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Title: Dark Radiation Constraints on Heavy QCD Axions

Journal Article · · Journal of High Energy Physics (Online)
ORCiD logo [1];  [1];  [2]
  1. University of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
  2. European Organization for Nuclear Research (CERN), Geneva (Switzerland)

The naturalness problem of PQ symmetry motivates study of the heavy QCD axion, with masses ma > 1 MeV generated at scales above the QCD scale, and low values of the PQ symmetry breaking scale, ƒa. We compute the abundance of such axions in a model-independent way, assuming only that they freeze-out after reheating from inflation, and are not subsequently diluted by new physics. If these axions decay between neutrino decoupling and the last scatter era of the Cosmic Microwave Background (CMB), they dilute the neutrinos and their abundance is constrained by CMB measurements of the energy density in dark radiation, Neff. We accurately compute this bound using a numerical code to evolve the axion momentum distribution, including many key processes and effects previously ignored. We assume that the only relevant axion decays are to final states involving Standard Model particles. We determine regions of (ma, ƒa) that will give a signal in Neff at CMB Stage 4 experiments. We similarly compute the Neff bound and CMB Stage 4 signal for heavy axions that can decay to light mirror photons. Finally, we compute the bounds on heavy axions with mass below 1 MeV that decay after the era of CMB last scatter, from their contribution to cold or hot dark matter or Neff at this era.

Research Organization:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), High Energy Physics (HEP); USDOE Office of Science (SC), Nuclear Physics (NP)
Grant/Contract Number:
AC02-05CH11231
OSTI ID:
2407309
Journal Information:
Journal of High Energy Physics (Online), Journal Name: Journal of High Energy Physics (Online) Journal Issue: 4 Vol. 2024; ISSN 1029-8479
Publisher:
Springer NatureCopyright Statement
Country of Publication:
United States
Language:
English

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