Underlying-event properties in pp and p$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

We report about the properties of the underlying event measured with ALICE at the LHC in pp and p$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. The event activity, quantified by charged-particle number and summed-$p_{\rm T}$ densities, is measured as a function of the leading-particle transverse momentum ($p_{\rm T}^{\rm trig}$). These quantities are studied in three azimuthal-angle regions relative to the leading particle in the event: toward, away, and transverse. Results are presented for three different $p_{\rm T}$ thresholds (0.15, 0.5, and 1 GeV/$c$) at mid-pseudorapidity ($|\eta|<~0.8$). The event activity in the transverse region, which is the most sensitive to the underlying event, exhibits similar behaviour in both pp and p$-$Pb collisions, namely, a steep increase with $p_{\rm T}^{\rm trig}$ for low $p_{\rm T}^{\rm trig}$, followed by a saturation at $p_{\rm T}^{\rm trig} \approx 5$ GeV/$c$. The results from pp collisions are compared with existing measurements at other centre-of-mass energies. The quantities in the toward and away regions are also analyzed after the subtraction of the contribution measured in the transverse region. The remaining jet-like particle densities are consistent in pp and p$-$Pb collisions for $p_{\rm T}^{\rm trig}>10$ GeV/$c$, whereas for lower $p_{\rm T}^{\rm trig}$ values the event activity is slightly higher in p$-$Pb than in pp collisions. The measurements are compared with predictions from the PYTHIA 8 and EPOS LHC Monte Carlo event generators.

 

Submitted to: JHEP
e-Print: arXiv:2204.10389 | PDF | inSPIRE
CERN-EP-2022-041
Figure group

Figure 1

Illustration of the toward, transverse, and away regions in the azimuthal plane with respect to theleading particle direction. The figure has been taken from Ref.~..

Figure 2

Charged-particle number density as a function of \ptt measured in the transverse region for \pp (left) and \pPb collisions (right) at $\sqrt{s_{\rm NN}}=5.02$\,\TeV. Measurements were performed considering three \pt thresholds for associated charged particles: $\pt >0.15$\,\GeVc, $\pt >0.5$\,\GeVc, and $\pt >1$\,\GeVc. Data are compared with \py and \ep predictions. The coloured boxes and the error bars represent the systematic and statistical uncertainties, respectively..

Figure 3

Charged-particle number density as a function of \ptt measured in \pp (left) and \pPb collisions (right) at $\sqrt{s_{\rm NN}}=5.02$\,\TeV. Measurements were performed considering associated charged particles with $\pt>0.5$\,\GeVc. Results for the toward, transverse, and away regions are displayed. The coloured boxes and the error bars represent the systematic and statistical uncertainties, respectively..

Figure 4

The charged-particle number (left) and summed-$p_{\rm T}$ (right) densities as a function of \ptt in \pp collisions at $\sqrt{s}=5.02$\,\TeV are displayed. Results for the transverse (top), away (middle), and toward (bottom) regions were obtained for the transverse momentum threshold $\pt>0.5$\,\GeV/$c$. The shaded area and the error bars around the data points represent the systematic and statistical uncertainties, respectively. Data are compared with \py/Monash (solid line) and \ep (dashed line) predictions. The data-to-model ratios are displayed in the bottom panel of each plot. The boxes around unity represent the statistical and systematic uncertainties added in quadrature. .

Figure 5

The charged-particle number (left) and summed-$p_{\rm T}$ (right) densities as a function of \ptt in \pPb collisions at $\sqrt{s_{\rm NN}}=5.02$\,\TeV are displayed. Results for the transverse (top), away (middle), and toward (bottom) regions were obtained for the transverse momentum threshold $\pt>0.5$\,\GeV/$c$. The shaded area and the error bars around the data points represent the systematic and statistical uncertainties, respectively. Data are compared with \py/Angantyr (solid line) and \ep (dashed line) predictions. The data-to-model ratios are displayed in the bottom panel of each plot. The boxes around unity represent the statistical and systematic uncertainties added in quadrature..

Figure 6

Upper panels: charged-particle number (left) and summed-$p_{\rm T}$ (right) densities as a function of \ptt in \pp (blue) and \pPb (red) collisions at $\sqrt{s_{\rm NN}}=5.02$\,\TeV. Results for data and comparison with models \py (green) and \ep (red) predictions for the away (upper) and toward (bottom) regions, after the subtraction of the charged-particle number (left) and summed-$p_{\rm T}$ (right) densities in the transverse region, are shown. Bottom panels: charged-particle number and summed-$p_{\rm T}$ densities measured in \pp collisions divided by those in \pPb collisions are displayed for both data and models..