Millicharged particles

In view of the lack of any firm theoretical evidence for the principle of charge quantization, we propose testing its validity experimentally by looking for particles with small electric charges (Q∼${10}^{\mathrm{-}3}$e). Toward this aim we investigate various constraints on their masses and charges and discuss the problem of the experimental search for them. As a byproduct we find that such particles may solve the orthopositronium-lifetime puzzle.