Measurement of the jet mass in highly boosted t(t)over-bar events from pp collisions at root s=8TeV

The first measurement of the jet mass mjet of top quark jets produced in $t \overline{t}$ events from pp collisions at $\sqrt{s}=8$ TeV is reported for the jet with the largest transverse momentum $p_T$ in highly boosted hadronic top quark decays. The data sample, collected with the CMS detector, corresponds to an integrated luminosity of $19.7fb^{−1}$. The measurement is performed in the lepton + jets channel in which the products of the semileptonic decay t→bW with W→lν where l is an electron or muon, are used to select $t \overline{t}$ events with large Lorentz boosts. The products of the fully hadronic decay t→bW with W→$q \overline q′$ are reconstructed using a single Cambridge–Aachen jet with distance parameter R=1.2, and $p_T$>400GeV. The $t \overline{t}$ cross section as a function of $m_{jet}$ is unfolded at the particle level and is used to test the modelling of highly boosted top quark production. The peak position of the $m_{jet}$ distribution is sensitive to the top quark mass $m_t$, and the data are used to extract a value of $m_t$ to assess this sensitivity.