Search for dark photons from neutral meson decays in p0p and $d$$+$Au collisions at $\sqrt{s_{{}_{NN}}}$=200 GeV

The standard model (SM) of particle physics is spectacularly successful, yet the measured value of the muon anomalous magnetic moment $(g-2{)}_{\mu }$ deviates from SM calculations by 3.6$\sigma$. Several theoretical models attribute this to the existence of a “dark photon,” an additional U(1) gauge boson, which is weakly coupled to ordinary photons. The PHENIX experiment at the Relativistic Heavy Ion Collider has searched for a dark photon, $U$, in $\pi^0,\eta \rightarrow \gamma e^+e^-$decaysandobtainedupperlimitsof$$\mathcal{O}$(2\times10^{-6})$on$U$-$\gamma$ mixing at 90% CL for the mass range $30<m_U<90$ MeV/$c^2$. Combined with other experimental limits, the remaining region in the $U$-$\gamma$ mixing parameter space that can explain the $(g-2{)}_{\mu }$ deviation from its SM value is nearly completely excluded at the 90% confidence level, with only a small region of $29<m_U<32$ MeV/$c^2$ remaining.