• Centrality dependence of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at √ s N N = 5.02  TeV.

      ALICE Collaboration; Barnby, Lee; University of Birmingham (APS Physics, 2016-06-03)
      The pseudorapidity density of charged particles, d N ch / d η , at midrapidity in Pb-Pb collisions has been measured at a center-of-mass energy per nucleon pair of √ s N N = 5.02     TeV . For the 5% most central collisions, we measure a value of 1943 ± 54 . The rise in d N ch / d η as a function of √ s N N is steeper than that observed in proton-proton collisions and follows the trend established by measurements at lower energy. The increase of d N ch / d η as a function of the average number of participant nucleons, ⟨ N part ⟩ , calculated in a Glauber model, is compared with the previous measurement at √ s N N = 2.76     TeV . A constant factor of about 1.2 describes the increase in d N ch / d η from √ s N N = 2.76 to 5.02 TeV for all centrality classes, within the measured range of 0%–80% centrality. The results are also compared to models based on different mechanisms for particle production in nuclear collisions.
    • Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb–Pb collisions at √sNN = 2.76 TeV.

      ALICE Collaboration; Barnby, Lee; University of Birmingham (Elsevier, 2016-01-26)
      The centrality dependence of the charged-particle pseudorapidity density measured with ALICE in Pb–Pb collisions at √sNN = 2.76 TeV over a broad pseudorapidity range is presented. This Letter extends the previous results reported by ALICE to more peripheral collisions. No strong change of the overall shape of charged-particle pseudorapidity density distributions with centrality is observed, and when normalised to the number of participating nucleons in the collisions, the evolution over pseudorapidity with centrality is likewise small. The broad pseudorapidity range (−3.5 < η < 5) allows precise estimates of the total number of produced charged particles which we find to range from 162 ± 22(syst.) to 17170 ± 770(syst.) in 80–90% and 0–5% central collisions, respectively. The total charged-particle multiplicity is seen to approximately scale with the number of participating nucleons in the collision. This suggests that hard contributions to the charged-particle multiplicity are limited. The results are compared to models which describe dNch/dη at mid-rapidity in the most central Pb–Pb collisions and it is found that these models do not capture all features of the distributions
    • Correlated event-by-event fluctuations of flow harmonics in Pb-Pb Collisions at √sNN = 2.76 TeV

      ALICE Collaboration; Barnby, Lee; European Organization for Nuclear Research (CERN); University of Birmingham (American Physical Society, 2016-10-28)
      We report the measurements of correlations between event-by-event fluctuations of amplitudes of anisotropic flow harmonics in nucleus-nucleus collisions, obtained for the first time using a new analysis method based on multiparticle cumulants in mixed harmonics. This novel method is robust against systematic biases originating from nonflow effects and by construction any dependence on symmetry planes is eliminated. We demonstrate that correlations of flow harmonics exhibit a better sensitivity to medium properties than the individual flow harmonics. The new measurements are performed in Pb-Pb collisions at the center-of-mass energy per nucleon pair of √sNN=2.76  TeV by the ALICE experiment at the Large Hadron Collider. The centrality dependence of correlation between event-by-event fluctuations of the elliptic v2 and quadrangular v4 flow harmonics, as well as of anticorrelation between v2 and triangular v3 flow harmonics are presented. The results cover two different regimes of the initial state configurations: geometry dominated (in midcentral collisions) and fluctuation dominated (in the most central collisions). Comparisons are made to predictions from Monte Carlo Glauber, viscous hydrodynamics, ampt, and hijing models. Together with the existing measurements of the individual flow harmonics the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions.
    • D-meson production in p-Pb collisions at √sNN = 5.02 TeV and in pp collisions at √s = 7 TeV

