p0pgamma

p0pgamma is a shorthand notation used in particle physics to denote the decay of the neutral pion into two photons, written as π0 → γγ. The neutral pion is a light, neutral meson with a mass of about 135 MeV/c^2 and a very short lifetime, roughly 8×10^-17 seconds. Its two-photon decay is the dominant decay mode, with a branching fraction of about 98–99%, while rarer channels such as the Dalitz decay (π0 → e+ e- γ) contribute only a small fraction.

The π0 → γγ decay is a classic example of a process governed by the chiral anomaly in quantum chromodynamics (QCD). In leading-order theory, the decay width is predicted by the axial anomaly and is related to fundamental constants, including the electromagnetic coupling and the pion decay constant. The observed rate closely agrees with these anomaly-driven predictions, making the channel a clean test of low-energy QCD.

In experimental practice, p0pgamma serves as a standard label for simulations and data analyses, appearing in event generators and reconstruction pipelines to identify the neutral-pion two-photon final state. Accurate modeling of this channel is important for calibrating electromagnetic calorimeters, validating detector performance, and studying QCD in the non-perturbative regime.

Related topics include the Dalitz decay π0 → e+ e- γ, other rare π0 decays, and the broader role of neutral pions in hadronic physics. References are typically found in particle-physics data compilations and the Particle Data Group review of mesons.