Measurement of the Jet Mass Distribution and Top Quark Mass in Hadronic Decays of Boosted Top Quarks in $pp$ Collisions at $\sqrt{s}=13\mathrm{TeV}$

A measurement is reported of the jet mass distribution in hadronic decays of boosted top quarks produced in $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The measurement is performed in the $\mathrm{lepton}+\text{jets}$ channel of $t\overline{t}$ events, where the lepton is an electron or muon. The products of the hadronic top quark decay $t\to bW\to bq{\overline{q}}^{\prime }$ are reconstructed as a single jet with transverse momentum larger than 400 GeV. The $t\overline{t}$ 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 $172.6\pm 2.5\mathrm{GeV}$. 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.

Figure 1

Reconstructed distribution of $m_{\text{jet}}$ after the full event selection in the $\ell +\text{jets}$ channel. The vertical bars on the points show the statistical uncertainty. The hatched region shows the total uncertainty in the simulation, including the statistical and experimental systematic uncertainties. The lower panel shows the ratio of the data to the simulation. The uncertainty band includes the statistical and experimental systematic uncertainties, where the statistical (light gray) and total (dark gray) uncertainties are shown separately in the ratio.

Figure 2

The particle-level $t\overline{t}$ differential cross section in the fiducial region as a function of the XCone-jet mass (top). The measurement is compared to predictions from powheg and madgraph5_amc@nlo with $m_t=172.5\mathrm{GeV}$. Theoretical uncertainties are shown as bands for the predictions from powheg. The normalized differential cross section (bottom) is compared to predictions from powheg for different values of $m_t$. The vertical bars represent the statistical (inner) and the total (outer) uncertainties. The horizontal bars reflect the bin widths. The lower panels show the ratios of theoretical predictions to data.