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JLab Experiment Pins Down Neutral Pion Lifetime

A new measurement of the lifetime of the neutral pion is twice as precise as previous measurements, researchers from the PrimEx collaboration reported at the APS April Meeting.

The result agrees with previous measurements and confirms our understanding of fundamental symmetry breaking, said PrimEx collaboration spokesman Ashot Gasparian of North Carolina A&T State University.

The new result is the most precise measurement of the neutral pion lifetime to date. The experiment was carried out at the Thomas Jefferson National Accelerator Facility by the PrimEx collaboration, a group of over 70 researchers from 21 institutions.

The new mean lifetime value, about 82 attoseconds, is more than twice as precise as previous measurements, Gasparian said. 

The neutral pion, the lightest quark-anti-quark meson, is made up of a superposition an up, anti-up pair and a down, anti-down pair.

The lifetime of the chargeless pion is one of the few quantities that can be calculated directly from QCD, said PrimEx collaboration member Liping Gan of the University of North Carolina, Wilmington.

Lawrence Cardman, Jefferson Lab’s Associate Director for Experimental Nuclear Physics commented that the neutral pion is a simple system that provides a good test of fundamental theory. He called the system the “positronium of QCD.”

The JLab experiment produces pions using the Primakoff effect. In this effect, a beam of photons is aimed at a target nucleus. The nucleus generates a cloud of virtual photons, one of which interacts with a photon from the beam to produce a neutral pion.

The pion then quickly decays back into two photons. Using a sensitive calorimeter, the researchers detected the energy and position of both of these decay photons. They used these measurements to calculate the pion’s lifetime.

The PrimEx collaboration came up with a mean value of (8.2 ±0.24) x 10-17 s. The Particle Data Group’s Particle Physics Booklet’s value, based on the average of several previous experiments, is (8.4 ±0.6) x10-17s.

The PrimEx group was able to obtain a more precise measurement than previous Primakoff experiments because the photon beam is tagged so that the number and energy of photons aimed at the target nucleus can be tracked, and the decay photons are measured by an advanced hybrid calorimeter. 

More data from the PrimEx experiment remains to be analyzed, and the collaboration expects to announce an even more precise value for the neutral pion lifetime after completing that analysis.

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