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Precision calculation of N_eff in the Standard Model

Subject Area Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
Term since 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 549385911
 
The effective number of neutrinos N_eff is a critical input parameter in all precision cosmological computations. Its expected value in the Standard Model (SM) of particle physics is three for three generations, plus percent-level corrections due to residual energy transfer between the quantum electrodynamic (QED) plasma and the neutrino sector during neutrino decoupling as well as deviations of the QED plasma itself from an ideal gas. Unfortunately, recent SM precision calculations of N_eff are inconclusive. Furthermore, many Beyond-the-SM scenarios predict N_eff-like effects, including light sterile neutrinos, axions, gravitational waves, and solutions to the discrepancies in observations of the Hubble parameter. The next-generation (Stage-4) CMB experiment CMB-S4 and other planned observatories will measure N_eff with even higher precision than the Planck satellite. There are thus strong theoretical and experimental motivations for further precision calculations of N_eff in the SM. Building on a collaboration established during a sabbatical at UNSW Sydney in 2022 and documented in a recent publication, we will compute quantum and finite-temperature corrections to the weak interaction rates governing neutrino energy transport and decoupling from the quantum electrodynamics (QED) plasma. Time permitting, we will also consider the formation of positronium bound states and its effects on the QED equation of state as well as instabilities from anisotropic neutrino-neutrino forward scattering and fast flavour conversion and their impact on neutrino energy transport. The influence of these calculations on N_eff will be evaluated with different methods, giving a handle on the remaining uncertainty, and ultimately by implementing them in the public tool FortEPiaNO. The expected outcome is a new benchmark prediction for N_eff in the SM that is accurate to four significant digits.
DFG Programme Research Grants
International Connection Australia, Belgium
 
 

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