Number 372, May 20, 1998 by Phillip F. Schewe and Ben Stein
PROTONS AND ANTIPROTONS HAVE THE SAME MASS to within one part in 10 billion. Harvard physicists Gerald Gabrielse (617-495-4381) and Anton Khabbaz and their Bonn collaborators are able to make this comparison by loading a single antiproton and a single proton (saddled with two electrons, in order to make the proton into a negatively charged object) and lets them orbit (simultaneously) around an ion trap under the influence of a strong magnetic field. One has no reason to believe the proton and antiproton masses would be different, but this stringent new measurement constitutes the best test yet (by a factor of 10; see Update 218) of the CPT theorem (C stands for charge conjugation, P for parity inversion, and T for time reversal), which says that physics should not discriminate between particles, on the one hand, and antiparticles moving backwards in time on the other. These new results will be reported next week (paper I6.05) at the APS Division of Atomic, Molecular, and Optical Physics meeting in Santa Fe, NM.
RECORD LOW TEMPERATURES FOR ELEMENTARY PARTICLES. In a separate ion trap experiment (paper DP.72 at the APS Santa Fe meeting), Gerald Gabrielse and his Harvard colleagues chilled electrons down to only 70 mK, making this the first time elementary particles had ever been stored at temperatures below 4 K; previously only atoms, which are much heavier composite structures, had been cooled so low. Moreover, one can consider such an electron plus the ion trap itself as forming an "atom," on which interesting "quantum nondemolition" measurements (the quantum state of the electron probed without affecting its energy) of the electron's cyclotron motion around the trap can be carried out in the absence of blackbody photons.
HOW THREE HYDROGEN IONS SHARE THEIR ENERGY and how they position themselves with respect to each other has been experimentally measured for the first time, shedding light on the infamous "three-body problem" in the realm of electrically charged particles. Although physics can make exact predictions on the forces between two objects (such as the Earth and the Sun), it can only approximate the considerably more complicated case of three interacting objects (like the Earth-Sun-Moon) or, similarly, three electrically charged particles experiencing the Coulomb force, the "electrostatic" force that particles exert on each other because of their electrical charge. Previous experiments have investigated the Coulomb interactions among two electrons and a positive ion; however, such situations can be easily approximated as a two-body problem because the much heavier ion remains relatively stationary. At next week's APS meeting (paper K5.03), Lisa Wiese of the University of Nebraska (402-472-2786) will describe a technically difficult study of the Coulomb interactions between three charged particles with roughly equal mass. Smashing the molecular ion H3+ against a helium target produces three ions: H+, H-, and H+. Measuring the energies of all three particles and their angles as they emerged from the target, the physicists deduced that the H- tended to reside in between the two H+ ions, from near the "Coulomb saddle point" (where the forces from the other hydrogen ions balance out) to the near vicinity of an H+ ion. Interestingly--and in contrast to all theoretical assumptions--the H- was never found at the saddle point itself. (See the figure at Physics News Graphics; see also L.M. Wiese, Phys. Rev. Lett., 22 Dec. 1997).