Abstract: 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.

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