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ALICE Collaboration

Publications and source records attributed to ALICE Collaboration.

At least 37 records · Page 2Linked to original sources
arXiv2026

Time resolution of the ALICE Time-Of-Flight detector with the first Run 3 pp collisions at ${\bf \sqrt{\textit{s}} = 13.6}$ TeV

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 ($0.5 < p_{\rm T} < 4$ GeV/$c$). The ALICE TOF detector performed successfully during the first ten 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 $\sqrt{s} = 13.6$ 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.

physics.ins-det↗
arXiv2026

Space-time evolution of particle emission in p$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}}=~5.02}$ TeV with 3D kaon femtoscopy

The measurement of three-dimensional femtoscopic correlations between identical charged kaons (K$^\pm$K$^\pm$) produced in p$-$Pb collisions at center-of-mass energy per nucleon pair $\sqrt{s{_{\rm NN}}} = 5.02$ 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$^\pm$K$^\pm$ are similar to the ones observed earlier in identical charged-pion and K$_{\rm s}^{0}$K$_{\rm s}^{0}$ correlations in Pb$-$Pb collisions at various energies and in $π^\pm π^\pm$ correlations in p$-$Pb collisions at $\sqrt{s{_{\rm NN}}} = 5.02$ 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.

nucl-ex↗
arXiv2026

Observation of flow vector fluctuations in p$-$Pb collisions at $\mathbf{\sqrt{\textit{s}_{_{\bf NN}}}}=$ 5.02 TeV

Measurements of transverse momentum ($p_{\rm T}$) and pseudorapidity ($η$) dependent flow vector fluctuations in p$-$Pb collisions at $\sqrt{s_{_{\rm NN}}} = 5.02$ 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 $p_{\rm T}$- 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.

nucl-ex↗
arXiv2026

Strangeness enhancement at its extremes: multiple (multi-)strange hadron production in pp collisions at $\mathbf{\sqrt{\textit{s}} = 5.02}$ TeV

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 K$^{0}_{\rm s}$, 5 for $Λ$ and $\barΛ$, 4 for $Ξ^-$ and $Ξ^+$, and 2 for $Ω^-$ and $Ω^+$. 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 $\it{u}$ and $\it{d}$ 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.

nucl-ex↗
arXiv2026

Multiplicity dependence of prompt and non-prompt J/$ψ$ production at midrapidity in pp collisions at $\sqrt{s} = 13$ TeV

The yields of prompt and non-prompt \jpsi 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 D$^0$ 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.

hep-ex↗
arXiv2026

Study of the interaction between $Ξ$ baryons and light mesons via femtoscopy at the LHC

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 $Ξ-{\rm K}$ and $Ξ-π$ pairs obtained in high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV at the LHC are presented. For the first time, high-precision data on the $Ξ-{\rm K}$ interaction are delivered at small relative momenta. The scattering lengths, extracted via the Lednický-Lyuboshits expression of the pair wavefunction, indicate a repulsive and a shallow attractive strong interaction for the $Ξ-{\rm K}$ and $Ξ-π$ systems, respectively. The $Ξ(1620)$ and $Ξ(1690)$ states are observed in the $Ξ-π$ 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.

nucl-ex↗
arXiv2026

Measurement of the p$-Σ^+$ correlation function in pp collisions at $\sqrt{\textit{s}}=13$ TeV

In this letter, the first measurement of the femtoscopic correlation of protons and $Σ^+$ hyperons is presented and used to study the p$-Σ^+$ interaction. The measurement is performed with the ALICE detector in high-multiplicity triggered pp collisions at $\sqrt{s} = 13$ TeV. The $Σ^+$ hyperons are reconstructed using a missing-mass approach in the decay channel to $\textrm{p} + π^0$ with $π^0\rightarrowγγ$, while both $Σ^+$ 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$-Σ^+$ correlation function for the first time. Thanks to the high significance achieved in the p$-Σ^+$ correlation signal, it is possible to discriminate between the predictions of different models of the N$-Σ$ interaction and to accomplish a first determination of the p$-Σ^+$ scattering parameters.

nucl-ex↗
arXiv2026

Multiplicity dependence of K$^*(892)^{\pm}$ production in pp collisions at $\sqrt{s}$ = 13 TeV

