412 research outputs found

    Azimuthal dependence of pion source radii in Pb+Au collisions at 158 A GeV

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    We present results of a two-pion correlation analysis performed with the Au+Pb collision data collected by the upgraded CERES experiment in the fall of 2000. The analysis was done in bins of the reaction centrality and the pion azimuthal emission angle with respect to the reaction plane. The pion source, deduced from the data, is slightly elongated in the direction perpendicular to the reaction plane, similarly as was observed at the AGS and at RHIC.Comment: 5 pages, 2 figure

    Enhanced production of multi-strange hadrons in high-multiplicity proton-proton collisions

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    At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the quark-gluon plasma (QGP)(1). Such an exotic state of strongly interacting quantum chromodynamics matter is produced in the laboratory in heavy nuclei high-energy collisions, where an enhanced production of strange hadrons is observed(2-6). Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions(7), is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton-proton (pp) collisions(8,9), but the enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity proton-proton collisions. We find that the integrated yields of strange and multi-strange particles, relative to pions, increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with the p-Pb collision results(10,11), indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb-Pb collisions, where a QGP is formed.Peer reviewe

    Measurement of jet radial profiles in Pb\u2013Pb collisions at 1asNN = 2.76 TeV

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    The jet radial structure and particle transverse momentum (pT) composition within jets are presented in centrality-selected Pb\u2013Pb collisions at 1asNN = 2.76 TeV. Track-based jets, which are also called charged jets, were reconstructed with a resolution parameter of R = 0.3 at midrapidity |\u3b7ch jet| < 0.6 for transverse momenta pT, ch jet = 30\u2013120 GeV/c. Jet\u2013hadron correlations in relative azimuth and pseudorapidity space (\u3c6,\u3b7) 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\u2013Pb 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\u2013Pb 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\u2013Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb\u2013Pb 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\u2013Pb collisions, are compatible with both previous jet\u2013hadron-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

    Study of the \u39b\u2013\u39b interaction with femtoscopy correlations in pp and p\u2013Pb collisions at the LHC

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    This work presents new constraints on the existence and the binding energy of a possible \u39b\u2013\u39b bound state, the H-dibaryon, derived from \u39b\u2013\u39b femtoscopic measurements by the ALICE collaboration. The results are obtained from a new measurement using the femtoscopy technique in (Figure presented.) collisions at s=13 TeV and p\u2013Pb collisions at sNN=5.02 TeV, combined with previously published results from (Figure presented.) collisions at s=7 TeV. The \u39b\u2013\u39b scattering parameter space, spanned by the inverse scattering length f0 121 and the effective range d0, is constrained by comparing the measured \u39b\u2013\u39b correlation function with calculations obtained within the Lednick\ufd model. The data are compatible with hypernuclei results and lattice computations, both predicting a shallow attractive interaction, and permit to test different theoretical approaches describing the \u39b\u2013\u39b interaction. The region in the (f0 121,d0) plane which would accommodate a \u39b\u2013\u39b bound state is substantially restricted compared to previous studies. The binding energy of the possible \u39b\u2013\u39b bound state is estimated within an effective-range expansion approach and is found to be B\u39b\u39b=3.2 122.4+1.6(stat) 121.0+1.8(syst) MeV

    Spin alignment measurements using vector mesons with ALICE detector at the LHC

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    We present new measurements related to spin alignment of K*(0) vector mesons at mid-rapidity for Pb-Pb collisions at root s(NN) = 2.76 and 5.02 TeV. The spin alignment measurements are carried out with respect to production plane and 2nd order event plane. At low p(T) the spin density matrix element rho(00) for K*(0) is found to have values slightly below 1/3, while it is consistent with 1/3, i.e. no spin alignment, at high p(T). Similar values of rho(00) are observed with respect to both production plane and event plane. Within statistical and systematic uncertainties, rho(00) values are also found to be independent of root s(NN). rho(00) also shows centrality dependence with maximum deviation from 1/3 for mid-central collisions with respect to both the kinematic planes. The measurements for K*(0) in pp collisions at root s = 13 TeV and for K-s(0) (a spin 0 hadron) in 20-40\% central Pb-Pb collisions at root s(NN) = 2.76 TeV are consistent with no spin alignment

