Studies of axion-like particles (ALPs) commonly focus on a single type of interaction, for example couplings only to photons. Most ALP models however predict correlations between different couplings, which change the phenomenology in important ways. For example, an MeV-scale ALP coupled to Standard Model gauge bosons at high energies will in general interact with photons, W± and Z bosons as well as mesons and nucleons at low energies. We study the implications of such scenarios and point out that astrophysical constraints, in particular from SN1987A, may be substantially relaxed, open- ing up new regions of parameter space that may be explored with laboratory experiments such as NA62.

New light pseudoscalars, such as axion-like particles, appear in many well-motivated extensions of the Standard Model and provide an exciting target for present and future experiments. We study the experimental sensitivity for such particles by revising the CHARM exclusion contour, updating bounds from LHCb and presenting prospects for NA62 and SHiP. We first consider a simplified model of a light pseudoscalar A and then propose a model-independent approach applicable to any spin-0 boson light enough to be produced in B-meson decays. As illustration, we provide upper bounds on BR(B → K A)×BR(A → µ + µ − ) as a function of the boson lifetime and mass for models that satisfy minimal flavour violation. Our results demonstrate the important complementarity between different experiments resulting from their different geometries.

We investigate direct detection signatures of dark matter particles interacting with quarks via a light spin-0 mediator with general CP phases. Since tree-level scattering may be strongly suppressed in the non-relativistic limit, loop contributions play an important role and can lead to observable signals in near-future experiments. We study the phenomenology of different mediator masses and CP phases with an emphasis on scenarios with maximal CP violation and Higgs portal models. Intriguingly, the sum of the rates obtained at tree-and loop-level can give a characteristic recoil spectrum not obtainable from a single type of interaction. We furthermore develop a novel method for decomposing the two-loop contribution to effective interactions between dark matter and gluons into two separate one-loop diagrams, which in our case substantially simplifies the calculation of the important top-quark contribution.

The low-energy electronic recoil spectrum in XENON1T provides an intriguing hint for potential new physics. At the same time, observations of horizontal branch stars favor the existence of a small amount of extra cooling compared to the one expected from the Standard Model particle content. In this note, we argue that a hidden photon with a mass of ∼ 2.5 keV and a kinetic mixing of ∼ 10−15 allows for a good fit to both of these excesses. In this scenario, the signal detected in XENON1T is due to the absorption of hidden photon dark matter particles, whereas the anomalous cooling of horizontal branch stars arises from resonant production of hidden photons in the stellar interior.

Stochastic gravitational wave (GW) backgrounds from first-order phase transitions are an exciting target for future GW observatories and may enable us to study dark sectors with very weak couplings to the Standard Model. In this work we show that such signals may be significantly enhanced for hot dark sectors with a temperature larger than the one of the SM thermal bath. The need to transfer the entropy from the dark sector to the SM after the phase transition can however lead to a substantial dilution of the GW signal. We study this dilution in detail, including the effect of number-changing processes in the dark sector (so-called cannibalism), and show that in large regions of parameter space a net enhancement remains. We apply our findings to a specific example of a dark sector containing a dark Higgs boson and a dark photon and find excellent detection prospects for LISA and the Einstein telescope.