Measurement of the jet mass distribution and top quark mass in hadronic decays of boosted top quarks in pp collisions at sqrt(s) = 13 TeV

A measurement is reported of the jet mass distribution in hadronic decays of boosted top quarks produced in pp collisions at √s=13  TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 35.9  fb−1. The measurement is performed in the lepton+jets channel of t¯t events, where the lepton is an electron or muon. The products of the hadronic top quark decay t→bW→bq¯q′ are reconstructed as a single jet with transverse momentum larger than 400 GeV. The t¯t cross section as a function of the jet mass is unfolded at the particle level and used to extract a value of the top quark mass of 172.6±2.5  GeV. A novel jet reconstruction technique is used for the first time at the LHC, which improves the precision by a factor of 3 relative to an earlier measurement. This highlights the potential of measurements using boosted top quarks, where the new technique will enable future precision measurements.