  A measurement is presented of the primary Lund jet plane (LJP) density in
inclusive jet production in proton-proton collisions. The analysis uses 138
fb$^{-1}$ of data collected by the CMS experiment at $\sqrt{s}$ = 13 TeV. The
LJP, a representation of the phase space of emissions inside jets, is
constructed using iterative jet declustering. The transverse momentum
$k_\mathrm{T}$ and the splitting angle $\Delta R$ of an emission relative to
its emitter are measured at each step of the jet declustering process. The
average density of emissions as function of $\ln(k_\mathrm{T}$/GeV) and
$\ln(R/\Delta R)$ is measured for jets with distance parameters $R$ = 0.4 or
0.8, transverse momentum $p_\mathrm{T} \gt$ 700 GeV, and rapidity $\vert y\vert
\lt $ 1.7. The jet substructure is measured using the charged-particle tracks
of the jet. The measured distributions, unfolded to the level of stable
particles, are compared with theoretical predictions from simulations and with
perturbative quantum chromodynamics calculations. Due to the ability of the LJP
to factorize physical effects, these measurements can be used to improve
different aspects of the physics modeling in event generators.

High Energy Physics - Experiment