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Charged-particle multiplicity distributions over a wide pseudorapidity range in p–Pb collisions at √ˢɴɴ=5.02 TeV
(Springer Nature, 2025)
This paper presents the primary charged-particle multiplicity distributions in proton–lead collisions at a centre-of-mass energy per nucleon–nucleon collision of √ˢɴɴ = 5.02 TeV. The distributions are reported for non-single diffractive collisions in different pseudorapidity ranges. The measurements are performed using the combined information from the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE. The multiplicity distributions are parametrised with a double negative binomial distribution function which provides satisfactory descriptions of the distributions for all the studied pseudorapidity intervals. The data are compared to models and analysed quantitatively, evaluating the first four moments (mean, standard deviation, skewness, and kurtosis). The shape evolution of the measured multiplicity distributions is studied in terms of KNO variables and it is found that none of the considered models reproduces the measurements. This paper also reports on the average charged-particle multiplicity, normalised by the average number of participating nucleon pairs, as a function of the collision energy. The multiplicity results are then compared to measurements made in proton–proton and nucleus–nucleus collisions across a wide range of collision energies.
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System size and energy dependence of the mean transverse momentum fluctuations at the LHC
(Springer Nature, 2025)
Event-by-event fluctuations of the event-wise mean transverse momentum, ⟨Pᴛ⟩, of charged particles produced in proton–proton (pp) collisions at √s = 5.02 TeV, Xe–Xe collisions at √ˢɴɴ = 5.44 TeV, and Pb–Pb collisions at √ˢɴɴ = 5.02 TeV are studied using the ALICE detector based on the integral correlator ⟨⟨ΔPᴛ ΔPᴛ⟩⟩ . 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.
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Measurements of differential two-particle number and transverse momentum correlation functions in pp collisions at s = 13 TeV
(Springer Nature, 2025)
Differential two-particle normalized cumulants (R₂) and transverse momentum correlations (P₂) are measured as a function of the relative pseudorapidity and azimuthal angle difference (Δ η , Δ φ) of charged particle pairs in minimum bias pp collisions at √s = 13 TeV. The measurements use charged hadrons in the pseudorapidity region of |η| < 0.8 and the transverse momentum range 0.2 < pT < 2.0 GeV/c in order to focus on soft multiparticle interactions and to complement prior measurements of these correlation functions in p–Pb and Pb–Pb collisions. The correlation functions are reported for both unlike-sign and like-sign pairs and their charge-independent and charge-dependent combinations. Both the R₂ and P₂ measured in pp collisions exhibit features qualitatively similar to those observed in p–Pb and Pb–Pb collisions. The Δη and Δφ root mean square widths of the near-side peak of the correlation functions are evaluated and compared with those observed in p–Pb and Pb–Pb collisions and show smooth evolution with the multiplicity of charged particles produced in the collision. The comparison of the measured correlation functions with predictions from PYTHIA8 shows that this model qualitatively captures their basic structure and characteristics but feature important differences. In addition, the R₂ᶜᶛ is used to determine the charge balance function of hadrons produced within the detector acceptance of the measurements. The integral of the balance function is found to be compatible with those reported by a previous measurement in Pb–Pb collisions.
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Measurement of the inclusive isolated-photon production cross section in pp and Pb–Pb collisions at √ˢɴɴ=5.02 TeV
(Springer Nature, 2025)
The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum ( Pʸᴛ ), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of √ˢɴɴ = 5.02 TeV. The photon transverse momentum range is between 10–14 and 40–140 GeV/c , depending on the collision system and on the Pb–Pb centrality class. The result extends to lower Pʸᴛ than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is |ηʸ| < 0.67 . The isolation selection is based on a charged particle isolation momentum threshold Pⁱˢᵒ′ᶜʰᴛ = 1.5 GeV/c within a cone of radii R = 0.2 and 0.4. The nuclear modification factor is calculated and found to be consistent with unity in all centrality classes, and also consistent with the HG-PYTHIA model, which describes the event selection and geometry biases that affect the centrality determination in peripheral Pb–Pb collisions. The measurement is compared to next-to-leading order perturbative QCD calculations and to the measurements of isolated photons and Z⁰ bosons from the CMS experiment, which are all found to be in agreement.
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Common femtoscopic hadron-emission source in pp collisions at the LHC
(Springer Nature, 2025)
The femtoscopic study of pairs of identical pions is particularly suited to investigate the effective source function of particle emission, due to the resulting Bose–Einstein correlation signal. In small collision systems at the LHC, pp in particular, the majority of the pions are produced in resonance decays, which significantly affect the profile and size of the source. In this work, we explicitly model this effect in order to extract the primordial source in pp collisions at √s = 13 TeV from charged π – π correlations measured by ALICE. We demonstrate that the assumption of a Gaussian primordial source is compatible with the data and that the effective source, resulting from modifications due to resonances, is approximately exponential, as found in previous measurements at the LHC. The universality of hadron emission in pp collisions is further investigated by applying the same methodology to characterize the primordial source of K –p pairs. The size of the primordial source is evaluated as a function of the transverse mass ( mₜ ) of the pairs, leading to the observation of a common scaling for both π – π and K – p, suggesting a collective effect. Further, the present results are compatible with the mT scaling of the p – p and p - Λ primordial source measured by ALICE in high multiplicity pp collisions, providing additional evidence for the presence of a common emission source for all hadrons in small collision systems at the LHC. This will allow the determination of the source function for any hadron–hadron pairs with high precision, granting access to the properties of the possible final-state interaction among pairs of less abundantly produced hadrons, such as strange or charmed particles.
