Study of charged particle production at high $p_{\rm T}$ using event topology in pp, p$-$Pb and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

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.

 

Submitted to: PLB
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 $

Figure 5

colour online) Comparison of the measured the It X (left) and Ia X (right) in 4 < pT < 6 GeV/c with model predictions. The results are shown as a function of 〈NTch〉 for different multiplicity classes in pp (top panel), p–Pb (middle panel) and Pb–Pb (bottom panel) collisions at √sNN = 5.02 TeV. The red and magenta lines show the PYTHIA 8 (Monash) [28] and PYTHIA 8/Angantyr [28] predictions, respectively. The blue lines show the EPOS-LHC [21] results. The statistical and systematic uncertainties are shown by bars and boxes, respectively.