What particle that is found outside the nucleus?
particles found outside a nucleus are the electrons.
The electron is the negatively charged particle found in the orbitals surrounding the nucleus in conventional matter. The muon is a negatively charged particle that can be produced in particle accelerators and can substitute for electrons in the orbitals surrounding the nucleus in muonic matter. In this case the orbitals shrink in diameter so much that the nuclei can occasionally touch at room temperature and pressure (which is not possible with electrons in those orbitals)!