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Investigating the p-π± and p-p-π± dynamics with femtoscopy in pp collisions at √s =13 TeV
(Springer, 2025)
The interaction between pions and nucleons plays a crucial role in hadron physics. It represents a fundamental building block of the low-energy QCD dynamics and is subject to several resonance excitations. This work studies the p–π± dynamics using femtoscopic correlations in high-multiplicity pp collisions at √s =13 TeV measured by ALICE at the LHC. As the final-state interaction between protons and pions is well constrained by scattering experiments and the study of pionic hydrogen, the results give access to information on the particle-emitting source in pp collisions using the femtoscopy methods. The scaling of the source size of primordial protons and pions against their pair transverse mass is extracted. The results are compared with the source sizes studied with p–p, p–K⁺, and π±–π± pairs by ALICE in the same collision system and are found to be in agreement for the different particle pairs. This reinforces recent findings by ALICE of a common emission source for all hadron-pairs in pp collisions at LHC energies. Furthermore, the p–p–π± systems are studied using three-particle femtoscopy in pp collisions at √s =13 TeV. The presence of three-body effects is analyzed utilizing the cumulant expansion method. In this formalism, the known two-body interactions are subtracted in order to isolate the three-body effects. For both, p–p–π⁺ and p–p–π⁻, a non-zero cumulant is found, indicating effects beyond pairwise interactions. These results give information on the coupling of the pion to multiple nucleons.
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First Measurement of A = 4 Hypernuclei and Antihypernuclei at the LHC
(American Physical Society, 2025)
In this Letter, the first evidence of the 4/Λ H ̅ⅇ 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/Λ H and (anti) 4/Λ He. In addition, the antiparticle-to-particle ratios for both hypernuclei (4/Λ H ̅/4/Λ H = and 4/Λ H ̅ⅇ = 4/Λ 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 √ˢɴɴ = 5.02 TeV. The yields measured for the average of the charge-conjugated states are found to be [0.78 ± 0.19(stat) ± 0.17(syst)] × 10⁻⁶ for the (anti) 4/Λ H, and [1.08 ± 0.34(stat) ± 0.20(syst)] × 10⁻⁶ for the (anti) 4/Λ He, and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (anti) 4/Λ H and (anti) 4/Λ He excited states is expected to strongly enhance the production yield of these hypernuclei. The yield values exhibit a combined deviation of 3.3σ from the theoretical ground-state-only expectation, while the inclusion of the excited states in the calculations leads to an agreement within 0.6σ with the present measurements. Additionally, the measured (anti) 4/Λ H and (anti) 4/Λ He masses are compatible with the world-average values within the uncertainties.
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Probing Strangeness Hadronization with Event-by-Event Production of Multistrange Hadrons
(American Physical Society, 2025)
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 √ˢɴɴ = 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.
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First measurement of Dₛ₁(1⁺)(2536)⁺ and D*ₛ₂(2⁺)(2573)⁺ production in proton-proton collisions at √s = 13 TeV at the LHC
(American Physical Society, 2025)
The production yields of the orbitally excited charm-strange mesons Dₛ₁(1⁺)(2536)⁺ and D*ₛ₂(2⁺)(2573)⁺ were measured for the first time in proton-proton (pp) collisions at a center-of-mass energy of √s = 13 TeV with the ALICE experiment at the LHC. The D⁺ₛ₁ and D*⁺ₛ₂ mesons were measured at midrapidity (|y| < 0.5) in minimum-bias and high-multiplicity pp collisions in the transverse-momentum interval 2 < Pᴛ < 24 GeV/c. Their production yields relative to the D⁺ₛ ground-state yield were found to be compatible between minimum-bias and high-multiplicity collisions, as well as with previous measurements in e±p and e⁺e¯ collisions. The measured D⁺ₛ₁/D⁺ₛ and D*⁺ₛ₂/D⁺ₛ yield ratios are described by statistical hadronization models and can be used to tune the parameters governing the production of excited charm-strange hadrons in Monte Carlo generators, such as PYTHIA 8.
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Search for Quasiparticle Scattering in the Quark-Gluon Plasma with Jet Splittings in pp and Pb-Pb Collisions at √ˢɴɴ =5.02 TeV
(American Physical Society, 2025)
The ALICE Collaboration reports measurements of the large relative transverse momentum ( Kᴛ ) component of jet substructure in p p and Pb-Pb collisions at center-of-mass energy per nucleon pair √ˢɴɴ = 5.02 TeV . Enhancement in the yield of such large- Kᴛ 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- Kᴛ algorithm with resolution parameter R = 0.2 in the transverse-momentum interval 60 < Pᴛ‚ch‚jet < 80 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 p p collisions shows medium-induced narrowing, corresponding to yield suppression of high- Kᴛ 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.
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