Jet fragmentation function and groomed substructure of bottom quark jets in proton-proton collisions at 5.02 TeV

TL;DR

This paper presents a detailed measurement of bottom quark jet substructure in proton-proton collisions at 5.02 TeV, using CMS data, to test QCD predictions and observe the dead-cone effect.

Contribution

It introduces a novel algorithm to identify and cluster charged decay daughters of b hadrons for substructure analysis, enabling the first such measurement of b jets.

Findings

Observed suppression of small $R_g$ emissions in b jets, consistent with dead-cone effect.

Measured jet fragmentation function $z_{b,ch}$ for b hadrons.

Compared b jet substructure with light-quark and gluon jets, confirming mass effects.

Abstract

A measurement of the substructure of bottom quark jets (b jets) in proton-proton (pp) collisions is presented. The measurement uses data collected in pp collisions at $\sqrt{s}$ = 5.02 TeV, with a low number of simultaneous interactions per bunch crossing, recorded by the CMS experiment in 2017, corresponding to an integrated luminosity of 301 pb$^{-1}$. An algorithm to identify and cluster the charged decay daughters of b hadrons is developed for this analysis, which facilitates the exposure of the gluon radiation pattern of b jets using iterative Cambridge$-$Aachen declustering. The soft-drop-groomed jet radius, $R_\mathrm{g}$, and momentum balance, $z_\mathrm{g}$, of b quark jets are presented. These observables can be used to test perturbative quantum chromodynamics predictions that account for mass effects. Because the b hadron is partially reconstructed from its charged decay daughters, only charged particles are used for the jet substructure studies. In addition, a jet fragmentation function, $z_\text{b,ch}$, is measured, which is defined as the distribution of the ratio of the transverse momentum ($p_\mathrm{T}$) of the partially reconstructed b hadron with respect to the charged-particle component of the jet $p_\mathrm{T}$. The substructure variable distributions are unfolded to the charged-particle level. The b jet substructure is compared to the substructure of jets in an inclusive jet sample that is dominated by light-quark and gluon jets in order to assess the role of the b quark mass. A strong suppression of emissions at small $R_\mathrm{g}$ values is observed for b jets when compared to inclusive jets, consistent with the dead-cone effect. The measurement is also compared with theoretical predictions from Monte Carlo event generators. This is the first substructure measurement of b jets that clusters together the b hadron decay daughters.