The masses of light mesons, K-mesons and hypersons in 1956

Abstract

A number of independent accurate measurements of the masses of light mesons are critically reviewed. Assuming the positive-and negative-charged mesons to have equal masses, best values for the pion and muon masses can be calculated. It is found that the negative π- and μ-mesic X-ray energies measured by K absorption edges can be combined with the mass difference between the positive pion and muon to give a result with a standard deviation approximately one third of the assigned errors of the best previous measurement. All the results are in satisfactory agreement and can be combined in the usual way. The mass results are as follows:Mμ=(206.86±0.11) me;Mπ==(273.27±0.11) me. Experimental data on the mean lives, spins, parities, and decay modes are also reviewed and the results are given. The measurements of the masses of the various K-mesons are obtained in several ways:MKπ2=(966.0±1.5) me;MKπ2=(965.3±1.9) me;MΣ=(966.8±0.4) me. There is at present no evidence for a difference in the masses to a precision of 1 ÷ 2 me. Results on the Σ+-Σ− and τ+ masses and lifetimes and the analysis of τ+ decay are discussed. At present the experimental equality of these particles seems to be contradictory to assigning a spin of less than 2. The cosmic-ray data on the ϑ0 lifetime are reviewed and recent results on artificially-produced ϑ0’s are discussed. The hyperon mass data are reviewed.MΔδ=(2181.74±0.35) me;MΣδ=(2 327.4±1.0) me. The latest data on a Σ+ − Σ− mass difference and their mean lives are presented.