Abstract The probability to observe a specific number of strange and multi-strange hadrons (n S), denoted as P(n S), is measured by ALICE at midrapidity (|y| < 0.5) in $$\sqrt{s}=5.02$$ TeV proton-proton (pp) collisions, dividing events into several multiplicity-density classes. Exploiting, for the first time, a technique based on counting the number of strange-particle candidates event-by-event, this measurement allows one to extend the study of strangeness production beyond the mean of the distribution. This constitutes a new test bench for production mechanisms, probing events with a large imbalance between strange and non-strange content. The analysis of a large-statistics data sample makes it possible to extract P(n S) up to a maximum n S of 7 for $${\text{K}}_{\text{S}}^{0}$$ , 5 for Λ and $$\overline{\Lambda }$$ , 4 for Ξ − and $${\overline{\Xi } }^{+}$$ , and 2 for Ω − and $${\overline{\Omega } }^{+}$$ . From this, the probability of producing strange hadron multiplets per event is calculated, thereby enabling the extension of the study of strangeness enhancement to extreme situations where several strange quarks hadronize in a single event at midrapidity. Moreover, comparing hadron combinations with different u and d quark compositions and equal overall s quark content, the contribution to the enhancement pattern coming from non-strangeness related mechanisms is isolated. The results are compared with state-of-the-art phenomenological models implemented in commonly used Monte Carlo event generators, including PYTHIA 8 Monash 2013, PYTHIA 8 with QCD-based Color Reconnection and Rope Hadronization (QCD-CR + Ropes), and EPOS LHC, which incorporates both partonic interactions and hydrodynamic evolution. These comparisons show that the new approach dramatically enhances the sensitivity to the different underlying physics mechanisms modeled by each generator.

The transverse momentum spectra and integrated yields of anti-Sigma hyperons ((Sigma) over bar (+/-)) have been measured in pp and p-Pb collisions at root s(NN) = 5.02 TeV with the ALICE experiment. Measurements are performed via the newly accessed decay channel (Sigma) over bar (+/-) -> (n) over bar pi(+/-). A new method of antineutron reconstruction with the PHOS electromagnetic spectrometer is developed and applied to this analysis. The p(T) spectra of (Sigma) over bar (+/-) are measured in the range 0.5 < p(T) < 3 GeV/c and compared to predictions of the PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4 models. The EPOS LHC and EPOS4 models provide the best descriptions of the measured spectra both in pp and p-Pb collisions, while models which do not account for multiparton interactions provide a considerably worse description at high p(T). The total yields of (Sigma) over bar (+/-) in both pp and p-Pb collisions are compared to predictions of the Thermal-FIST model and dynamical models PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4. All models reproduce the total yields in both colliding systems within uncertainties. The nuclear modification factors R-pPb for both (Sigma) over bar (+) and (Sigma) over bar (-) are evaluated and compared to those of protons, Lambda and Xi hyperons, and predictions of EPOS LHC and EPOS4 models. No deviations of R-pPb for (Sigma) over bar (+/-) from the model predictions or measurements for other hadrons are found within uncertainties.

The first results of K*(892)(+/-) production at midrapidity (|y| < 0.5) in pp collisions at root s = 13 TeV as a function of the event multiplicity are presented. The K*(892)+/- has been reconstructed via its hadronic decay channel K*(892)(+/-) -> pi +/- + K-S(0) using the ALICE detector at the LHC. For each multiplicity class, the differential transverse momentum (p(T)) spectrum, the mean transverse momentum < p(T)>, the p(T)-integrated yield (dN/dy), and the ratio of the K*(892)(+/-) to K-S(0) yields are reported. These are consistent with previous K*(892)0 resonance results with a higher level of precision. Comparisons with phenomenological models such as PYTHIA6, PYTHIA8, EPOS-LHC, and DIPSY are also discussed. For the first time, a significant K*(892)(+/-)/K-S(0) suppression in pp collisions is observed at a 7.. level passing from low to high multiplicity events. The ratios of the p(T)-differential yields of K*(892)(+/-) and K-S(0) in high and low multiplicity events are also presented along with their double ratio. For p(T) less than or similar to 2 GeV/c this double ratio persists below unity by more than 3 sigma suggesting that the suppression affects mainly low p(T) resonances. The measured decreasing trend of the K*(892)(+/-)/K-S(0) ratio with increasing multiplicity, which in heavy-ion collisions is typically attributed to the rescattering of decay particles of the short-lived resonances, is reproduced by the EPOS-LHC model without the use of hadronic afterburners

Abstract 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.