The first results of K$^*$(892)$^{\pm}$ production at midrapidity ($|y| < 0.5$) in pp collisions at $\sqrt{s} = 13$ TeV as a function of the event multiplicity are presented. The K$^*$(892)$^{\pm}$ has been reconstructed via its hadronic decay channel K$^*$(892)$^{\pm} \rightarrow~π^{\pm}~+$K$_{\rm S}^0$ using the ALICE detector at the LHC. For each multiplicity class, the differential transverse momentum ($p_{\rm T}$) spectrum, the mean transverse momentum $\langle p_{\rm T} \rangle$, the $p_{\rm T}$-integrated yield (d$N$/d$y$), and the ratio of the K$^*$(892)$^{\pm}$ to K$_{\rm S}^0$ 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)$^{\pm}$/K$_{\rm S}^0$ suppression in pp collisions is observed at a 7$σ$ level passing from low to high multiplicity events. The ratios of the $p_{\rm T}$-differential yields of K$^*$(892)$^{\pm}$ and K$_{\rm S}^0$ in high and low multiplicity events are also presented along with their double ratio. For $p_{\rm T} \lesssim 2$ GeV/$c$ this double ratio persists below unity by more than $3σ$ suggesting that the suppression affects mainly low \pt resonances. The measured decreasing trend of the K$^*$(892)$^{\pm}$/K$_{\rm S}^0$ 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.

nucl-ex↗
arXiv2026

Investigating charm quark energy loss in medium with the nuclear modification factor of D$^0$-tagged jets

The nuclear modification factor $R_\mathrm{AA}$ 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 $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE detector at the LHC. The D$^0$ mesons and their charge conjugates are reconstructed from the hadronic decay D$^0\to \mathrm K^{-}π^{+}$. Jets are reconstructed from D$^0$-meson candidates and charged particles using the anti-$k_\mathrm{T}$ algorithm with jet resolution parameter $R=0.3$, in the jet transverse momentum ($p_\mathrm{T}$) range 5 $< p_\mathrm{T}^\mathrm{ch~jet}<$ 50 GeV/$c$ and pseudorapidity $|η^\mathrm{ch~jet}| < 0.6$. A hint of reduced suppression in the charm-jet $R_\mathrm{AA}$ is observed in comparison to inclusive jets in central Pb-Pb collisions with a significance of about 2$σ$ in 20 $< p_\mathrm{T}^\mathrm{ch~jet}<$ 50 GeV/$c$, suggesting the in-medium energy loss to depend on both the difference between quark and gluon coupling strength (Casimir colour-charge effect) and quark mass (dead-cone effect). The data are compared with model calculations that include mass effects in the in-medium energy loss. Several state-of-the-art models are consistent with the data, with the LIDO model providing the best description of the data in the common kinematic range of inclusive and D$^0$-tagged jets, highlighting the role of mass effects in interpreting the results.

nucl-ex↗
arXiv2026

Observation of partonic flow in proton-proton and proton-nucleus collisions

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.

nucl-ex↗
arXiv2026

Measurement of the angle between jet axes in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

This paper presents the first measurement of the angle between different jet axes (denoted as $ΔR$) in Pb$-$Pb collisions. The measurement is carried out in the 0$-$10% most-central events at $\sqrt{s_{\rm NN}} = 5.02$ TeV. Jets are assembled by clustering charged particles at midrapidity using the anti-$k_{\rm T}$algorithm with resolution parameters $R=0.2$ and $0.4$ and transverse momenta in the intervals $40 < p_{\rm T}^{\rm ch jet} < 140$ GeV/$c$ and $80 < p_{\rm T}^{\rm ch jet} < 140$ 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 intra-jet $p_{\rm T}$ broadening as implemented in a simple model calculation with the BDMPS 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.

nucl-ex↗
arXiv2026

Medium-induced modification of azimuthal correlations of electrons from heavy-flavor hadron decays with charged particles in Pb--Pb collisions at $\sqrt{s_{\rm{NN}} = 5.02}$ TeV

The azimuthal-correlation distributions between electrons from the decays of heavy-flavor hadrons and associated charged particles in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ 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 $4 < p_\mathrm{T}^\mathrm{e} < 16~{\rm GeV}/c$, considering associated particles within the transverse-momentum range $1 < p_\mathrm{T}^\mathrm{assoc} < 7$ ${\rm GeV}/c$, and a pseudorapidity difference of $|Δη|<1$ between the trigger electron and associated particles. The per-trigger nuclear modification factor ($I_\mathrm{AA}$) is calculated to compare the near- and away-side peak yields to those in pp collisions at $\sqrt{s} = 5.02$ TeV. In 0$-$10% central collisions, the $I_\mathrm{AA}$ indicates a hint of enhancement of associated-particle yields with $p_\mathrm{T}<3$ GeV/$c$ on the near side, and a suppression of yields with $p_\mathrm{T}>4$ GeV/$c$ on the away side. The $I_\mathrm{AA}$ 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.

nucl-ex↗
arXiv2026

Multiplicity dependence of two-particle angular correlations of identified particles in pp collisions at $\mathbf{\sqrt{s} = 13}$ TeV

