87 Search Results

The nuclear modification factor R AA of charm jets, identified by the presence of a D0 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 s NN = 5.02 TeV with the ALICE detector at the LHC. The D0 mesons and their charge conjugates are reconstructed from the hadronic decay D 0 → K − π + . Jets are reconstructed from D0-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)more » range 5 < p T ch jet < 50 GeV/c and pseudorapidity |η ch jet| < 0.6. A hint of reduced suppression in the charm-jet R AA is observed in comparison to inclusive jets in central Pb–Pb collisions with a significance of about 2σ in 20 < p T 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 D0-tagged jets, highlighting the role of mass effects in interpreting the results.« less

The transverse momentum spectra and integrated yields of anti-$$Σ$$ hyperons ($$\overline{\Sigma}^{\pm}$$) have been measured in and collisions at $$\sqrt{s_{\textrm{NN}}} = 5.02$$ TeV with the ALICE experiment. Measurements are performed via the newly accessed decay channel $$\overline{\Sigma}^{\pm}$$ → $$\bar{\textrm{n}}π^±$$. A new method of antineutron reconstruction with the PHOS electromagnetic spectrometer is developed and applied to this analysis. The p_T spectra of $$\overline{\Sigma}^{\pm}$$ 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 measuredmore » 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 $$\overline{\Sigma }^{\pm }$$ 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 $$\overline{\Sigma}^{+}$$ and $$\overline{\Sigma}^{-}$$ are evaluated and compared to those of protons, $$Λ$$ and $$Ξ$$ hyperons, and predictions of EPOS LHC and EPOS4 models. No deviations of R_pPb for $$\overline{\Sigma}^{\pm}$$ from the model predictions or measurements for other hadrons are found within uncertainties.« less

The dependence of f₀ (980) production on the final-state charged-particle multiplicity is reported for proton–proton (pp) collisions at the centre-of-mass energy, $$\mathbf{\sqrt{s}}$$ = 13 TeV. The production of (980) is measured with the ALICE detector via the decay channel in a midrapidity region of |y| < 0.5. The evolution of the integrated yields and mean transverse momentum of f₀ (980) as a function of charged-particle multiplicity measured in pp at $$\mathbf{\sqrt{s}}$$ = 13 TeV follows the trends observed in pp at $$\mathbf{\sqrt{s}}$$ = 5.02 TeV and in proton–lead (p–Pb) collisions at$$\mathbf{\sqrt{s_{NN}}}$$ = 5.02 TeV. Particle yield ratios of f₀ (980)more » to π^± and K* (892) 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 f₀ (980) is assumed, which suggests that the data do not support significant hidden strangeness in the f₀ (980).« less

This Letter presents measurements of long-range transverse-momentum correlations using a new observable, 𝑣_0⁡(𝑝_T), serving as a probe of event-by-event radial-flow fluctuations, the underlying radial expansion, and the medium’s properties in heavy-ion collisions. Results are reported for inclusive charged particles, pions, kaons, and protons across various centrality intervals in Pb-Pb collisions at $$\sqrt{𝑠_{\textrm{NN}}}$$ = 5.02 TeV, recorded by the ALICE detector. A pseudorapidity-gap technique, similar to that used in anisotropic-flow studies, is employed to suppress short-range correlations. At low 𝑝_T, a characteristic mass ordering consistent with hydrodynamic collective flow is observed. At higher 𝑝_T (>3 GeV/𝑐), protons exhibit larger 𝑣₀⁡(𝑝_T) thanmore » pions and kaons, in agreement with expectations from quark-recombination models. Comparisons to viscous hydrodynamic calculations with varying bulk viscosity and equation of state demonstrate the sensitivity of the 𝑣₀⁡(𝑝_T) observable to these key medium properties. The findings establish 𝑣₀⁡(𝑝_T) as a valuable addition to the set of observables used in Bayesian analyses for extracting the transport properties and constraining the equation of state of strongly interacting matter, while also helping to systematically explore its sensitivity and impact within such global studies.« less

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 QGPmore » 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.« less

High-energy hadronic collisions generate environments characterized by temperatures above 100 MeV (refs.), about 100,000 times hotter than the centre of the Sun. At present, it is therefore unclear how light (anti)nuclei with mass number A of a few units, such as the deuteron, ³He or ⁴He, each bound by only a few MeV, can emerge from these collisions. Here, the ALICE Collaboration reports that deuteron–pion momentum correlations in proton–proton (pp) collisions provide model-independent evidence that about 90% of the observed (anti)deuterons are produced in nuclear reactions following the decay of short-lived resonances, such as the Δ(1232). These findings, obtained by themore » ALICE Collaboration at the Large Hadron Collider, resolve a gap in our understanding of nucleosynthesis in ultrarelativistic hadronic collisions. Apart from offering insights on how (anti)nuclei are formed in hadronic collisions, the results can be used in the modelling of the production of light and heavy nuclei in cosmic rays and dark-matter decays.« less

