The inscrutable subatomic particles called quarks--which make up all matter---are well known from high-energy collisions that smash larger particles like protons or neutrons. But it's never been clear how a whole nucleus is affected by the quarks that make up its protons and neutrons. Now an experiment has for the first time detected the effect of quarks in a nuclear reaction. The new work should help physicists better understand the nucleus.
Physicists have long researched the nucleus without needing to consider quarks, a bit like doctors studying how the body works by examining its organs, but not their component cells. Yet cells ultimately determine how we live and breathe, and nuclear physicists assume a comparable importance for quarks within the nucleus. After all, quarks interact with each other through the strong nuclear force--just as do neutrons and protons.
Hunting for a quark effect, physicists using the electron accelerator at the Jefferson Lab in Newport News, Virginia, directed energetic photons at a target composed of simple nuclei called deuterons, each comprising one proton plus one neutron. "The photon comes in and whacks the deuteron, the neutron and proton interact with each other, and we are able to detect the outgoing proton," says Argonne National Lab's Elaine Schulte, lead author on the group's paper in the 3 September issue of Physical Review Letters. Crucially, the incoming photon breaks up the deuteron without smashing its constituent parts. The proton's exit from the collision is determined in part by how it was bound to its partner neutron, including any ties between their component quarks. The group counted outgoing protons while varying both the detection angle and photon energy, and they found hallmarks of quarks.
"This represents a first," says Gerald Miller, a physicist at the University of Washington, Seattle. The new observations show a process that begins and ends with protons and neutrons but which can only proceed via quarks, he explains, though there are plenty of details still to thrash out. What next? "The theorists, to a certain extent, will take it from here," Schulte says.