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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. 

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. 

This Letter presents measurements of long-range transverse-momentum correlations using a new observable, $v_0\left(p_{\mathrm{T}}\right)$, serving as a probe of event-by-event radial-flow fluctuations, the underlying radial expansion, and the medium’s properties in heavy-ion collisions. Results are reported for inclusive charged particles, pions, kaons, and protons across various centrality intervals in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$, recorded by the ALICE detector. A pseudorapidity-gap technique, similar to that used in anisotropic-flow studies, is employed to suppress short-range correlations. At low $p_{\mathrm{T}}$, a characteristic mass ordering consistent with hydrodynamic collective flow is observed. At higher $p_{\mathrm{T}}$( $>3\mathrm{GeV}/c$), protons exhibit larger $v_0\left(p_{\mathrm{T}}\right)$than pions and kaons, in agreement with expectations from quark-recombination models. Comparisons to viscous hydrodynamic calculations with varying bulk viscosity and equation of state demonstrate the sensitivity of the $v_0\left(p_{\mathrm{T}}\right)$observable to these key medium properties. The findings establish $v_0\left(p_{\mathrm{T}}\right)$as a valuable addition to the set of observables used in Bayesian analyses for extracting the transport properties and constraining the equation of state of strongly interacting matter, while also helping to systematically explore its sensitivity and impact within such global studies. 

Abstract High-energy hadronic collisions generate environments characterized by temperatures above 100 MeV (refs. ^1,2), about 100,000 times hotter than the centre of the Sun. At present, it is therefore unclear how light (anti)nuclei with mass numberAof a few units, such as the deuteron,³He or⁴He, each bound by only a few MeV, can emerge from these collisions^3,4. Here, the ALICE Collaboration reports that deuteron–pion momentum correlations in proton–proton (pp) collisions provide model-independent evidence that about 90% of the observed (anti)deuterons are produced in nuclear reactions⁵following the decay of short-lived resonances, such as the Δ(1232). These findings, obtained by the ALICE Collaboration at the Large Hadron Collider, resolve a gap in our understanding of nucleosynthesis in ultrarelativistic hadronic collisions. Apart from offering insights on how (anti)nuclei are formed in hadronic collisions, the results can be used in the modelling of the production of light and heavy nuclei in cosmic rays⁶and dark-matter decays^7,8. 

Abstract This paper presents the measurement of the isolated prompt photon inclusive production cross section in pp and p–Pb collisions by the ALICE Collaboration at the LHC. The measurement is performed in p–Pb collisions at centre-of-mass energies per nucleon pair of$$\sqrt{s_{\text {NN}}}={5.02}\,\textrm{TeV}$$ $\sqrt{s_{\text{NN}}}=5.02\text{TeV}$and 8.16 TeV, as well as in pp collisions at$$\sqrt{s}={5.02}\,\textrm{TeV}$$ $\sqrt{s}=5.02\text{TeV}$and 8 TeV. The cross section is obtained at midrapidity$$(|y|<0.7)$$ $\left(\right|y|<0.7)$using a charged-track based isolation momentum$$p_\textrm{T}^{\text {iso,~ch}}<{1.5}\,\textrm{GeV}/c$$ $p_{\text{T}}^{\text{iso, ch}}<1.5\text{GeV}/c$in a cone with radius$$R=0.4$$ $R=0.4$. The data for both collision systems are well reproduced by perturbative QCD (pQCD) calculations at next-to-leading order (NLO) using recent parton distribution functions for free (PDF) and bound (nPDF) nucleons. Furthermore, the nuclear modification factor$$R_{\text {pA}}$$ $R_{\text{pA}}$for both collision energies is consistent with unity for$$p_{\textrm{T}}$$ $p_{\text{T}}$$$>{20}\,\textrm{GeV}/c$$ $>20\text{GeV}/c$. However, deviations from unity ($$R_\textrm{pA}<1$$ $R_{\text{pA}}<1$) of up to 20% are observed for$$p_{\textrm{T}}$$ $p_{\text{T}}$$$<{20}\,\textrm{GeV}/c$$ $<20\text{GeV}/c$with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision. The suppression increases with decreasing$$p_{\textrm{T}}$$ $p_{\text{T}}$with a significance of$$2.3\upsigma $$ $2.3\sigma$for a non-zero slope and yields$$R_{\textrm{pA}}<1$$ $R_{\text{pA}}<1$with a significance of$$1.8\upsigma $$ $1.8\sigma$at$$\sqrt{s_{\textrm{NN}}}={8.16}\,\textrm{TeV}$$ $\sqrt{s_{\text{NN}}}=8.16\text{TeV}$for$$p_{\textrm{T}}$$ $p_{\text{T}}$$$<{20}\,\textrm{GeV}/c$$ $<20\text{GeV}/c$. In addition, a significance of$$1.1\upsigma $$ $1.1\sigma$is observed for$$R_{\textrm{pA}}<1$$ $R_{\text{pA}}<1$at the lower collision energy$$\sqrt{s_{\textrm{NN}}}={5.02}\,\textrm{TeV}$$ $\sqrt{s_{\text{NN}}}=5.02\text{TeV}$for$$p_{\textrm{T}} < {14}\,\textrm{GeV}/c$$ $p_{\text{T}}<14\text{GeV}/c$. The magnitude and shape of the suppression are consistent with pQCD predictions at NLO using nPDFs that incorporate nuclear shadowing effects in the Pb nucleus. 

