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 fb−1 of proton-proton collisions collected at √s =13 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.
