Measurement of the jet mass distribution and top quark mass in hadronic decays of boosted top quarks in pp collisions at $\sqrt{s} =$ 13 TeV

TL;DR

This paper reports a measurement of the jet mass distribution in boosted top quark decays at 13 TeV, using a novel reconstruction technique that improves precision and enables future detailed studies of the top quark.

Contribution

Introduction of a new jet reconstruction method at the LHC that enhances the precision of top quark mass measurements in boosted decay events.

Findings

Top quark mass measured as 172.6 ± 2.5 GeV.

Jet reconstruction technique improves measurement precision by a factor of three.

First application of this method at the LHC for boosted top quark analysis.

Abstract

A measurement is reported of the jet mass distribution in hadronic decays of boosted top quarks produced in pp collisions at $\sqrt{s} =$ 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 $\mathrm{t\bar{t}}$ events, where the lepton is an electron or muon. The products of the hadronic top quark decay t $\to$ bW $\to$ bq$\mathrm{\bar{q}}'$ are reconstructed as a single jet with transverse momentum larger than 400 GeV. The $\mathrm{t\bar{t}}$ 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 GeV. A novel jet reconstruction technique is used for the first time at the LHC, which improves the precision by a factor of three relative to an earlier measurement. This highlights the potential of measurements using boosted top quarks, where the new technique will enable future precision measurements.