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First measurement of Λ+c production down to pT = 0 in pp and p-Pb collisions at √𝑠NN = 5.02 TeV
(2023)
The production of prompt +c baryons has been measured at midrapidity in the transverse momentum interval 0 < pT < 1 GeV/c for the first time, in pp and p–Pb collisions at a center-of-mass energy per nucleon-nucleon collision √sNN = 5.02 TeV. The measurement was performed in the decay channel +c → pK0S by applying new decay reconstruction techniques using a Kalman-Filter vertexing algorithm and adopting a machine-learning approach for the candidate selection. The pT -integrated +c production cross sections in both collision systems were determined and used along with the measured yields in Pb–Pb collisions to compute the pT -integrated nuclear modification factors RpPb and RAA of +c baryons, which are compared to model calculations that consider nuclear modification of the parton distribution functions. The +c /D0 baryon-to-meson yield ratio is reported for pp and p–Pb collisions. Comparisons with models that include modified hadronization processes are presented, and the implications of the results on the understanding of charm hadronization in hadronic collisions are discussed. A significant (3.7σ) modification of the mean transverse momentum of + c baryons is seen in p–Pb collisions with respect to pp collisions, while the pT -integrated +c /D0 yield ratio was found to be consistent between the two collision systems within the uncertainties.
Experimental results are presented on event-by-event net-proton fluctuation measurements in Pb–Pb collisions at √sNN = 2.76 TeV, recorded by the ALICE detector at the CERN LHC. These measurements have as their ultimate goal an experimental test of Lattice QCD (LQCD) predictions on second and higher order cumulants of net-baryon distributions to search for critical behavior near the QCD phase boundary. Before confronting them with LQCD predictions, account has to be taken of correlations stemming from baryon number conservation as well as fluctuations of participating nucleons. Both effects influence the experimental measurements and are usually not considered in theoretical calculations. For the first time, it is shown that event-by-event baryon number conservation leads to subtle long-range correlations arising from very early interactions in the collisions.
The striking similarities that have been observed between high-multiplicity proton-proton (pp) collisions and heavy-ion collisions can be explored through multiplicity-differential measurements of identified hadrons in pp collisions. With these measurements, it is possible to study mechanisms such as collective flow that determine the shapes of hadron transverse momentum (pT) spectra, to search for possible modifications of the yields of short-lived hadronic resonances due to scattering effects in an extended hadron-gas phase, and to investigate different explanations provided by phenomenological models for enhancement of strangeness production with increasing multiplicity. In this paper, these topics are addressed through measurements of the K∗(892)0 and φ(1020) mesons at midrapidity in pp collisions at √s = 13 TeV as a function of the charged-particle multiplicity. The results include the pT spectra, pT-integrated yields, mean transverse momenta, and the ratios of the yields of these resonances to those of longer-lived hadrons. Comparisons with results from other collision systems and energies, as well as predictions from phenomenological models, are also discussed.
The inclusive J/ψ meson production in Pb–Pb collisions at a center-of-mass energy per nucleon–nucleon collision of sNN=5.02 TeV at midrapidity (|y|<0.9) is reported by the ALICE Collaboration. The measurements are performed in the dielectron decay channel, as a function of event centrality and J/ψ transverse momentum pT, down to pT=0. The J/ψ mean transverse momentum 〈pT〉 and rAA ratio, defined as 〈pT2〉PbPb/〈pT2〉pp, are evaluated. Both observables show a centrality dependence decreasing towards central (head-on) collisions. The J/ψ nuclear modification factor RAA exhibits a strong pT dependence with a large suppression at high pT and an increase to unity for decreasing pT. When integrating over the measured momentum range pT<10 GeV/c, the J/ψ RAA shows a weak centrality dependence. Each measurement is compared with results at lower center-of-mass energies and with ALICE measurements at forward rapidity, as well as to theory calculations. All reported features of the J/ψ production at low pT are consistent with a dominant contribution to the J/ψ yield originating from charm quark (re)combination.
This paper presents the first measurements of the charge independent (CI) and charge dependent (CD) two-particle transverse momentum correlators GCI 2 and GCD 2 in Pb–Pb collisions at √sNN = 2.76 TeV by the ALICE collaboration. The two-particle transverse momentum correlator G2 was introduced as a measure of the momentum current transfer between neighboring system cells. The correlators are measured as a function of pair separation in pseudorapidity (Δη) and azimuth (Δφ) and as a function of collision centrality. From peripheral to central collisions, the correlator GCI 2 exhibits a longitudinal broadening while undergoing a monotonic azimuthal narrowing. By contrast, GCD 2 exhibits a narrowing along both dimensions. These features are not reproduced by models such as HIJING and AMPT. However, the observed narrowing of the correlators from peripheral to central collisions is expected to result from the stronger transverse flow profiles produced in more central collisions and the longitudinal broadening is predicted to be sensitive to momentum currents and the shear viscosity per unit of entropy density η/s of the matter produced in the collisions. The observed broadening is found to be consistent with the hypothesized lower bound of η/s and is in qualitative agreement with values obtained from anisotropic flow measurements.
