The measured mass of electrons in solids is always larger than the value predicted by theory. The reason for this is that theoretical calculations do not account properly for various interactions with other electrons or lattice vibrations – that “dress” the electrons. EPFL scientists have now carried out a study on a lithium-containing copper oxide and have found that its electrons are 2.5 times lighter than was predicted by theoretical calculations. The work is published in Physical Review Letters and has made the cover.