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You're probably thinking of the SU(5) Grand Unified Theory by Georgi-Glashow. This has, in addition to the usual particles, a pair of superheavy bosons named X and Y. The X and Y carry color charge and form a weak doublet, and therefore couple to other particles through both the strong and weak interactions. Counting three colors for each, this would work out to six additional particles. The X boson has a charge of -4/3, while the Y boson has a charge of -1/3. The mass of each would be in the range of 1015 GeV/c2. They would cause proton decays, such as p → π0 + e+, which have not been observed.
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