A theory of extra radiation in the Universe |
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Authors: | Kazunori Nakayama Fuminobu Takahashi Tsutomu T Yanagida |
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Institution: | 1. Theory Center, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan;2. Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa 277-8568, Japan;3. Department of Physics, University of Tokyo, Tokyo 113-0033, Japan |
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Abstract: | Recent cosmological observations, such as the measurement of the primordial 4He abundance, CMB, and large scale structure, give preference to the existence of extra radiation component, ΔNν>0. The extra radiation may be accounted for by particles which were in thermal equilibrium and decoupled before the big bang nucleosynthesis. Broadly speaking, there are two possibilities: (1) there are about 10 particles which have very weak couplings to the standard model particles and decoupled much before the QCD phase transition; (2) there is one or a few light particles with a reasonably strong coupling to the plasma and it decouples after the QCD phase transition. Focusing on the latter case, we find that a light chiral fermion is a suitable candidate, which evades astrophysical constraints. Interestingly, our scenario predicts a new gauge symmetry at TeV scale, and therefore may be confirmed at the LHC. As a concrete example, we show that such a light fermion naturally appears in the E6-inspired GUT. |
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Keywords: | Extra radiation Grand unified theory |
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