University of Rochester

Currents--University of Rochester newspaper

Physicist, student uncover cosmic clue

A precise measurement taken by Kevin McFarland, professor of physics, and doctoral student Ben Kilminster using the particle accelerator at Fermilab has shed new light on the limits of the standard model of physics. Reported in the February 10 issue of Physical Review D, the work reveals certain characteristics of how the heaviest particle in known physics, called a top quark, decays.

"No one has made this kind of measurement as precisely, and the findings are laying another brick in our knowledge of how the universe works," says McFarland.

The findings suggest there is no connection between the top quark and the weak nuclear force--an idea that had been attractive because the unusually high and similar energies of the top quark and the weak force's particle, the W-boson, stood out from the rest of the known particles.

"People are trying desperately to understand why the weak force is weak. At the beginning of the universe, it and the force that is responsible for light, among other things, were essentially one and the same; but now, light can cross the cosmos, but the weak force can't even cross an atom. We've come up with a lot of theories as to why this is, but these new findings mean that a lot of those theories are going to have to be crossed off," adds McFarland.

To understand the critical connection between the top quark and the weak force's W-boson, McFarland and Kilminster designed a test to show that the tremendously heavy top quark had a certain characteristic known as left-handedness--measuring the handedness of the top quark with precision had never been accomplished before.

Because the weak force only affects particles that are considered "left-handed," the force would likely act on the top quark the same way it did on all the other known quarks. Since there is no known way to make the measurement of the top quark directly, the Rochester team decided to look at the particles the top quark decays into.

One of the principal functions of the weak force is to "break down" heavier particles, like the top quark, into lighter quarks from which nearly everything in the universe is constructed. In the Fermilab accelerator, the team let a soup of top quarks decay into their constituent, lighter particles. Those particles would shoot out in certain directions. Connecting which particles came from which top quarks in the ensuing collision had always been the stumbling block for physicists, but Kilminster developed a program that essentially picked out all the particles that might have been produced by a top quark decay and statistically figured out which top quark they came from.

The results showed the top quark's decay spattered its resulting particles in a pattern that strongly suggested the weak force likely interacts with the top quark in the same basic manner that it interacts with all particles, raising serious doubts about theories that assume the top quark and the weak force were linked to each other in a unique fashion.

"Models that rely on a link between the two are becoming more and more implausible. The theories are really a last ditch effort to make do with the fundamentally flawed Standard Model of physics. If these theories keep getting disproved, we're going to have to go on to an entirely new model of the universe's workings," says McFarland.

This research was funded by the National Science Foundation and the U. S. Department of Energy.

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