2024/04/15 by Dilip Kumar Ghosh, Ghosh, Dilip Kumar, Purusottam Ghosh +5
Physics and Astronomy · #Cosmology and Gravitation Theories #FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph) #Particle physics theoretical and experimental studies #Quantum Chromodynamics and Particle Interactions
paper · pdf · doi:10.48550/arxiv.2404.10077
openalex publication_date 2024/04/15 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
The relativistic degrees of freedom (N\rm eff) is one of the crucial cosmological parameters. The precise measurement of N\rm eff at the time of cosmic microwave background formation, by Planck 2018 can be used to understand the new fundamental interactions, in particular involving light mediators. Presence of any new particle with sufficient energy density and sizeable interactions with Standard Model particles at the temperature around ∼ MeV can significantly alter the neutrino decoupling and hence N\rm eff. Thus the bound on N\rm eff can place stringent constraints on various beyond Standard Model paradigms involving light particles. U(1)X models are among such scenarios and are widely studied in several aspects. In this work, we consider several popular U(1)X models with light Z' boson like U(1)B-L, U(1)B - 3Li, U(1)Bi - 3 Lj, U(1)Li - Lj; i,j =1,2,3 being the flavour indices and study their impact on N\rm eff. We also examine the constraints from ground based experiments like Xenon1T, Borexino, trident, etc. Our analysis shows that for light mass MZ' \lesssim O (\rmMeV) the N\rm eff provides the most stringent constraints on the Z' mass and coupling, far exceeding the existing constraints from other experiments.