Measurement of the jet mass in highly boosted events from pp collisions at

The first measurement of the jet mass of top quark jets produced in events from pp collisions at is reported for the jet with the largest transverse momentum in highly boosted hadronic top quark decays. The data sample, collected with the CMS detector, corresponds to an integrated luminosity of 19.7. The measurement is performed in the lepton+jets channel in which the products of the semileptonic decay with where is an electron or muon, are used to select events with large Lorentz boosts. The products of the fully hadronic decay with are reconstructed using a single Cambridge–Aachen jet with distance parameter , and . The cross section as a function of is unfolded at the particle level and is used to test the modelling of highly boosted top quark production. The peak position of the distribution is sensitive to the top quark mass , and the data are used to extract a value of to assess this sensitivity.