Constraints on the Coupling of Weakly-Interacting Particles to Matter from Stellar Evolution

Abstract

Axions couple to nucleons in both the KSVZ and DFSZ models.¹ The matrix elements for nucleon-nucleon bremsstrahlung (nn → nna, pp → ppa, and np → npa) are calculated and the energy loss rates from these processes have been obtained in the case where the nucleons are degenerate with different neutron and proton Fermi momenta.² The neutron-proton bremsstrahlung rate thus obtained differs from the previous results^3,4: The phase space integrals are carried out explicitly for different neutron and proton Fermi momenta as compared with the approximate expression in Ref. 3. Furthermore, the rate vanishes when the proton concentration approaches zero, which is to be contrasted with the results in Ref. 4. The neutron-proton rate is found to be larger than the neutron-neutron rate by a factor 2–5 for typical values of the proton concentration in neutron star matter. Requiring the energy loss rate due to axion emission to be less than that of neutrino emission, one obtains the upper bound on the axion-nucleon coupling. This upper bound is slightly lower than the previous bound⁵ due to the enhanced axion energy loss rate, which results from the inclusion of the neutron-proton process.

Supported in part by the U.S. National Science Foundation under Grant No. PHY90–08475.