In quark world, a strange discovery
Like a grocery shopper peering at a jar of spaghetti sauce in search of flecks of basil, physicists have taken a long look at the proton and have found an extra, if long-suspected, ingredient inside.
Physicists know that the positively charged protons in atomic nuclei are made primarily of two types of elementary particles, the so-called up and down quarks. But now an experiment called G-Zero at Jefferson Lab in Newport News, Va., has confirmed the presence of a third type, or "flavor" of quark, the strange quark.
To find that protons contain strange quarks isn't a major surprise, said Brian Quinn, a Carnegie Mellon University physicist who is part of the international G-Zero collaboration. But what was somewhat startling was the degree to which strange quarks actually affect the structure and behavior of the proton, he added.
"We're showing that they could be a significant part of what makes up the proton," agreed Gregg Franklin, another Carnegie Mellon physicist among the 108 scientists on the G-Zero team.
That is a bit surprising because the strange quark isn't always there. That is, unlike the dependable up and down quarks, the strange quark and its anti-matter counterpart, the anti-strange quark, are constantly appearing and disappearing in the proton.
This now-you-see-us-now-you-don't behavior only makes sense according to the weird rules of quantum physics, which come into play at atomic scales. Though the strange quark is about 100 times more massive than either the up or down quark, the strange quark's brief appearances in the proton made it hard to predict just how it might affect the proton's structure.
"Nobody would have been surprised if the result had been zero," Quinn said, noting the negatively charged strange quark and the positively charged anti-strange quark could have canceled each other out if they were close to each other in the proton.
But just as a small amount of basil is noticeable in spaghetti sauce, the ghostly, but massive, strange quarks contribute several percent of the proton's charge structure, he added.
The results were announced earlier this month at a Jeff Lab seminar by Doug Beck, a University of Illinois physicist and G-Zero spokesman, and a formal scientific paper is being prepared.
Though six flavors of quarks have been found — up, down, strange, charm, bottom and top — all but the first two have been seen only fleetingly in exotic, short-lived particles such as mesons.
Evidence of the strange quark in the proton was gleaned from experiments last year using the Continuous Electron Beam Accelerator Facility at the Department of Energy's Jeff Lab. The high-energy beam of electrons was directed at a target of hydrogen. The hydrogen nucleus contains a single proton.
Rather than bust the nuclei apart, the electron beam dislodged some nuclei, sending them careening away from the target. By detecting these scattered protons and applying some "mathematical trickery," the researchers were able to discern the presence of the strange quark, Quinn said.
Ten Carnegie Mellon students have participated in G-Zero since it was initiated in 1991 and have helped construct the custom electronics necessary for detecting the scattered protons. A second and final run of the experiment is scheduled to begin in December.