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Abstract Quantum Chromodynamics predicts a phase transition from hadronic matter to quark–gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are no longer confined within hadrons. The QGP forms in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the azimuthal expansion of produced matter, probe QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum – a feature associated with flow imparted at the quark level, confirming QGP existence. The observation of QGP-like features in proton–proton and proton–ion collisions has sparked debate about QGP formation in smaller systems. For the first time, we demonstrate the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton–lead and proton–proton collisions at the Large Hadron Collider (LHC). These results are described by a model including hydrodynamic flow followed by hadron formation via quark coalescence, consistent with the formation of partonic flowing systems in these collisions. 

A<sc>bstract</sc> The measurement of three-dimensional femtoscopic correlations between identical charged kaons (K^±K^±) produced in p–Pb collisions at center-of-mass energy per nucleon pair$$\sqrt{{s}_{\text{NN}}}=5.02$$TeV with ALICE at the LHC is presented for the first time. This measurement, supplementary to those in pp and Pb–Pb collisions, allows understanding the particle-production mechanisms at different charged-particle multiplicities and provides information on the dynamics of the source of particles created in p–Pb collisions, for which a general consensus does not yet exist. It is shown that the measured source sizes increase with charged-particle multiplicity and decrease with increasing pair transverse momentum. These trends for K^±K^±are similar to the ones observed earlier in identical charged-pion and$${\text{K}}_{\text{s}}^{0}{\text{K}}_{\text{s}}^{0}$$correlations in Pb–Pb collisions at various energies and inπ^±π^±correlations in p–Pb collisions at$$\sqrt{{s}_{\text{NN}}}=5.02$$TeV. At comparable multiplicity, the source sizes measured in p–Pb collisions agree within uncertainties with those observed in pp collisions, and there is an indication that they are smaller than those observed in Pb–Pb collisions. The obtained results are also compared with predictions from the hadronic interaction model EPOS 3, which tends to underestimate the source size for the most central collisions and agrees with the data for semicentral and peripheral events. Furthermore, the time of maximal emission for kaons is extracted. It turns out to be comparable with the value obtained in highly peripheral Pb–Pb collisions at the same energy, indicating that the kaon emission evolution is similar to that in p–Pb collisions. 

This paper presents the first measurement of the angle between different jet axes (denoted as $\mathrm{\Delta }{R}_{\mathrm{axis}}$) in $\text{Pb-Pb}$collisions. The measurement is carried out in the 0–10 % most-central events at $\sqrt{s_{NN}}=5.02$TeV. Jets are assembled by clustering charged particles at midrapidity using the $\text{anti-}{k}_{\mathrm{T}}$algorithm with resolution parameters $R=0.2$and 0.4 and transverse momenta in the intervals $40<p_{\mathrm{T}}^{\mathrm{ch}\mathrm{jet}}<140\mathrm{GeV}/c$and $80<p_{\mathrm{T}}^{\mathrm{ch}\mathrm{jet}}<140\mathrm{GeV}/c$, respectively. Measurements at these low transverse momenta enhance the sensitivity to quark-gluon plasma (QGP) effects. A comparison to models implementing various mechanisms of jet energy loss in the QGP suggests that the observed narrowing of the $\text{Pb-Pb}$distribution relative to $\mathit{pp}$can be explained if quark-initiated jets are more likely to emerge from the medium than gluon-initiated jets. These new measurements disfavor intrajet $p_{\mathrm{T}}$broadening as implemented in a simple model calculation with the Baier-Dokshitzer-Mueller-Peigne-Schiff formalism for energy loss in the QGP. The comparison of $\text{Pb-Pb}$and $\mathit{pp}$collisions shows sensitivity to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss. 

The first measurement of the $e^{+}{e}^{-}$pair production at midrapidity and low invariant mass in central Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$at the Large Hadron Collider is presented. The yield of $e^{+}{e}^{-}$pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass ( $m_{ee}$) and pair transverse momentum ( $p_{\mathrm{T},ee}$) ranges $m_{ee}<3.5\mathrm{GeV}/c^2$and $p_{\mathrm{T},ee}<8\mathrm{GeV}/c$. For $0.18<m_{ee}<0.5\mathrm{GeV}/c^2$the ratio of data to the cocktail of hadronic contributions amounts to $1.40\pm 0.11\left(\mathrm{stat}.\right)\pm 0.23\left(\mathrm{syst}.\right)\pm 0.16\left(\mathrm{cocktail}\right)$and $1.42\pm 0.11\left(\mathrm{stat}.\right)\pm 0.23{\left(\mathrm{syst}.\right)}_{-0.29}^{+0.24}\left(\mathrm{cocktail}\right)$, 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 ( $1.2<m_{ee}<2.6\mathrm{GeV}/c^2$), the pair transverse impact parameter of the $e^{+}{e}^{-}$pairs ( ${\mathrm{DCA}}_{ee}$, where “DCA” denotes “distance of closest approach”) 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 $c\overline{c}$and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$is reported via the study of virtual direct photons in the transverse momentum range $1<p_{\mathrm{T}}<5\mathrm{GeV}/c$. A model including prompt photons, as well as photons from the preequilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties. 