      ALICE Collaboration; Barnby, Lee; European Organization for Nuclear Research (CERN); University of Birmingham (American Physical Society, 2016-11-23)
      Background: In the context of the investigation of the quark gluon plasma produced in heavy-ion collisions, hadrons containing heavy (charm or beauty) quarks play a special role for the characterization of the hot and dense medium created in the interaction. The measurement of the production of charm and beauty hadrons in proton– proton collisions, besides providing the necessary reference for the studies in heavy-ion reactions, constitutes an important test of perturbative quantum chromodynamics (pQCD) calculations. Heavy-flavor production in proton–nucleus collisions is sensitive to the various effects related to the presence of nuclei in the colliding system, commonly denoted cold-nuclear-matter effects. Most of these effects are expected to modify open-charm production at low transverse momenta (pT) and, so far, no measurement of D-meson production down to zero transverse momentum was available at mid-rapidity at the energies attained at the CERN Large Hadron Collider (LHC). Purpose: The measurements of the production cross sections of promptly produced charmed mesons in p-Pb collisions at the LHC down to pT = 0 and the comparison to the results from pp interactions are aimed at the assessment of cold-nuclear-matter effects on open-charm production, which is crucial for the interpretation of the results from Pb-Pb collisions. Methods: The prompt charmed mesons D0, D+, D∗+, and Ds+ were measured at mid-rapidity in p-Pb collisions at a center-of-mass energy per nucleon pair √sNN = 5.02 TeV with the ALICE detector at the LHC.D mesons were reconstructed from their decays D0 → K−π+, D+ → K−π+π+, D∗+ → D0π+, Ds+ → φπ+ → K−K+π+, and their charge conjugates, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. In addition, the prompt D0 production cross section was measured in pp collisions at √s = 7 TeV and p-Pb collisions at √sNN = 5.02 TeV down to pT = 0 using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D0 decay vertex. Results: The production cross section in pp collisions is described within uncertainties by different implementations of pQCD calculations down to pT = 0. This allowed also a determination of the total cc ̄ production cross section in pp collisions, which is more precise than previous ALICE measurements because it is not affected by uncertainties owing to the extrapolation to pT = 0. The nuclear modification factor RpPb(pT), defined as the ratio of the pT-differential D meson cross section in p-Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium. Conclusions: These measurements add experimental evidence that the modification of the D-meson transverse momentum distributions observed in Pb–Pb collisions with respect to pp interactions is due to strong final-state effects induced by the interactions of the charm quarks with the hot and dense partonic medium created in ultrarelativistic heavy-ion collisions. The current precision of the measurement does not allow us to draw conclusions on the role of the different cold-nuclear-matter effects and on the possible presence of additional hot-medium effects in p-Pb collisions. However, the analysis technique without decay-vertex reconstruction, applied on future larger data samples, should provide access to the physics-rich range down to pT = 0
    • Elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity in Pb-Pb collisions at √sNN = 2.76 TeV

      ALICE Collaboration; Barnby, Lee; European Organization for Nuclear Research (CERN); University of Birmingham (Springer Berlin Heidelberg, 2016-09-06)
      The elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity (|y| < 0.7) is measured in Pb-Pb collisions at √sNN=2.76 TeV with ALICE at the LHC. The particle azimuthal distribution with respect to the reaction plane can be parametrized with a Fourier expansion, where the second coefficient (v2) represents the elliptic flow. The v2 coefficient of inclusive electrons is measured in three centrality classes (0–10%, 10–20% and 20–40%) with the event plane and the scalar product methods in the transverse momentum (pT) intervals 0.5–13 GeV/c and 0.5–8 GeV/c, respectively. After subtracting the background, mainly from photon conversions and Dalitz decays of neutral mesons, a positive v2 of electrons from heavy-flavour hadron decays is observed in all centrality classes, with a maximum significance of 5.9σ in the interval 2 < pT < 2.5 GeV/c in semi-central collisions (20–40%). The value of v2 decreases towards more central collisions at low and intermediate pT (0.5 < pT < 3 GeV/c). The v2 of electrons from heavy-flavour hadron decays at mid-rapidity is found to be similar to the one of muons from heavy-flavour hadron decays at forward rapidity (2.5 < y < 4). The results are described within uncertainties by model calculations including substantial elastic interactions of heavy quarks with an expanding strongly-interacting medium.
    • J/ψ production and nuclear effects in p-Pb collisions at √sNN=5.02 TeV.

