Focus: The Difference between Protons and Neutrons
Nuclear physicists think of protons and neutrons as practically identical. When they aim low-energy particle beams at nuclear targets, they find that the two behave in almost exactly the same way in the nucleus. Physicists have always assumed that with high energy collisions, which break protons and neutrons into their constituent quarks, they would see the same charge symmetry–quarks inside protons should behave very much like their counterparts inside neutrons. But charge symmetry is harder to test accurately at high energies, and only the most recent experiments have had enough precision to do so. Physicists publishing in the 9 November PRL analyzed these recent results and found that there is a clear violation of charge symmetry at high energies, even after they accounted for all known sources of uncertainty. The authors conclude that more experiments are needed to verify the existence of this violation, which is inconsistent with the current understanding of quark properties.
In low-energy collisions, nuclei are best described as collections of protons and neutrons, but as the collision energy is increased, the nucleons eventually break up into their constituent particles. In the highest energy experiments, protons and neutrons appear to consist of three main “valence” quarks–each of which carries a significant amount of the nucleon’s momentum–plus a “sea” of virtual quarks that have relatively little momentum. These experiments use a beam of particles, such as muons, to measure the distribution of the various quarks inside the nucleus. Based on their detector data, researchers label each quark that is struck with a value of x, which gives the fraction of the nucleon’s momentum found in that quark.
The three valence quarks in the proton include two up quarks and a down quark, while the neutron valence quarks include two downs and an up. The quark sea contains additional quark flavors. According to charge symmetry, an up quark in the proton acts like a down quark in the neutron, and the most direct high-energy test of this property involves scattering off nuclei with equal numbers of protons and neutrons. The idea is to compare scattering of muons, which interact with all quarks, with that of neutrinos, which interact only with negatively charged quarks. For nuclear targets with equal numbers of protons and neutrons, the two probes should see very similar distributions of quarks, if charge symmetry is respected.
J. Timothy Londergan is part of the team that examined data from two recent experiments of this type. The CCFR collaboration at the Fermi National Accelerator Laboratory scattered neutrinos from iron nuclei, and the NMC collaboration at CERN in Geneva scattered muons from deuterium. The experiments involved energy transfers of up to severalhundred GeV.
The experimental groups reported “structure functions,” which give information on the number of quarks they found with each value of fractional momentum x. Before comparing the neutrino and muon data, Londergan and his colleagues corrected them for differences in the way muons and neutrinos react with quarks inside nuclei, as compared with their simpler interactions with free quarks. After these corrections, the team found that for high x the muon and neutrino structure functions agreed to within 2%, but for x < 0.1 the two experiments appeared to disagree by as much as 10%. Even after including the effects of the occasional strange quarks that appear in the virtual quark sea, the team could not account for more than a 1% violation of charge symmetry. According to Londergan, as long as the experimental uncertainties are accurate, “There appears to be a significant violation of charge symmetry at small x.” If this result holds up to further tests, physicists will have to revise their models of the quark structure of nuclei.
Michael Shaevitz of Columbia University is one of the CCFR collaboration leaders, and he says that some researchers had expected that the corrections would eliminate the apparent charge symmetry violation implied by the data. But these new calculations reveal that a glaring violation remains. “It’s most likely that we’re missing something, either experimentally or in calculating the quark distributions, but right now we don’t know what it is,” says Shaevitz. He suggests that some independent test of charge symmetry should be possible, especially given the size of the apparent violation.