Measurement of the top quark mass in events with a single reconstructed top quark at $\sqrt{s}=13\, \mathrm{TeV}$

A measurement of the top quark mass is performed in a sample enriched with single top quark events produced in the $t$ channel using $35.9\,\mathrm{fb}^{-1}$ of proton-proton collision data recorded at $\sqrt{s} \,=\,13$ TeV by the CMS experiment in 2016. Candidate events are selected by requiring an isolated energetic lepton (muon or electron) and exactly two jets. One of the jets is identified to originate from a bottom quark, whereas the other stems from the hadronization of a light-flavor quark. Multivariate discriminators are designed to separate signal from backgrounds, and the thresholds on the discriminator outputs are optimized to ensure an event sample with high signal purity. The mass of the top quark is found to be $172.13^{+0.76}_{-0.77}\, \mathrm{GeV}$, reaching a sub-GeV precision for the first time in this particular phase space. The masses of top quark and antiquark are also measured separately based on the charge of the lepton in the final state, from which the mass ratio and difference are determined to be $0.995^{+0.005}_{-0.006}$ and $0.83^{+0.77}_{-1.01}\, \mathrm{GeV}$, respectively. These results are consistent with CPT invariance.