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

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 ${\pi }^0,\eta \to \gamma e^{+}{e}^{-}$ decays and obtained upper limits of $\mathcal{O}(2\times {10}^{-6})$ on $U-\gamma$ mixing at 90% C.L. for the mass range $30<m_U<90$ $\mathrm{MeV}/c^2$. 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 $29<m_U<32$ $\mathrm{MeV}/c^2$ remaining.

Figure 1

The raw spectra of $e^{+}{e}^{-}$ pairs for the 2006 $p+p$, 2008 $d+\text{Au}$, and 2009 $p+p$ data sets. The contributions of various background components to the measured invariant mass spectra are shown. The 2009 $p+p$ data have a significant contribution to the conversion background coming from the material of the HBD that is not present in the 2006 and 2008 data sets.

Figure 2

The best fit to the three mass spectra with the physics-motivated function describing the $e^{+}{e}^{-}$ distributions from hadron decays.

Figure 3

The experimental sensitivity and observed limit on the number of dark photon candidates as a function of the assumed dark photon mass. The $\pm 1\sigma$ and $\pm 2\sigma$ bands of the combined statistical and systematic uncertainties around the experimental sensitivity are shown in green and yellow, respectively.

Figure 4

A compilation of the limits on the $U-\gamma$ mixing parameter, showing the PHENIX results. Also shown are the limits at 90% C.L. from WASA [29], HADES [30], KLOE [31], A1(MAMI) [32], and BaBar [33] experiments and the band indicating the range of mass and coupling parameters favored by the ${(g-2)}_{\mu }$ anomaly at 90% C.L. Also shown is the $2\sigma$ upper limit obtained from ${(g-2)}_e$ [34].

Figure 5

Limits on the $U-\gamma$ mixing parameters from PHENIX at different confidence levels, together with the 90% C.L. limits from BaBar [33] and A1(MAMI) [32], the $2\sigma$ upper limit derived from ${(g-2)}_e$ [34], and the region favored by ${(g-2)}_{\mu }$.