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   from the issue of June 10, 2004

Top quark found to be massive


One step closer in the search for the holy grail: That’s how Greg Snow and Dan Claes describe their field, high-energy physics, with the publication this week of the surprisingly large mass of the top quark.

Snow and Claes, both associate professors of physics and astronomy at UNL, have been part of the long-running DZero experiment at Fermi National Accelerator Laboratory outside of Chicago that proved the existence of the top quark in 1995 and now has established its mass at about 178 billion electron volts, give or take 4.3 billion electron volts. Scientists use energy measurements to express the mass of subatomic particles because energy and mass are interchangeable in Albert Einstein’s famous equation. In this case, mass equals energy divided by the square of the speed of light, m = e/c2.

The result will be published in the June 10 issue of Nature, the international weekly journal of science, and it’s quite a surprise to physicists. Quarks come in six varieties and are the fundamental building blocks of matter - they’re what make up the more familiar atomic particles such as protons and neutrons. The top quark has long been known to be the largest of the quarks, so large that it is unstable, which is why it was the last of the six to be observed. Still, it seemed logical that it would be smaller than a proton or a neutron. Not so, according to the Fermilab scientists.

“A quark is supposed to be a fundamental particle,” Claes said. “It’s supposed to be essentially of no dimension, a point-like object, smaller than anything we can imagine - smaller than a proton, smaller than an electron. And yet the top quark is about 180 times heavier than a proton, it’s enormously much more massive, which is a surprise. It’s about as heavy as the nucleus of a gold atom.”

Snow said finding the mass of the top quark is important in itself, but the discovery’s real significance is in narrowing the search for a particle called the Higgs boson. Finding the Higgs is necessary if physicists are to validate the Standard Model of particle physics, which describes the fundamental structure of matter.

“The mass of the top quark is related to the next sort of holy grail in our field, which is looking for and hopefully discovering the Higgs boson,” Snow said. “How heavy the top quark is influences how heavy this Higgs boson needs to be in order for all the pieces of this Standard Model to hang together. The Higgs boson plays a very special and odd role in the Standard Model in that it’s the entity which we think is responsible for the fact that all the different particles we observe have mass at all.

“The top quark is right around 180 billion electron volts, and this Higgs boson should be somewhere in the vicinity of 117. We’ve narrowed down the possible window in which the Higgs mass should exist and we know better how to look for it in the collisions that we see at Fermilab.”

Snow said Fermilab experiments may discover the Higgs boson in the next few years, but if they don’t, he said he’s certain the Higgs will turn up sometime after 2007 in the next generation of high-energy experiments at the Large Hadron Collider, under construction near Geneva, Switzerland. He and Claes are also involved in the LHC.

However, if the Higgs isn’t found after all, Snow and Claes said, it may be back to the theoretical drawing board for high-energy physics, a development that could have rich rewards of its own.

“The Standard Model is an elaborate mathematical construct,” Claes said. “Up until this point, it has been enormously self-consistent and very powerful in its predictive ability, but its validity hinges on the existence of this Higgs particle. If it’s not found, then maybe high-energy physics is all the more interesting because that would be a bigger surprise than finding it.”

The DZero experiment that discovered and measured the top quark is run by a team of scientists from nearly 40 U.S. universities and 40 foreign institutions on Fermilab’s Tevatron.



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