# Underlying Event properties in pp collisions at $\sqrt{s}$ = 13 TeV

This article reports measurements characterizing the Underlying Event (UE) associated with hard scatterings at midrapidity ($|\eta| <~ 0.8$) in pp collisions at $\sqrt{s}$ = 13 TeV. The hard scatterings are identified by the leading particle, the charged particle with the highest transverse momentum ($p_{\rm{T}}^{\rm{leading}}$) in the event. Charged-particle numbers and summed transverse-momentum densities are measured in different azimuthal regions defined with respect to the leading particle direction: Toward, Transverse, and Away. The Toward and Away regions contain the fragmentation products of the hard scatterings in addition to the UE contribution, whereas particles in the Transverse region are expected to originate predominantly from the UE. The study is performed as a function of $p_{\rm{T}}^{\rm{leading}}$ with three different $p_{\rm{T}}$ thresholds for the associated particles, $p_{\rm{T}}^{\rm{track}}>$ 0.15, 0.5, and 1.0 GeV/$c$. The charged-particle density in the Transverse region rises steeply for low values of $p_{\rm{T}}^{\rm{leading}}$ and reaches a plateau. The results confirm the trend observed at lower collision energies that the charged-particle density in the Transverse region shows a stronger increase with $\sqrt{s}$ than the inclusive charged-particle density at midrapidity. The plateau in the Transverse region ($5 <~ p_{\rm{T}}^{\rm{leading}} <~ 40$ GeV/$c$) is further characterized by the probability distribution of its charged-particle multiplicity normalized to its average value (relative transverse activity, $R_{\rm{T}}$) and the mean transverse momentum as a function of $R_{\rm{T}}$. Experimental results are compared to model calculations obtained using PYTHIA 8 and EPOS LHC. The overall agreement between models and data is within 30%. These measurements provide new insights on the interplay between hard scatterings and the associated UE in pp collisions.

Submitted to: JHEP
e-Print: arXiv:1910.14400 | PDF | inSPIRE
CERN-EP-2019-235

Figures

## Figure 1

 Mean \Rtm\ and standard deviation $\sigma$ of the \Rtm\ distribution as a function of \Rtt The solid lines represent the fits to the points and the extrapolation to higher \Rtt, resulting in the parameterized response matrix used for unfolding.

## Figure 2

 Number density $\Nch$ (left) and $\sum \pt$ (right) distributions as a function of \ptlead\ in Toward, Transverse, and Away regions for $\ptmin > 0.15 \; \gmom$. The shaded areas represent the systematic uncertainties and vertical error bars indicate statistical uncertainties

## Figure 3

 Number density $\Nch$ (left) and $\sum \pt$ (right) distributions as a function of \ptlead\, along with the MC predictions in Toward (top), Transverse (middle), and Away (bottom) regions for the threshold of $\ptmin > 0.15 \; \gmom$. The shaded areas in the upper panels represent the systematic uncertainties and vertical error bars indicate statistical uncertainties. In the lower panels, the shaded areas are the sum in quadrature of statistical and systematic uncertainties from upper panels. No uncertainties are shown for the MC calculations.

## Figure 4

 Number density $\Nch$ (left) and $\sum \pt$ (right) distributions as a function of \ptlead\, along with the MC predictions in Transverse region for three transverse momentum thresholds of $\ptmin\ > 0.15$, $0.5$, and $1.0 \; \gmom$ The shaded areas represent the systematic uncertainties and vertical error bars indicate statistical uncertainties

## Figure 5

 Left: Number density $\Nch$ in the Transverse region as a function of \ptlead\ ($\ptmin > 0.15 \; \gmom$ threshold) for $\sqrt{s} =$ 0.9, 7 and 13~$\TeV$. A constant function is used to fit the data in the range $5 < \ptlead < ~ 10 \; \gmom$ and the results are shown as solid lines Right: Number densities $\Nch$ scaled by the pedestal values obtained from the fit in order to compare the shapes. The open boxes represent the systematic uncertainties and vertical error bars indicate statistical uncertainties

## Figure 6

 Comparison of number density in the plateau of the Transverse region with lower energy data taken from~ and $dN_{\rm ch}/d\eta$ in minimum-bias events (scaled by $1/2\pi$)~. Both are for charged-particles with $\pt >$ 0.5~\gmom. Error bars represent statistical and systematic uncertainties summed in quadrature. The straight lines show the results of fitting data points with the functional form $a+b\ln{s}$.

## Figure 7

 Number density $\Nch$ (left) and $\sum \pt$ density (right) in the Transverse region for $\ptmin > 0.5$ ~\gmom\ at $\sqrt{s}=13~\TeV$ from ALICE ($|\eta| < 0.8$) and ATLAS ($|\eta| < 2.5$) measurements~. The results are compared to \textsc{Pythia}~8 Monash-2013 calculations. The shaded areas represent the systematic uncertainties and vertical error bars indicate statistical uncertainties. No uncertainties are shown for the MC calculations.

## Figure 8

 \Rt probability distribution in the Transverse region for $\ptmin > 0.15 \; \gmom$ and $|\eta | < 0.8$. The result (solid circles) is compared to the \textsc{Pythia}~8 and \textsc{Epos} LHC calculations (lines) The red line represents the result of the NBD fit, where the multiplicity is scaled by its mean value, $m$. The parameter $k$ is related to the standard deviation of the distribution via $\sigma = \sqrt{\frac{1}{m}+\frac{1}{k}}$ The open boxes represent the systematic uncertainties and vertical error bars indicate statistical uncertainties. No uncertainties are shown for the MC calculations. The bottom panel shows the ratio between the NBD fit, as well as those of the MC to the data

## Figure 9

 $\avpt$ in the Transverse region as a function of \Rt\ for $\ptmin > 0.15 \; \gmom$ and $|\eta | < 0.8$. Data (solid circles) are compared to the results of \textsc{Pythia}~8 and \textsc{Epos} LHC calculations (lines). The open boxes represent the systematic uncertainties and vertical error bars indicate statistical uncertainties. No uncertainties are shown for the MC calculations. The bottom panel shows the ratio of the MC to data.