Abstract Event-by-event fluctuations of the event-wise mean transverse momentum,$$\langle p_{\textrm{T}}\rangle $$ $⟨p_{\text{T}}⟩$, of charged particles produced in proton–proton (pp) collisions at$$\sqrt{s}$$ $\sqrt{s}$= 5.02 TeV, Xe–Xe collisions at$$\sqrt{s_{\textrm{NN}}}$$ $\sqrt{s_{\text{NN}}}$= 5.44 TeV, and Pb–Pb collisions at$$\sqrt{s_{\textrm{NN}}}$$ $\sqrt{s_{\text{NN}}}$= 5.02 TeV are studied using the ALICE detector based on the integral correlator$$\langle \!\langle \Delta p_\textrm{T}\Delta p_\textrm{T}\rangle \!\rangle $$ $⟨⟨\Delta p_{\text{T}}\Delta p_{\text{T}}⟩⟩$. The correlator strength is found to decrease monotonically with increasing produced charged-particle multiplicity measured at midrapidity in all three systems. In Xe–Xe and Pb–Pb collisions, the multiplicity dependence of the correlator deviates significantly from a simple power-law scaling as well as from the predictions of the HIJING and AMPT models. The observed deviation from power-law scaling is expected from transverse radial flow in semicentral to central Xe–Xe and Pb–Pb collisions. In pp collisions, the correlation strength is also studied by classifying the events based on the transverse spherocity,$$S_0$$ $S_0$, of the particle production at midrapidity, used as a proxy for the presence of a pronounced back-to-back jet topology. Low-spherocity (jetty) events feature a larger correlation strength than those with high spherocity (isotropic). The strength and multiplicity dependence of jetty and isotropic events are well reproduced by calculations with the PYTHIA 8 and EPOS LHC models. 

The ALICE Collaboration reports measurements of the large relative transverse momentum ( $k_T$) component of jet substructure in $pp$and Pb-Pb collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Enhancement in the yield of such large- $k_{\mathrm{T}}$emissions in head-on Pb-Pb collisions is predicted to arise from partonic scattering with quasiparticles of the quark-gluon plasma. The analysis utilizes charged-particle jets reconstructed by the anti- $k_{\mathrm{T}}$algorithm with resolution parameter $R=0.2$in the transverse-momentum interval $60<p_{\mathrm{T},\mathrm{ch},\mathrm{jet}}<80\mathrm{GeV}/c$. The soft drop and dynamical grooming algorithms are used to identify high transverse momentum splittings in the jet shower. Comparison of measurements in Pb-Pb and $pp$collisions shows medium-induced narrowing, corresponding to yield suppression of high- $k_T$splittings, in contrast to the expectation of yield enhancement due to quasiparticle scattering. The measurements are compared to theoretical model calculations incorporating jet modification due to jet-medium interactions (“jet quenching”), both with and without quasiparticle scattering effects. These measurements provide new insight into the underlying mechanisms and theoretical modeling of jet quenching. 

A<sc>bstract</sc> We report on the measurement of inclusive, non-prompt, and prompt J/ψ-hadron correlations by the ALICE Collaboration at the CERN Large Hadron Collider in pp collisions at a center-of-mass energy of 13 TeV. The correlations are studied at midrapidity (|y| <0.9) in the transverse momentum rangesp_T<40 GeV/cfor the J/ψand 0.15< p_T<10 GeV/cand |η|<0.9 for the associated hadrons. The measurement is based on minimum bias and high multiplicity data samples corresponding to integrated luminosities ofL_int= 34 nb⁻¹andL_int= 6.9 pb⁻¹, respectively. In addition, two more data samples are employed, requiring, on top of the minimum bias condition, a threshold on the tower energy ofE= 4 and 9 GeV in the ALICE electromagnetic calorimeters, which correspond to integrated luminosities ofL_int= 0.9 pb⁻¹andL_int= 8.4 pb⁻¹, respectively. The azimuthally integrated near and away side yields of associated charged hadrons per J/ψtrigger are presented as a function of the J/ψand associated hadron transverse momentum. The measurements are discussed in comparison to PYTHIA calculations.