APS News

Scientists Observe Charge Symmetry Breaking in Separate Experiments

In separate experiments at the Indiana University Cyclotron Facility (IUCF) and the TRIUMF cyclotron in Canada, researchers have made groundbreaking new measurements of charge symmetry breaking (CSB), according to results presented at the APS April meeting in Philadelphia.

Such 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 differences 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.

In the 1930s, 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 the phenomenon known as charge symmetry breaking.

One effect of this charge symmetry violation is that the neutron is slightly heavier than its charged partner, the proton. Thus, isolated neutrons decay into protons in about 10 minutes.

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.

Over a two-month period, researchers observed this rare reaction several dozen times, giving physicists enough data to test theories of CSB.

"Scientists have searched for this rare fusion process since the 1950s," said Stephenson. "And the process would not happen at all if nature did not allow a small violation of charge symmetry." In fact, if the symmetry violation had occurred in other direction-that is, if the proton had been slightly heavier than the neutron-hydrogen would not have survived after the Big Bang, and the universe would not have the hydrogen fuel that keeps stars shining, including the sun, which makes human life possible. "Sometimes large consequences hang on delicate balances in nature," he said.

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) in which the proton and neutron meet at the center, the reaction-repeated many times-produces a small excess of pions (about 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 converts into a small amount of mass. "The rate of the process will tell scientists how much of the violation comes from the fact that quarks carry small electrical charges, and how much comes from the difference in mass between the two types of quarks found inside neutrons and protons," said Stephenson.


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