New BBN limits on Physics Beyond the Standard Model from He4
Richard H. Cyburt, Brian D. Fields, Keith A. Olive, Evan Skillman
TL;DR
The paper re-evaluates Big Bang Nucleosynthesis limits in light of a revised, larger uncertainty for the Helium-4 abundance from extragalactic H II regions. By fixing the baryon density with the WMAP measurement, it compares BBN predictions to the updated He-4 data to derive bounds on physics beyond the Standard Model, including the effective number of relativistic species, potential variations of fundamental constants, and decaying particles. It adopts updated nuclear reaction-rate inputs (NACRE) with a sample-variance treatment and uses deuterium observations as a cross-check, finding that He-4 is broadly consistent with BBN at the WMAP eta and that the enlarged Yp uncertainty relaxes some prior constraints. The work outlines a robust framework for translating cosmological and nuclear-physics inputs into quantitative limits on new physics.
Abstract
A recent analysis of the He4 abundance determined from observations of extragalactic HII regions indicates a significantly greater uncertainty for the He4 mass fraction. The derived value is now in line with predictions from big bang nucleosynthesis when the baryon density determined by WMAP is assumed. Based on this new analysis of He4, we derive constraints on a host of particle properties which include: limits on the number of relativistic species at the time of BBN (commonly taken to be the limit on neutrino flavors), limits on the variations of fundamental couplings such as alpha_{em} and G_N, and limits on decaying particles.