In 1956, theoretical physicist David Pines predicted that electrons in a solid can do something strange. While they normally have a mass and an electric charge, Pines asserted that they can combine to form a composite particle that is massless, neutral, and does not interact with light. He called this particle a “demon.” Since then, it has been speculated to play an important role in the behaviors of a wide variety of metals. Unfortunately, the same properties that make it interesting have allowed it to elude detection since its prediction.
Click here for original story, Demon hunting: Physicists confirm 67-year-old prediction of massless, neutral composite particle
Source: Phys.org