Number 282 (Story #1), August 2, 1996 by Phillip F. Schewe and Ben Stein LASER LIGHT CAN COOL PARTICLE BEAMS IN 3 DIMENSIONS. In accelerators, it is usually desirable for a beam to be carefully controlled. That is, one generally wants all the particles to travel down the beam pipe with the same longitudinal velocity and very little motion perpendicular to the beam axis. "Cooling" the beam---getting rid of unwanted motion---can be achieved by injecting a co-moving stream of electrons, which interact with and absorb excess energy in the beam particles. Proton-antiproton collision experiments at CERN and Fermilab depend on cooling techniques which allows swarms of unruly antiprotons, created by smashing protons into a small target, to be gathered into tidy bunches. Laser light can also be used to soak up unwanted motion. In atom traps lasers have cooled populations of atoms to within a fraction of a kelvin. At accelerators so far, lasers have only succeeded in cooling longitudinal motion. Now a group of scientists at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany has used lasers to indirectly cool transverse motions as well through laser-induced intrabeam interactions among 7-MeV beryllium ions circulating around the Heidelberg Test Storage Ring. The effectiveness of the process can be characterized by specifying an equivalent temperature for the beam particles. Thus the starting longitudinal and transverse "temperatures" for the ions are 1500 and 15,000 K, respectively. Electron cooling reduces these values to 300 and 3500 K. The result with lasers is even better: 10 and 2500 K. Recently, still lower temperatures have been obtained. The laser cooling process will only be pertinent for certain species, but this may still allow for studying novel phenomena. For example, team leader Rudolf Grimm (grimm@mickey.mpi-hd.mpg.de) says that it may soon be possible to cool a beam to such an extent that it "liquefies." (H.-J. Miesner et al., Physical Review Letters, 22 July 1996.)