Charge symmetry breaking

Charge symmetry breaking (CSB) has been observed in two experiments, reported in April at the American Physical Society meeting in Philadelphia. In the 1930s, Werner Heisenberg proposed that the neutron and proton are slightly different manifestations of the same particle, the “nucleon.” Modern nuclear physics endorses this view: Many nuclear reactions proceed exactly the same if a proton replaces a neutron, or vice versa. However, the similarity of protons and neutrons breaks down in some cases, leading to CSB. Edward Stephenson of Indiana University announced the first unambiguous identification of a process that is forbidden if charge symmetry is exact: the fusion of two deuterium nuclei to form a helium nucleus and a neutral pion. The experiment was conducted over a two-month period at the Indiana University Cyclotron Facility. Allena Opper of Ohio University discussed the other result: the fusion of a proton and neutron to form a deuteron and a neutral pion. The experiment—a collaboration at the TRIUMF cyclotron in Canada—revealed a hallmark of CSB: A small excess of deuterons emerged in a preferred direction. CSB has been observed many times before, but the new results promise a wealth of information on such things as the slightly different electromagnetic fields inside each nucleon and why the neutron and proton have slightly different masses. The results can also potentially yield more precise values of the mass difference between the up and down quarks that make up protons and neutrons.