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E-08-023: An updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect

Broken symmetries in the early universe account for the presence of stars, planets and people. Physicists study modern-day symmetry breaking 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..

To study pions, scientists in the PrimEx collaboration produce them via the Primakoff Effect, where two interacting photons produce a pion, which lives for billionths of a billionth of a second before decaying back into two photons. Both decay photons are then measured in a novel hybrid calorimeter.

The first run of this experiment, called PrimEx, produced a measurement of the lifetime of the neutral pion that is a factor of two-and-a-half times more precise than the current average value of this fundamental parameter quoted in the Particle Physics Booklet. PrimEx achieved a total experimental uncertainty of 2.8 percent and is the most precise Primakoff measurement of the 0→ decay width to date.

In order to reach the final goal to test the chiral anomaly and its corrections, the collaboration will run a second experiment (PrimEx-II). The projected 1.4 percent precision for PrimEx-II will be achieved by two major improvements from the previous run: (1) reduced statistical uncertainty by taking more experimental data; and (2) a better understanding of systematical uncertainty by reducing background with a newly modified the photon beam line and additional timing information from HYCAL.

E08-023 Technical Paper