262 research outputs found

    First measurement of the vertical bar t vertical bar-dependence of coherent J/psi photonuclear production

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    The first measurement of the cross section for coherent J/Psi photoproduction as a function of vertical bar t vertical bar, the square of the momentum transferred between the incoming and outgoing target nucleus, is presented. The data were measured with the ALICE detector in ultra-peripheral Pb-Pbcollisions at a centre-of-mass energy per nucleon pair root s(NN) = 5.02 TeV with the J/Psi produced in the central rapidity region vertical bar y vertical bar < 0.8, which corresponds to the small Bjorken-xrange (0.3 - 1.4) x 10(-3). The measured vertical bar t vertical bar-dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky-Kovchegov equation. These new results are therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken- x. (C) 2021 The Author(s). Published by Elsevier B.V.Peer reviewe

    Measurements of mixed harmonic cumulants in Pb-Pb collisions at root s(NN)=5.02 TeV

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    Correlations between moments of different flow coefficients are measured in Pb-Pb collisions at root s(NN) = 5.02 TeV recorded with the ALICE detector. These new measurements are based on multiparticle mixed harmonic cumulants calculated using charged particles in the pseudorapidity region vertical bar eta vertical bar < 0.8 with the transverse momentum range 0.2 < p(T) < 5.0 GeV/c. The centrality dependence of correlations between two flow coefficients as well as the correlations between three flow coefficients, both in terms of their second moments, are shown. In addition, a collection of mixed harmonic cumulants involving higher moments of v(2) and v(3) is measured for the first time, where the characteristic signature of negative, positive and negative signs of four-, six- and eight-particle cumulants are observed, respectively. The measurements are compared to the hydrodynamic calculations using iEBE-VISHNU with AMPT and TRENTo initial conditions. It is shown that the measurements carried out using the LHC Run 2 data in 2015 have the precision to explore the details of initial-state fluctuations and probe the nonlinear hydrodynamic response of v(2) and v(3) to their corresponding initial anisotropy coefficients epsilon(2) and epsilon(3). These new studies on correlations between three flow coefficients as well as correlations between higher moments of two different flow coefficients will pave the way to tighten constraints on initial-state models and help to extract precise information on the dynamic evolution of the hot and dense matter created in heavy-ion collisions at the LHC. (C) 2021 The Author. Published by Elsevier B.V.Peer reviewe

    Elliptic Flow of Electrons from Beauty-Hadron Decays in Pb-Pb Collisions at root s(NN)=5.02 TeV

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    The elliptic flow of electrons from beauty hadron decays at midrapidity (vertical bar y vertical bar < 0.8) is measured in Pb-Pb collisions at root s(NN) = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parametrized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, v(2). The v(2) coefficient of electrons from beauty hadron decays is measured for the first time in the transverse momentum (p(T)) range 1.3- 6 GeV/c in the centrality class 30%-50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length c tau approximate to 500 mu m compared to that of charm hadrons and most of the other background sources. The v(2) of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75 sigma. The results provide insights into the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavored by the measurement at high p(T), but is in agreement with the results at low p(T). Transport models including substantial interactions of beauty quarks with an expanding strongly interacting medium describe the measurement within uncertainties.Peer reviewe

    Measurement of the production cross section of prompt Xi(0)(c) baryons at midrapidity in pp collisions at root s=5.02 TeV

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    The transverse momentum (p(T)) differential cross section of the charm-strange baryon Xi(0)(c) is measured at midrapidity (vertical bar y vertical bar < 0.5) via its semileptonic decay into e(+)Xi(-)nu(e) in pp collisions at root s = 5.02TeV with the ALICE detector at the LHC. The ratio of the p(T)-differential Xi(0)(c)-baryon and D-0-meson production cross sections is also reported. The measurements are compared with simulations with different tunes of the PYTHIA 8 event generator, with predictions from a statistical hadronisation model (SHM) with a largely augmented set of charm-baryon states beyond the current lists of the Particle Data Group, and with models including hadronisation via quark coalescence. The pT-integrated cross section of prompt Xi(0)(c)-baryon production at midrapidity is also reported, which is used to calculate the baryon-to-meson ratio Xi(0)(c)/D-0 = 0.20 +/- 0.04 (stat.)(-0.07)(+0.08) (syst.). These results provide an additional indication of a modification of the charm fragmentation from e(+) e(-) and e-p collisions to pp collisions.Peer reviewe

