Underlying Event measurements in pp collisions at $\sqrt{s} =$ 0.9 and 7 TeV with the ALICE experiment at the LHC

We present measurements of Underlying Event observables in pp collisions at $\sqrt{s} =$ 0.9 and 7 TeV. The analysis is performed as a function of the highest charged-particle transverse momentum $p_{T,LT}$ in the event. Different regions are defined with respect to the azimuthal direction of the leading (highest transverse momentum) track: Toward, Transverse and Away. The Toward and Away regions collect the fragmentation products of the hardest partonic interaction. The Transverse region is expected to be most sensitive to the Underlying Event activity. The study is performed with charged particles above three different $\pt$ thresholds: 0.15, 0.5 and 1.0 GeV/$c$. In the Transverse region we observe an increase in the multiplicity of a factor 2-3 between the lower and higher collision energies, depending on the track $\pt$ threshold considered. Data are compared to Pythia 6.4, Pythia 8.1 and Phojet. On average, all models considered underestimate the multiplicity and summed $\pt$ in the Transverse region by about 10-30%.

Figures

Figure 1

Definition of the regions Toward, Transverse and Away w.r.t. leading track direction.
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Figure 2

Tracking efficiency vs. track $\pt$ (left, $|\eta|< 0.8$) and $\eta$ (right, $\pt > 0.5 \gev/c$) from a PYTHIA 6.4 and GEANT3 simulation.
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Figure 3

Contamination correction: correction factor $vs.$ track $\pt$ (left, $|\eta|< 0.8$) and $\eta$ (right, $\pt > 0.5 \gev/c$) from a PYTHIA 6.4 and GEANT3 simulation.
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Figure 4

Number density in Toward region at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Right and left vertical scales differ by a factor 2. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 5

Number density in Toward (top), Transverse (middle) and Away (bottom) regions at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Right and left vertical scales differ by a factor 2. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 6

Number density in Toward (top), Transverse (middle) and Away (bottom) regions at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Right and left vertical scales differ by a factor 2. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 7

Comparison of number density in the plateau of the Transverse region (see Table 8) and $dN_{\rm ch}/d\eta$ in minimum-bias events (scaled by $1/2\pi$) [30]. Both are for charged particles with $\pt > 0.5 \gev/c$. For this plot, statistical and systematic uncertainties have been summed in quadrature. The lines show fits with the functional form $a+b\ln{s}$.
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Figure 8

Ratio between $\sqrt{s}= 0.9$ TeV and $\sqrt{s} = 7$ TeV for number density (left) and summed $\pt$ (right) distributions in the Transverse region. Statistical uncertainties only.
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Figure 9

Summed $\pt$ in Toward (top), Transverse (middle) and Away (bottom) regions at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Right and left vertical scales differ by a factor 4 (2) in the top (middle and bottom) panel. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 10

Summed $\pt$ in Toward (top), Transverse (middle) and Away (bottom) regions at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Right and left vertical scales differ by a factor 4 (2) in the top (middle and bottom) panel. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 11

Summed $\pt$ in Toward (top), Transverse (middle) and Away (bottom) regions at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Right and left vertical scales differ by a factor 4 (3) in the top (middle and bottom) panel. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 12

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 0.5 $< p_{T,LT}< $ 1.5 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 13

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 2.0 $< p_{T,LT}< $ 4.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 14

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 4.0 $< p_{T,LT}< $ 6.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 15

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 6.0 $< p_{T,LT}< $ 10.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 16

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 0.5 $< p_{T,LT}< $ 1.5 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 17

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 2.0 $< p_{T,LT}< $ 4.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 18

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 4.0 $< p_{T,LT}< $ 6.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 19

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 6.0 $< p_{T,LT}< $ 10.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 20

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 2.0 $< p_{T,LT}< $ 4.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 21

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 4.0 $< p_{T,LT}< $ 6.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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Figure 22

Azimuthal correlation at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Leading-track: 6.0 $< p_{T,LT}< $ 10.0 GeV/$c$. For visualization purposes the $\Delta \phi$ axis is not centered around 0. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.
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