23 research outputs found

    Particle-yield modification in jet-like azimuthal di-hadron correlations in Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 2.76 TeV

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    The yield of charged particles associated with high-pTp_{\rm T} trigger particles (8<pT<158 < p_{\rm T} < 15 GeV/cc) is measured with the ALICE detector in Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 2.76 TeV relative to proton-proton collisions at the same energy. The conditional per-trigger yields are extracted from the narrow jet-like correlation peaks in azimuthal di-hadron correlations. In the 5% most central collisions, we observe that the yield of associated charged particles with transverse momenta pT>3p_{\rm T}> 3 GeV/cc on the away-side drops to about 60% of that observed in pp collisions, while on the near-side a moderate enhancement of 20-30% is found.Comment: 15 pages, 2 captioned figures, 1 table, authors from page 10, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/350

    Evolution of the longitudinal and azimuthal structure of the near-side jet peak in Pb-Pb collisions at 1asNN = 2.76 TeV

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    In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher-pT trigger particle. Measurements of these correlations as a function of pseudorapidity ( \u3b7) and azimuthal ( \u3c6) differences are used to extract the centrality and pT dependence of the shape of the near-side peak in the pT range 1 < pT < 8 GeV/c in Pb-Pb and pp collisions at 1asNN = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the \u3c6 direction is almost independent of centrality, a significant broadening in the \u3b7 direction is found from peripheral to central collisions. This feature is prominent for the low-pT region and vanishes above 4 GeV/c. The widths measured in peripheral collisions are equal to those in pp collisions in the \u3c6 direction and above 3 GeV/c in the \u3b7 direction. Furthermore, for the 10% most central collisions and 1 < pT,assoc < 2 GeV/c, 1 < pT,trig < 3 GeV/c, a departure from a Gaussian shape is found: a depletion develops around the center of the peak. The results are compared to A Multi-Phase Transport (AMPT) model simulation as well as other theoretical calculations indicating that the broadening and the development of the depletion are connected to the strength of radial and longitudinal flow

    Centrality dependence of \u3c0, K, and p production in Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV

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    In this paper measurements are presented of \u3c0\ub1, K\ub1, p, and p(bar) production at midrapidity (|y| < 0.5), in Pb-Pb collisions at 1asNN= 2.76 TeV as a function of centrality. The measurement covers the transverse-momentum (pT ) range from 100, 200, and 300 MeV/c up to 3, 3, and 4.6 GeV/c for \u3c0, K, and p, respectively. The measured pT distributions and yields are compared to expectations based on hydrodynamic, thermal and recombination models. The spectral shapes of central collisions show a stronger radial flow than measured at lower energies, which can be described in hydrodynamic models. In peripheral collisions, the pT distributions are not well reproduced by hydrodynamic models. Ratios of integrated particle yields are found to be nearly independent of centrality. The yield of protons normalized to pions is a factor 3c1.5 lower than the expectation from thermal models

    Transverse Momentum Distribution and Nuclear Modification Factor of Charged Particles in p+Pb Collisions at sqrt[s_{NN}]=5.02\u2009\u2009TeV

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    The transverse momentum (pT) distribution of primary charged particles is measured in minimum bias (non-single-diffractive) p+Pb collisions at 1asNN=5.02 TeV with the ALICE detector at the LHC. The p T spectra measured near central rapidity in the range 0.5<p T<20 GeV/c exhibit a weak pseudorapidity dependence. The nuclear modification factor RpPb is consistent with unity for pT above 2 GeV/c. This measurement indicates that the strong suppression of hadron production at high pT observed in Pb+Pb collisions at the LHC is not due to an initial-state effect. The measurement is compared to theoretical calculations

    J/\u3c8 Elliptic Flow in Pb-Pb Collisions at sqrt[s_{NN}]=2.76\u2009\u2009TeV

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    We report on the first measurement of inclusive J/\u3c8 elliptic flow v2 in heavy-ion collisions at the LHC. The measurement is performed with the ALICE detector in Pb-Pb collisions at 1asNN=2.76 TeV in the rapidity range 2,5<y<4,0. The dependence of the J=c v2 on the collision centrality and on the J/\u3c8 transverse momentum is studied in the range 0 64pT 64 10 GeV/c. For semicentral Pb-Pb collisions at 1asNN=2.76 TeV, an indication of nonzero v2 is observed with a largest measured value of v2=0.116 \ub1 0.046 (stat) \ub1 0:029 (syst) for J/\u3c8 in the transverse momentum range 2 64 pT 64 4 GeV/c. The elliptic flow measurement complements the previously reported ALICE results on the inclusive J/\u3c8 nuclear modification factor and favors the scenario of a significant fraction of J/\u3c8 production from charm quarks in a deconfined partonic phase

    Event-by-event mean pT \ua0fluctuations in pp and Pb\u2013Pb collisions at the LHC

