Search for the lepton number violating decay $\eta\to \pi^+\pi^+e^-e^- + c.c.$ via $J/\psi\to\phi\eta$

Abstract

Based on a sample of $ (10.087\pm 0.044)\times 10^{9} J/\psi$ events collected by the BESIII detector at the BEPCII collider, we perform the first search for the lepton number violating decay $\eta \to \pi^+\pi^+ e^-e^- + \text{c.c.}$ No signal is found, and an upper limit on the branching fraction of $\eta \to \pi^+\pi^+ e^-e^- + c.c.$ is set to be $4.6 \times 10^{-6}$ at the 90\% confidence level.