Abstract 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 5σ 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.

(Multi-)strange particle production rates and transverse momentum distributions are measured at midrapidity ( | y | < 0.5 ) as a function of the charged-particle multiplicity density by the ALICE experiment at the CERN Large Hadron Collider (LHC), using proton-proton collisions at a center-of-mass energy of s = 5.02 TeV. This study extends similar studies performed at s = 7 TeV and s = 13 TeV to a lower-energy regime, improving the statistical precision and extending the measurement to previously unexplored low-multiplicity regions. While K S 0 , Λ , and Ξ yields can be described with a linear multiplicity dependence within uncertainties, the Ω yields follow a significantly faster than linear increasing trend. For all analyzed particles, the overall production rate is consistent with that observed at higher energy and at similar multiplicity densities. Transverse momentum distributions are observed to evolve with multiplicity. Several state-of-the-art QCD-inspired Monte Carlo models have been compared with the data, testing some recently introduced features to address the findings at higher energies. Models can qualitatively describe the transverse momentum spectra and the Λ / K S 0 spectral ratio only if collectivity is introduced in the evolution of the system.

In this letter, the first measurement of the femtoscopic correlation of protons and Sigma(+) hyperons is presented and used to study the p-Sigma(+) interaction. The measurement is performed with the ALICE detector in high-multiplicity triggered pp collisions at root s = 13 TeV. The Sigma(+) hyperons are reconstructed using a missing-mass approach in the decay channel to p + pi(0) with pi(0) -> gamma gamma, while both Sigma(+) and protons are identified using a machine learning approach. These techniques result in a high reconstruction efficiency and purity, which allows the measurement of the p-Sigma(+) correlation function for the first time. Thanks to the high significance achieved in the p-Sigma(+) correlation signal, it is possible to discriminate between the predictions of different models of the N-Sigma interaction and to accomplish a first determination of the p-Sigma(+) scattering parameters.

Per-trigger yields of π 0 − hadron correlations were measured in semicentral Pb-Pb collisions at s NN = 5.02 TeV in ALICE at the CERN Large Hadron Collider. The reconstructed π 0 → γ γ , with a transverse momentum of 11 GeV / c < p T ( π 0 ) < 14 GeV / c , is used as the trigger particle to calculate yields of associated charged particles on the near- and away-side. The photons are reconstructed using the ALICE Electromagnetic Calorimeter, and the charged particles are measured in the ALICE central barrel within a pseudorapidity range of | η | < 0.8 . The yields are reported relative to the orientation of the π 0 with the second-order event plane and are background subtracted using the reaction-plane fit method. The data give an indication of a suppression of the associated charged-particle yields near p T ≈ 2 GeV / c when comparing out-of-plane to in-plane trigger particles. At associated charged-particle p T > 3 GeV/ c , no significant event-plane dependence is observed within uncertainties. The results are compared with predictions from the JEWEL model, which implements jet energy loss in an expanding medium. JEWEL predicts no significant modification of either the near- or away-side associated yields, independent of whether medium recoils are included. The observed behavior may indicate the presence of additional energy-loss mechanisms beyond those governed by path-length dependence.

