Measurements of charged-particle production in pp, p$-$Pb, and Pb$-$Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum ($p_{\mathrm{T}}^{\rm trig}$) in the range $8<~p_{\mathrm{T}}^{\rm trig}<~15$ GeV$/c$. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, $R_{\mathrm{T}}=N_{\mathrm{ch}}^{\mathrm{T}}/\langle N_{\mathrm{ch}}^{\mathrm{T}}\rangle$, is used to group events according to their UE activity, where $N_{\mathrm{ch}}^{\mathrm{T}}$ is the charged-particle multiplicity per event in the transverse region and $\langle N_{\mathrm{ch}}^{\mathrm{T}}\rangle$ is the mean value over the whole analysed sample. The energy dependence of the $R_{\mathrm{T}}$ distributions in pp collisions at $\sqrt{s}=2.76$, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle $p_{\rm T}$ spectra as a function of $R_{\mathrm{T}}$ in the three azimuthal regions in pp, p$-$Pb, and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p$-$Pb).
Submitted to: JHEP
e-Print: arXiv:2310.07490 | PDF | inSPIRE
CERN-EP-2023-212
Figure group
Figure 1
Schematic representation of the toward, transverse, and away regions in the azimuthal plane with respect to the leading particle, i.e.\,the particle with the highest \pt in the event. Figure taken from Ref. . |  |
Figure 3
Top: \rt distributions in \pp collisions for different centre-of-mass energies $\s=2.76$, 5.02, 7, and 13\,TeV. Bottom: \rt distributions normalised to that for \pp collisions at $\s=7$\,TeV. The ratio is calculated using a linear interpolation between adjacent points. The boxes and bars represent the systematic and statistical uncertainties, respectively. The pp sample at $\sqrt{s}=13$\,TeV is smaller than that used for pp collisions at $\sqrt{s}=7$\,TeV. |  |
Figure 4
Top panel: charged-particle transverse momentum spectra as a function of \rt for different topological regions in \pp collisions at $\sqrt{s}=5.02$\,TeV. Data are compared with \py and \ep predictions. Middle panel: the ratio of the \pt spectra in different \rt intervals to the \rt-integrated ones. The boxes and bars represent the systematic and statistical uncertainties, respectively. Bottom panel: the ratio of the \pt spectra for each \rt interval to the corresponding PYTHIA8 and EPOS-LHC predictions. The shaded area represents the sum in quadrature of the systematic and statistical uncertainties. |  |
Figure 5
Top panel: charged-particle transverse momentum spectra as a function of \rt for different topological regions in \pPb collisions at $\sqrt{s_{\rm NN}}=5.02$\,TeV. Data are compared with \py and \ep predictions. Middle panel: the ratio of the \pt spectra in different \rt intervals to the \rt-integrated ones. The boxes and bars represent the systematic and statistical uncertainties, respectively. Bottom panel: the ratio of the \pt spectra for each \rt interval to the corresponding PYTHIA8 Angantyr and EPOS-LHC predictions. The shaded area represents the sum in quadrature of the systematic and statistical uncertainties. |  |
Figure 6
Top panel: charged-particle transverse momentum spectra as a function of \rt for different topological regions in \pPb collisions at $\sqrt{s_{\rm NN}}=5.02$\,TeV. Data are compared with \py Angantyr and \ep predictions. Middle panel: the ratio of the \pt spectra in different \rt intervals to the \rt-integrated ones. The boxes and bars represent the systematic and statistical uncertainties, respectively. Bottom panel: the ratio of the \pt spectra for each \rt interval to the corresponding PYTHIA8 Angantyr and EPOS-LHC predictions. The shaded area represents the sum in quadrature of the systematic and statistical uncertainties. |  |
Figure 8
Top: \mpt of charged particles in \pp (left), \pPb (middle), and \PbPb (right) collisions at $\snn=5.02$\,TeV as a function of \rt for different topological regions compared with predictions from \ep and \py. Bottom: ratio of MC to data. The band around unity in the ratio depicts the experimental uncertainties. |  |
Figure 9
Top: integrated yield of charged particles in \pp (left), \pPb (middle), and \PbPb (right) collisions at $\snn=5.02$\,TeV as a function of \rt for toward and away regions compared with predictions from \ep and \py. Bottom: ratio of model predictions to data. The band around unity in the ratio depicts the experimental uncertainties. |  |