Measurement of the jet mass in highly boosted

The first measurement of the jet mass $$m_{\text {jet}}$$ of top quark jets produced in $${\mathrm{t}}\overline{\mathrm{t}} $$ events from pp collisions at $$\sqrt{s}=8$$ $$\,\text {TeV}$$ is reported for the jet with the largest transverse momentum $$p_{\mathrm{T}}$$ in highly boosted hadronic top quark decays. The data sample, collected with the CMS detector, corresponds to an integrated luminosity of 19.7$$\,\text {fb}^{-1}$$. The measurement is performed in the lepton+jets channel in which the products of the semileptonic decay $${\mathrm{t}} \rightarrow \mathrm{b} \mathrm {W}$$ with $$\mathrm {W}\rightarrow \ell \nu $$ where $$\ell $$ is an electron or muon, are used to select $${\mathrm{t}}\overline{\mathrm{t}} $$ events with large Lorentz boosts. The products of the fully hadronic decay $${\mathrm{t}} \rightarrow \mathrm{b} \mathrm {W}$$ with $$\mathrm {W}\rightarrow \mathrm{q} \overline{\mathrm{q}} '$$ are reconstructed using a single Cambridge–Aachen jet with distance parameter $$R=1.2$$, and $$p_{\mathrm{T}} >400$$ $$\,\text {GeV}$$. The $${\mathrm{t}}\overline{\mathrm{t}} $$ cross section as a function of $$m_{\text {jet}}$$ is unfolded at the particle level and is used to test the modelling of highly boosted top quark production. The peak position of the $$m_{\text {jet}}$$ distribution is sensitive to the top quark mass $$m_{{\mathrm{t}}}$$, and the data are used to extract a value of $$m_{{\mathrm{t}}}$$ to assess this sensitivity