Ever since physicists at Fermi National Accelerator Laboratory announced in April that they had detected "strong evidence" for the final particle in the subatomic puzzle, physics watchers have been waiting for the other shoe to drop. This week, the sound of shoes dropping will be heard worldwide with the expected announcement that 900 collaborators in two simultaneous experiments have found the long-sought "top quark."
Scientists at Fermilab, outside of Chicago, are expected to declare Thursday that the top quark--the only member of the quark family as yet unseen--has been unequivocally spotted.
Because everything, quite literally, is made of quarks, confirmation of the top quark would validate the understanding of all matter. For example, it might point the way toward a solution to perhaps the most basic mystery in physics: the origin and nature of mass.
The new confirmation "closes the last loophole," said William C. Carithers Jr., a physicist at Lawrence Berkeley Laboratory. "If the Standard Model (the current picture of forces and particles) is correct, there must be a top quark. This means the missing link has indeed been found."
Quarks lurk at the very heart of atoms. Strip an atom of its electron cloud, and what's left is a tight wad of nuclear particles called protons and neutrons. Both were thought to be the most basic, indivisible particles until the 1970s, when quarks were discovered.
"It's like peeling an onion," Carithers said. "There are a lot of tears along the way."
The tears come partly from the struggle to get 900 physicists to agree on anything, and partly from the difficulty of "seeing" a quark--which is many orders of magnitude more difficult than finding a needle in the haystack. All too frequently, "discoveries" of new particles evaporate under the glare of more careful and complete analysis. The top has been found, and lost, before.
But this time, "there's no question anymore," said Fermilab Nobel laureate Leon Lederman. "The difficulty is getting hundreds of people to agree where all the commas go."
The new finding comes after months of laborious search for this final piece to the subatomic puzzle. The scientists describe sightings of top quarks that burst briefly into existence in the intense energy of head-on collisions between protons and antiprotons traveling at nearly light speed in Fermilab's Tevatron accelerator.
Because the speed of the colliding particles converts directly into matter (according to Einstein's famous recipe E equals MC squared), the quarks are far heavier than the protons that produced them. The enormous energy required to produce such heavy particles has made the top the hardest of all quarks to create.
Moreover, the more mass/energy a particle has, the more quickly it disintegrates into more stable generations of particles. The top can transform into "daughter particles" in less than a billionth of a second. They, in turn, beget stable granddaughter particles such as electrons and muons that leave tracks in detectors.
But these tracks are buried in an avalanche of subatomic spaghetti, the tracks of other particles generated in collisions that rival--on a miniature scale--the explosive power of the Big Bang.
Other, more ordinary events can also produce the same daughter particles, mimicking the top quark's signature. "That happens maybe a thousand times more often than a top quark," Carithers said.
Knowing how to spot the real signal in the noise is a matter of playing the odds. "The processes we're looking for are inherently statistical," he said. April's careful announcement of "evidence for" the top came with a disclaimer of a 1-in-400 chance that the experiment was wrong. Now, armed with four times more data, the physicists are confident that they have their quarry in the bag.
Quarks come in three different generations, each heavier than the next. Consummately gregarious, they are never seen alone. The harder one tries to pull them apart, the stronger the force between them, so they rebound together like balls attached by a spring.
Ordinary matter--the protons and neutrons that make up everything from people to newsprint to stars--is composed of "up" and "down" quarks, which were spotted by experiments at Stanford Linear Accelerator Center in the early 1970s. A heavier "strange" quark--so-called because it explained the odd disintegration patterns of a particle called the K meson--was found at the same time.
After a fiercely competitive search for the strange quark's middleweight partner--the "charmed" quark--both Brookhaven National Laboratory and the Stanford center claimed credit for the discovery of a particle made up of a tightly bound pair of charmed and anti-charmed quarks.
The race was then on to find the third and final quark pair, initially called "truth" and "beauty," although now more prosaically known as "bottom" and "top." (Although physicists have no problem living with a particle named after a laundry detergent--the axion--they balked at the quasi-mystical appellations.)
Lederman found evidence for the bound pair of bottom quarks--called the upsilon--at Fermilab in the 1980s, but did not have enough data to substantiate his claim. That "discovery" quickly went down in physics history as the "oops, Leon." Some years later, the discovery of the upsilon was confirmed.
The top quark completes the picture of elementary particles that are the building blocks of all matter. The most pressing question for physics now is why particles have mass at all, where mass comes from, and why some things weigh more than others. Because the top weighs in at a theoretical limit to the heaviness of quarks, it might provide important clues.
Not everyone thinks that quarks are the most fundamental particles. Some physicists, such as Haim Harari of the Weizmann Institute of Science in Israel, speculate that quarks too might have an internal structure containing even smaller components. The existence of three quark varieties with different masses suggests a simpler explanation might be hidden underneath. "Everything repeats three times for some unknown reason," Harari said. "Every time that's happened in history, it's pointed to a more fundamental substructure."
Even if the top quark turns out to be the last in its line, as most physicists think it will, it holds out the promise of exciting new physics simply because its properties are so peculiar. Like all quarks, it is a dimensionless point in space-time, an indivisible quantum speck of matter/energy. Yet it weighs more than a silver atom.
"That makes it a very unusual object," said Nicholas Hadley, a physicist at the University of Maryland. "It's so strange it means you would expect unusual things to happen." Strange happenings in physics have a way of opening the doors to new discoveries.
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The Top Quark
Finding the last member of the quark family is like finding an exotic long lost ancestor whose qualities explain quirky family traits. The top quark should shed light on the behavior of more ordinary particles that make up people, newsprint and stars. Since it is the last member of the quark family to be found, it also gives physicists confidence that their picture of matter is correct.
WHAT IS IT?
A quark is the most elementary parcel of matter known; all other matter is made of quarks.
A contradiction in terms, the top quark is a particle with no dimension that is as heavy as a silver atom.
HUNTING THE TOP QUARK
Top quarks are created in pairs from the energy of protons colliding at nearly the speed of light, according to the recipe E = mc squared. They disintegrate into more ordinary particles in less than a millisecond.
THE QUARK FAMILY
* Up and Down: The constituents of protons and neutrons that make up stable atoms.
* Strange and Charmed: Heavier quarks that make up various unstable particles.
* Bottom and Top: The heaviest members of the quark family, found only at the origins of the universe and perhaps in violent cosmic events, such as the collision of neutron stars.