We have found a bubble-chamber picture which shows three contemporaneous high-energy cosmic-ray tracks. One of the tracks has ionization which is that expected of a charge-$\frac{2}{3}$ particle if the mass of this particle is less than 6.5 GeV. The observed ionization is also consistent with that which is expected of a charge-$\frac{1}{3}$ particle with a mass of 8.0±3.0 GeV.

• Received 11 February 1970

DOI:https://doi.org/10.1103/PhysRevLett.24.917