The mean lives of the ir + and TT~ mesons are determined by measurements of the attenuation in flight of a 73-Mev meson beam, using a scintillation counter telescope of variable length. The decay scheme, ir^->/z ± +^, is assumed to be isotropic in the rest system of the 7r-meson. The data, when the Lorentz time dilation is taken into account, give for the mean lives at rest r_=(2.55±0.19)XlO~8sec, r+=(2.44±0.18)XlO-8 sec.

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T HE study of the absorption of positive pions by deuterons, which, together with its inverse, has been used to determine the spin of the meson, 1 -2 has been extended. We present here results on the energy dependence of the angular distribution and of the total cross section, in the hope that they will prove useful in the attempt to understand the pion-nucleon interaction.The reaction is -K + -\-d-*p-\-p. The reaction rate is determined by the rate of recoil proton pairs in coincidence with incoming mesons, incident successively on water and heavy water targets. The experimental technique has already been described. 1 The water cells are 1" thick along the beam direction. The energy dispersion due to target thickness and meson beam inhomogeneity is approximately ±7 Mev. The angular dispersion is approximately ±14°. Despite this poor resolution the coincidence rate of recoil protons is only ^1/minute in the Nevis meson beam of ~500/sec. The uncertainties in the result are almost entirely statistical; the geometrical factors have been calculated with greater accuracy, and the combined uncertainties in counting efficiency and beam composition are less than 10 percent.The results are given in Table I. All parameters as well as differential cross sections are in the center of mass system. The recoil angles listed are averaged over the angular dispersion. This affects only the angles near 90°, since, because of the symmetry, 90° is the extreme angle, and larger angles are recorded as smaller angles. When the proton counter axes are set at 90° relative to the meson beam, the average detection angle is 83°.It is possible to represent the angular dependence as a+/3 cos 2 0, since meson angular momenta with respect to the deuteron of more than two Planck units should not contribute appreciably at these energies. The best fits to such a distribution are given in Table II, together with absolute cross sections.In Table II we have also included the results of Cartwright, Richman, Wilcox, and Whitehead. 3 The angular distribution found at Berkeley and our results are only in fair agreement.There is no large change in the angular distribution in the energy range 25-53 Mev for the incident meson in the center of mass system. The total cross section increases by a factor 2.25±0.32.To see the implication of this result on the meson nucleon interaction, it is necessary to separate the effects of the nuclear binding in initial and final states as well as the kinematical factors. The effects of the binding on the angular distribution are complicated, and are discussed in the following note. 4 The kinematical factors are TABLE I. Differential cross sections for the absorption of pions by deuterons in the center of mass system. Rms statistical errors are given. £«=25 Mev Em =40 Mev Em =53 Mev 6cm dtr/dQ cmVsterad 6cm d

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