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Studying strangeness and baryon production mechanisms through angular correlations between charged Ξ baryons and identified hadrons in pp collisions at √s = 13 TeV
(Springer Science and Business Media Deutschland GmbH, 2024-09-01)
The angular correlations between charged Ξ baryons and associated identified hadrons (pions, kaons, protons, Λ baryons, and Ξ baryons) are measured in pp collisions at s = 13 TeV with the ALICE detector to give insight into the particle production mechanisms and balancing of quantum numbers on the microscopic level. In particular, the distribution of strangeness is investigated in the correlations between the doubly-strange Ξ baryon and mesons and baryons that contain a single strange quark, K and Λ. As a reference, the results are compared to Ξπ and Ξp correlations, where the associated mesons and baryons do not contain a strange valence quark. These measurements are expected to be sensitive to whether strangeness is produced through string breaking or in a thermal production scenario. Furthermore, the multiplicity dependence of the correlation functions is measured to look for the turn-on of additional particle production mechanisms with event activity. The results are compared to predictions from the string-breaking model Pythia 8, including tunes with baryon junctions and rope hadronisation enabled, the cluster hadronisation model Herwig 7, and the core-corona model Epos-lhc. While some aspects of the experimental data are described quantitatively or qualitatively by the Monte Carlo models, no model can match all features of the data. These results provide stringent constraints on the strangeness and baryon number production mechanisms in pp collisions.
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Investigating strangeness enhancement with multiplicity in pp collisions using angular correlations
(Springer Science and Business Media Deutschland GmbH, 2024-09-01)
A study of strange hadron production associated with hard scattering processes and with the underlying event is conducted to investigate the origin of the enhanced production of strange hadrons in small collision systems characterised by large charged-particle multiplicities. For this purpose, the production of the single-strange meson KS0 and the double-strange baryon Ξ± is measured, in each event, in the azimuthal direction of the highest-pT particle (“trigger” particle), related to hard scattering processes, and in the direction transverse to it in azimuth, associated with the underlying event, in pp collisions at s = 5.02 TeV and s = 13 TeV using the ALICE detector at the LHC. The per-trigger yields of KS0 and Ξ± are dominated by the transverse-to-leading production (i.e., in the direction transverse to the trigger particle), whose contribution relative to the toward-leading production is observed to increase with the event charged-particle multiplicity. The transverse-to-leading and the toward-leading Ξ±/KS0 yield ratios increase with the multiplicity of charged particles, suggesting that strangeness enhancement with multiplicity is associated with both hard scattering processes and the underlying event. The relative production of Ξ± with respect to KS0 is higher in transverse-to-leading processes over the whole multiplicity interval covered by the measurement. The KS0 and Ξ± per-trigger yields and yield ratios are compared with predictions of three different phenomenological models, namely Pythia8.2 with the Monash tune, Pythia8.2 with ropes and EPOS LHC. The comparison shows that none of them can quantitatively describe either the transverse-to-leading or the toward-leading yields of KS0 and Ξ±.
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Azimuthal anisotropy of jet particles in p-Pb and Pb-Pb collisions at √sNN = 5.02 TeV
(Springer Science and Business Media Deutschland GmbH, 2024-08-01)
The azimuthal anisotropy of particles associated with jets (jet particles) at midrapidity is measured for the first time in p-Pb and Pb-Pb collisions at sNN = 5.02 TeV down to transverse momentum (pT) of 0.5 GeV/c and 2 GeV/c, respectively, with ALICE. The results obtained in p-Pb collisions are based on a novel three-particle correlation technique. The azimuthal anisotropy coefficient v2 in high-multiplicity p-Pb collisions is positive, with a significance reaching 6.8σ at low pT, and its magnitude is smaller than in semicentral Pb-Pb collisions. In contrast to the measurements in Pb-Pb collisions, the v2 coefficient is also found independent of pT within uncertainties. Comparisons with the inclusive charged-particle v2 and with AMPT calculations are discussed. The predictions suggest that parton interactions play an important role in generating a non-zero jet-particle v2 in p-Pb collisions, even though they overestimate the reported measurement. These observations shed new insights on the understanding of the origin of the collective behaviour of jet particles in small systems such as p-Pb collisions, and provide significant stringent new constraints to models.
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Measurements of long-range two-particle correlation over a wide pseudorapidity range in p–Pb collisions at √sNN = 5.02 TeV
(Springer Science and Business Media Deutschland GmbH, 2024-01-01)
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 ∆η ~ 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 ∆η ~ 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(η), 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 with 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.
