Scientists Try to Put Right Spin on Quarks To Understand Matter
Talk about accounting problems. In a quest that has its roots 2,400 years ago in Democritus' search for the smallest bit of matter, physicists thought they were doing pretty well when, in the 1960s, they discovered that the protons in atomic nuclei are each made of three even-smaller subatomic particles, which were given the whimsical name quarks.
But it quickly became clear that the numbers "don't add up," says physicist Douglas Beck of the University of Illinois, Urbana-Champaign. The total mass of the three quarks, for instance, is a mere 1.5% of the proton's. Try as they might to balance the books, no amount of creative accounting has turned up the sources of the missing mass, casting doubt on science's understanding of how the basic building blocks of the physical world are assembled into matter.
That is frustrating in its own right. But it also suggests that physicists may be missing out on potential new technologies. Other advances in understanding the esoterica of subatomic particles led to transistors, CT scanning, MRI machines and other marvels. Cracking the proton's remaining enigmas might bring undreamed-of wonders.
Protons are made of two "up" quarks and one "down" quark (the identification doesn't mean much), all of which zip around the proton at the speed of light. "People expected most of a proton's properties to come from the three quarks," says theorist Werner Vogelsang of Brookhaven National Lab on Long Island.
But no. The three not only fail to bring enough mass to the party, but they also fall short in spin. Protons spin like Earth on its axis. But adding up the spins of the three resident quarks, you get no more than 20% to 30% of the proton's spin, physicists discovered at CERN, the European physics lab in Switzerland, in 1989. It was a "spin crisis."
Protons also have a property called a magnetic moment, which is like the magnetism you'd have if you carried around a little bar magnet, and is the basis for magnetic resonance imaging (MRI). But the total of the three quarks' magnetic moments is only one-third the proton's.
"Everyone believed they knew everything there was to know about the theory [governing subatomic particles, such as quarks]," says physicist Abhay Deshpande of Stony Brook University, New York, and Brookhaven National Lab. When experiments showed that things didn't add up, "it created excitement for experimentalists and shock for theorists."
Luckily, when it comes to subatomic particles, saying one thing is "made of" smaller things is not like saying a Reuben sandwich is made of corned beef, sauerkraut and Swiss cheese. Instead, if a Reuben were a proton, slices of salami and dollops of coleslaw would pop onto and off the sandwich in the blink of an eye. (Bite it while you can.)
According to standard theory, within the proton roils a sea of "virtual" quarks. Virtual means they pop into and out of existence as the spirit moves them. (Or to be more scientific, they pop into and out of existence in accordance with the Heisenberg uncertainty principle, which lets things move from virtual to real as long as they don't stay real too long.) Also within a proton are particles called gluons, which keep the quarks close to each other.
Virtual quarks were supposed to balance the books, through cameo appearances that make up the shortfall in the proton's mass, spin and magnetism. But experiments keep showing otherwise. Last year, at the Jefferson Lab in Newport News, Va., Prof. Beck and 107 colleagues (not atypical for an accelerator experiment) detected strange quarks flitting in and out of existence within the proton -- the first time that had been done convincingly.
Strange quarks, the next-heaviest kind after up and down, were expected to account for 10% of the proton's magnetic moment. But they contribute no more than 5%, researchers found.
In April, physicist Paul Souder of Syracuse University and colleagues reported that their Jefferson Lab experiment found that strange quarks might contribute nothing at all to the proton's magnetic moment. "There is some sort of conspiracy [among the proton's constituents] to make the magnetic moment and mass as big as they are, and we have no clear idea what that conspiracy is," says Prof. Beck.
Things are not going much better in experiments to account for the proton's spin. At Brookhaven, five years of experiments at the Relativistic Heavy Ion Collider have tried to determine how much gluons contribute. So far, the answer seems to be no more than 40%, and perhaps zero. "If the answer turns out to be at the low end of this range," says Prof. Deshpande, "we're going to have spin crisis No. 2."
Fundamental theory is no help. "There are no first principles to explain how much gluons or virtual quarks should contribute to the proton's properties," says Dr. Vogelsang. "Ultimately, there has to be a final theory that does."
Until then, however, Prof. Souder wonders: "Can we say we understand the proton if we can't answer these basic questions?" Or more fundamentally, that we understand the physical world -- and have milked it for all its wonders?