The lifetime of the chargeless pion, the lightest particle made of quark-antiquark pairs, has been determined to higher levels of precision in a new experiment at Jefferson Laboratory in Virginia. According to experimentalist Liping Gan (Univ of North Carolina-Wilmington), speaking at the APS meeting, the neutral pion lifetime is one of the few quantities that can be directly calculated (to about 1% precision) in quantum chromodynamics (QCD), the theory of the strong force, which holds together quarks and quark-containing objects.
In Jefferson Labís Primakoff Experiment, the researchers aim a gamma-ray beam at nuclei, which perpetually has a cloud of photons around it. Through a phenomenon known as the Primakoff effect, two photons (one from the target nucleus and another from the photon beam) interact and make a chargeless pion, which decays into two daughter photons.
Measuring the daughter photons reconstructs the details of the decay and provides lifetime information about the pions. The new experiment is more precise than past Primakoff effect experiments because the incident photons (produced from the deceleration of Jlabís electron beam) are "tagged," meaning that the researchers can keep track of the numbers of incoming photons hitting the nuclear targets, as well as their energies.
When the photons emerge from the decay, an advanced calorimeter (called HyCal) is able to measure the daughter photons' trajectories and energies to high precision. Ashot Gasparian of North Carolina A&T State University said the calculated lifetime of the pion is 82 attoseconds with about a 2.9% error [(8.20+/-0.24)x10^-17 sec].
The new, preliminary result is two times more precise than the present value published in particle data tables [8.4+/-(0.6)x10^-17s], and the precision can potentially double as researchers analyze all of their data and finalize their result.