Constraints on the new particle in $\Sigma^+\to p\mu^+\mu^-$
  The HyperCP collaboration has presented the branching ratio of $\Sigma^+\to p\mu^+\mu^-$ to be $(8.6^{+6.6}_{-5.4}\pm 5.5)\times 10^{-8}$ and suggested a new boson $P^0$ with a mass of $214.3\pm 0.5$ MeV to induce the flavor changing transition of $s\to d\mu^+\mu^-$. We demonstrate that to explain the data, the new particle cannot be a scalar but pseudoscalar based on the direct constraints from $K^+\to\pi^+\mu^+\mu^-$ and $K_L\to\mu^+\mu^-$, respectively. Moreover, we determine that the decay width of the pseudoscalar should be in the range of $10^{-7}$ MeV with the lifetime of $10^{-14}$ sec. 
C. Q. Geng and Y. K. Hsiao