Search for fractionally charged particles in proton-proton collisions at $\sqrt{s}$ = 13 TeV

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