Are physical laws the same if you reflect an interaction between two particles in a hypothetical mirror, or if you run the movie backwards?

Many kinships, or to use the preferred term, "symmetries," have broken down as the universe has expanded and cooled. Thus a left-right asymmetry developed during the early universe, at least for those interactions mediated by the weak nuclear force.

Another symmetry or kinship thought to have broken down over the course of time is the supposed kinship between fermions (particles with a half-integer value of spin -- examples being quarks and electrons) and bosons (particles with integer spin values, such as the force-carrying particles -- the photon, gluon, and Z boson).

This particular kinship, embodied in the theory of "supersymmetry," specifies that all of the known bosons have supersymmetric (SUSY) fermion partners (named by adding an "ino" to the end; e.g., the gluon's partner would be the "gluino") and the known fermions have boson counterparts (named by adding an "s" to the beginning (e.g., SUSY version of a quark is a squark).

Just as the Neanderthals disappeared while Homo Sapiens survived, so something in the early universe favored some particles (such as the up and down quarks and electrons) while others (such as most of those promulgated in SUSY) became extinct.

Except, perhaps, at a place like Fermilab where, amid fiery proton-antiproton collisions, the earlier conditions favoring supersymmetry can be reconstructed.

Looking for events in which three jets of energetic particles stream out of the reaction zone, physicists at the CDF detector have performed the most authoritative search yet for SUSY particles.

Finding no positive evidence, the Fermilab scientists have established a new lower limit (195 GeV) on the mass of one prominent SUSY particle, the gluino.

The data base for this painstaking analysis was actually gathered several years ago; with the new, m beam, five times as much data is expected within a year, and this will aid the search for these very rare events.

If and when a gluino were produced, it would promptly decay into a hypothetical lightest supersymmetric particle (LSP), a stable but neutrino-like entity which interacts so ineffectually that its presence would be inferred only by its absence; with a mass of at least 40 GeV, it would presumably carry off a large chunk of energy that would be missing from the overall accounting of interaction energy.

This situation is not unlike Lavoisier's early analytic studies of the chemistry of combustion, which helped to establish our modern notions of atoms and energy.

The LSP belongs not just to particle physics; in some theories, it accounts for the bulk of cold dark matter in the universe.

(Reference: Affolder et al., Physical Review Letters, 28 Jan 2002.)

(This story, with minor editing, is based on PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 574, January 23, 2002, by Phillip F. Schewe, Ben Stein, and James Riordon.)

[Contact: Maria Spiropulu]

25-Jan-2002