This Letter presents the first direct investigation of the p–0 interaction, using the femtoscopy technique in high-multiplicity pp collisions at √s = 13 TeV measured by the ALICE detector. The 0 is reconstructed via the decay channel to Λγ, and the subsequent decay of Λ to pπ−. The photon is detected via the conversion in material to e+e− pairs exploiting the capability of the ALICE detector to measure electrons at low transverse momenta. The measured p–0 correlation indicates a shallow strong interaction. The comparison of the data to several theoretical predictions obtained employing the Correlation Analysis Tool using the Schrödinger Equation (CATS) and the Lednický–Lyuboshits approach shows that the current experimental precision does not yet allow to discriminate between different models, as it is the case for the available scattering and hypernuclei data. Nevertheless, the p–0 correlation function is found to be sensitive to the strong interaction, and driven by the interplay of the different spin and isospin channels. This pioneering study demonstrates the feasibility of a femtoscopic measurement in the p–0 channel and with the expected larger data samples in LHC Run 3 and Run 4, the p–0 interaction will be constrained with high precision.
ϒ production in p–Pb interactions is studied at the centre-of-mass energy per nucleon–nucleon collision √sNN = 8.16 TeV with the ALICE detector at the CERN LHC. The measurement is performed reconstructing bottomonium resonances via their dimuon decay channel, in the centre-of-mass rapidity intervals 2.03 < ycms < 3.53 and −4.46 < ycms < −2.96, down to zero transverse momentum. In this work, results on the ϒ(1S) production cross section as a function of rapidity and transverse momentum are presented. The corresponding nuclear modification factor shows a suppression of the ϒ(1S) yields with respect to pp collisions, both at forward and backward rapidity. This suppression is stronger in the low transverse momentum region and shows no significant dependence on the centrality of the interactions. Furthermore, the ϒ(2S) nuclear modification factor is evaluated, suggesting a suppression similar to that of the ϒ(1S). A first measurement of the ϒ(3S) has also been performed. Finally, results are compared with previous ALICE measurements in p–Pb collisions at √sNN = 5.02 TeV and with theoretical calculations.
Measurements of K∗(892)0 and φ(1020) resonance production in Pb–Pb and pp collisions at √sNN = 5.02 TeV with the ALICE detector at the Large Hadron Collider are reported. The resonances are measured at midrapidity (|y| < 0.5) via their hadronic decay channels and the transverse momentum (pT) distributions are obtained for various collision centrality classes up to pT = 20 GeV/c. The pT-integrated yield ratio K∗(892)0/K in Pb–Pb collisions shows significant suppression relative to pp collisions and decreases towards more central collisions. In contrast, the φ(1020)/K ratio does not show any suppression. Furthermore, the measured K∗(892)0/K ratio in central Pb–Pb collisions is significantly suppressed with respect to the expectations based on a thermal model calculation, while the φ(1020)/K ratio agrees with the model prediction. These measurements are an experimental demonstration of rescattering of K∗(892)0 decay products in the hadronic phase of the collisions. The K∗(892)0/K yield ratios in Pb–Pb and pp collisions are used to estimate the time duration between chemical and kinetic freeze-out, which is found to be ∼ 4–7 fm/c for central collisions. The pT-differential ratios of K∗(892)0/K, φ(1020)/K, K∗(892)0/π , φ(1020)/π , p/K∗(892)0 and p/φ(1020) are also presented for Pb–Pb and pp collisions at √sNN = 5.02 TeV. These ratios show that the rescattering effect is predominantly a low-pT phenomenon.
Data from the first physics run at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory, Au+Au collisions at sqrt[sNN]=130 GeV, have been analyzed by the STAR Collaboration using three-pion correlations with charged pions to study whether pions are emitted independently at freeze-out. We have made a high-statistics measurement of the three-pion correlation function and calculated the normalized three-particle correlator to obtain a quantitative measurement of the degree of chaoticity of the pion source. It is found that the degree of chaoticity seems to increase with increasing particle multiplicity.
The jet radial structure and particle transverse momentum (pT) composition within jets are presented in centrality-selected Pb–Pb collisions at √sNN = 2.76 TeV. Track-based jets, which are also called charged jets, were reconstructed with a resolution parameter of R = 0.3 at midrapidity |ηch jet| < 0.6 for transverse momenta pT, ch jet = 30–120 GeV/c. Jet–hadron correlations in relative azimuth and pseudorapidity space (Δϕ, Δη) are measured to study the distribution of the associated particles around the jet axis for different pT,assoc-ranges between 1 and 20 GeV/c. The data in Pb–Pb collisions are compared to reference distributions for pp collisions, obtained using embedded PYTHIA simulations. The number of high-pT associate particles (4 < pT,assoc < 20 GeV/c) in Pb–Pb collisions is found to be suppressed compared to the reference by 30 to 10% depending on centrality. The radial particle distribution relative to the jet axis shows a moderate modification in Pb–Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb–Pb collisions compared to the reference, while low-pT associate particles tend to be broadened. The results, which are presented for the first time down to pT, ch jet = 30 GeV/c in Pb–Pb collisions, are compatible with both previous jet–hadron-related measurements from the CMS Collaboration and jet shape measurements from the ALICE Collaboration at higher pT, and add further support for the established picture of in-medium parton energy loss.