Neff as a new physics probe in the precision era of cosmology

We perform a global fit to the electroweak vertices and four-fermion operators of the standard model effective field theory in this work using <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>N</a:mi><a:mi>eff</a:mi></a:msub></a:math> from cosmological probes, as well as datasets from colliders and low-energy experiments. We find <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>N</c:mi><c:mi>eff</c:mi></c:msub></c:math>, both its current measurement and future projections, can only marginally improve the fit in both the flavor universal and the most general flavor scenarios. The resulting <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mn>1</e:mn><e:mi>σ</e:mi></e:math> bound on <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msub><g:mi>N</g:mi><g:mi>eff</g:mi></g:msub></g:math> is significantly improved from the global fit and becomes comparable to its current theoretical uncertainty, such that the latter will become important for this study at next generation experiments like future lepton colliders. <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi>N</i:mi><i:mi>eff</i:mi></i:msub></i:math> from the global fit is also adopted to predict the primordial helium abundance <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msub><k:mi>Y</k:mi><k:mi>P</k:mi></k:msub></k:math>, which significantly reduces the parameter space on the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msub><m:mi>Y</m:mi><m:mi>P</m:mi></m:msub><m:mo>−</m:mo><m:msub><m:mi>N</m:mi><m:mi>eff</m:mi></m:msub></m:math> plane. Through error propagation, we also conclude that reducing the experimental uncertainty of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msub><o:mi>Y</o:mi><o:mi>P</o:mi></o:msub></o:math> from metal-poor galaxies down below 0.2% could play an important role in deepening our understanding of the free neutron lifetime anomaly. Published by the American Physical Society 2024