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    Event-by-event fluctuations of the mean transverse momentum of charged particles produced in pp collisions at 1as = 0.9, 2.76 and 7 TeV, and Pb\u2013Pb collisions at 1asNN = 2.76 TeV are studied as a function of the charged-particle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations indicative of correlated particle emis- sion are observed in all systems. The results in pp collisions show little dependence on collision energy. The Monte Carlo event generators PYTHIA and PHOJET are in qualitative agreement with the data. Peripheral Pb\u2013Pb data exhibit a similar multiplicity dependence as that observed in pp. In central Pb\u2013Pb, the results deviate from this trend, featuring a significant reduction of the fluctuation strength. The results in Pb\u2013 Pb are in qualitative agreement with previous measurements in Au\u2013Au at lower collision energies and with expectations from models that incorporate collective phenomena

    Studies of J/\u3c8 production at forward rapidity in Pb\u2013Pb collisions at sNNsqrtsmathrmNN sqrts_mathrmNN = 5.02 TeV

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    The inclusive J/\u3c8 production in Pb\u2013Pb collisions at the center-of-mass energy per nucleon pair sNN 12 12 12 1a = 5.02 TeV, measured with the ALICE detector at the CERN LHC, is reported. The J/\u3c8 meson is reconstructed via the dimuon decay channel at forward rapidity (2.5 < y < 4) down to zero transverse momentum. The suppression of the J/\u3c8 yield in Pb\u2013Pb collisions with respect to binary-scaled pp collisions is quantified by the nuclear modification factor (RAA). The RAA at sNN 12 12 12 1a = 5.02 TeV is presented and compared with previous measurements at sNN 12 12 12 1a = 2.76 TeV as a function of the centrality of the collision, and of the J/\u3c8 transverse momentum and rapidity. The inclusive J/\u3c8 RAA shows a suppression increasing toward higher transverse momentum, with a steeper dependence for central collisions. The modification of the J/\u3c8 average transverse momentum and average squared transverse momentum is also studied. Comparisons with the results of models based on a transport equation and on statistical hadronization are carried out

    Multiparticle azimuthal correlations in p -Pb and Pb-Pb collisions at the CERN Large Hadron Collider

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    Measurements of multiparticle azimuthal correlations (cumulants) for charged particles in p-Pb at root s(NN) = 5.02 TeV and Pb-Pb at root s(NN) = 2.76 TeV collisions are presented. They help address the question of whether there is evidence for global, flowlike, azimuthal correlations in the p-Pb system. Comparisons are made to measurements from the larger Pb-Pb system, where such evidence is established. In particular, the second harmonic two-particle cumulants are found to decrease with multiplicity, characteristic of a dominance of few-particle correlations in p-Pb collisions. However, when a vertical bar Delta eta vertical bar gap is placed to suppress such correlations, the two-particle cumulants begin to rise at high multiplicity, indicating the presence of global azimuthal correlations. The Pb-Pb values are higher than the p-Pb values at similar multiplicities. In both systems, the second harmonic four-particle cumulants exhibit a transition from positive to negative values when the multiplicity increases. The negative values allow for a measurement of v(2){4} to be made, which is found to be higher in Pb-Pb collisions at similar multiplicities. The second harmonic six-particle cumulants are also found to be higher in Pb-Pb collisions. In Pb-Pb collisions, we generally find v(2){4} similar or equal to v(2){6} not equal 0 which is indicative of a Bessel-Gaussian function for the v(2) distribution. For very high-multiplicity Pb-Pb collisions, we observe that the four-and six-particle cumulants become consistent with 0. Finally, third harmonic two-particle cumulants in p-Pb and Pb-Pb are measured. These are found to be similar for overlapping multiplicities, when a vertical bar Delta eta vertical bar > 1.4 gap is placed

    Neutral pion production at midrapidity in pp and Pb\u2013Pb collisions at 1asNN = 2.76 TeV

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    Invariant yields of neutral pions at midrapidity in the transverse momentum range 0.6 < pT < 12 GeV/c measured in Pb\u2013Pb collisions at 1asNN = 2.76 TeV are presented for six centrality classes. The pp reference spectrum was measured in the range 0.4 < pT < 10 GeV/c at the same center-of-mass energy. The nuclear modification factor, RAA , shows a suppression of neutral pions in central Pb\u2013Pb collisions by a factor of up to about 8 1210 for 5 pT 7GeV/c. The presented measurements are compared with results at lower center-of-mass energies and with theoretical calculations

    Production of (anti-)He-3 and (anti-)H-3 in p-Pb collisions at root s(NN)=5.02 TeV

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    The transverse momentum (p(T)) differential yields of (anti-)He-3 and (anti-)H-3 measured in p-Pb collisions at root s(NN) = 5.02 TeV with ALICE at the Large Hadron Collider (LHC) are presented. The ratios of the p(T)-integrated yields of (anti-)He-3 and (anti-)H-3 to the proton yields are reported, as well as the pT dependence of the coalescence parameters B-3 for (anti-)He-3 and (anti-)H-3. For (anti-)He-3, the results obtained in four classes of the mean charged-particle multiplicity density are also discussed. These results are compared to predictions from a canonical statistical hadronization model and coalescence approaches. An upper limit on the total yield of (4)(He) over bar is determined
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