Rutherford inferred the existence of the atomic nucleus by firing alpha particles at gold atoms. Most of the alpha particles blasted straight through as expected, but a few rebounded at sharp angles indicating that they had struck something at least as substantial as the alpha particle itself and that the atom was not a uniform cloud of stuff. No similar effect has ever been observed with electrons (and innumerable experiments over the years have involved highly energetic electron collisions); there is no reason, either theoretical or experimental, to believe that the electron has any internal structure or can be further decomposed.
An experimental measurement of the electric dipole moment of the electron with sensitivity an order of magnitude better than previous studies increases the accessible mass range of fundamental particles by a factor of three.
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