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Anisotropic Flow and Flow Fluctuations of Identified Hadrons in Pb–Pb Collisions at √sNN = 5.02 TeV
(Springer Science and Business Media Deutschland GmbH, 2023-06-01)
The first measurements of elliptic flow of π ±, K ±, p + p ¯ , KS0 , Λ + Λ ¯ , ϕ, Ξ −+ Ξ ¯ + , and Ω −+ Ω ¯ + using multiparticle cumulants in Pb–Pb collisions at sNN = 5.02 TeV are resented. Results obtained with two- (v 2{2}) and four-particle cumulants (v 2{4}) are shown as a function of transverse momentum, p T, for various collision centrality intervals. Combining the data for both v 2{2} and v 2{4} also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous p T-differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low p T and the approximate scaling with the number of constituent quarks at intermediate p T, are similarly present in the four-particle correlations and the combinations of v 2{2} and v 2{4}. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide p T range. [Figure not available: see fulltext.].
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Study of and its higher moments, and extraction of the speed of sound in Pb-Pb collisions with ALICE
(Springer Nature, 2025-11-01)
A bstract Ultrarelativistic heavy-ion collisions produce a state of hot and dense strongly interacting QCD matter called quark-gluon plasma (QGP). On an event-by-event basis, the volume of the QGP in ultracentral collisions is mostly constant, while its total entropy can vary significantly due to quantum fluctuations, leading to variations in the temperature of the system. Exploiting this unique feature of ultracentral collisions allows for the interpretation of the correlation of the mean transverse momentum $$(\langle {p}_{\text{T}}\rangle )$$ of produced charged hadrons and the number of charged hadrons as a measure for the speed of sound, c s . This speed is related to the rate at which compression waves travel in the QGP and is determined by fitting the relative increase in $$\langle {p}_{\text{T}}\rangle $$ with respect to the relative change in the average charged-particle density $$(\langle \text{d}{N}_{\text{ch}}/\text{d}\eta \rangle )$$ measured at mid-rapidity. This study reports the event-average $$\langle {p}_{\text{T}}\rangle $$ of charged particles as well as the variance, skewness, and kurtosis of the event-by-event transverse momentum per charged particle $$([{p}_{\text{T}}])$$ distribution in ultracentral Pb-Pb collisions at a center-of-mass energy of 5 . 02 TeV per nucleon pair using the ALICE detector. Different centrality estimators based on charged-particle multiplicity or the transverse energy of the event are used to select ultracentral collisions. By ensuring a pseudorapidity gap between the region used to define the centrality and the region used to perform the measurement, the influence of biases and their potential effects on the rise of the mean transverse momentum is tested. The measured $${c}_{\text{s}}^{2}$$ is found to strongly depend on the exploited centrality estimator and ranges between 0 . 1146±0 . 0028 (stat . )±0 . 0065 (syst . ) and 0 . 4374±0 . 0006 (stat . )±0 . 0184 (syst . ) in natural units. The self-normalized variance shows a steep decrease towards ultracentral collisions, while the self-normalized skewness variables show a maximum, followed by a fast decrease. These non-Gaussian features are understood in terms of the vanishing of the impact-parameter fluctuations contributing to the event-to-event [ p T ] distribution.
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Space-time evolution of particle emission in p–Pb collisions at TeV with 3D kaon femtoscopy
(Springer Nature, 2026-04-01)
A bstract 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 in π ± 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.
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Observation of flow vector fluctuations in p–Pb collisions at TeV
(Springer Nature, 2026-06-01)
A bstract Measurements of transverse momentum ( p T ) and pseudorapidity ( η ) dependent flow vector fluctuations in p–Pb collisions at $$\sqrt{{s}_{NN}}=5.02$$ TeV at the CERN Large Hadron Collider are presented. By studying long-range two-particle correlations with a template fit method, potential biases from non-flow effects such as jets and resonance decays are effectively suppressed. Significant p T - and η -dependent fluctuations of the second-harmonic flow vector are observed with more than 5 σ confidence in p–Pb collisions, similar to the observations in Pb–Pb collisions. The influence of residual non-flow effects has been evaluated and cannot account for the observed fluctuations, thereby confirming the observation of flow vector fluctuations in small collision systems at the LHC. Comparisons to model calculations from 3DGlauber+MUSIC+UrQMD and the parton transport model from AMPT are also presented. The measurements provide constraints on the theoretical modelling of the three-dimensional initial geometry and its event-by-event fluctuations, offering critical insights into the origin of collective flow in small collision systems at the LHC.
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J/ψ-hadron correlations at midrapidity in pp collisions at s = 13 TeV
(Springer Nature, 2025-07-01)
A bstract 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 ranges p T < 40 GeV /c for the J/ ψ and 0 . 15 < p T < 10 GeV /c and | η | < 0 . 9 for the associated hadrons. The measurement is based on minimum bias and high multiplicity data samples corresponding to integrated luminosities of L int = 34 nb − 1 and L 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 of E = 4 and 9 GeV in the ALICE electromagnetic calorimeters, which correspond to integrated luminosities of L int = 0 . 9 pb − 1 and L int = 8 . 4 pb − 1 , respectively. The azimuthally integrated near and away side yields of associated charged hadrons per J/ ψ trigger are presented as a function of the J/ ψ and associated hadron transverse momentum. The measurements are discussed in comparison to PYTHIA calculations.
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