Two-particle angular correlations probe particle production mechanisms and the underlying event-wide phenomena present in hadronic collisions. The correlations are examined as a function of rapidity and azimuthal-angle differences ($Δy, Δφ$) for pairs of like- and unlike-sign pions, kaons, and (anti-)protons produced in pp collisions at $\sqrt{s}$ = 13 TeV, measured by the ALICE experiment. Two-particle correlation functions are provided together with $Δy$ and $Δφ$ projections and compared to Monte Carlo (MC) model predictions. For the first time, the measurement is performed as a function of the event's charged-particle density. Previous studies conducted for pp collisions at $\sqrt{s}$ = 7 TeV at ALICE revealed a near-side anticorrelation for baryon$-$baryon and antibaryon$-$antibaryon pairs, whose origin remains unresolved. Here, an additional approach is introduced to study the multiplicity dependence and the expected inverse multiplicity scaling of the correlation function. This method highlights qualitative differences in the underlying sources of correlations, such as quantum-statistics effects, final-state interactions, and resonance decays. The puzzling near-side anticorrelation in baryon$-$baryon measurements is observed across all multiplicity classes and continues to challenge current particle-production models. Furthermore, the multiplicity dependence of the correlations between mesons provides an independent probe of the sensitivity of current MC models to soft-QCD effects and hadronization dynamics. The presented measurements, together with the baryon results, enrich the experimental picture of two-particle correlations in pp collisions and serve as valuable input for ongoing theoretical developments.

nucl-ex↗
arXiv2026

Centrality dependence of charged-particle pseudorapidity density at midrapidity in Pb-Pb collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}} = 5.36}$ TeV

The ALICE Collaboration reports its first LHC Run 3 measurements of charged-particle pseudorapidity density at midrapidity in Pb-Pb collisions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV. Particle multiplicity in high-energy collisions characterises the system geometry, constrains particle-production mechanisms, and is used to estimate initial energy density. Multiplicity also acts as a reference for subsequent measurements as a function of centrality. In this letter, for the first time, charged particles are reconstructed using the upgraded ALICE Inner Tracking System and Time Projection Chamber, while the collision centrality is determined by measuring charged-particle multiplicities with the Fast Interaction Trigger system. Pseudorapidity density, ${\rm d}N_{\rm ch}/{\rm d}η$, is presented, averaged over events, for various centrality classes. Results are shown as a function of pseudorapidity and the average number of participating nucleons ($\langle N_{\mathrm{part}}\rangle$) in the collision. The average charged-particle pseudorapidity density ($\langle {\rm d}N_{\rm ch}/{\rm d}η\rangle$) at midrapidity ($|η|<0.5$) is 2010 $\pm$ 53 for the 5% most central collisions. The value of $\langle {\rm d}N_{\rm ch}/{\rm d}η\rangle$ normalised to $\langle N_{\mathrm{part}}\rangle/2$ as a function of $\sqrt{s_{\mathrm{NN}}}$ follows the trend established in previous measurements in heavy-ion collisions. Theoretical models based on mechanisms for particle production in nuclear collisions that involve the formation of quark-gluon plasma medium and models based on individual nucleon-nucleon interactions are compared to the data.

nucl-ex↗
arXiv2026

Three-baryon femtoscopy as an effective 3$\rightarrow$3 scattering experiment

Scattering experiments have long been the gold standard for constraining hadron$-$hadron interactions, providing direct information on the angular momentum and spin dependence over a wide range of kinematic configurations. However, experimental constraints on three-body dynamics remain limited, specifically for unbound systems and systems involving short-lived hadrons. In this work, the three-proton correlation function is measured in pp collisions at $\sqrt{s}=13.6$ TeV with ALICE at the LHC and presented as a novel approach to access hadronic interactions in three-body systems. A new analysis strategy is employed to isolate the p$-$p$-$p contribution to the correlation function by correcting for background channels and experimental effects, and enabling a direct comparison with state-of-the-art three-body continuum calculations. The extracted correlation function provides the first direct access to the isospin $3/2$ three-body system. The measured observable is found to be sensitive to the partial-wave structure of the nucleon$-$nucleon interaction and indicates that the nuclear interaction acts even at high angular momentum and parity states of the three-body system, revealing an effective long-range attractive component, observed experimentally for the first time in a three-proton continuum system. Hence, three-hadron femtoscopy emerges as an effective 3$\rightarrow$3 scattering experiment with three unbound hadrons in initial and final states. The copious production of hyperons at the modern high-energy colliders ensures the possibility of extending such measurements beyond nucleons, opening a new avenue for future precision studies of three-body dynamics in the strangeness sector.

nucl-ex↗
arXiv2026

First measurement of $\mathbf{\rm Ξ_{\rm c}^{0}}$ production in $\mathbf{Pb}-\mathbf{Pb}$ collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}}}$ = 5.02 TeV