Particle identification (PID) is a fundamental aspect of the ALICE detector system, central to its heavy-ion and proton–proton physics programs. Among the different PID strategies, ALICE uses the time-of-flight (TOF) detector to identify particles at intermediate momenta ( 0.5< p_T <4 GeV/c ). The ALICE TOF detector performed successfully during the first 10 years of LHC operations. During the Long Shutdown 2, many ALICE sub-detectors, including TOF, were upgraded to fully leverage the targeted 50 kHz interaction rate of Pb–Pb collisions, which required the implementation of a continuous readout scheme. The TOF detector electronics were upgraded and refurbished, while processing algorithms for data quality control, reconstruction, calibration, and analysis were rewritten. This paper presents the upgraded TOF detector operation and calibration procedures and its performance in terms of timing resolution, a key factor for particle separation in ALICE analyses. Using 2022 pp collision data at √(s) = 13.6 TeV from Run 3, the time resolution of the detector was estimated with two independent methods, both yielding consistent results, better than 80 ps. Despite the excellent performance already achieved, further improvements are expected after additional detector commissioning and refined calibration procedures, thus enhancing the ALICE PID capabilities for Run 3 and beyond.

This paper presents the first measurement of the angle between different jet axes (denoted as Delta R-axis) in Pb-Pb collisions. The measurement is carried out in the 0-10% most-central events at root s(NN) = 5.02 TeV. Jets are assembled by clustering charged particles at midrapidity using the anti-kT algorithm with resolution parameters R = 0.2 and 0.4 and transverse momenta in the intervals 40 < p(T)(ch jet) < 140 GeV/c and 80 < p(T)(ch jet) < 140 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 Pb-Pb distribution relative to 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(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 Pb-Pb and 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 identical charged-kaon correlations induced by quantum-statistics effects and final-state interactions are measured in Pb-Pb collisions at s N N = 5.02 TeV . The results of one- (1D) and three-dimensional (3D) analyses show that the obtained system-size parameters (radii) are smaller for more peripheral collisions and decrease with increasing pair transverse momentum k T . The 1D parameters agree within uncertainties with those obtained in Pb-Pb collisions at s N N = 2.76 TeV . The observed power-law dependence of the extracted 3D radii as a function of the pair transverse momentum is a signature of the collective flow in the particle-emitting system created in Pb-Pb collisions. This dependence is well reproduced by the integrated hydrokinetic model calculations except for the outward projection of the radius (measured in the longitudinally comoving system) for the most central collisions. The time of maximal emission for kaons is extracted from the 3D analysis in a wide collision centrality range from 0 to 90%. Its reduction with decreasing charged-particle multiplicity is well reproduced by the hydrokinetic model predictions, and means that kaons are emitted earlier in more peripheral events.

The dependence of $$\textrm{f}_{0}$$ f 0 (980) production on the final-state charged-particle multiplicity is reported for proton–proton (pp) collisions at the centre-of-mass energy, $$\sqrt{s}=$$ s = 13 TeV. The production of $$\textrm{f}_{0}$$ f 0 (980) is measured with the ALICE detector via the $$\textrm{f}_0 (980) \rightarrow \pi ^{+}\pi ^{-}$$ f 0 ( 980 ) → π + π - decay channel in a midrapidity region of $$|y|<$$ | y | < 0.5. The evolution of the integrated yields and mean transverse momentum of f $$_{0}$$ 0 (980) as a function of charged-particle multiplicity measured in pp at $$\sqrt{s}=$$ s = 13 TeV follows the trends observed in pp at $$\sqrt{s}=$$ s = 5.02 TeV and in proton–lead (p–Pb) collisions at $$\sqrt{s_{\textrm{NN}}}=$$ s NN = 5.02 TeV. Particle yield ratios of $$\textrm{f}_{0}$$ f 0 (980) to $$\pi ^{\pm }$$ π ± and $$\textrm{K}^{*}$$ K ∗ (892) $$^{0}$$ 0 are found to decrease with increasing charged-particle multiplicity. These particle ratios are compared with calculations from the canonical statistical thermal model as a function of charged-particle multiplicity. The thermal model calculations provide a better description of the decreasing trend of particle ratios when no strange or antistrange quark composition for f $$_{0}$$ 0 (980) is assumed, which suggests that the data do not support significant hidden strangeness in the $$\textrm{f}_{0} (980)$$ f 0 ( 980 ) .

