Is the θ[over ¯] Parameter of QCD Constant?

Testing the cosmological variation of fundamental constants of nature can provide valuable insights into new physics scenarios. While many such constraints have been derived for standard model coupling constants and masses, the θ[over ¯] parameter of QCD has not been as extensively examined. In this Letter, we discuss potentially promising paths to investigate the time dependence of the θ[over ¯] parameter. While laboratory searches for CP-violating signals of θ[over ¯] yield the most robust bounds on today's value of θ[over ¯], we show that CP-conserving effects provide constraints on the variation of θ[over ¯] over cosmological timescales. We find no evidence for a variation of θ[over ¯] that could have implied an "iron-deficient" Universe at higher redshifts. By converting recent atomic clock constraints on a variation of constants, we infer d(θ[over ¯]^{2})/dt≤6×10^{-15} yr^{-1}, at 1σ. Finally, we also sketch an axion model that results in a varying θ[over ¯] and could lead to excess diffuse gamma ray background, from decays of axions produced in high redshift supernova explosions.

Supermassive Black Holes, Ultralight Dark Matter, and Gravitational Waves from a First Order Phase Transition.

The formation of ultrarare supermassive black holes (SMBHs), with masses of O(10^{9} M_{⊙}), in the first billion years of the Universe remains an open question in astrophysics. At the same time, ultralight dark matter (DM) with mass in the vicinity of O(10^{-20} eV) has been motivated by small scale DM distributions. Though this type of DM is constrained by various astrophysical considerations, certain observations could be pointing to modest evidence for it. We present a model with a confining first order phase transition at ∼10 keV temperatures, facilitating production of O(10^{9} M_{⊙}) primordial SMBHs. Such a phase transition can also naturally lead to the implied mass for a motivated ultralight axion DM candidate, suggesting that SMBHs and ultralight DM may be two sides of the same cosmic coin. We consider constraints and avenues to discovery from superradiance and a modification to N_{eff}. On general grounds, we also expect primordial gravitational waves-from the assumed first order phase transition-characterized by frequencies of O(10^{-12}-10^{-9} Hz). This frequency regime is largely uncharted, but could be accessible to pulsar timing arrays if the primordial gravitational waves are at the higher end of this frequency range, as could be the case in our assumed confining phase transition.

CP-Violating Neutrino Nonstandard Interactions in Long-Baseline-Accelerator Data.

Neutrino oscillations in matter provide a unique probe of new physics. Leveraging the advent of neutrino appearance data from NOvA and T2K in recent years, we investigate the presence of CP-violating neutrino nonstandard interactions in the oscillation data. We first show how to very simply approximate the expected NSI parameters to resolve differences between two long-baseline appearance experiments analytically. Then, by combining recent NOvA and T2K data, we find a tantalizing hint of CP-violating NSI preferring a new complex phase that is close to maximal: ϕ_{eµ} or ϕ_{eτ}≈3π/2 with |ε_{eµ}| or |ε_{eτ}|∼0.2. We then compare the results from long-baseline data to constraints from IceCube and COHERENT.