7 Search Results

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 $$\sqrt{s}$$ = 13 TeV from charged π–π correlations measured by ALICE. We demonstrate that the assumption of a Gaussian primordial source is compatible withmore » 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 (m_T) 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 m_T 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.« less

The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton–proton (pp) collisions is reported. The mean charged particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at center-of-mass energy $$\sqrt{s}$$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region (|η| < 0.9) using the sequential recombination anti-k_T algorithm with jet resolution parameters R = 0.2, 0.3, and 0.4 for the trans verse momentum (p_T) interval 5–110 GeV/c. The high multiplicity events are selected by the forward V0 scintilla tor detectors.more » The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet p_T in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables z^ch and ξ^ch are measured for different jet-p_T intervals. Jet-p_T independent fragmentation of leading jets is observed for wider jets except at high- and low-z^ch values. The observed “hump-backed plateau” structure in the ξ^ch distribution indicates suppression of low-p_T particles. In high-multiplicity events, an enhancement of the fragmen tation probability of low-z^ch particles accompanied by a suppression of high-zch particles is observed compared to minimum-bias events. This behavior becomes more promi nent for low-p_T jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet p_T. These measurements provide important constraints to models of jet fragmentation.« less

This Letter presents the most precise measurement to date of the matter-antimatter imbalance at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair $$\sqrt{s_{NN}}$$ = 5.02 TeV. Using the Statistical Hadronization framework, it is possible to obtain the value of the electric charge and baryon chemical potentials, μ_Q = -0.18 ± 0.90 MeV and μ_B = 0.71 ± 0.45 MeV, with unprecedented precision. A centrality-differential study of the antiparticle-to-particle yield ratios of charged pions, protons, Ω baryons, and light (hyper)nuclei is performed. These results indicate that the system created in Pb-Pb collisions at the LHC is on averagemore » baryon-free and electrically neutral at midrapidity.« less

The inclusive production of the charm-strange baryon $$Ω^0_c$$ is measured for the first time via its semileptonic decay into $$Ω$$^-e⁺ν_e at midrapidity (|y| < 0.8) in proton-proton (pp) collisions at the center-of mass energy $$\sqrt{s}$$ = 13 TeV with the ALICE detector at the LHC. The transverse momentum (p_T) differential cross section multiplied by the branching ratio is presented in the interval 2 < p_T < 12 GeV=c. The branching-fraction ratio BR($$Ω^0_c$$ → $$Ω$$^-e⁺ν_e)/BR($$Ω^0_c$$→ Ω^-π⁺) is measured to be 1.12 ± 0.22 (stat) ± 0.27 (syst). Comparisons with other experimental measurements, as well as with theoretical calculations, are presented

The two-particle momentum correlation functions between charm mesons ( ${\mathrm{D}}^{*\pm }$ and ${\mathrm{D}}^{\pm }$ ) and charged light-flavor mesons ( ${\pi }^{\pm }$ and ${\mathrm{K}}^{\pm }$ ) in all charge combinations are measured for the first time by the ALICE Collaboration in high-multiplicity proton–proton collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ . For DK and mathvariant="normal">D * K pairs, the experimental results are in agreement with theoretical predictions of the residual strong interaction based on quantum chromodynamics calculations on the lattice and chiral effective field theory. In the case of $\mathrm{D}\pi$ and ${\mathrm{D}}^{*}\pi$ pairs, tension between the calculations including strong interactions and the measurement is observed. For all particle pairs, the data can be adequately described by Coulomb interaction only, indicating a shallow interaction between charm and light-flavor mesons. Finally, the scattering lengths governing the residual strong interaction of the $\mathrm{D}\pi$ and ${\mathrm{D}}^{*}\pi$ systems are determined by fitting the experimental correlation functions with a model that employs a Gaussian potential. The extracted values are small and compatible with zero. © 2024 CERN, for the ALICE Collaboration 2024 CERN« less

Measurements of the 𝑝_T-dependent flow vector fluctuations in Pb–Pb collisions at $$\sqrt{S_{NN}}$$ = 5.02 TeV using azimuthal correlations with the ALICE experiment at the Large Hadron Collider are presented. A four-particle correlation approach [ALICE Collaboration, Phys. Rev. C 107, L051901 (2023)] is used to quantify the effects of flow angle and magnitude fluctuations separately. This paper extends previous studies to additional centrality intervals and provides measurements of the 𝑝_T-dependent flow vector fluctuations at $$\sqrt{S_{NN}}$$ = 5.02 TeV with two-particle correlations. Significant 𝑝_T-dependent fluctuations of the $$\vec{𝑉}$$₂ flow vector in Pb–Pb collisions are found across different centrality ranges, with the largestmore » fluctuations of up to ∼15% being present in the 5% most central collisions. In parallel, no evidence of significant 𝑝_T-dependent fluctuations of $$\vec{𝑉}$$₃ or $$\vec{𝑉}$$₄ is found. Additionally, evidence of flow angle and magnitude fluctuations is observed with more than 5⁢𝜎 significance in central collisions. These observations in Pb–Pb collisions indicate where the classical picture of hydrodynamic modeling with a common symmetry plane breaks down. This has implications for hard probes at high 𝑝_T, which might be biased by 𝑝_T-dependent flow angle fluctuations of at least 23% in central collisions. Given the presented results, existing theoretical models should be reexamined to improve our understanding of initial conditions, quark–gluon plasma properties, and the dynamic evolution of the created system.« less

This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ . Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of $1.4<\left|\mathrm{\Delta }\eta \right|<1.8$ and a transverse momentum of $1<p_{\mathrm{T}}<2\mathrm{GeV}/c$ , asmore » a function of the charged-particle multiplicity measured at midrapidity. This Letter extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly interacting medium is unlikely to be formed. The precision of the new low multiplicity results allows for the first direct quantitative comparison with the results obtained in ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ collisions at $\sqrt{s}=91\mathrm{GeV}$ and $\sqrt{s}=183–209\mathrm{GeV}$ , where initial-state effects such as preequilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range $8\lesssim ⟨N_{\mathrm{ch}}⟩\lesssim 24$ where the ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ results have good precision, the measured ridge yields in $pp$ collisions are substantially larger than the limits set in ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ annihilations do not contribute significantly to the emergence of long-range correlations in $pp$ collisions. © 2024 CERN, for the ALICE Collaboration 2024 CERN« less