Underlying event characteristics and their dependence on jet size of charged-particle jet events in $pp$ collisions at $\sqrt{\mathbf{(}}s\mathbf{)}\mathbf{=}7\mathrm{TeV}$ with the ATLAS detector

Distributions sensitive to the underlying event are studied in events containing one or more charged-particle jets produced in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ with the ATLAS detector at the Large Hadron Collider (LHC). These measurements reflect $800\mu {\mathrm{b}}^{-1}$ of data taken during 2010. Jets are reconstructed using the anti-$k_t$ algorithm with radius parameter $R$ varying between 0.2 and 1.0. Distributions of the charged-particle multiplicity, the scalar sum of the transverse momentum of charged particles, and the average charged-particle $p_{\mathrm{T}}$ are measured as functions of $p_{\mathrm{T}}^{\mathrm{jet}}$ in regions transverse to and opposite the leading jet for $4\mathrm{GeV}<p_{\mathrm{T}}^{\mathrm{jet}}<100\mathrm{GeV}$. In addition, the $R$ dependence of the mean values of these observables is studied. In the transverse region, both the multiplicity and the scalar sum of the transverse momentum at fixed $p_{\mathrm{T}}^{\mathrm{jet}}$ vary significantly with $R$, while the average charged-particle transverse momentum has a minimal dependence on $R$. Predictions from several Monte Carlo tunes have been compared to the data; the predictions from Pythia 6, based on tunes that have been determined using LHC data, show reasonable agreement with the data, including the dependence on $R$. Comparisons with other generators indicate that additional tuning of soft-QCD parameters is necessary for these generators. The measurements presented here provide a testing ground for further development of the Monte Carlo models.

Figure 1

Definition of the transverse and away regions with respect to the leading jet.

Figure 2

The mean value, per unit $\eta \mathrm{\text{−}}\varphi$, of $\Sigma p_{\mathrm{T}}$ in the transverse region, as a function of $p_{\mathrm{T}}^{\mathrm{jet}}$ for (a) $R=0.2$, (b) $R=0.4$, (c) $R=0.6$, (d) $R=0.8$, and (e) $R=1.0$. The shaded band shows the combined statistical and systematic uncertainty on the data. The data are compared to the predictions obtained with Pythia 6 using the AUET2B and Z1 tunes. The bottom insert shows the ratio of MC predictions to data for several recent MC tunes.

Figure 3

The mean value, per unit $\eta \mathrm{\text{−}}\varphi$, of $N_{\mathrm{ch}}$ in the transverse region, as a function of $p_{\mathrm{T}}^{\mathrm{jet}}$ for (a) $R=0.2$, (b) $R=0.4$, (c) $R=0.6$, (d) $R=0.8$, and (e) $R=1.0$. The shaded band shows the combined statistical and systematic uncertainty on the data. The data are compared to the predictions obtained with Pythia 6 using the AUET2B and Z1 tunes. The bottom insert shows the ratio of MC predictions to data for several recent MC tunes.

Figure 4

The mean value of ${\overline{p}}_{\mathrm{T}}$ in the transverse region, as a function of $p_{\mathrm{T}}^{\mathrm{jet}}$ for (a) $R=0.2$, (b) $R=0.4$, (c) $R=0.6$, (d) $R=0.8$, and (e) $R=1.0$. The shaded band shows the combined statistical and systematic uncertainty on the data. The data are compared to the predictions obtained with Pythia 6 using the AUET2B and Z1 tunes. The bottom insert shows the ratio of MC predictions to data for several recent MC tunes.

Figure 5

Distributions of ${\Sigma }_{p\mathrm{T}}$ in the transverse region for (a) $R=0.2$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (b) $R=0.2$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (c) $R=0.6$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (d) $R=0.6$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (e) $R=1.0$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, and (f) $R=1.0$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$. The shaded band shows the combined statistical and systematic uncertainty on the data. The histograms show the predictions of several MC models with the same legend as in (a) and also Figs. 2, 3, 4.

Figure 6

Distributions of $N_{\mathrm{ch}}$ in the transverse region for (a) $R=0.2$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (b) $R=0.2$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (c) $R=0.6$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (d) $R=0.6$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (e) $R=1.0$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, and (f) $R=1.0$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$. The shaded band shows the combined statistical and systematic uncertainty on the data. The histograms show the predictions of several MC models with the same legend as in (a) and also Figs. 2, 3, 4.

