Measurement of underlying event characteristics using charged particles in $pp$ collisions at $\sqrt{s}=900\mathrm{GeV}$ and 7 TeV with the ATLAS detector

Measurements of charged particle distributions, sensitive to the underlying event, have been performed with the ATLAS detector at the LHC. The measurements are based on data collected using a minimum-bias trigger to select proton-proton collisions at center-of-mass energies of 900 GeV and 7 TeV. The “underlying event” is defined as those aspects of a hadronic interaction attributed not to the hard scattering process, but rather to the accompanying interactions of the rest of the proton. Three regions are defined in azimuthal angle with respect to the highest transverse momentum charged particle in the event, such that the region transverse to the dominant momentum-flow is most sensitive to the underlying event. In each of these regions, distributions of the charged particle multiplicity, transverse momentum density, and average $p_{\mathrm{T}}$ are measured. The data show generally higher underlying event activity than that predicted by Monte Carlo models tuned to pre-LHC data.

Figure 1

Definition of regions in the azimuthal angle with respect to the leading track.

Figure 2

Difference in $\varphi$ between the leading and the subleading track in PYTHIA, PHOJET and uncorrected data. The left plot is for 900 GeV and the right is for 7 TeV. The MC curves are shown after the full detector simulation.

Figure 3

ATLAS data at 900 GeV (left plots) and at 7 TeV (right plots) corrected back to particle level, showing the density of the charged particles $⟨d^2{N}_{\mathrm{ch}}/d\eta d\varphi ⟩$ with $p_{\mathrm{T}}>0.5\mathrm{GeV}$ and $|\eta |<2.5$, as a function of $p_{\mathrm{T}}^{\mathrm{lead}}$. The data are compared with PYTHIA ATLAS MC09, DW and Perugia0 tunes, HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET predictions. The top, middle and the bottom rows, respectively, show the transverse, toward and away regions defined by the leading charged particle. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty.

Figure 4

ATLAS data at 900 GeV (left plots) and at 7 TeV (right plots) corrected back to particle level, showing the scalar $\sum _{}^{}{p}_{\mathrm{T}}$ density of the charged particles $⟨d^2\sum _{}^{}{p}_{\mathrm{T}}/d\eta d\varphi ⟩$ with $p_{\mathrm{T}}>0.5\mathrm{GeV}$ and $|\eta |<2.5$, as a function of $p_{\mathrm{T}}^{\mathrm{lead}}$. The data are compared with PYTHIA ATLAS MC09, DW and Perugia0 tunes, HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET predictions. The top, middle and the bottom rows, respectively, show the transverse, toward and away regions defined by the leading charged particle. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty.

Figure 5

ATLAS data at 900 GeV (left plots) and at 7 TeV (right plots) corrected back to the particle level, showing the standard deviation of the density of the charged particles $⟨d^2{N}_{\mathrm{ch}}/d\eta d\varphi ⟩$ (top row) and the standard deviation of the scalar $\sum _{}^{}{p}_{\mathrm{T}}$ density of charged particles $⟨d^2\sum _{}^{}{p}_{\mathrm{T}}/d\eta d\varphi ⟩$ (bottom row) with $p_{\mathrm{T}}>0.5\mathrm{GeV}$ and $|\eta |<2.5$, as a function of $p_{\mathrm{T}}^{\mathrm{lead}}$, for the transverse region defined by the leading charged particle and compared with PYTHIA ATLAS MC09, DW and Perugia0 tunes, HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET predictions. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty.

Figure 6

ATLAS data at 900 GeV (left plots) and at 7 TeV (right plots) corrected back to particle level, showing the mean $p_{\mathrm{T}}$ of the charged particles with $p_{\mathrm{T}}>0.5\mathrm{GeV}$ and $|\eta |<2.5$, as a function of $p_{\mathrm{T}}^{\mathrm{lead}}$. The data are compared with PYTHIA ATLAS MC09, DW and Perugia0 tunes, HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET predictions. The top, middle and the bottom rows, respectively, show the transverse, toward and away regions defined by the leading charged particle. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty.

Figure 7

ATLAS data at 900 GeV (left plots) and at 7 TeV (right plots) corrected back to particle level, showing the mean $p_{\mathrm{T}}$ of the charged particles against the charged multiplicity, for charged particles with $p_{\mathrm{T}}>0.5\mathrm{GeV}$ and $|\eta |<2.5$. The data are compared with PYTHIA ATLAS MC09, DW and Perugia0 tunes, HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET predictions. The top, middle and the bottom rows, respectively, show the transverse, toward and away regions defined by the leading charged particle. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty.