Femtoscopy of nonidentical particle pairs has been instrumental for precision measurements of both two-particle sources and the final-state interactions in high-energy elementary and heavy-ion collisions. The majority of measurements assessing the source properties are based on identical particle pairs, providing direct access to the characteristics of the single-particle source. The work in this paper demonstrates, via femtoscopy measurements of charged pion-deuteron pairs in Pb-Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV, the feasibility of accessing the characteristics of the single-particle femtoscopic source by using particle pairs with large mass differences such as pions and deuterons. The first experimental results of themore » measurement of deuteron source sizes in ultrarelativistic heavy-ion collisions are presented. The results show good agreement with the trend derived from other charged hadrons such as pions, kaons, and protons as a function of transverse mass, indicating similar source properties.« less

This paper presents the measurement of the isolated prompt photon inclusive production cross section in pp and p–Pb collisions by the ALICE Collaboration at the LHC. The measurement is performed in p–Pb collisions at centre-of-mass energies per nucleon pair of $$\sqrt{s_{NN}}$$ = 5.02 Te and 8.16 TeV, as well as in pp collisions at $$\sqrt{s}$$ = 5.02 TeV and 8 TeV. The cross section is obtained at midrapidity (|γ| < 0.7) using a charged-track based isolation momentum p$$^{iso,~ch}_{T}$$ < 1.5 GeV/c in a cone with radius R = 0.4. The data for both collision systems are well reproduced by perturbativemore » QCD (pQCD) calculations at next-to-leading order (NLO) using recent parton distribution functions for free (PDF) and bound (nPDF) nucleons. Furthermore, the nuclear modification factor R_pA for both collision energies is consistent with unity for p_T > 20 GeV/c. However, deviations from unity (R_pA < 1) of up to 20% are observed for p_T < 20 GeV/c with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision.« less

The first measurement at midrapidity (|y| < 0.5) of the production yield of the strange-charm baryons $$Ξ$$$^{+}_{c}$$ and $$Ξ$$$^{0}_{c}$$ as a function of transverse momentum (p_T) in different charged-particle multiplicity classes in proton-proton collisions at $$\sqrt{s}$$ = 13 TeV with the ALICE experiment at the LHC is reported. The $$Ξ$$$^{+}_{c}$$ baryon is reconstructed via the $$Ξ$$$^{+}_{c}$$ → $$Ξ$$^– π⁺π⁺ decay channel in the range 4 < p_T < 12 GeV/c, while the $$Ξ$$$^{0}_{c}$$ baryon is reconstructed via both the $$Ξ$$$^{0}_{c}$$ → $$Ξ$$^–π⁺ and $$Ξ$$$^{0}_{c}$$ → $$Ξ$$^–e⁺ν_e decay channels in the range 2 < p_T < 12 GeV/c. The baryon-to-meson ($$Ξ$$$^{0}_{c}$$⁺/D⁰)more » and the baryon-to-baryon ($$Ξ$$$^{0}_{c}$$⁺/Λ$$^{+}_{c}$$) production yield ratios show no significant dependence on multiplicity. In addition, the observed yield ratios are not described by theoretical predictions that model charm-quark fragmentation based on measurements at e⁺e⁻ and e⁻p colliders, indicating differences in the charm-baryon production mechanism in pp collisions. A comparison with different event generators and tunings, including different modelling of the hadronisation process, is also discussed.« less

Heavy-flavor quarks produced in proton-proton (pp) collisions provide a unique opportunity to investigate the evolution of quark-initiated parton showers from initial hard scatterings to final-state hadrons. By examining jets that contain heavy-flavor hadrons, this study explores the effects of both perturbative and nonperturbative QCD on jet formation and structure. The angular differences between various jet axes, Δ⁢𝑅_axis, offer insight into the radiation patterns and fragmentation of charm quarks. The first measurement of 𝐷⁰-tagged jet axes differences in pp collisions at $$\sqrt{s}$$ =5.02  TeV by the ALICE experiment at the LHC is presented for jets with transverse momentum 𝑝$$^{ch jet}_{T}$$ ≥ 10 GeV/𝑐more » and 𝐷⁰ mesons with 𝑝$$^{D^0}_{T}$$ ≥ 5  GeV/𝑐. In this 𝐷⁰-meson-tagged jet measurement, three jet axis definitions, each with different sensitivities to soft, wide-angle radiation, are used: the standard axis, soft drop groomed axis, and winner-takes-all axis. Measurements of the radial distributions of 𝐷⁰ mesons with respect to the jet axes, Δ⁢𝑅_axis−D⁰, are reported, along with the angle, Δ⁢𝑅_axis, between the three jet axes. The 𝐷⁰ meson emerges as the leading particle in these jets, closely aligning with the winner-takes-all axis and diverging from the standard jet axis. The results also examine how varying the sensitivity to soft radiation with grooming influences the orientation of the soft drop jet axis and uncover that charm-jet structure is more likely to survive grooming when the soft drop axis is further from the 𝐷⁰ direction, providing further evidence of the dead-cone effect recently measured by ALICE.« less