Odd particle left out in the cold
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The old saying birds of a feather flock together may also be true for the smallest bits of matter. According to a study recently published in Physical Review Letters, like particles inside protons and neutrons band together, leaving the odd one out.
Inside every proton and neutron is a triplet of smaller particles called quarks.
The neutron has two down quarks and one up quark. In the neutron, it's the down quarks that hunker together, with the up quark getting the cold shoulder.
On the other hand, the proton has two up quarks and one down quark. In the proton, it's the up quarks that pal around together, leaving the down quark out.
"The pair of quarks are bound more strongly than, and are also spread over a much smaller space than, the lone quark," says Bogdan Wojtsekhowski, a Jefferson Lab staff scientist and spokesperson for the experiment. "In the proton, the up quarks are concentrated, and the down is diffuse."
The result comes from research that focused on measuring the internal structure of the neutron. Most experiments that investigate the neutron in the nucleus of the atom do so by blowing it apart. The idea is to learn what the neutron looked like by studying the pieces. But this time, researchers delved deep inside an intact neutron, revealing clues about its internal structure. It was the deepest scan of the innards of an intact neutron ever successfully attempted. "This is the first time anyone has looked at the three quarks in the neutron," Wojtsekhowski says. "It is a step to the territory where the three quarks dominate: We should only see three quarks."
Studying the neutron is fraught with difficulty, he explains. Pull the neutron out of the nucleus of the atom, and it'll survive for only a few minutes on its own. So, scientists devised a way to set the neutron apart while it's still bound inside the nucleus.
In the Jefferson Lab experiment, the researchers studied a unique source of neutrons called helium-3. The nucleus of an atom of helium-3 has two protons and one neutron. Set this nucleus to spinning, and the protons will spin in opposite directions, effectively canceling out. That leaves the neutron to carry the spin of the helium-3 nucleus, clearly setting the neutrons' spin direction.
The spinning neutrons were bombarded with electrons from Jefferson Lab's CEBAF accelerator. Some of these neutrons were knocked cleanly out of the nucleus and into a detector for measurement, called BigHAND (the Big Hall A Neutron Detector). Another detector, the BigBite spectrometer, collected and measured the electrons that had knocked neutrons out of the target.
"When we probed the neutron in this experiment, it allowed us to look at the three quarks with great clarity. We were probing forces between the three quarks – examining forces that exist before and after the electron-neutron interaction," Wojtsekhowski explains.
By combining information gathered from these measurements, similar measurements of the proton and theoretical calculations, the researchers were able to piece together a new picture of how the quarks distribute themselves inside protons and neutrons.
"This type of data was impossible to see before this measurement. It was not possible before to see how the pair of quarks behave in relation to the other. We now know how the quarks are distributed in these particles," Wojtsekhowski says. "What we have learned here, is that the dual quarks are more tightly bound than the lone quark."
Experiment E02-013 was first approved to run in 2002. The experiment was carried out in Jefferson Lab's Experimental Hall A in the spring of 2006, collecting 125 times more data than previously gathered in similar experiments with a polarized target. More than 100 scientists from 33 institutions worked on the experiment.
Wojtsekhowski credits the dedication of his collaborators for the experiment's success. Innovative target design by the University of Virginia researchers, led by Gordon Cates, and unique detector development, led by Nilanga Liyanage, Richard Lindgren and Donal Day, and persistent effort by Albert Shahinyan provided the ingredients for success, along with work to integrate the various systems by numerous engineers.
Credit also goes to an army of undergraduates who constructed thousands of connections for BigHAND – the largest dedicated neutron detector - and six diligent graduate students who helped make the experiment happen.