Investigating strangeness enhancement with multiplicity in pp collisions using angular correlations

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±.publishedVersio

System size and energy dependence of the mean transverse momentum fluctuations at the LHC

Event-by-event fluctuations of the event-wise mean transverse momentum, ⟨pT⟩, of charged particles produced in proton–proton (pp) collisions at s = 5.02 TeV, Xe–Xe collisions at sNN = 5.44 TeV, and Pb–Pb collisions at sNN = 5.02 TeV are studied using the ALICE detector based on the integral correlator ⟨⟨ΔpTΔpT⟩⟩. 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, S0, 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

Investigating strangeness enhancement with multiplicity in pp collisions using angular correlations

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±

Measurement of HΛ3 production in Pb–Pb collisions at sNN=5.02 TeV

The first measurement of HΛ3 and H‾Λ‾3 differential production with respect to transverse momentum and centrality in Pb–Pb collisions at sNN=5.02 TeV is presented. The HΛ3 has been reconstructed via its two-charged-body decay channel, i.e., HΛ3→3He+π−. A Blast-Wave model fit of the pT-differential spectra of all nuclear species measured by the ALICE collaboration suggests that the HΛ3 kinetic freeze-out surface is consistent with that of other nuclei. The ratio between the integrated yields of HΛ3 and He3 is compared to predictions from the statistical hadronisation model and the coalescence model, with the latter being favoured by the presented measurements

Probing Strangeness Hadronization with Event-by-Event Production of Multistrange Hadrons

This Letter presents the first measurement of event-by-event fluctuations of the net number (difference between the particle and antiparticle multiplicities) of multistrange hadrons Ξ- and Ξ¯+ and its correlation with the net-kaon number using the data collected by the ALICE Collaboration in pp, p-Pb, and Pb-Pb collisions at a center-of-mass energy per nucleon pair sNN=5.02 TeV. The statistical hadronization model with a correlation over three units of rapidity between hadrons having the same and opposite strangeness content successfully describes the results. On the other hand, string-fragmentation models that mainly correlate strange hadrons with opposite strange quark content over a small rapidity range fail to describe the data

Medium-induced modification of groomed and ungroomed jet mass and angularities in Pb–Pb collisions at sNN=5.02 [Figure presented]

The ALICE Collaboration presents a new suite of jet substructure measurements in Pb–Pb and pp collisions at a center-of-mass energy per nucleon pair sNN=5.02 [Figure presented]. These measurements provide access to the internal structure of jets via the momentum and angle of their constituents, probing how the quark–gluon plasma modifies jets, an effect known as jet quenching. Jet grooming additionally removes soft wide-angle radiation to enhance perturbative accuracy and reduce experimental uncertainties. We report the groomed and ungroomed jet mass mjet and jet angularities λακ using κ=1 and α>0. Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with resolution parameter R=0.2. A narrowing of the jet mass and angularity distributions in Pb–Pb collisions with respect to pp is observed and is enhanced for groomed results, confirming modification of the jet core. By using consistent jet definitions and kinematic cuts between the mass and angularities for the first time, previous inconsistencies in the interpretation of quenching measurements are resolved, rectifying a hurdle for understanding how jet quenching arises from first principles and highlighting the importance of a well-controlled baseline. These results are compared with a variety of theoretical models of jet quenching, providing constraints on jet energy-loss mechanisms in the quark–gluon plasma

First Measurement of <math display="inline"><mi>A</mi><mo>=</mo><mn>4</mn></math> Hypernuclei and Antihypernuclei at the LHC

