Browsing by Subject "FREEZE-OUT"

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  • Niemi, H.; Eskola, K. J.; Paatelainen, R. (2016)
    We introduce an event-by-event perturbative-QCD + saturation + hydro ("EKRT") framework for ultrarelativistic heavy-ion collisions, where we compute the produced fluctuating QCD-matter energy densities from next-to-leading-order perturbative QCD using a saturation conjecture to control soft-particle production and describe the space-time evolution of the QCD matter with dissipative fluid dynamics, event by event. We perform a simultaneous comparison of the centrality dependence of hadronic multiplicities, transverse momentum spectra, and flow coefficients of the azimuth-angle asymmetries against the LHC and RHIC measurements. We compare also the computed event-by-event probability distributions of relative fluctuations of elliptic flow and event-plane angle correlations with the experimental data from Pb + Pb collisions at the LHC. We show how such a systematic multienergy and multiobservable analysis tests the initial-state calculation and the applicability region of hydrodynamics and, in particular, how it constrains the temperature dependence of the shear viscosity-to-entropy ratio of QCD matter in its different phases in a remarkably consistent manner.
  • The ALICE collaboration; Acharya, S.; Brucken, E. J.; Hilden, T. E.; Kim, D. J.; Krizek, F.; Litichevskyi, V.; Novitzky, Norbert; Orava, R.; Parkkila, J. E.; Rak, J.; Rytkonen, H.; Räsänen, Sami; Saarimäki, Oskari Antti Matti; Slupecki, M.; Snellman, T. W.; Trzaska, W. H.; Zhou, Z. (2020)
    Measurements of K*(892)(0) and phi(1020) resonance production in Pb-Pb and pp collisions at root s(NN) = 5.02 TeV with the ALICE detector at the Large Hadron Collider are reported. The resonances are measured at midrapidity (vertical bar y vertical bar <0.5) via their hadronic decay channels and the transverse momentum (p(T)) distributions are obtained for various collision centrality classes up to p(T) = 20 GeV/c. The p(T)-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 phi(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 phi(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 similar to 4-7 fm/c for central collisions. The p(T)-differential ratios of K*(892)(0)/K, phi(1020)/K, K*(892)(0)/pi, phi(1020)/pi, p/K*(892)(0) and p/phi(1020) are also presented for Pb-Pb and pp collisions at root s(NN) = 5.02 TeV. These ratios show that the rescattering effect is predominantly a low-p(T) phenomenon. (C) 2020 The Author(s). Published by Elsevier B.V.
  • Gallmeister, K.; Niemi, H.; Greiner, C.; Rischke, D. H. (2018)
    Background: Experimental data from heavy-ion experiments at RHIC-BNL and LHC-CERN are quantitatively described using relativistic fluid dynamics. Even p + A and p + p collisions show signs of collective behavior describable in the same manner. Nevertheless, small system sizes and large gradients strain the limits of applicability of fluid-dynamical methods. Purpose: The range of applicability of fluid dynamics for the description of the collective behavior, and in particular of the elliptic flow, of small systems needs to be explored. Method: Results of relativistic fluid-dynamical simulations are compared with solutions of the Boltzmann equation in a longitudinally boost-invariant picture. As the initial condition, several different transverse energy-density profiles for equilibrated matter are investigated. Results: While there is overall a fair agreement of energy- and particle-density profiles, components of the shear-stress tensor are more sensitive to details of the implementation. The highest sensitivity is exhibited by quantities influenced by properties of the medium at freeze-out. Conclusions: For some quantities, like the shear-stress tensor, agreement between fluid dynamics and transport theory extends into regions of Knudsen numbers and inverse Reynolds numbers where relativistic fluid dynamics is believed to fail.
  • The ALICE collaboration; Acharya, S.; Brücken, E. J.; Chang, B.; Hilden, T. E.; Kim, D. J.; Litichevskyi, V.; Mieskolainen, M. M.; Orava, R.; Parkkila, J. E.; Rak, J.; Räsänen, S. S.; Saarinen, S.; Slupecki, M.; Snellman, T. W.; Trzaska, W. H.; Vargyas, M.; Viinikainen, J. (2019)
    Comprehensive results on the production of unidentified charged particles, pi(+/-), K-+/-, K-s(0), K*(892)(0), p, (p) over bar, phi(1020), Lambda, (Lambda) over bar, Xi(-) , (Xi) over bar (+), Omega(-), and (Omega) over bar (+) hadrons in proton-proton (pp) collisions at root s = 7 TeV at midrapidity (vertical bar y vertical bar < 0.5) as a function of charged-particle multiplicity density are presented. In order to avoid autocorrelation biases, the actual transverse momentum (p(T)) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particleto-pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at Large Hadron Collider energies. The obtained p(T), distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions.
