Particle Physics Bounds from the Hulse-Taylor Binary

The orbital period of the binary pulsar PSR 1913+16 has been observed to decrease at the rate of $2.40\times 10^{-12}$ 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 like 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 $g_s~\phi~\bar\psi~\psi$ we find that $g_s< 3\times 10^{-19}$. From the radiation loss of massless Goldstone bosons we establish the upper bound $\theta /f< 7.5\times 10^{-16}$ GeV$^{-1}$ on the QCD vacuum angle $\theta$ and the scale $f$ at which the baryon number symmetry can be broken spontaneously.