Increasing the effective number of neutrinos with decaying particles

Kazuhide Ichikawa, Masahiro Kawasaki, Kazunori Nakayama, Masato Senami, Fuminobu Takahashi

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61 Citations (Scopus)


We present models of decaying particles for increasing the effective number of neutrinos Nν after big bang nucleosynthesis but before the structure formation begins. We point out that our scenario not only solves the discrepancy between the constraints on Nν from these two epochs, but also provides a possible answer to deeper inconsistency in the estimation of the matter power spectrum amplitude at small scales, represented by σ8, between the WMAP and some small scale matter power measurements such as the Lyman-α forest and weak lensing. We consider (a) saxion decay into two axions; (b) gravitino decay into axino and axion; (c) Dirac right-handed sneutrino decay into gravitino and right-handed neutrino.

Original languageEnglish
Article number008
JournalJournal of Cosmology and Astroparticle Physics
Issue number5
Publication statusPublished - 2007 May 1


  • Axions
  • Cosmological neutrinos
  • Cosmology of theories beyond the SM


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