- Number 376 |
- November 19, 2012
Quarks pair up in protons (and neutrons)
Researchers have found intriguing new
evidence on how the different kinds of
quarks behave inside protons and neutrons.
In the proton, one of the up quarks and the
only down quark appear to combine into a
diquark, while the remaining up quark hangs
out on its own. Measurements with higher
precision are planned to test the idea.
It's often been said that two's company, but three's a crowd. Now, scientists have found that old saw may be true for the smallest bits of matter: quarks.
Nearly everything in our visible universe is built of quarks. Quarks are the smallest, indivisible particles of matter that bind together in threes to build protons and neutrons. Protons and neutrons form the nuclei that reside at the center of every atom of every molecule of every thing around us.
Now, researchers have found intriguing new evidence on how the different kinds of quarks behave inside protons and neutrons. The data and insights, which were published in the journal Physical Review Letters, have recently received further support and scrutiny from theory.
Gordon Cates, a physics professor at the University of Virginia, is a coauthor on the paper. He says the result comes out of data from different experiments at DOE's Jefferson Lab that measured the electric form factors of the proton and of the neutron. The electric form factor reveals where the charge - which is contributed by the individual quarks - resides in the proton and neutron.
"People like to call the electric form factor the closest thing there is to a snapshot. Because if you take what's called a Fourier Transform, what you get is literally the charge distribution. And after all, the charge distribution is where the stuff is, so it really is like a picture," he explains.
Another piece of information that's needed to connect the experiments is recognition of a symmetry between protons and neutrons. Both particles are built of different combinations of up- and down-flavored quarks. The proton has two up quarks and one down quark, while the neutron sports the symmetrically opposite two down quarks and one up quark.
"The only difference between a proton and neutron, is you take every up quark and turn it into a down quark, and you take every down quark and you turn it into an up quark. Otherwise, in many respects, they're the same object," Cates says.
Combining these two bits of information - the location of charge within protons and neutrons along with the flavor symmetry between these particles - then allowed researchers to tease out where the two different flavors of quark reside inside protons and neutrons. Cates offers a metaphor for how the research allows physicists to view the insides of a proton.
"Imagine that you had a picture of the proton, and said, okay, now I'm going to turn off the down quarks and see what it looks like. And then you said, okay, now I'm going to turn off the up quarks and see what it looks like. That's sort of what it's doing. You can turn on your up quark filter or your down quark filter and see what's there," he explains.
Still, the experimentalists needed insight from theorists to resolve the picture into something that made sense. What they actually had were two curved lines, the fit of the experimental data, on a graph, which, for the rest of us, isn't a very compelling snapshot.
Theorists from Argonne National Lab and the University of Washington tested different possibilities in the theories of proton structure to figure out how the quarks would have to be situated to produce the graph. What they found was unexpected.
The scientists concluded that two quarks buddy up into what physicists call a diquark, leaving the last quark hanging out on its own nearby as a kind of frenetic outcast.
"In the proton, for example, you might have one of the up quarks and the only down quark combined into a diquark, with the remaining up quark off on its own," Cates says.
This is surprising, because the diquark had been predicted to exist inside protons and neutrons decades ago. However, no conclusive evidence has confirmed their existence.
"The original diquark that was conceived by people was really a point-like object, of two point-like quarks sitting right next to each other, almost indistinguishable. And that's probably not what's going on. The diquarks that are being suggested here are a little on the fuzzy side, not quite the point-like things that were imagined years ago," Cates explains. "There are at least two serious calculations that very clearly suggest the diquark is real. So we see these behaviors, and they're unexpected. And they do seem to have some very interesting implications."
What some physicists consider to be the most important implication is that if it holds up, this interpretation would also tidy up another mystery. In one sense, subatomic particles are similar to guitar strings. If you 'pluck' them by giving them extra energy, they resonate with that energy in a predictable way, by becoming so-called excited states.
But when scientists attempted to pluck protons and neutrons, they were unable to produce many of the excited states that had been predicted. They recognized that by wrapping up two of the quarks in a diquark, they would limit how those two quarks would contribute to the formation of excited states. Instead of three quarks contributing to the formation of these states, you would have one quark and a quark pair contributing, reducing the number of possible states. It was a natural way to explain the missing excited states. Unfortunately, however, other than the missing states, there did not seem to be any other smoking guns.
Perhaps until now. "If the idea of diquarks is a reality, you quickly and efficiently explain why half the states are missing. And all of a sudden, everything makes perfect sense," Cates says. "And in fact, even now, there's even evidence that they might have found one or two of these missing states. That's actually consistent with everything I'm saying, so long as the diquarks are a little on the fuzzy side."
He says the next step is to follow up the high-precision measurements of the form factors of the proton and of the neutron with an upgraded CEBAF accelerator, more sensitive detectors and better targets to add information to the curved lines on the graph that first indicated this funny behavior."The spirit that's important to emphasize is that it's very suggestive, and that if true, it would represent an exciting shift in the picture that we would draw of the structure," Cates concludes.
Submitted by DOE’s Jefferson Lab