Neutral pions (π0) have a lifetime of around 80 attoseconds, decaying into two photons. In new research, an international team of physicists has measured this lifetime with an uncertainty that was half that of the previous most precise result.
Two photons collide in this artist’s rendering of the pion experiment; the result is a charge-neutral pion shown as a green sphere; just 83 attoseconds later, the pion disintegrates and emits two photons, shown as purple spheres. Image credit: Jefferson Lab.
Pions, also known as pi mesons, are the lightest mesons and, more generally, the lightest hadrons.
These subatomic particles consist of a quark and an antiquark and are extremely unstable.
They carry the strong force that binds protons and neutrons together in nuclei. This force accounts for 98% of the mass of our visible Universe.
As simple particles, pions are described well by the theory of quantum chromodynamics (QCD). This theory describes how the visible Universe is built from the interactions of subatomic particles.
Measuring the charge-neutral pion’s lifetime and comparing it to QCD thus gives insight into the particles and forces that shape the Universe.
“Physicists predicted the existence of the Higgs boson decades before its discovery in 2012 because of symmetries in the building blocks of the Universe,” said Jefferson Lab physicist Ashot Gasparian and colleagues.
“Similarly, rules about a special type of symmetry called chiral symmetry predict the existence of pions.”
“The lifetime of a neutrally charged pion is tied to breaking of chiral symmetry. This means we can use theory to precisely calculate this lifetime.”
“Measurements of this lifetime have been much less precise than calculations from theory.”
Dr. Gasparian and his colleagues from the PrimEx Collaboration precisely measured the charge-neutral pion’s lifetime using the Continuous Electron Beam Accelerator Facility (CEBAF).
Their final result of 83.37 attoseconds has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in QCD.
“The result helps test other aspects of the theory of QCD,” the physicists said.
“It also provides critical information for carrying out a precision test of the Standard Model of particle physics, the theory that describes how three of nature’s forces govern the particles that make up matter.”
The team’s paper was published in the journal Science.
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I. Larin et al. 2020. Precision measurement of the neutral pion lifetime. Science 368 (6490): 506-509; doi: 10.1126/science.aay6641
