Search for Fractionally Charged Particles in Proton-Proton Collisions at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$

A search is presented for fractionally charged particles with charges below $ 1e$, using their small energy loss in the tracking detector as a key variable to observe a signal. The analyzed dataset corresponds to an integrated luminosity of $ 138\text{}\text{}{\mathrm{fb}}^{-1}$ of proton-proton collisions collected at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$ in 2016–2018 at the CERN LHC. This is the first search at the LHC for new particles with a charge between $ e/3$ and $ 0.9e$, including an extension of previous results at a charge of $ 2e/3$. Masses up to 640 GeV and charges as low as $ e/3$ are excluded at 95% confidence level. These are the most stringent limits to date for the considered Drell-Yan-like production mode.