Figure 8

ATLAS data at 900 GeV (top plots) and at 7 TeV (bottom plots) corrected back to the particle level, showing the $\varphi$ distribution of charged particle densities $d^2{N}_{\mathrm{ch}}/d\eta d\Delta \varphi$ with respect to the leading charged particle (at $\Delta \varphi =0$), for $p_{\mathrm{T}}>0.5\mathrm{GeV}$ and $|\eta |<2.5$. The leading charged particle is excluded. The data are compared to MC predictions by the PYTHIA ATLAS MC09, DW and Perugia0 tunes, the HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET. The distributions obtained by restricting the minimum leading charged particle $p_{\mathrm{T}}$ to different values are shown separately. The plots have been symmetrized by reflecting them about $\Delta \varphi =0$. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty corresponding to each $p_{\mathrm{T}}$ lower cut value.

Figure 9

ATLAS data at 900 GeV (top plots) and at 7 TeV (bottom plots) corrected back to the particle level, showing the $\varphi$ distribution of charged particle $p_{\mathrm{T}}$ densities $d^2{p}_{\mathrm{T}}/d\eta d\Delta \varphi$ with respect to the leading charged particle (at $\Delta \varphi =0$), for $p_{\mathrm{T}}>0.5\mathrm{GeV}$ and $|\eta |<2.5$. The leading charged particle is excluded. The data are compared to MC predictions by the PYTHIA ATLAS MC09, DW and Perugia0 tunes, the HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET. The distributions obtained by restricting the minimum leading charged particle $p_{\mathrm{T}}$ to different values are shown separately. The plots have been symmetrized by reflecting them about $\Delta \varphi =0$. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty corresponding to each $p_{\mathrm{T}}$ lower cut value.

Figure 10

ATLAS data at 900 GeV (left plots) and at 7 TeV (right plots) corrected back to particle level, showing the density of the charged particles $⟨d^2{N}_{\mathrm{ch}}/d\eta d\varphi ⟩$ with $p_{\mathrm{T}}>0.1\mathrm{GeV}$ and $|\eta |<2.5$, as a function of $p_{\mathrm{T}}^{\mathrm{lead}}$. The data are compared with PYTHIA ATLAS MC09, DW and Perugia0 tunes, HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET predictions. The top, middle and the bottom rows, respectively, show the transverse, toward and away regions defined by the leading charged particle. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty.

Figure 11

ATLAS data at 900 GeV (left) and at 7 TeV (right) corrected back to particle level, showing the scalar $\sum _{}^{}{p}_{\mathrm{T}}$ density of the charged particles $⟨d^2\sum _{}^{}{p}_{\mathrm{T}}/d\eta d\varphi ⟩$ with $p_{\mathrm{T}}>0.1\mathrm{GeV}$ and $|\eta |<2.5$, as a function of $p_{\mathrm{T}}^{\mathrm{lead}}$. The data are compared with PYTHIA ATLAS MC09, DW and Perugia0 tunes, HERWIG$+$JIMMY ATLAS MC09 tune and PHOJET predictions. The top, middle and the bottom rows, respectively, show the transverse, toward and away regions defined by the leading charged particle. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty.

Figure 12

ATLAS data at 7 TeV corrected back to the particle level, showing the density of the charged particles $⟨d^2{N}_{\mathrm{ch}}/d\eta d\varphi ⟩$ (left plot) and the scalar $\sum _{}^{}{p}_{\mathrm{T}}$ density of charged particles $⟨d^2\sum _{}^{}{p}_{\mathrm{T}}/d\eta d\varphi ⟩$ (right plot) with $p_{\mathrm{T}}>0.1\mathrm{GeV}$ and $|\eta |<2.5$, as a function of the leading charged particle $|\eta |$, for the transverse region plateau ($p_{\mathrm{T}}^{\mathrm{lead}}>5\mathrm{GeV}$), defined by the leading charged particle and compared with PYTHIA ATLAS MC09, DW and Perugia0 tunes, and HERWIG$+$JIMMY ATLAS MC09 tune, and PHOJET predictions. The error bars show the statistical uncertainty while the shaded areas show the combined statistical and systematic uncertainty.