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

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}^{-}+\mathrm{c}.\mathrm{c}$. No signal is found, and an upper limit on the branching fraction of $ \eta \to {\pi }^{+}{\pi }^{+}{e}^{-}{e}^{-}+\mathrm{c}.\mathrm{c}.$ is set to be $ 4.6\times {10}^{-6}$ at the 90% confidence level.