Search for a dark photon in the $\pi^0 \to e^+e^-\gamma$ decay

TL;DR

This study searches for a hypothetical dark photon in pi0 decays using a large dataset, setting new limits on its properties and constraining theories related to the muon magnetic moment anomaly.

Contribution

First search for dark photon in pi0 decay with the largest dataset, establishing new upper limits on mixing strength in the 20-100 MeV mass range.

Findings

Set an upper limit on epsilon^2 of 5e-6 at 90% CL for 20-100 MeV mass range.

Significantly constrains the parameter space for dark photon models related to muon g-2.

Reduces viable models explaining the muon anomalous magnetic moment.

Abstract

The presently world largest data sample of pi0 --> gamma e+e- decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 --> gamma U --> gamma e+e- decay from the pp-->pp\pi^0 reaction was carried out. An upper limit on the square of the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained for the mass range 20 MeV <M_U< 100 MeV. This result together with other recent experimental limits significantly reduces the M_U vs. \epsilon^2 parameter space preferred by the measured value of the muon anomalous magnetic moment.