  • The ALICE collaboration; Acharya, S.; Brücken, E. J.; Chang, B.; Kim, D. J.; Litichevskyi, V.; Mieskolainen, M. M.; Orava, R.; Rak, J.; Räsänen, S. S.; Saarinen, S.; Slupecki, M.; Snellman, T. W.; Trzaska, W. H.; Vargyas, M.; Viinikainen, J. (2018)
    Neutral pion and eta meson production in the transverse momentum range 1 < p(T) < 20 GeV/c have been measured at midrapidity by the ALICE experiment at the Large Hadron Collider (LHC) in central and semicentral Pb-Pb collisions at root S-NN = 2.76 TeV. These results were obtained using the photon conversion method as well as the Photon Spectrometer (PHOS) and Electromagnetic Calorimeter detectors. The results extend the upper p(T) reach of the previous ALICE pi(0) measurements from 12 to 20 GeV/c and present the first measurement of eta meson production in heavy-ion collisions at the LHC. The eta/pi(0) ratio is similar for the two centralities and reaches at high p(T) a plateau value of 0.457 +/- 0.013(stat) +/- 0.018(syst) A suppression of similar magnitude for pi(0) and eta meson production is observed in Pb-Pb collisions with respect to their production in pp collisions scaled by the number of binary nucleon-nucleon collisions. We discuss the results in terms of Next to Leading Order (NLO) pQCD predictions and hydrodynamic models. The measurements show a stronger suppression than observed at lower center-of-mass energies in the p T range 6 < p(T) < 10 GeV/c. For p(T) < 3 GeV/c, hadronization models describe the pi(0) results while for the eta some tension is observed.
  • The ALICE collaboration; Acharya, S.; Adamova, D.; Kim, D. J.; Krizek, F.; Parkkila, J. E.; Rak, J.; Rytkönen, Heidi Maria; Räsänen, Sami; Saarimäki, Oskari Antti Matti; Slupecki, M.; Trzaska, W. H.; Zhou, Zhipeng (2021)
    In this paper, the first femtoscopic analysis of pion-kaon correlations at the LHC is reported. The analysis was performed on the Pb-Pb collision data at root(S)(NN) = 2.76 TeV recorded with the ALICE detector. The non-identical particle correlations probe the spatio-temporal separation between sources of different particle species as well as the average source size of the emitting system. The sizes of the pion and kaon sources increase with centrality, and pions are emitted closer to the centre of the system and/or later than kaons. This is naturally expected in a system with strong radial flow and is qualitatively reproduced by hydrodynamic models. ALICE data on pion-kaon emission asymmetry are consistent with (3+1)-dimensional viscous hydrodynamics coupled to a statistical hadronisation model, resonance propagation, and decay code THERMINATOR 2 calculation, with an additional time delay between 1 and 2 fm/c for kaons. The delay can be interpreted as evidence for a significant hadronic rescattering phase in heavy-ion collisions at the LHC. (C) 2020 The Author. Published by Elsevier B.V.
  • Adamova, D.; Brucken, E. J.; Chang, B.; Kim, D. J.; Litichevskyi, V.; Mieskolainen, M. M.; Orava, R.; Rak, J.; Räsänen, S. S.; Snellman, T. W.; Trzaska, W. H.; Viinikainen, J.; The ALICE collaboration (2017)
    The transverse momentum distributions of the strange and double-strange hyperon resonances ( Sigma ( 1385)(+/-), Xi ( 1530)(0)) produced in p-Pb collisions at root s(NN) = 5.02 TeV were measured in the rapidity range -0.5 <yCMS <0 for event classes corresponding to different charged-particle multiplicity densities, <dN(ch)/d eta(lab)> . The mean transverse momentumvalues are presented as a function of <dNch/d eta(lab)>, as well as a function of the particle masses and compared with previous results on hyperon production. The integrated yield ratios of excited to ground- state hyperons are constant as a function of <dN(ch)/d eta(lab)>. The equivalent ratios to pions exhibit an increase with <dN(ch)/ d eta(lab)>, depending on their strangeness content.
  • The ALICE collaboration; Acharya, S.; Brücken, E. J.; Chang, B.; Hilden, T. E.; Kim, D. J.; Litichevskyi, V.; Mieskolainen, M. M.; Orava, R.; Parkkila, J. E.; Rak, J.; Räsänen, S. S.; Saarinen, S.; Slupecki, M.; Snellman, T. W.; Trzaska, W. H.; Vargyas, M.; Viinikainen, J. (2019)
    The production yield of the Lambda(1520) baryon resonance is measured at midrapidity in Pb-Pb collisions at root S-NN = 2.76 TeV with the ALICE detector at the Large Hadron Collider (LHC). The measurement is performed in the Lambda(1520) -> pK(-) (and charge conjugate) hadronic decay channel as a function of the transverse momentum (p(T)) and collision centrality. The ratio of the P-T-integrated production of Lambda(1520) baryons relative to A baryons in central collisions is suppressed by about a factor of 2 with respect to peripheral collisions. This is the first observation of the suppression of a baryonic resonance at the LHC and the first 3 sigma evidence of Lambda(1520) suppression within a single collision system. The measured Lambda(1520)/Lambda ratio in central collisions is smaller than the value predicted by the statistical hadronization model calculations. The shape of the measured p(T) distribution and the centrality dependence of the suppression are reproduced by the EPOS3 Monte Carlo event generator. The measurement adds further support to the formation of a dense hadronic phase in the final stages of the evolution of the fireball created in heavy-ion collisions, lasting long enough to cause a significant reduction in the observable yield of short-lived resonances.