This letter reports measurements which characterize the underlying event associated with hard scatterings at mid-pseudorapidity ($|\eta|<~0.8$) in pp, p$-$Pb and Pb$-$Pb collisions at centre-of-mass energy per nucleon pair, $\sqrt{s_{\rm NN}}=5.02$ TeV. The measurements are performed with ALICE at the LHC. The hard scatterings are identified by the leading particle defined as the charged particle with the largest transverse momentum ($p_{\rm T}$) in the collision and having $8 <~ p_{\rm T} <~ 15$ GeV/$c$. The $p_{\rm T}$ spectra of associated particles ($0.5 \leq p_{\rm T}<~6$ GeV/$c$) are measured in different azimuthal regions defined with respect to the leading particle direction: toward, transverse, and away. The associated charged particle yields in the transverse region are subtracted from those of the away and toward regions. The remaining jet-like yields are reported as a function of the multiplicity measured at forward rapidities. The measurements show a suppression of the jet-like yield in the away region and an enhancement of high-$p_{\rm T}$ associated particles in the toward region in central Pb$-$Pb collisions, as compared to minimum-bias pp collisions. These observations are consistent with previous measurements that used two-particle correlations, and with an interpretation in terms of parton energy loss in a high-density quark gluon plasma. These yield modifications vanish in peripheral Pb$-$Pb collisions and are not observed in either high-multiplicity pp or p$-$Pb collisions.
Accepted by: PLB
HEP Data
e-Print: arXiv:2204.10157 | PDF | inSPIRE
CERN-EP-2022-059
Figure group
Figure 1
Illustration of toward, away and transverse regions with respect to the leading particle in a collision. |  |
Figure 2
Top panel: \pt spectra of charged particles in the transverse region for different multiplicity classes in \pp (left), \pPb (middle) and \PbPb (right) collisions at $\sqrt{s_{\rm NN}}=5.02$\,\TeV. Lower panel: Ratio of \pt spectra in different multiplicity classes to \pt spectrum in 0-100\% multiplicity class for the corresponding collision systems. The statistical and systematic uncertainties are shown by bars and boxes, respectively. |  |
Figure 3
\pt spectra of charged particles in Toward-Transverse, ${\rm{d}}N_{\rm{ch}}^{\rm{st}}/{\rm{d}}p_{\rm T}$ (top plot) and Away-Transverse, ${\rm{d}}N_{\rm{ch}}^{\rm{sa}}/{\rm{d}}p_{\rm T}$ (bottom plot) regions for different multiplicity classes in \pp (left), \pPb (middle) and \PbPb (right) collisions at $\sqrt{s_{\rm NN}}=5.02$\,\TeV. The lower panels of both plots show the ratio to minimum bias \pp collisions. The statistical and systematic uncertainties are shown by bars and boxes, respectively. |   |
Figure 4
The $I_{X}^{\rm t}$ (left) and $I_{X}^{\rm a}$ (right) as a function of $\langle N_{\rm ch}^{\rm T} \rangle$ in 4 $< p_{\rm T} < $ 6 \GeVc for different multiplicity classes in \pp, \pPb and \PbPb collisions at \snn $=$ 5.02\,\TeV. \PbPb results are shown assuming a flat background (filled markers), and assuming a $v_{2}$-modulated background (empty markers). The statistical and systematic uncertainties are shown by bars and boxes, respectively. |  |
Figure 5
Comparison of the measured the $I_{X}^{\rm t}$ (left) and $I_{X}^{\rm a}$ (right) in 4 $< p_{\rm T} < $ 6 \GeVc with model predictions. The results are shown as a function of $\langle N_{\rm ch}^{\rm T} \rangle$ for different multiplicity classes in \pp (top panel), \pPb (middle panel) and \PbPb (bottom panel) collisions at \snn $=$ 5.02\,\TeV. The red and magenta lines show the \textsc{PYTHIA} 8 (Monash) and \textsc{PYTHIA} 8/Angantyr predictions, respectively. The blue lines show the EPOS-LHC results. The statistical and systematic uncertainties are shown by bars and boxes, respectively. |  |