Charge Symmetry Breaking

Charge symmetry breaking has been observed in two experiments reported at the recent American Physical Society meeting in Philadelphia. In the 1930s, physicist Werner Heisenberg proposed that the neutron and proton are simply slightly different manifestations of the same particle, called the "nucleon." Modern nuclear physics endorses this view: plenty of nuclear reactions proceed exactly the same way if a proton takes the place of a neutron, or vice versa. However, this close similarity breaks down in some cases, leading to a situation known as "charge symmetry breaking" (CSB). In separate experiments at the Indiana University Cyclotron Facility (IUCF) and the TRIUMF cyclotron in Canada, researchers have made groundbreaking new measurements of CSB (which, incidentally, is a nuclear-physics phenomenon completely different from charge [C] conjugation in particle physics). Such CSB measurements can provide deep insights into why nature gave the neutron and proton slightly different masses. At an even more fundamental level, the CSB measurements can potentially yield more precise values of the mass difference between the up and down quarks that make up protons and neutrons. Nuclear theorists are busily analyzing these new experimental results to put tighter constraints on the up-down mass difference.

At the APS meeting, Ed Stephenson of Indiana University announced the first unambiguous identification of a rare process: the fusion of two nuclei of heavy hydrogen to form a nucleus of helium and an uncharged pion, one of the subatomic particles responsible for the strong force that binds nuclei together. This process would not exist at all were it not that nature allowed a small violation of charge symmetry. Over a two-month period, researchers observed this rare reaction several dozen times, giving physicists enough data to test theories of charge-symmetry breaking.

Representing a collaboration at TRIUMF, Allena Opper of Ohio University discussed the detection of CSB in another nuclear reaction: the fusion of a proton and neutron, which produces a charged pion as one of its products. Viewed from a perspective or ("reference frame") at which the proton and neutron meet at the center, the reaction, repeated man times, produces a small excess of pions (0.17%) in a preferred direction. Such an asymmetry is a hallmark of CSB. Taken together, these new CSB results promise a wealth of information on such things as the slightly different electromagnetic fields inside each nucleon. As it turns out, such fields may contribute to the proton-neutron mass difference, as they carry energy which convert into a small amount of mass.