Charged-particle production as a function of the relative transverse activity classifier in pp, p−-Pb, and Pb−-Pb collisions at the LHC

International audienceMeasurements 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)

Dielectron production in central Pb−-Pb collisions at sNN\sqrt{s_\mathrm{NN}} = 5.02 TeV

International audienceThe first measurement of the e$^+$e$^-$ pair production at midrapidity and low invariant mass in central Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV at the LHC is presented. The yield of e$^+$e$^-$ pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass ($m_{\rm ee}$) and pair transverse momentum ($p_{\rm T,ee}$) ranges $m_{\rm ee} < 3.5$ GeV$/c^2$ and $p_{\rm T,ee} < 8$ GeV$/c$. For $0.18 < m_{\rm ee} < 0.5$ GeV$/c^2$ the ratio of data to the cocktail of hadronic contributions without $\rho$ mesons amounts to $1.42 \pm 0.12 \ ({\rm stat.}) \pm 0.17 \ ({\rm syst.}) \pm 0.12 \ ({\rm cocktail})$ and $1.44 \pm 0.12 \ ({\rm stat.}) \pm 0.17 \ ({\rm syst.}) ^{+0.17}_{-0.21} \ ({\rm cocktail})$, including or not including medium effects in the estimation of the heavy-flavor background, respectively. It is consistent with predictions from two different models for an additional contribution of thermal e$^+$e$^-$ pairs from the hadronic and partonic phases. In the intermediate-mass range ($1.2 < m_{\rm ee} < 2.6$ GeV$/c^2$), the pair transverse impact parameter of the e$^+$e$^-$ pairs (DCA$_{\rm ee}$) is used for the first time in Pb$-$Pb collisions to separate displaced dielectrons from heavy-flavor hadron decays from a possible (thermal) contribution produced at the interaction point. The data are consistent with a suppression of e$^+$e$^-$ pairs from ${\rm c\overline{c}}$ and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is reported via the study of virtual direct photons in the transverse momentum range $1 < p_{\rm T} < 5$ GeV$/c$. A model including prompt photons, as well as photons from the pre-equilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties

Dielectron production in central Pb−-Pb collisions at sNN\sqrt{s_\mathrm{NN}} = 5.02 TeV

International audienceThe first measurement of the e$^+$e$^-$ pair production at midrapidity and low invariant mass in central Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV at the LHC is presented. The yield of e$^+$e$^-$ pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass ($m_{\rm ee}$) and pair transverse momentum ($p_{\rm T,ee}$) ranges $m_{\rm ee} < 3.5$ GeV$/c^2$ and $p_{\rm T,ee} < 8$ GeV$/c$. For $0.18 < m_{\rm ee} < 0.5$ GeV$/c^2$ the ratio of data to the cocktail of hadronic contributions without $\rho$ mesons amounts to $1.42 \pm 0.12 \ ({\rm stat.}) \pm 0.17 \ ({\rm syst.}) \pm 0.12 \ ({\rm cocktail})$ and $1.44 \pm 0.12 \ ({\rm stat.}) \pm 0.17 \ ({\rm syst.}) ^{+0.17}_{-0.21} \ ({\rm cocktail})$, including or not including medium effects in the estimation of the heavy-flavor background, respectively. It is consistent with predictions from two different models for an additional contribution of thermal e$^+$e$^-$ pairs from the hadronic and partonic phases. In the intermediate-mass range ($1.2 < m_{\rm ee} < 2.6$ GeV$/c^2$), the pair transverse impact parameter of the e$^+$e$^-$ pairs (DCA$_{\rm ee}$) is used for the first time in Pb$-$Pb collisions to separate displaced dielectrons from heavy-flavor hadron decays from a possible (thermal) contribution produced at the interaction point. The data are consistent with a suppression of e$^+$e$^-$ pairs from ${\rm c\overline{c}}$ and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is reported via the study of virtual direct photons in the transverse momentum range $1 < p_{\rm T} < 5$ GeV$/c$. A model including prompt photons, as well as photons from the pre-equilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties

Multiplicity and event-scale dependent flow and jet fragmentation in pp collisions at s\sqrt{s} = 13 TeV and in p-Pb collisions at sNN\sqrt{s_\mathrm{NN}} = 5.02 TeV

Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at ${\sqrt{{\textit s}}}=13$ TeV and p-Pb collisions at ${\sqrt{s_\mathrm{NN}}} = 5.02$ TeV. The correlation functions are measured as a function of relative azimuthal angle $\Delta\varphi$ and pseudorapidity separation $\Delta\eta$ for pairs of primary charged particles within the pseudorapidity interval $|\eta| < 0.9$ and the transverse-momentum interval $1 < p_{\rm T} <4$ GeV/$c$. Flow coefficients are extracted for the long-range correlations ($1.6 < |\Delta\eta| <1.8$) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at $\sqrt{s}=13$ TeV and p$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV. The correlation functions are measured as a function of relative azimuthal angle $\Delta\varphi$ and pseudorapidity separation $\Delta\eta$ for pairs of primary charged particles within the pseudorapidity interval $|\eta| < 0.9$ and the transverse-momentum interval $1 < p_{\rm T} < 4$ GeV/$c$. Flow coefficients are extracted for the long-range correlations ($1.6 < |\Delta\eta| <1.8$) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events

Probing the Chiral Magnetic Wave with charge-dependent flow measurements in Pb-Pb collisions at the LHC

International audienceThe Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic ($v_{2}$) and triangular ($v_{3}$) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope $r_{3}^{\rm Norm}$ is found to be larger than zero and to have a magnitude similar to $r_{2}^{\rm Norm}$, thus pointing to a large background contribution for these measurements. Furthermore, $r_{2}^{\rm Norm}$ can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW ($f_{\rm CMW}$) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for $f_{\rm CMW}$, and in the 10-60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level

Charged-particle production as a function of the relative transverse activity classifier in pp, p−-Pb, and Pb−-Pb collisions at the LHC

International audienceMeasurements 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)

Light-flavor particle production in high-multiplicity pp collisions at s\sqrt{s} = 13 TeV as a function of transverse spherocity

Results on the transverse spherocity dependence of light-flavor particle production ($\pi$, K, p, $\phi$, ${\rm K^{*0}}$, ${\rm K}^{0}_{\rm{S}}$, $\Lambda$, $\Xi$) at midrapidity in high-multiplicity pp collisions at $\sqrt{s}$ = 13 TeV were obtained with the ALICE apparatus. The transverse spherocity estimator ($S_{\text{O}}^{p_{\rm T}=1}$) categorizes events by their azimuthal topology. Utilizing narrow selections on $S_{\text{O}}^{p_{\rm T}=1}$, it is possible to contrast particle production in collisions dominated by many soft initial interactions with that observed in collisions dominated by one or more hard scatterings. Results are reported for two multiplicity estimators covering different pseudorapidity regions. The $S_{\text{O}}^{p_{\rm T}=1}$ estimator is found to effectively constrain the hardness of the events when the midrapidity ($\left | \eta \right |< 0.8$) estimator is used. The production rates of strange particles are found to be slightly higher for soft isotropic topologies, and severely suppressed in hard jet-like topologies. These effects are more pronounced for hadrons with larger mass and strangeness content, and observed when the topological selection is done within a narrow multiplicity interval. This demonstrates that an important aspect of the universal scaling of strangeness enhancement with final-state multiplicity is that high-multiplicity collisions are dominated by soft, isotropic processes. On the contrary, strangeness production in events with jet-like processes is significantly reduced. The results presented in this article are compared with several QCD-inspired Monte Carlo event generators. Models that incorporate a two-component phenomenology, either through mechanisms accounting for string density, or thermal production, are able to describe the observed strangeness enhancement as a function of $S_{\text{O}}^{p_{\rm T}=1}$.Results on the transverse spherocity dependence of light-flavor particle production ($\pi$, K, p, $\phi$, ${\rm K^{*0}}$, ${\rm K}^{0}_{\rm{S}}$, $\Lambda$, $\Xi$) at midrapidity in high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV were obtained with the ALICE apparatus. The transverse spherocity estimator ($S_{{\rm O}}^{{\it p}_{\rm T}=1}$) categorizes events by their azimuthal topology. Utilizing narrow selections on $S_{\text{O}}^{{\it p}_{\rm T}=1}$, it is possible to contrast particle production in collisions dominated by many soft initial interactions with that observed in collisions dominated by one or more hard scatterings. Results are reported for two multiplicity estimators covering different pseudorapidity regions. The $S_{{\rm O}}^{{\it p}_{\rm T}=1}$ estimator is found to effectively constrain the hardness of the events when the midrapidity ($\left | \eta \right |< 0.8$) estimator is used. The production rates of strange particles are found to be slightly higher for soft isotropic topologies, and severely suppressed in hard jet-like topologies. These effects are more pronounced for hadrons with larger mass and strangeness content, and observed when the topological selection is done within a narrow multiplicity interval. This demonstrates that an important aspect of the universal scaling of strangeness enhancement with final-state multiplicity is that high-multiplicity collisions are dominated by soft, isotropic processes. On the contrary, strangeness production in events with jet-like processes is significantly reduced. The results presented in this article are compared with several QCD-inspired Monte Carlo event generators. Models that incorporate a two-component phenomenology, either through mechanisms accounting for string density, or thermal production, are able to describe the observed strangeness enhancement as a function of $S_{{\rm O}}^{{\it p}_{\rm T}=1}$

