Measurement of the underlying event activity in pp collisions at sqrt(s) = 0.9 and 7 TeV with the novel jet-area/median approach

TL;DR

This work presents the first charged-component underlying event measurement in pp collisions using the jet-area/median observable $\rho'$ with CMS data at $\sqrt{s}=0.9$ and 7 TeV. By clustering tracks with the $k_{\mathrm{T}}$ algorithm and using ghost particles to define jet areas, the authors compute $\rho$ and the adjusted $\rho'$ to isolate soft activity, unfolding the results to compare to multiple MC tunes. The findings show substantial discrepancies between data and all tested tunes across energies and event scales, underscoring the need for improved UE modeling; the approach itself proves robust and extendable to other topologies. Overall, the study demonstrates the sensitivity of the jet-area/median method to soft hadronic activity and its potential for guiding future tune development in high-energy hadron collisions.

Abstract

The first measurement of the charged component of the underlying event using the novel "jet-area/median" approach is presented for proton-proton collisions at centre-of-mass energies of 0.9 and 7 TeV. The data were recorded in 2010 with the CMS experiment at the LHC. A new observable, sensitive to soft particle production, is introduced and investigated inclusively and as a function of the event scale defined by the transverse momentum of the leading jet. Various phenomenological models are compared to data, with and without corrections for detector effects. None of the examined models describe the data satisfactorily.

Figures (5)

• Figure 1: Uncorrected inclusive $\rho'$ distributions for data and simulation (upper row), and ratios of the pythia 6 tunes Z1, Z2, D6T, and the pythia 8 tune 4C relative to data (lower row) at $\sqrt{s} = 0.9\,\text{Te\spaceV}\xspace$ (left) and $\sqrt{s} = 7\,\text{Te\spaceV}\xspace$ (right). The dark grey shaded band corresponds to the systematic uncertainty and the light grey shaded band to the quadratic sum of the systematic and statistical uncertainty. The reach in $\rho'$ is different at the two centre-of-mass energies.
• Figure 2: Uncorrected $\rho'$ distributions in the two slices of leading track jet transverse momentum, $3 < p_\mathrm{T,leading} < 6\,\text{Ge\spaceV}\xspace$ (left) and $9 < p_\mathrm{T,leading} < 12\,\text{Ge\spaceV}\xspace$ (right) at $\sqrt{s} = 7\,\text{Te\spaceV}\xspace$. The reach in $\rho'$ is different for the two slices in leading track jet $p_{\mathrm{T}}$. The lower plots show the ratios of the different generator tunes to the reconstructed data. The dark grey shaded band corresponds to the systematic uncertainty and the light grey shaded band to the quadratic sum of the systematic and statistical uncertainty.
• Figure 3: Mean values of the uncorrected $\rho'$ distributions versus leading track jet transverse momentum at $\sqrt{s} = 0.9\,\text{Te\spaceV}\xspace$ (left) and $\sqrt{s} = 7\,\text{Te\spaceV}\xspace$ (right) in comparison to the predictions by the different generator tunes. The error bars, which are mostly smaller than the symbol sizes, correspond to the quadratic sum of the systematic and statistical uncertainty.
• Figure 4: Unfolded inclusive $\rho'$ distributions for data and simulation (upper row), and ratios of the pythia 6 tunes Z1, Z2, D6T, and the pythia 8 tune 4C relative to data (lower row) at $\sqrt{s} = 0.9\,\text{Te\spaceV}\xspace$ (left) and $\sqrt{s} = 7\,\text{Te\spaceV}\xspace$ (right). The quadratic difference between the total uncertainty, as given by the light grey band, and the dark grey band corresponds to the unfolding uncertainty, which inherently also comprises the statistical uncertainty.
• Figure 5: Mean values of the corrected $\rho'$ distributions versus leading charged-particle jet transverse momentum at $\sqrt{s} = 0.9\,\text{Te\spaceV}\xspace$ (left) and $\sqrt{s} = 7\,\text{Te\spaceV}\xspace$ (right) in comparison to the predictions by the different generator tunes. The $\rho'$ distributions in each slice are unfolded with the Bayesian method. The error bars, which are mostly smaller than the symbol sizes, correspond to the total uncertainty.