Neutrino experiments and data as a bridge to new physics

Project Information

NuBridge

Grant agreement ID: 101068791

DOI

10.3030/101068791

Project closed

EC signature date 26 July 2022

Start date 1 December 2022

End date 30 April 2026

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 288 859,20

Coordinated by

ALMA MATER STUDIORUM - UNIVERSITA DI BOLOGNA
Italy

Periodic Reporting for period 1 - NuBridge (Neutrino experiments and data as a bridge to new physics)

Reporting period: 2022-12-01 to 2025-04-30

Summary of the context and overall objectives of the project

The Standard Model of particle physics is the theory that incorporates our current knowledge about the nature of fundamental particles and their interactions. Despite being an incredibly successfully predictive framework, the Standard Model appears to be incomplete, as several observations cannot be addressed within it (massive nature of neutrinos, existence of dark matter and matter-antimatter asymmetry of the Universe). This motivates the search for New Physics beyond the Standard Model. In the past, this search has mostly focused on exploring the high-energy frontier, by looking for new particles that can be produced in the interaction of Standard Model particles at increasingly high values of energy (this is carried out at collider facilities such as the LHC). This activity provided decisive confirmation tests of the validity of the Standard Model, but currently no hints for the nature of the beyond the Standard Model physics.
The aim of NuBridge is to explore the possibility that instead the New Physics particles lie at a lower energy scale, but have very feeble couplings with the Standard Model ones, effectively forming a "Dark Sector". The test of this hypothesis requires a different kind of experiments, where a very large number of interactions is generated in fixed-target experiments, thus exploring the high-intensity frontier. NuBridge employs neutrinos as a possible bridge between the Standard Model and New Physics, following three main research axes. First, it plans to use data from the current and next generation neutrino experiments to test the dark sector hypothesis; neutrino experiments can provide powerful beams of primary particles, and large and sensitive detectors, making them ideal sets to look for feebly interacting particles. Second, it looks for possible indirect manifestations of New Physics in precision neutrino data, by performing global fits of experimental data and looking for deviations with respect to the Standard Model framework. Third, it provides public and open-source tools that can be used by third-party researchers to study the dark sector and neutrino physics, to enhance the impact of the project and keep providing additional research results beyond its end-date.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We explicitly verified that a supercooled dark scalar phase transition in the early Universe can account for the NANOGrav data, by addressing the challenges previously raised in the literature that did put under question the feasibility of this explanation, and going beyond previous studies by carefully considering the effects of a vacuum domination phase and explicitly tracking the phase transition from its onset to its completion. We showed that a conformal-like potential is required to reproduce the observed data, and that this feature imposes a strong correlation on the mass spectrum of the theory that can be targeted by dedicated experiments.
We greatly improved the algorithm for the numerical computation of the tunnelling action in first order phase transitions, which represents the most numerically demanding task in the process, by developing a code that amounts to approximately 10 milliseconds for each tunnelling computation on consumer grade computers. The code implementing the algorithm will be released free and open-source for use by third-parties.

We identified a new solution to the dark matter problem within the minimal seesaw framework (a minimal extensions of the Standard Model originally conceived to generate neutrino masses). We computed for the first time the dark matter production rate in this framework by coherently taking into account thermal effects, employing thermal quantum field theory in the real time formalism. We thus determined the parameter space of solutions and the observables that experiments can use to test this simultaneous dark matter and neutrino solution.

We published PEANUTS, a new free and open-source code for the computation of oscillation probabilities of solar neutrinos, based on a semi-analytical approach and thus greatly improving the numerical efficiency of the computation, allowing for effective implementations of large statistical studies. PEANUTS has been already interfaced with the public code GAMBIT, whose full module for neutrino oscillation analysis will soon be released for public access to the community.