The main research aim of NuBridge, which is the study and exploration of low scale dark sectors, received a strong motivation at the very start of the project in June 2023, when the the NANOGrav collaboration published the analysis of their 15 years data det, providing for the first time evidence for a stochastic gravitational-wave background in the Universe. Given that the observed signal is in tension with common astrophysical predictions, the project focused on exploring possible new physics explanations for its origin. The analysis identified a minimal dark sector model that can reproduce the observed data from a primordial first order phase transition in the early Universe, carefully taking into account many subtleties that are often overlooked in the literature, and highlighted strong phenomenological relations that can be used by experiments to test the model.
In order to effectively study the proposed models, we developed a scientific software aimed at computing the bubble nucleation rates by using the so called tunnelling potential formalism, that allows for a fast numerical computation of the tunnelling action, as an alternative to existing tools that are instead based on the more computationally demanding bounce equation solving. The software has been extensively tested privately, and will soon be released for public use as free and open-source code.
On the connections between neutrinos and physics beyond the Standard Model, we identified a new solution to the dark matter problem within the minimal seesaw mechanism, that is the minimal extension of the Standard Model that can account for massive neutrinos. In a series of works, we first proposed that dark matter can be produced in the form of sterile neutrinos in the decay of heavier neutral leptons, then carefully computed the dark matter production rates taking into account the significant thermal effects, and finally verified that simultaneous solutions exist for reproducing both neutrino masses and dark matter properties, as well as identified the observables that experiments can use to test the proposed model.
We are also working on a global fit of neutrino data to extract the values of oscillation parameters from the combination of all available experiments worldwide. While other analysis of this kind exist, they all use private algorithms. Within NuBridge we are extending the public code GAMBIT to include neutrino oscillations analysis, providing the first study of such that employs a fully open-source software, making the analysis totally transparent and reproducible. As part of this ongoing work, we developed and publicly released the new public software PEANUTS, for the fast computation of oscillation probabilities of solar neutrinos, based on a semi-analytical algorithm that greatly improves performance with respect to simple numerical solutions. PEANUTS has been used to reproduce the results of the SNO experiment and is currently implemented as backend in the GAMBIT pipeline.