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

Based on a sample of $(10.087\pm 0.044)\times {10}^{9}J\mathrm{/}\psi$ 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 $\eta \to {\pi }^{+}{\pi }^{+}{e}^{-}{e}^{-}+c\mathrm{.}c\mathrm{.}$ is set to be $4.6\times {10}^{-6}$ at the 90% confidence level.