Thermal Axion Production at Low Temperatures: A Smooth Treatment of the QCD Phase Transition.

We study thermal axion production around the confinement scale. At higher temperatures, we extend current calculations to account for the masses of heavy quarks, whereas we quantify production via hadron scattering at lower temperatures. Matching our results between the two opposite regimes provides us with a continuous axion production rate across the QCD phase transition. We employ such a rate to quantify the axion contribution to the effective number of neutrino species.

Long-lived particles at the energy frontier: the MATHUSLA physics case.

We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of standard model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the [Formula: see text]m scale up to the Big Bang Nucleosynthesis limit of [Formula: see text] m. Neutral LLPs with lifetimes above [Formula: see text]100 m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. We study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC, assuming backgrounds can be rejected as expected. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

Sub-GeV Dark Matter Shining at Future MeV γ-Ray Telescopes.

We propose a novel framework where light (sub-GeV) dark matter (DM) is detectable with future MeV γ-ray telescopes without conflicting with cosmic microwave background (CMB) data. The stable DM particle χ has a very low thermal relic abundance due to its large pair-annihilation cross section. The DM number density is stored in a slightly heavier, metastable partner ψ with suppressed pair-annihilation rates, that does not perturb the CMB, and whose late-time decays ψ→χ fill the Universe with χ DM particles. We provide explicit, model-independent realizations for this framework, and discuss constraints on late-time decays, and thus on parameters of this setup, from CMB, big bang nucleosynthesis, and large scale structure.

New constraints on dark matter effective theories from standard model loops.

We consider an effective field theory for a gauge singlet Dirac dark matter particle interacting with the standard model fields via effective operators suppressed by the scale Λ â‰³ 1 TeV. We perform a systematic analysis of the leading loop contributions to spin-independent Dirac dark matter-nucleon scattering using renormalization group evolution between Λ and the low-energy scale probed by direct detection experiments. We find that electroweak interactions induce operator mixings such that operators that are naively velocity suppressed and spin dependent can actually contribute to spin-independent scattering. This allows us to put novel constraints on Wilson coefficients that were so far poorly bounded by direct detection. Constraints from current searches are already significantly stronger than LHC bounds, and will improve in the near future. Interestingly, the loop contribution we find is isospin violating even if the underlying theory is isospin conserving.