Abstract The yields of prompt and non-prompt J/ψ and the fraction of non-prompt J/ψ are measured at midrapidity (|y| < 0.9) via the dielectron decay channel as a function of the midrapidity charged-particle multiplicity (|η| < 0.9) in pp collisions at $$\sqrt{s}=13$$ TeV. The J/ψ yields and the multiplicity are normalized by their average value in inelastic collisions. The multiplicity-dependent yield ratio between prompt J/ψ and D0 is reported. The multiplicity is further divided into three azimuthal regions with respect to the J/ψ momentum: toward the J/ψ emission direction, transverse, or opposite to it. A stronger-than-linear increase of the self-normalized yields is observed for both prompt and non-prompt J/ψ production, with similar trends. This behaviour is also observed in the toward region, while a weaker increase is observed in the transverse and away regions.

The measurement of Sigma(+) production in pp collisions at root s = 13 TeV is presented. The measurement is performed at midrapidity in both minimum-bias and high-multiplicity pp collisions at root s = 13 TeV. The Sigma(+) is reconstructed via its weak-decay topology in the decay channel Sigma(+) -> p + pi(0) with pi(0) -> gamma + gamma. In a novel approach, the neutral pion is reconstructed by combining photons that convert in the detector material with photons measured in the calorimeters. The transverse-momentum (p(T)) distributions of the Sigma(+) and its rapidity densities dN/dy in both event classes are reported. The p(T) spectrum in minimum-bias collisions is compared to QCD-inspired event generators. The ratio of Sigma(+) to previously measured Lambda baryons is in good agreement with calculations from the Statistical Hadronization Model. The high efficiency and purity of the novel reconstruction method for Sigma(+) presented here will enable future studies of the interaction of Sigma(+) with protons in the context of femtoscopic measurements, which could be crucial for understanding the equation of state of neutron stars.

The azimuthal-correlation distributions between electrons from the decays of heavy-flavor hadrons and associated charged particles in Pb-Pb collisions at root s(NN) = 5.02 TeV are reported for the 0-10% and 30-50% centrality classes. This measurement provides access to the jet-like correlation observables in the heavy-flavor sector in Pb-Pb collisions. The analysis is performed for trigger electrons from heavy-flavor hadron decays with transverse momentum 4 < p(T)(e) < GeV/c, considering associated particles within the transverse-momentum range 1< p(T)(assoc) < 7 GeV/c, and a pseudorapidity difference of |Delta eta |<1 between the trigger electron and associated particles. The per-trigger nuclear modification factor (I-AA) is calculated to compare the near- and away-side peak yields to those in pp collisions at root s = 5.02 TeV. In 0-10% central collisions, the I(AA )indicates a hint of enhancement of associated-particle yields with p(T)<3 GeV/c on the near side, and a suppression of yields with p(T)>4 GeV/c on the away side. The I-AA for electron triggers from heavy-flavor hadron decays is compared with that for light-flavor and strange-particle triggers to investigate the dependence on different fragmentation processes and parton-medium dynamics, and is found to be the same within uncertainties.

Meson-baryon systems with strangeness content provide a unique laboratory for investigating the strong interaction and testing theoretical models of hadron structure and dynamics. In this work, the measured correlation functions for oppositely charged Xi-K and Xi-pi pairs obtained in high-multiplicity pp collisions at root s = 13 TeV at the LHC are presented. For the first time, high-precision data on the Xi-K interaction are delivered at small relative momenta. The scattering lengths, extracted via the Lednicky-Lyuboshits expression of the pair wavefunction, indicate a repulsive and a shallow attractive strong interaction for the Xi-K and Xi-pi systems, respectively. The Xi(1620) and Xi(1690) states are observed in the Xi-pi correlation function and their properties, mass and width, are determined. These measurements are in agreement with other available results. Such high-precision data can help refine the understanding of these resonant states, provide stronger constraints for chirally motivated potentials, and address the key challenge of describing the coupled-channel dynamics that may give rise to molecular configurations.

