Search for the dark photon in $π^0$ decays

TL;DR

The paper performs a targeted search for dark photons produced in π0 decays by analyzing a large, clean π0_D-dominated dataset from NA48/2. It models the irreducible π0_D background with data-driven, radiatively corrected simulations and performs a mass scan for A'→e+e−, deriving 90% CL upper limits on B(π0→γA') and the kinetic mixing ε^2 that improve previous bounds in the 9–70 MeV/c^2 range. No DP signal is observed, constraining the DP parameter space and challenging its role as an explanation for the muon g−2 anomaly under quark couplings. The study also assesses the NA48/2 sensitivity for the K±→π±A' channel, providing a complementary but less competitive probe of the dark photon hypothesis.

Abstract

A sample of $1.69\times 10^7$ fully reconstructed $π^0\toγe^+e^-$ decay candidates collected by the NA48/2 experiment at CERN in 2003--2004 is analysed to search for the dark photon ($A'$) production in the $π^0\toγA'$ decay followed by the prompt $A'\to e^+e^-$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_{A'}$ and mixing parameter $\varepsilon^2$ is established. The obtained upper limits on $\varepsilon^2$ are more stringent than the previous limits in the mass range $9~{\rm MeV}/c^2<m_{A'}<70~{\rm MeV}/c^2$. The NA48/2 sensitivity to the dark photon production in the $K^\pm\toπ^\pm A'$ decay is also evaluated.

Figures (4)

• Figure 1: Invariant mass distributions of data and MC events passing the $K_{2\pi D}$ (top row) and $K_{\mu3 D}$ (bottom row) selections. The signal mass regions are indicated with vertical arrows. A dark photon signal would correspond to a spike in the $m_{ee}$ distributions (right column).
• Figure 2: a) Numbers of observed data events ($N_{\rm obs}$) and expected $\pi^0_D$ background events ($N_{\rm exp}$) passing the joint DP selection (indistinguishable in a logarithmic scale), estimated uncertainties $\delta N_{\rm obs}=\sqrt{N_{\rm exp}}$ and $\delta N_{\rm exp}$, and obtained upper limits at 90% CL on the numbers of DP candidates ($N_{\rm DP}$) for each DP mass value $m_{A'}$. The contribution to $\delta N_{\rm exp}$ from the MC statistical uncertainty is shown separately ($\delta N_{\exp}^{\rm MC}$). The remaining and dominant component is due to the statistical errors on the trigger efficiencies measured in the DP signal region. b) Estimated local significance of the DP signal for each $A'$ mass value. All presented quantities are strongly correlated for neighbouring DP masses as the mass step of the scan is about 6 times smaller than the signal window width.
• Figure 3: a) Acceptances of the joint DP selection for $K_{2\pi}$, $K_{\mu 3}$ and $K_{3\pi}$ decays followed by the prompt decay chain $\pi^0\to\gamma A'$, $A'\to e^+e^-$ depending on the assumed DP mass, evaluated with MC simulations. The $K_{3\pi}$ acceptance is scaled by a factor of 10 for visibility. b) Obtained upper limits on ${\cal B}(\pi^0\to\gamma A')$ at 90% CL for each DP mass value $m_{A'}$.
• Figure 4: Obtained upper limits at 90% CL on the mixing parameter $\varepsilon^2$ versus the DP mass $m_{A'}$, compared to other published exclusion limits from meson decay, beam dump and $e^+e^-$ collider experiments an12bab13ad13ag14me14ab11le14. Also shown is the band where the inconsistency of theoretical and experimental values of muon $(g-2)$ reduces to less than 2 standard deviations, as well as the region excluded by the electron $(g-2)$ measurement po09en12da14.