International audienceIn this Letter, the first evidence of the $\overline{{}^4_\Lambda\mathrm{He}}$ antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number $A = 4$, specifically (anti)$^4_\Lambda \mathrm{H}$ and (anti)$^4_\Lambda \mathrm{He}$. In addition, the antiparticle-to-particle ratios for both hypernuclei ($\overline{{}^4_\Lambda \mathrm{H}} / {}^4_\Lambda \mathrm{H}$ and $\overline{{}^4_\Lambda\mathrm{He}} / {}^4_\Lambda\mathrm{He}$) are shown, which are sensitive to the baryochemical potential of the strongly interacting matter created in heavy-ion collisions. The results are obtained from a data sample of central Pb--Pb collisions, collected during the 2018 LHC data taking at a center-of-mass energy per nucleon pair of \sqrt{s_{\mathrm{NN}}} = \SI{5.02}{\tera\electronvolt}. The yields measured for the average of the charge-conjugated states are found to be $[0.78 \pm 0.19\,\text{(stat)} \pm 0.17\,\text{(syst)}] \times 10^{-6}$ for the (anti)$^4_\Lambda \mathrm{H}$ and $[1.08 \pm 0.34\,\text{(stat)} \pm 0.20\,\text{(syst)}] \times 10^{-6}$ for the (anti)$^4_\Lambda \mathrm{He}$, and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (anti)$^4_\Lambda \mathrm{H}$ and (anti)$^4_\Lambda \mathrm{He}$ excited states is expected to strongly enhance the production yield of these hypernuclei. The yield values exhibit a combined deviation of $3.3\,\sigma$ from the theoretical ground-state-only expectation, while the inclusion of the excited states in the calculations leads to an agreement within $0.6\,\sigma$ with the present measurements. Additionally, the measured (anti)$^4_\Lambda \mathrm{H}$ and (anti)$^4_\Lambda \mathrm{He}$ masses are compatible with the world-average values within the uncertainties

Measurement of f1(1285) production in pp collisions at s = 13 TeV

This study presents the first measurement of the f1(1285) resonance using the ALICE detector in inelastic proton–proton collisions at a center-of-mass energy of 13 TeV. The resonance is reconstructed at midrapidity (|y|< 0.5) through the hadronic decay channel f1(1285)→KS0K±π∓. Key measurements include the determination of its mass, transverse-momentum integrated yield, and average transverse momentum. Additionally, the ratio of the transverse-momentum integrated yield of f1(1285) to pion is compared with calculations from the canonical statistical hadronization model. The model calculation, assuming a zero total strangeness content for f1(1285), reproduces the data within 1σ deviation, shedding light on the quark composition of f1(1285)

Multimuons in cosmic-ray events as seen in ALICE at the LHC

ALICE is a large experiment at the CERN Large Hadron Collider. Located 52 meters underground, its detectors are suitable to measure muons produced by cosmic-ray interactions in the atmosphere. In this paper, the studies of the cosmic muons registered by ALICE during Run 2 (2015–2018) are described. The analysis is limited to multimuon events defined as events with more than four detected muons (Nµ &gt; 4) and in the zenith angle range 0◦ &lt; θ &lt; 50◦. The results are compared with Monte Carlo simulations using three of the main hadronic interaction models describing the air shower development in the atmosphere: QGSJET-II-04, EPOS-LHC, and SIBYLL 2.3d. The interval of the primary cosmic-ray energy involved in the measured muon multiplicity distribution is about 4×1015 &lt; Eprim &lt; 6×1016 eV. In this interval none of the three models is able to describe precisely the trend of the composition of cosmic rays as the energy increases. However, QGSJET-II-04 is found to be the only model capable of reproducing reasonably well the muon multiplicity distribution, assuming a heavy composition of the primary cosmic rays over the whole energy range, while SIBYLL 2.3d and EPOS-LHC underpredict the number of muons in a large interval of multiplicity by more than 20% and 30%, respectively. The rate of high muon multiplicity events (Nµ &gt; 100) obtained with QGSJET-II-04 and SIBYLL 2.3d is compatible with the data, while EPOS-LHC produces a significantly lower rate (55% of the measured rate). For both QGSJET-II-04 and SIBYLL 2.3d, the rate is close to the data when the composition is assumed to be dominated by heavy elements, an outcome compatible with the average energy Eprim ∼ 1017 eV of these events. This result places significant constraints on more exotic production mechanisms. © 2025 The Author(s)

Rapidity dependence of antideuteron coalescence in pp collisions at s=13 TeV with ALICE

The production yields of antideuterons and antiprotons are measured in pp collisions at a center-of-mass energy of s=13 TeV, as a function of transverse momentum (pT) and rapidity (y), for the first time rapidity-differentially up to |y|=0.7. The measured spectra are used to study the pT and rapidity dependence of the coalescence parameter B2, which quantifies the coalescence probability of antideuterons. The pT and rapidity dependence of the obtained B2 is extrapolated for pT>1.7 GeV/c and |y|>0.7 using the phenomenological antideuteron production model implemented in PYTHIA 8.3 as well as a baryon coalescence afterburner model based on EPOS 3. Such measurements are of interest to the astrophysics community, since they can be used for the calculation of the flux of antinuclei from cosmic rays, in combination with coalescence models