The pion is one of the subatomic particles responsible for the strong force that holds every nucleus together. The achievement was announced Saturday (April 5) at the meeting of the American Physical Society in Philadelphia.

"Scientists have searched for this rare fusion process since the 1950s," said IU physicist Edward Stephenson, the leader of the research team.

"The process would not happen at all if nature did not allow a small violation of what is known as charge symmetry. If this symmetry violation had happened to be in the other direction, hydrogen would not have survived after the Big Bang, and the universe would not have the hydrogen fuel that keeps stars shining, including our sun, making human life possible. Sometimes large consequences hang on delicate balances in nature."

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

"If the charge symmetry violation had been in the other direction instead, and if the proton had been heavier than the neutron by the same slight amount, protons would have decayed into neutrons and hydrogen could not have survived," Stephenson explained.

The rate at which the rare fusion process occurs is expected to be a key piece of information in finding the cause for this violation of charge symmetry, he said. Theorists have proposed that the violation originates with quarks, the small particles that are found inside protons and neutrons.

"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," Stephenson said.

The IU team used the electron-cooled storage ring at the cyclotron laboratory to focus a beam of heavy hydrogen onto a target of the same material. The high precision of the beam allowed them to use just enough energy to make the uncharged pion without producing unwanted heavier particles. Sensitive detectors tracked the helium nuclei and captured the two photons or particles of light that are produced when the pion decays.

The team worked around the clock for two months, seeing at most only five of the rare events per day, Stephenson said. However, the several dozen events that they collected will be enough to allow scientists to test their theories about the violation of charge symmtery.

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