      ALICE Collaboration; Barnby, Lee; University of Birmingham (Springer, 2014-02-18)
      Inclusive J/ψ production has been studied with the ALICE detector in p-Pb collisions at the nucleon-nucleon center of mass energy sNN−−−√ = 5.02 TeV at the CERN LHC. The measurement is performed in the center of mass rapidity domains 2.03 < y cms < 3.53 and −4.46 < y cms < −2.96, down to zero transverse momentum, studying the μ + μ − decay mode. In this paper, the J/ψ production cross section and the nuclear modification factor R pPb for the rapidities under study are presented. While at forward rapidity, corresponding to the proton direction, a suppression of the J/ψ yield with respect to binary-scaled pp collisions is observed, in the backward region no suppression is present. The ratio of the forward and backward yields is also measured differentially in rapidity and transverse momentum. Theoretical predictions based on nuclear shadowing, as well as on models including, in addition, a contribution from partonic energy loss, are in fair agreement with the experimental results.
    • Measurement of D-meson production versus multiplicity in p-Pb collisions at √sNN = 5.02 TeV

      ALICE Collaboration; Barnby, Lee; University of Birmingham (Springer Berlin Heidelberg, 2016-08-11)
      The measurement of prompt D-meson production as a function of multiplicity in p-Pb collisions at √sNN=5.02 TeV with the ALICE detector at the LHC is reported. D0, D+ and D*+ mesons are reconstructed via their hadronic decay channels in the centre-of-mass rapidity range −0.96 < ycms< 0.04 and transverse momentum interval 1
    • Measurement of transverse energy at midrapidity in Pb-Pb collisions at √sNN = 2.76 TeV

      ALICE Collaboration; Barnby, Lee; European Organization for Nuclear Research (CERN); University of Birmingham (American Physical Society, 2016-09-15)
      We report the transverse energy (ET) measured with ALICE at midrapidity in Pb-Pb collisions at √sNN=2.76 TeV as a function of centrality. The transverse energy was measured using identified single-particle tracks. The measurement was cross checked using the electromagnetic calorimeters and the transverse momentum distributions of identified particles previously reported by ALICE. The results are compared to theoretical models as well as to results from other experiments. The mean ET per unit pseudorapidity (η), 〈dET/dη〉, in 0%–5% central collisions is 1737±6(stat.)±97(sys.) GeV. We find a similar centrality dependence of the shape of 〈dET/dη〉 as a function of the number of participating nucleons to that seen at lower energies. The growth in 〈dET/dη〉 at the LHC energies exceeds extrapolations of low-energy data. We observe a nearly linear scaling of 〈dET/dη〉 with the number of quark participants. With the canonical assumption of a 1 fm/c formation time, we estimate that the energy density in 0%–5% central Pb-Pb collisions at √sNN=2.76 TeV is 12.3±1.0 GeV/fm3 and that the energy density at the most central 80fm2 of the collision is at least 21.5±1.7 GeV/fm3. This is roughly 2.3 times that observed in 0%–5% central Au-Au collisions at √sNN=200 GeV.
    • Multi-strange baryon production at mid-rapidity in Pb–Pb collisions at √sNN = 2.76 TeV.

      ALICE Collaboration; Barnby, Lee; University of Birmingham (Elsevier, 2013-11-26)
      The production of − and − baryons and their anti-particles in Pb–Pb collisions at √sNN = 2.76 TeV has been measured using the ALICE detector. The transverse momentum spectra at mid-rapidity (|y| < 0.5) for charged and hyperons have been studied in the range 0.6 < pT < 8.0 GeV/c and 1.2 < pT < 7.0 GeV/c, respectively, and in several centrality intervals (from the most central 0–10% to the most peripheral 60–80% collisions). These spectra have been compared with the predictions of recent hydrodynamic models. In particular, the Kraków and EPOS models give a satisfactory description of the data, with the latter covering a wider pT range. Mid-rapidity yields, integrated over pT, have been determined. The hyperon-to-pion ratios are similar to those at RHIC: they rise smoothly with centrality up to Npart ∼ 150 and saturate thereafter. The enhancements (yields per participant nucleon relative to those in pp collisions) increase both with the strangeness content of the baryon and with centrality, but are less pronounced than at lower energies.
    • Multi-strange baryon production in p–Pb collisions at √sNN = 5.02 TeV.