The dependence of f(0)(980) production on the final-state charged-particle multiplicity is reported for protonproton (pp) collisions at the centre-of-mass energy, root s = 13 TeV. The production of f(0)(980) is measured with the ALICE detector via the f(0)(980) -> pi(+)pi(-) decay channel in a midrapidity region of |y| < 0.5. The evolution of the integrated yields and mean transverse momentum of f(0)(980) as a function of charged-particle multiplicity measured in pp at root s = 13 TeV follows the trends observed in pp at root s = 5.02 TeV and in proton-lead (p-Pb) collisions at root s(NN) = 5.02 TeV. Particle yield ratios of f0(980) to p +/- and K*(892)(0) are found to decrease with increasing chargedparticle multiplicity. These particle ratios are compared with calculations from the canonical statistical thermalmodel as a function of charged-particle multiplicity. The thermal model calculations provide a better description of the decreasing trend of particle ratios when no strange or antistrange quark composition for f(0)(980) is assumed, which suggests that the data do not support significant hidden strangeness in the f(0)(980).

The centrality dependence of strange ( K S 0 , Λ + Λ ¯ ) and multistrange ( Ξ − + Ξ ¯ + , Ω − + Ω ¯ + ) hadron production is measured by ALICE in the Large Hadron Collider (LHC) lead-lead (Pb-Pb) collisions at a center-of-mass energy per nucleon pair s N N = 5.02 TeV, using the full dataset collected during the LHC Run 2 campaign in the years 2015 and 2018. This is the largest heavy-ion dataset analyzed to date at the LHC, and it allows for the extraction of transverse momentum ( p T ) spectra and p T -integrated yields with unprecedented precision, over a broad range of charged particle multiplicity densities ( 〈 d N ch / d η 〉 | η | < 0.5 ), probing regions where smaller collision system ( and p -Pb) results are also available. The p T spectra evolve with centrality, featuring higher 〈 p T 〉 in central events for all particles. The Λ / K S 0 ratio exhibits the distinctive baryon-to-meson enhancement in the intermediate p T region, with a maximum which is shifted to larger p T for more central collisions. The hadron-to-pion yield ratios are presented as a function of 〈 d N ch / d η 〉 | η | < 0.5 and compared to results from different collision systems and energies. A smooth connection from to Pb-Pb is observed, thus demonstrating that collision system or energy do not play a role in the multiplicity evolution of this observable. The previously reported enhancement of strangeness production in the multiplicity range probed in and p -Pb collisions saturates in the multiplicity range of Pb-Pb data. These results constitute a key test bench for theoretical models and a first comparison to the EPOS 4 generator is presented.

The nuclear modification factor R-AA of charm jets, identified by the presence of a D-0 meson among the jet constituents, has been measured for the first time in Pb-Pb collisions at a centre-of-mass energy per nucleon pair root s(NN) = 5.02 TeV with the ALICE detector at the LHC. The D-0 mesons and their charge conjugates are reconstructed from the hadronic decay D-0 -> K-pi(+). Jets are reconstructed from D-0-meson candidates and charged particles using the anti-k(T) algorithm with jet resolution parameter R = 0.3, in the jet transverse momentum (p(T)) range 5

The first measurements of the average transverse-momentum fraction (⟨z⟩) as a function of transverse momentum (pT) for strange baryons (Λ and Λ‾) and strange mesons (KS0), produced in mini-jets defined through angular correlations in pp collisions at s=13 TeV, are reported by the ALICE Collaboration at the LHC. The observable is obtained using a novel method, where the angular correlation between the strange hadrons and inclusive charged hadrons is weighted by the pT of correlated particles at small angular distance. As a function of strange particles’ pT, the results reveal a flat trend for strange mesons and a decreasing trend for strange baryons in the measured pT region, indicating distinct hadronization mechanisms for KS0 and Λ (Λ‾). The measurements are compared to Monte Carlo models, namely Pythia 8 (with both Monash and Color Rope tunes) and the AMPT (A Multi-Phase Transport) model with string melting. None of these models provides a satisfactory description of the ⟨z⟩ distributions at low and intermediate pT.