Number 369 (Story #1), April 29, 1998 by Phillip F. Schewe and Ben Stein A DENSE NUCLEAR ENVIRONMENT CAUSES PARTICLES TO LOSE WEIGHT, or at least act as if they have a lower mass, new experiments have suggested. In the world of room-temperature semiconductors, an electron moves through silicon more slowly than it does through gallium arsenide, even when subjected to the same forces. Physicists therefore say that an electron has a lower "effective mass" in gallium arsenide. At the April APS/AAPT meeting in Columbus, Johann Peter Wurm of the CERES collaboration at CERN announced experimental evidence for a similar effect in particles produced inside collisions between heavy nuclei. One such particle is the rho meson, an object consisting of an up quark and antidown quark bound together. In a vacuum, the rho has a mass of 780 MeV/c2, but in the fireball conditions of the collision, it acts as if it has a significantly lower mass (perhaps as low as 400 MeV/c2). In turn, the rho's lower effective mass changes its lifetime and its rate of decay into pairs of leptons such as electrons and positrons which were observed experimentally. According to Volker Koch of Lawrence Berkeley Laboratory, this effect can take place for particles inside any nuclear environment, from the most common atoms to superdense neutron stars.