In the context of the Standard Model of particle physics, which describes the fundamental particles and their interactions, quarks are the only known particles that carry fractional electric charges. Quarks have charges of either +2/3 or -1/3 times the elementary charge (approximately 1.602 x 10^-19 coulombs).
However, it's important to note that the quarks themselves are never observed in isolation due to color confinement, meaning they are always bound together in combinations of three to form baryons (such as protons and neutrons) or in pairs with their corresponding antiquarks to form mesons. As a result, the fractional charge of quarks is not directly observable.
Particles that are observed to carry electric charge in experiments, such as electrons, protons, and other charged particles, have charges that are integer multiples of the elementary charge. These charges are whole numbers, such as -1, -2, +1, +2, etc., and not fractions like the quarks.
It's worth mentioning that beyond the Standard Model, in some speculative theories and models, there are suggestions of the existence of particles with fractional charges other than quarks. However, to date, there is no experimental evidence to confirm the existence of such particles, and they remain purely hypothetical.
So, in the known framework of the Standard Model, quarks are the only particles with fractional charges, while all other observed particles carry charges that are integer multiples of the elementary charge.