Figure 7

Distributions of ${\overline{p}}_{\mathrm{T}}$ in the transverse region for (a) $R=0.2$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (b) $R=0.2$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (c) $R=0.6$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (d) $R=0.6$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (e) $R=1.0$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, and (f) $R=1.0$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$. The shaded band shows the combined statistical and systematic uncertainty on the data. The histograms show the predictions of several MC models with the same legend as in (a) and also Figs. 2, 3, 4.

Figure 8

The mean value of (a) ${\Sigma }_{p\mathrm{T}}$ (per unit $\eta \mathrm{\text{−}}\varphi$), (c) $N_{\mathrm{ch}}$ (per unit $\eta -\varphi$), and (e) ${\overline{p}}_{\mathrm{T}}$ in the transverse region as a function of $p_{\mathrm{T}}^{\mathrm{jet}}$ for $R=0.2$, $R=0.6$, and $R=1.0$; because the systematic uncertainties are correlated among the different $R$ values, only statistical uncertainties are shown, and the ratio of the mean value of (b) ${\Sigma }_{p\mathrm{T}}$, (d) $N_{\mathrm{ch}}$, and (f) ${\overline{p}}_{\mathrm{T}}$ in the transverse region measured for $R=0.2$ and $R=1.0$ to that measured for $R=0.6$. The shaded bands show the total uncertainty. The lines show the predictions of Pythia 6 with the Z1 tune; predictions for AUET2B show comparable agreement.

Figure 9

The mean value, per unit $\eta \mathrm{\text{−}}\varphi$, of $\Sigma p_{\mathrm{T}}$ in the away region, as a function of $p_{\mathrm{T}}^{\mathrm{jet}}$ (a) for $R=0.2$, (b) for $R=0.4$, (c) for $R=0.6$, (d) for $R=0.8$, and (e) for $R=1.0$. The shaded band shows the combined statistical and systematic uncertainty on the data. The data are compared to the predictions obtained with Pythia 6 using the AUET2B and Z1 tunes. The bottom insert shows the ratio of MC predictions to data for several recent MC tunes.

Figure 10

The mean value, per unit $\eta \mathrm{\text{−}}\varphi$, of $N_{\mathrm{ch}}$ in the away region, as a function of $p_{\mathrm{T}}^{\mathrm{jet}}$ (a) for $R=0.2$, (b) for $R=0.4$, (c) for $R=0.6$, (d) for $R=0.8$, and (e) for $R=1.0$. The shaded band shows the combined statistical and systematic uncertainty on the data. The data are compared to the predictions obtained with Pythia 6 using the AUET2B and Z1 tunes. The bottom insert shows the ratio of MC predictions to data for several recent MC tunes.

Figure 11

The mean value of ${\overline{p}}_{\mathrm{T}}$ in the away region, as a function of $p_{\mathrm{T}}^{\mathrm{jet}}$ (a) for $R=0.2$, (b) for $R=0.4$, (c) for $R=0.6$, (d) for $R=0.8$, and (e) for $R=1.0$. The shaded band shows the combined statistical and systematic uncertainty on the data. The data are compared to the predictions obtained with Pythia 6 using the AUET2B and Z1 tunes. The bottom insert shows the ratio of MC predictions to data for several recent MC tunes.

Figure 12

Distributions of $\Sigma p_{\mathrm{T}}$ in the away region for (a) $R=0.2$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (b) $R=0.2$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (c) $R=0.6$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (d) $R=0.6$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (e) $R=1.0$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, and (f) $R=1.0$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$. The shaded band shows the combined statistical and systematic uncertainty on the data. The histograms show the predictions of several MC models. The bottom insert shows the ratio of the MC predictions to the data.

Figure 13

Distributions of $N_{\mathrm{ch}}$ in the away region for (a) $R=0.2$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (b) $R=0.2$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (c) $R=0.6$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (d) $R=0.6$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (e) $R=1.0$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, and (f) $R=1.0$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$. The shaded band shows the combined statistical and systematic uncertainty on the data. The histograms show the predictions of several MC models. The bottom insert shows the ratio of the MC predictions to the data.

Figure 14

Distributions of ${\overline{p}}_{\mathrm{T}}$ in the away region for (a) $R=0.2$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (b) $R=0.2$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (c) $R=0.6$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, (d) $R=0.6$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$, (e) $R=1.0$ and $5\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<6\mathrm{GeV}$, and (f) $R=1.0$ and $31\mathrm{GeV}\le p_{\mathrm{T}}^{\mathrm{jet}}<50\mathrm{GeV}$. The shaded band shows the combined statistical and systematic uncertainty on the data. The histograms show the predictions of several MC models. The bottom insert shows the ratio of the MC predictions to the data.