    Muon physics at forward rapidity with the ALICE detector upgrade

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    ALICE is the experiment specifically designed to study the Quark-Gluon Plasma (QGP) in heavy-ion collisions at the CERN LHC. The ALICE detector will be upgraded during the Long Shutdown 2, planned for 2019-2020, in order to cope with the maximum interaction rate of 50 kHz of Pb-Pb collisions foreseen for Runs 3 and 4. The ambitious programme of high-precision measurements, expected for muon physics after 2020, requires an upgrade of the front-end and readout electronics of the existing Muon Spectrometer. This concerns the Cathode Pad Chambers (CPC) used for tracking and the Resistive Plate Chambers (RPC) used for triggering and for muon identification. The Muon Forward Tracker (MFT), an internal tracker added in front of the front absorber of the existing Muon Spectrometer, is also part of the ALICE detector upgrade programme. It is based on an assembly of circular planes made of Monolithic Active Pixel Sensors (MAPS), covering the pseudorapidity range 2.5 < eta < 3.6. The MFT will improve present measurements and enable new ones. A selection of results from physics performance studies will be presented, together with an overview of the technical aspects of the upgrade project

    Open heavy-flavour production and elliptic flow in p-Pb collisions at the LHC with ALICE

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    Measurements of open heavy flavour production in p-A collisions allow the investigation of Cold Nuclear Matter effects. In addition, they are an important tool for a complementary investigation of the long-range correlations found in small systems in the light flavour sector. In this work, production measurements of D mesons at mid-rapidity in p-Pb collisions at root S-NN = 5.02 TeV are reported. Production yields are also reported for the heavy-flavour hadron decay electrons at central rapidity at root(SNN) = 5.02 and 8.16 TeV. The elliptic flow (nu(2)) of heavy-flavour hadron decay electrons in high multiplicity p-Pb collisions at root(SNN) = 5.02 TeV is found to be positive with a significance larger than 5 sigma

    Unveiling the strong interaction among hadrons at the LHC

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    ALICE Collaboration., Acharya, S., Adamová, D. et al. Publisher Correction: Unveiling the strong interaction among hadrons at the LHC. Nature 590, E13 (2021). https://doi.org/10.1038/s41586-020-03142-2The study of (anti-)deuteron production in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper the production of (anti-)deuterons is studied as a function of the charged particle multiplicity in inelastic pp collisions at root s = 13 TeV using the ALICE experiment. Thanks to the large number of accumulated minimum bias events, it has been possible to measure (anti-)deuteron production in pp collisions up to the same charged particle multiplicity (dN(ch)/d eta similar to 26) as measured in p-Pb collisions at similar centre-of-mass energies. Within the uncertainties, the deuteron yield in pp collisions resembles the one in p-Pb interactions, suggesting a common formation mechanism behind the production of light nuclei in hadronic interactions. In this context the measurements are compared with the expectations of coalescence and statistical hadronisation models (SHM).Peer reviewe

    Light (anti-)nuclei production and elliptic flow at the LHC with ALICE

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    Results on the production of stable light nuclei, including deuterons, He-3, He-4 and the corresponding anti-nuclei, in Pb-Pb collisions at root s(NN) = 2.76 TeV and root s(NN) = 5.02 TeV are presented and compared with theoretical predictions and with the results in small systems to provide insight into the production mechanisms of (anti-)nuclei at colliders. The experimental results are presented giving a critical view of their comparison to the expectations from coalescence and hydrodynamic models that aim at describing both the p(T)-spectra and the elliptic flow

    Higher moment fluctuations of identified particle distributions from ALICE

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    Cumulants of conserved charges fluctuations are regarded as a potential tool to study the criticality in the QCD phase diagram and to determine the freeze-out parameters in a model-independent way. At LHC energies, the measurements of the ratio of the net-baryon (net-proton) cumulants can be used to test the lattice QCD predictions. In this work, we present the first measurements of cumulants of the net-proton number distributions up to 4th4^{th} order in Pb--Pb collisions at sNN\sqrt{s_{\mathrm{NN}}} = 2.76 and 5.02 TeV as a function of collision centrality. We compare our cumulant ratios results with the STAR experiment net-proton results measured in the first phase of the Beam Energy Scan program at RHIC. The results can be used to obtain the chemical freeze-out parameters at LHC.Comment: 4 pages, 3 figures, Proceedings of XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018
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