How Long Does a Pion Live?

Pions are some of matter's simplest particles. Built from the same building blocks as protons and neutrons, pions come in three types, designated by electric charge (positive, negative and chargeless). In particle physics, pion is the collective name for three subatomic particles. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force.

A new experiment at Jefferson Laboratory in Virginia has determined to higher levels of precision the lifetime of the chargeless pion, the lightest particle made of quark-antiquark pairs.

The new measurement of the lifetime of the chargeless pion conducted at the Department of Energy's Thomas Jefferson National Accelerator Facility is a factor of two more precise than the current world value. The preliminary result yields a pion mean lifetime of 82 attoseconds and is another step forward in understanding symmetry breaking in nature, a phenomenon that led to the evolution of matter in the early universe.

Broken symmetries in the early universe account for the presence of stars, planets and people. Nature continues to break the rules of symmetry, and physicists study modern-day symmetry breaking applications in hopes of shedding light on how the universe works. For instance, the spontaneous breaking of "chiral symmetry" gives rise to pions, particles that carry the strong force between protons and neutrons, binding them into the nuclei of atoms and accounting for 98% of the mass of the visible universe.

"This is the most precise measurement to date of the chargeless pion lifetime," says Ashot Gasparian, co-spokesman for Jefferson Lab's Primakoff Experiment (PrimEx) and associate professor of physics at North Carolina A&T State University. "The lifetime measurement agrees with the averaged value from previous measurements, but it is two-and-a-half times more precise," he adds. This result resolves a large uncertainty in the existing experimental measurements and confirms our present understanding of broken chiral symmetries.

The PrimEx experiment was carried out in Jefferson Lab's Experimental Hall B, which provides a high quality, tagged photon beam. The pions were produced via the Primakoff Effect, where two interacting photons produce a pion, which lives for billionths of a billionth of a second (82 attoseconds) before decaying back into two photons. Both decay photons were measured in a novel hybrid calorimeter.

The precision can potentially double as researchers analyze all of their data and finalize their result.