Old data yield new signs of extra force.

A pair of Pennsylvania physicists has found hints of an additional force of nature lurking in data from physics experiments, most of which were completed a few years ago.

If the hints withstand further scrutiny--a big "if," they and other physicists say--the researchers will have achieved something extraordinary and long sought. They will have identified a feature of the elementary-particle realm not predicted by the reigning theory of particle physics, the so-called standard model.

What's more, the presence of the force appears to best fulfill predictions of a theory in which incredibly tiny filaments or strings make up everything in C the universe (SN: 8/26/95, p. 140). A glaring deficiency of string theory to date has been its total lack of experimental verification.

In the Jan. 10 PHYSICAL REVIEW LETTERS, Jens Erler and Paul G. Langacker of the University of Pennsylvania in Philadelphia report indications of the additional force and a particle associated with it. They base their analysis on apparent anomalies in several studies.

Their findings "might be a really big step along the way to a whole new chapter of physics," comments Joseph Lykken of the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. However, a provocative deviation from the standard model appeared in 1996 and vanished under further analysis. "We've been burned before," he cautions.

The new clues, which could be confirmed in accelerator experiments as early as next year, are "not really compelling evidence," agrees William J. Marciano of Brookhaven National Laboratory in Upton, N.Y. When researchers measure scores of quantities, they expect some to differ from predictions because of random error.

An additional force would join the four already known: the electromagnetic, weak, strong, and gravitational forces. During much of the last century, physicists sought links between the four in hopes of better understanding the origin and development of the universe. Scientists have proposed that in the Big Bang, a single superforce ruled, then rapidly broke down into the forces observed today.

In the 1960s, theorists succeeded in reuniting the electromagnetic and weak forces into the so-called electroweak force. That theory predicted a new weakforce particle, called the Z boson, which physicists discovered in 1983.

In their new analysis, Langacker and Erler considered data in which three measurements are out of line with the standard model. They found that a theoretical model with an additional force best matches this data. Such a force, which is akin to the weak force but only about one-hundredth as strong, has been proposed at various times over the past 3 decades, Langacker says. A Z' particle, related to the Z boson but about 10 times as heavy, would carry the force.

The new report links disparate measurements--some new, some old--from large accelerator trials in Switzerland and California and a tabletop atomic physics experiment in Colorado. The accelerator work looks at Z boson decay. The atomic physics experiment probes an effect known as parity violation (SN: 5/6/95, p. 278).

In their report, Erler and Langacker calculate a likely mass range for Z'. Although previous attempts to create the particle in accelerators have failed, the new midrange estimate lies at an energy scale easily attainable in the next run of Fermilab's renovated Tevatron Collider, scheduled to begin March 2001, the researchers say. Otherwise, when the Large Hadron Collider in Switzerland starts operation in 5 years, the accelerator should be able to create Z', if it exists.

Although a Z' readily turns up in some versions of string theory, "in no way can you say if string theory is right or wrong" by proving the presence or absence of Z', remarks Brian R. Greene of Columbia University.

Langacker agrees but says that Z' may provide a "strong shot in the arm" for the theory.