The ALICE Collaboration reports the first measurement of the production of prompt $Ξ_{\mathrm c}^{0}$ baryons in nucleus$-$nucleus collisions by analyzing data from $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}~=5.02~\mathrm{TeV}$. The production and transverse-momentum ($p_{\mathrm T}$) differential spectra of $Ξ_{\mathrm c}^{0}$ are particularly sensitive to the hadronization process and to strangeness production in the quark--gluon plasma formed in high-energy heavy-ion collisions. The $Ξ_{\mathrm c}^{0}$ baryons are reconstructed at midrapidity ($|y|<0.5$) in the transverse-momentum intervals $3<p_{\mathrm T}~<12~\mathrm{GeV}/c$ and $4<p_{\mathrm T}~<12~\mathrm{GeV}/c$ in the 0$-$10\% and 30$-$50\% centrality intervals, respectively. The nuclear modification factor ($R_{\mathrm{AA}}$) reaches values up to 3 in the interval $3<p_{\mathrm T}~<4~\mathrm{GeV}/c$, which is the largest value measured so far for charm hadrons. Model predictions are compatible with the measured $R_{\mathrm{AA}}$, while, in the measured $p_{\mathrm T}$ intervals, they underestimate the production yield as well as the measured $Ξ_{\mathrm c}^{0}/\mathrm{D}^{0}$, $Ξ_{\mathrm c}^{0}/Λ_{\mathrm c}^{+}$, and $Ξ_{\mathrm c}^{0}/\mathrm{D}^{+}_{\mathrm s}$ yield ratios.

nucl-ex↗
arXiv2026

Multiplicity dependence of the size of the common hadron emission source in pp collisions at the LHC

Femtoscopic analysis can shed light on hadron production in pp collisions. In this paper, proton-proton correlations measured in collisions at $\sqrt{s}=13.6$ TeV recorded with the ALICE detector at the LHC are presented. The analysis is based on the minimum bias dataset collected in 2022 following the upgrade of the ALICE detector and corresponds to an integrated luminosity of $19.3$ pb$^{-1}$. The increased integrated luminosity allows us, for the first time, to simultaneously measure the multiplicity and transverse-mass ($m_{\rm T}$) dependence of the size of the hadron-emitting source. Precise knowledge of the femtoscopic source size in pp collisions is a crucial ingredient for using femtoscopy to study the residual strong interaction among stable and unstable hadrons at the LHC. In this light, the source radius was determined from the measured correlation functions by assuming several state-of-the-art models of the nucleon$-$nucleon interactions. The consistency among the extracted radii demonstrates the robustness of the measurement with respect to interaction model assumptions. A comparison to femtoscopic radii measured in Pb$-$Pb collisions at $\sqrt{s}=5.02$ TeV reveals a markedly different multiplicity dependence in similar $m_{\rm T}$ intervals, providing new insight into the system-size dependence of particle emission dynamics.

nucl-ex↗
arXiv2026

Evidence for parton energy loss in oxygen$-$oxygen collisions at $\mathbf{\sqrt{s_{\rm NN}}=5.36}$ TeV

Ultra-relativistic heavy-ion collisions create a hot and dense medium of deconfined quarks and gluons, the quark$-$gluon plasma (QGP), in which parton energy loss ("jet quenching") is a key probe of hot medium properties. While parton energy loss has been firmly established in large systems such as Pb$-$Pb and Au$-$Au collisions, no unambiguous direct evidence exists in smaller systems such as high-multiplicity p$-$Pb and pp collisions. To probe the onset of parton energy loss at intermediate system size, measurements of neutral-pion production are presented in this Letter for oxygen$-$oxygen (OO) and proton$-$oxygen (pO) collisions recorded with the ALICE detector in July 2025, relative to a pp baseline. The nuclear modification factor $R_{\rm OO}$ is suppressed relative to unity with a transverse-momentum dependence similar to that observed in Pb$-$Pb collisions, consistent with a previous CMS measurement in OO collisions with charged particles. As $R_{\rm OO}$ contains contributions from both cold and hot nuclear matter effects, $R_{\rm pO}$ is also presented in order to constrain cold nuclear matter (CNM) contributions. $R_{\rm pO}$ is found to be compatible with unity, indicating that CNM effects alone cannot account for the suppression observed in $R_{\rm OO}$. Final-state effects are isolated using the measured double ratio $R_{\rm OO} \left/ R_{\rm pO}^2 \right.$, which largely cancels CNM contributions and exhibits a significant suppression relative to expectations without energy loss at a 4.9$σ$ level. Theoretical models incorporating parton energy loss via different mechanisms predict a significant suppression of the $R_{\rm OO} \left/ R_{\rm pO}^2 \right.$ relative to unity, consistent with the data. These findings establish parton energy loss in OO collisions, extending experimental evidence for jet quenching to the smallest nuclear system studied to date.

nucl-ex↗