The orbital period of the binary pulsar PSR 1913+16 has been observed to decrease at the rate of 2.40×${10}^{-12\mathrm{}}$ s/s which agrees with the prediction of the quadropole formula for gravitational radiation to within one percent. The decrease in orbital period may also occur by radiation of other massless particles such as scalars and pseudoscalar Nambu-Goldstone bosons. Assuming that this energy loss is less than one percent of the gravitational radiation, we can establish bounds on couplings of these particles to nucleons. For a scalar nucleon coupling of the form $g_s\phi \overline{\psi }\psi$ we find that $g_s<3\mathrm{}\times {10}^{-19\mathrm{}}$. From the radiation loss of massless Gold-stone bosons we establish the upper bound $\theta g_p<{10}^{-16\mathrm{}}$ on the QCD vacuum angle $\theta$ and the pseudoscalar nucleon coupling constant $g_p$.

• Received 17 October 1994

DOI:https://doi.org/10.1103/PhysRevD.53.5723