Femtoscopy of nonidentical particle pairs has been instrumental for precision measurements of both two-particle sources and the final-state interactions in high-energy elementary and heavy-ion collisions. The majority of measurements assessing the source properties are based on identical particle pairs, providing direct access to the characteristics of the single-particle source. The work in this paper demonstrates, via femtoscopy measurements of charged pion-deuteron pairs in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$TeV, the feasibility of accessing the characteristics of the single-particle femtoscopic source by using particle pairs with large mass differences such as pions and deuterons. The first experimental results of the measurement of deuteron source sizes in ultrarelativistic heavy-ion collisions are presented. The results show good agreement with the trend derived from other charged hadrons such as pions, kaons, and protons as a function of transverse mass, indicating similar source properties. 

Heavy-flavor quarks produced in proton-proton (pp) collisions provide a unique opportunity to investigate the evolution of quark-initiated parton showers from initial hard scatterings to final-state hadrons. By examining jets that contain heavy-flavor hadrons, this study explores the effects of both perturbative and nonperturbative QCD on jet formation and structure. The angular differences between various jet axes, $\mathrm{\Delta }{R}_{\mathrm{axis}}$, offer insight into the radiation patterns and fragmentation of charm quarks. The first measurement of $D^0$-tagged jet axes differences in pp collisions at $\sqrt{s}=5.02\mathrm{TeV}$by the ALICE experiment at the LHC is presented for jets with transverse momentum $p_{\mathrm{T}}^{\text{ch jet}}\ge 10\mathrm{GeV}/c$and $D^0$mesons with $p_{\mathrm{T}}^{{\mathrm{D}}^0}\ge 5\mathrm{GeV}/c$. In this $D^0$-meson-tagged jet measurement, three jet axis definitions, each with different sensitivities to soft, wide-angle radiation, are used: the standard axis, soft drop groomed axis, and winner-takes-all axis. Measurements of the radial distributions of $D^0$mesons with respect to the jet axes, $\mathrm{\Delta }{R}_{\mathrm{axis}\text{−}{\mathrm{D}}^0}$, are reported, along with the angle, $\mathrm{\Delta }{R}_{\mathrm{axis}}$, between the three jet axes. The $D^0$meson emerges as the leading particle in these jets, closely aligning with the winner-takes-all axis and diverging from the standard jet axis. The results also examine how varying the sensitivity to soft radiation with grooming influences the orientation of the soft drop jet axis and uncover that charm-jet structure is more likely to survive grooming when the soft drop axis is further from the $D^0$direction, providing further evidence of the dead-cone effect recently measured by ALICE. 

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. 

A<sc>bstract</sc> The first measurement of prompt D^*+-meson spin alignment in ultrarelativistic heavy-ion collisions with respect to the direction orthogonal to the reaction plane is presented. The spin alignment is quantified by measuring the elementρ₀₀of the diagonal spin-density matrix for prompt D^*+mesons with 4< p_T<30 GeV/cin two rapidity intervals, |y|<0.3 and 0.3<|y|<0.8, in central (0–10%) and midcentral (30–50%) Pb–Pb collisions at$$\sqrt{{s}_{\text{NN}}}={5}.0{2}$$TeV. Evidence of spin alignmentρ₀₀>1/3 has been found forp_T>15 GeV/cand 0.3<|y|<0.8 with a significance of 3.1σ. The measured spin alignment of prompt D^*+mesons is compared with the one of inclusive J/ψ mesons measured at forward rapidity (2.5< y <4). 

We present the measurement of the $p_{\mathrm{T}}$-differential production cross section of $\omega$mesons in and $p$-Pb collisions at $\sqrt{s_{NN}}=5.02$TeV at midrapidity by ALICE. In addition, the first measurement of the nuclear modification factor $R_{p\mathrm{Pb}}$for $\omega$mesons at LHC energies is presented, complementing the existing measurements of lighter neutral mesons such as the ${\pi }^0$and $\eta$. Within the measured $p_{\mathrm{T}}$range, the $R_{p\mathrm{Pb}}$of $\omega$mesons is compatible with no cold nuclear matter effects within the uncertainties, consistent with previous measurements at lower energies. The $\omega /{\pi }^0$ratio is presented for both collision systems, showing no collision system dependence within the uncertainties. The comparison to previously published $\omega /{\pi }^0$ratios at lower and higher collision energies in collisions suggests a decreasing trend of the ratio above $p_{\mathrm{T}}$= 4 GeV/ $c$with increasing collision energy. The data in both collision systems are compared to predictions from PYTHIA 8, EPOS LHC, and DPMJET event generators, revealing significant shortcomings in these models’ ability to describe the production of $\omega$mesons.