Measurements of long-range two-particle correlation over a wide pseudorapidity range in p–Pb collisions at √sNN = 5.02 TeV

Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the "ridge" phenomenon, were discovered in heavy-ion collisions, and later found in pp and p−Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small-collision systems. In this Letter, measurements of the long-range correlations in p−Pb collisions at sNN−−−√=5.02 TeV are extended to a pseudorapidity gap of Δη∼8 between particles using the ALICE, forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of Δη∼8 for the first time in p−Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small-collision systems such as p−Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v2({\eta}), is extracted from the long-range correlations. The v2(η) results are presented for a wide pseudorapidity range of −3.1<η<4.8 in various centrality classes in p−Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small-collision systems, the v2(η) measurements are compared to hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small-collision systems

Prompt and non-prompt J/ψ/\psi production at midrapidity in Pb−-Pb collisions at sNN\sqrt{s_{\mathrm{NN}}} = 5.02 TeV

International audienceThe transverse momentum ($p_{\rm T}$) and centrality dependence of the nuclear modification factor $R_{\rm AA}$ of prompt and non-prompt J$/\psi$, the latter originating from the weak decays of beauty hadrons, have been measured by the ALICE collaboration in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. The measurements are carried out through the ${\rm e}^{+}{\rm e}^{-}$ decay channel at midrapidity ($|y|$ 5 GeV/$c$, which becomes stronger with increasing collision centrality. The results are consistent with similar LHC measurements in the overlapping $p_{\rm T}$ intervals, and cover the kinematic region down to $p_{\rm T}$ = 1.5 GeV/$c$ at midrapidity, not accessible by other LHC experiments. The suppression of prompt J$/\psi$ in central and semicentral collisions exhibits a decreasing trend towards lower transverse momentum, described within uncertainties by models implementing J$/\psi$ production from recombination of c and $\overline{\rm c}$ quarks produced independently in different partonic scatterings. At high transverse momentum, transport models including quarkonium dissociation are able to describe the suppression for prompt J$/\psi$. For non-prompt J$/\psi$, the suppression predicted by models including both collisional and radiative processes for the computation of the beauty-quark energy loss inside the quark$-$gluon plasma is consistent with measurements within uncertainties

Observation of medium-induced yield enhancement and acoplanarity broadening of low-pTp_\mathrm{T} jets from measurements in pp and central Pb−-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}}=5.02 TeV

International audienceThe ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high $p_{\rm T}$) hadron trigger in proton$-$proton and central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb$-$Pb collisions. Recoil jet distributions are reported for jet resolution parameter $R=0.2$, 0.4, and 0.5 in the range $7 < p_{\rm T,jet} < 140$ GeV$/c$ and trigger$-$recoil jet azimuthal separation $\pi/2 < \Delta\varphi < \pi$. The measurements exhibit a marked medium-induced jet yield enhancement at low $p_{\rm T}$ and at large azimuthal deviation from $\Delta\varphi\sim\pi$. The enhancement is characterized by its dependence on $\Delta\varphi$, which has a slope that differs from zero by 4.7$\sigma$. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation