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Abstract
We calculate the axion emission rate from reactions involving thermal pions in matter encountered in supernovae and neutron star mergers, identify unique spectral features, and explore their implications for astrophysics and particle physics. We find that it is about 2-5 times larger than nucleon-nucleon bremsstrahlung, which in past studies was considered to be the dominant process. The axion spectrum is also found be much harder. Together, the larger rates and higher axion energies imply a stronger bound on the mass of the QCD axion and better prospects for direct detection in a large underground neutrino detector from a nearby galactic supernova.
Highlights
The axion, a hypothetical particle initially introduced to explain the smallness of the observed CP-violating interactions in QCD [1,2] is a well-motivated dark matter (DM) candidate [3,4,5]
Axions produced during inflation would account for the totality of the dark matter in the Universe if their mass is in the range from a few μeV to a few tens of μeV [6,7], the exact value depending on unknown initial conditions
Axion masses ma ≳ 1–10 meV are interesting for astrophysics, since these axions can have a noticeable impact on stellar evolution, supernovae, and the cooling of white dwarfs and neutron stars [17,18,19,20,21,22,23]
Summary
Pierluca Carenza ,1,2,* Bryce Fore ,3,4,† Maurizio Giannotti ,5,‡ Alessandro Mirizzi,1,2,§ and Sanjay Reddy3,4,∥. We calculate the axion emission rate from reactions involving thermal pions in matter encountered in supernovae and neutron star mergers, identify unique spectral features, and explore their implications for astrophysics and particle physics We find that it is about 2–5 times larger than nucleon-nucleon bremsstrahlung, which in past studies was considered to be the dominant process. Subsequent improvements in the description of the axion emissivity from a SN core, over several years, demonstrated that the suppression of the neutrino luminosity due to axion emission would discernibly alter the observed neutrino events from SN 1987A to provide stringent bounds on the axion nucleon couplings [27,28,29,30,31,32,33,34] This bound excludes QCD axions with masses in the range 15 meV ≲ ma ≲ 10 keV [34]. A recent study demonstrated that the strong interactions enhance the abundance of negatively charged pions [35]
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