    Lambda K femtoscopy in Pb-Pb collisions at root s(NN)=2.76 TeV

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    The first measurements of the scattering parameters of Lambda K pairs in all three charge combinations (Lambda K+, Lambda K-, and Lambda K-S(0)) are presented. The results are achieved through a femtoscopic analysis of Lambda K correlations in Pb-Pb collisions at root s(NN) = 2.76 TeV recorded by ALICE at the Large Hadron Collider. The femtoscopic correlations result from strong final-state interactions and are fit with a parametrization allowing for both the characterization of the pair emission source and the measurement of the scattering parameters for the particle pairs. Extensive studies with the THERMINATOR 2 event generator provide a good description of the nonfemtoscopic background, which results mainly from collective effects, with unprecedented precision. Furthermore, together with HIJING simulations, this model is used to account for contributions from residual correlations induced by feed-down from particle decays. The extracted scattering parameters indicate that the strong force is repulsive in the Lambda K+ interaction and attractive in the Lambda K- interaction. The data hint that the Lambda K-S(0) interaction is attractive; however, the uncertainty of the result does not permit such a decisive conclusion. The results suggest an effect arising either from different quark-antiquark interactions between the pairs (s (s) over bar in Lambda K+ and u (u) over bar in Lambda K-) or from different net strangeness for each system (S = 0 for Lambda K+, and S = -2 for Lambda K-). Finally, the Lambda K systems exhibit source radii larger than expected from extrapolation from identical particle femtoscopic studies. This effect is interpreted as resulting from the separation in space-time of the single-particle Lambda and K source distributions.Peer reviewe

    Direct observation of the dead-cone effect in quantum chromodynamics

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    A Publisher Correction to this article was published on 07 July 2022: https://doi.org/10.1038/s41586-022-05026-zIn particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD)(1). These partons subsequently emit further partons in a process that can be described as a parton shower(2), which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m(Q) and energy E, within a cone of angular size m(Q)/E around the emitter(3). Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques(4,5) to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.Peer reviewe

    Measurement of K*(892)(+/-) production in inelastic pp collisions at the LHC

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    The first results on K*(892)(+/-) resonance production in inelastic pp collisions at LHC energies of root s = 5.02, 8, and 13 TeV are presented. The K*(892)(+/-) (;)has been reconstructed via its hadronic decay channel K*(892)(+/-) -> K-S(0) + pi(+/-) with the ALICE detector. Measurements of transverse momentum distributions, p(T)-integrated yields, and mean transverse momenta for charged K*(892) are found to be consistent with previous ALICE measurements for neutral K*(892) within uncertainties. For p(T) > 1 GeV/c the K*(892)(+/-) transverse momentum spectra become harder with increasing centre-of-mass energy from 5.02 to 13 TeV, similar to what previously observed for charged kaons and pions. For p(T) < 1 GeV/c the K*(892)(+/-) yield does not evolve significantly and the abundance of K*(892)(+/-) relative to K is rather independent of the collision energy. The transverse momentum spectra, measured for K*(892)(+/-) at midrapidity in the interval 0 < p(T) < 15 GeV/c, are not well described by predictions of different versions of PYTHIA 6, PYTHIA 8 and EPOS-LHC event generators. These generators reproduce the measured p(T)-integrated K*(+/-)/K ratios and describe well the momentum dependence for p(T) < 2 GeV/c. (c) 2022 European Organization for Nuclear Research. Published by Elsevier B.V.Peer reviewe

    Prompt D-0, D+, and D*(+) production in Pb-Pb collisions at root S-NN=5.02 TeV

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    The production of prompt D-0, D+, and D*(+) mesons was measured at midrapidity (vertical bar y vertical bar < 0.5) in Pb-Pb collisions at the centre-of-mass energy per nucleon-nucleon pair root S-NN = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decay channels and their production yields were measured in central (0-10%) and semicentral (30-50%) collisions. The measurement was performed up to a transverse momentum (p(T)) of 36 or 50 GeV/c depending on the D meson species and the centrality interval. For the first time in Pb-Pb collisions at the LHC, the yield of D-0 mesons was measured down to p(T) = 0, which allowed a model-independent determination of the p(T)-integrated yield per unit of rapidity (dN/dy). A maximum suppression by a factor 5 and 2.5 was observed with the nuclear modification factor (R-AA) of prompt D mesons at p(T) = 6-8 GeV/c for the 0-10% and 30-50% centrality classes, respectively. The D-meson R-AA is compared with that of charged pions, charged hadrons, and J/psi mesons as well as with theoretical predictions. The analysis of the agreement between the measured R-AA, elliptic (v(2)) and triangular (v(3)) flow, and the model predictions allowed us to constrain the charm spatial diffusion coefficient D-s. Furthermore the comparison of R-AA and v(2) with different implementations of the same models provides an important insight into the role of radiative energy loss as well as charm quark recombination in the hadronisation mechanisms.Peer reviewe

    Forward rapidity J/psi production as a function of charged-particle multiplicity in pp collisions at root s=5.02 and 13 TeV

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    The production of J/psi is measured as a function of charged-particle multiplicity at forward rapidity in proton-proton (pp) collisions at center-of-mass energies root s = 5.02 and 13 TeV. The J/psi mesons are reconstructed via their decay into dimuons in the rapidity interval (2.5 ), at both the colliding energies. Measurements are compared with available ALICE results at midrapidity and theoretical model calculations. First measurement of the mean transverse momentum () of J/psi in pp collisions exhibits an increasing trend as a function of dN(ch)/d eta/ showing a saturation towards high charged-particle multiplicities.Peer reviewe
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