Charged-Pion Lifetime and a Limit on a Fundamental Length

The lifetime of the charged pion has been determined in flight. The agreement of our result, 26.02 +/- 0.04 nsec, with experiments of comparable precision made with pions at rest provides the most precise verification of time dilation. In the model of Lundberg and Rédei this comparison sets an upper limit on a fundamental length of 3 × 10^-15 cm. Using the measurements of velocity and momentum for both π⁺ and π^-, we also obtain m(π⁺)m(π^-)=1.0002+/-0.0005.