document summarises the proposal of the LHC Dark Matter Working Group on how to present LHC results on s-channel simplified dark matter models and to compare them to direct (indirect) detection experiments.

It is a common belief that the last missing piece of the Standard Model of particles physics was found with the discovery of the Higgs boson at the Large Hadron Collider. However, there remains a major prediction of quantum tunnelling processes mediated by instanton solutions in the Yang-Mills theory, that is still untested in the Standard Model. The direct experimental observation of instanton-induced processes, which are a consequence of the non-trivial vacuum structure of the Standard Model and of quantum tunnelling in QFT, would be a major breakthrough in modern particle physics. In this paper, we present for the first time a full calculation of QCD instanton-induced processes in proton-proton collisions accounting for quantum corrections due to both initial and final state gluon interactions, a first implementation in an MC event generator as well as a basic strategy how to observe these effects experimentally.

We use the semiclassical formalism based on singular solutions in complex time to compute scattering rates for multiparticle production at high energies. In a weakly coupled λφ 4 scalar field theory in four dimensions, we consider scattering processes where the number of particles n in the final state approaches its maximal value n → E/m 1, where m is the particle mass. Quantum corrections to the known tree-level amplitudes in this regime are characterised by the parameter λn and we show that they become large at sufficiently high multiplicities. We compute full amplitudes in the large λn limit on multiparticle mass thresholds using the thin-wall realisation of the singular solutions in the WKB approach. We show that the scalar theory with spontaneous symmetry breaking, used here as a simplified model for the Higgs sector, leads to exponentially growing multiparticle rates within our regime which is likely to realise the high-energy Higgsplosion phenomenon. We also comment on realisation of Higgsplosion in dimensions lower than four.