      ALICE Collaboration; Barnby, Lee; University of Birmingham (Elsevier, 2016-05-12)
      The multi-strange baryon yields in Pb–Pb collisions have been shown to exhibit an enhancement relative to pp reactions. In this work, and production rates have been measured with the ALICE experiment as a function of transverse momentum, pT, in p–Pb collisions at a centre-of-mass energy of √sNN = 5.02 TeV. The results cover the kinematic ranges 0.6 GeV/c < pT < 7.2 GeV/c and 0.8 GeV/c < pT < 5 GeV/c, for and respectively, in the common rapidity interval −0.5 < yCMS < 0. Multi-strange baryons have been identified by reconstructing their weak decays into charged particles. The pT spectra are analysed as a function of event charged-particle multiplicity, which in p–Pb collisions ranges over one order of magnitude and lies between those observed in pp and Pb–Pb collisions. The measured pT distributions are compared to the expectations from a Blast-Wave model. The parameters which describe the production of lighter hadron species also describe the hyperon spectra in high multiplicity p–Pb collisions. The yield of hyperons relative to charged pions is studied and compared with results from pp and Pb–Pb collisions. A continuous increase in the yield ratios as a function of multiplicity is observed in p–Pb data, the values of which range from those measured in minimum bias pp to the ones in Pb–Pb collisions. A statistical model qualitatively describes this multiplicity dependence using a canonical suppression mechanism, in which the small volume causes a relative reduction of hadron production dependent on the strangeness content of the hyperon.
    • Multiplicity dependence of pion, kaon, proton and lambda production in p–Pb collisions at √sNN = 5.02 TeV.

      ALICE Collaboration; Barnby, Lee; University of Birmingham (Elsevier, 2014-01-20)
      In this Letter, comprehensive results on π±, K±, K0 S , p(p) and ¯ Λ(Λ)¯ production at mid-rapidity (0 < yCMS < 0.5) in p–Pb collisions at √sNN = 5.02 TeV, measured by the ALICE detector at the LHC, are reported. The transverse momentum distributions exhibit a hardening as a function of event multiplicity, which is stronger for heavier particles. This behavior is similar to what has been observed in pp and Pb–Pb collisions at the LHC. The measured pT distributions are compared to d–Au, Au–Au and Pb–Pb results at lower energy and with predictions based on QCD-inspired and hydrodynamic models.
    • Precision measurement of the mass difference between light nuclei and anti-nuclei.

      ALICE Collaboration; Barnby, Lee; University of Birmingham (Springer Nature, 2015-08-17)
      The measurement of the mass differences for systems bound by the strong force has reached a very high precision with protons and anti-protons. The extension of such measurement from (anti-)baryons to (anti-)nuclei allows one to probe any difference in the interactions between nucleons and anti-nucleons encoded in the (anti-)nuclei masses. This force is a remnant of the underlying strong interaction among quarks and gluons and can be described by effective theories, but cannot yet be directly derived from quantum chromodynamics. Here we report a measurement of the difference between the ratios of the mass and charge of deuterons (d) and anti-deuterons (d-bar ), and 3He and nuclei carried out with the ALICE (A Large Ion Collider Experiment)4 detector in Pb–Pb collisions at a centre-of-mass energy per nucleon pair of 2.76  TeV. Our direct measurement of the mass-over-charge differences confirms CPT invariance to an unprecedented precision in the sector of light nuclei. This fundamental symmetry of nature, which exchanges particles with anti-particles, implies that all physics laws are the same under the simultaneous reversal of charge(s) (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T).
    • Pseudorapidity dependence of the anisotropic flow of charged particles in Pb–Pb collisions at √sNN = 2.76 TeV

      ALICE Collaboration; Barnby, Lee; European Organisation for Nuclear Research (CERN); University of Birmingham (Elsevier, 2016-11-10)
      We present measurements of the elliptic (v2), triangular (v3) and quadrangular (v4) anisotropic azimuthal flow over a wide range of pseudorapidities (−3.5