Charged-particle production was studied in proton–proton collisions collected at the LHC with the ALICE detector at centre-of-mass energies 0.9 TeV and 2.36 TeV in the pseudorapidity range |η| < 1.4. In the central region (|η| < 0.5), at 0.9 TeV, we measure charged-particle pseudorapidity density dNch/dη = 3.02 ± 0.01(stat.) −0.05(syst.) for inelastic interactions, and dNch/dη = 3.58 ± 0.01(stat.) −0.12(syst.) for non-single-diffractive interactions. At 2.36 TeV, we find dNch/dη = 3.77 ± 0.01(stat.) −0.12(syst.) for inelastic, and dNch/dη = 4.43 ± 0.01(stat.) −0.12(syst.) for non-singlediffractive collisions. The relative increase in charged-particle multiplicity from the lower to higher energy is 24.7% ± 0.5%(stat.) −2.8%(syst.) for inelastic and 23.7% ± 0.5%(stat.) −1.1%(syst.) for nonsingle-diffractive interactions. This increase is consistent with that reported by the CMS collaboration for non-single-diffractive events and larger than that found by a number of commonly used models. The multiplicity distribution was measured in different pseudorapidity intervals and studied in terms of KNO variables at both energies. The results are compared to proton–antiproton data and to model

The yields of the ${K}^{*}{(892)}^{0}$ and $\ensuremath{\phi}(1020)$ resonances are measured in Pb-Pb collisions at $\sqrt{{s}_{NN}}=2.76$ TeV through their hadronic decays using the ALICE detector. The measurements are performed in multiple centrality intervals at mid-rapidity $(|y|<0.5)$ in the transverse-momentum ranges $0.3<{p}_{\mathrm{T}}<5 \mathrm{GeV}/c$ for the ${K}^{*}{(892)}^{0}$ and $0.5<{p}_{\mathrm{T}}<5 \mathrm{GeV}/c$ for the $\ensuremath{\phi}(1020)$. The yields of ${K}^{*}{(892)}^{0}$ are suppressed in central Pb-Pb collisions with respect to $pp$ and peripheral Pb-Pb collisions (perhaps due to rescattering of its decay products in the hadronic medium), while the longer-lived $\ensuremath{\phi}(1020)$ meson is not suppressed. These particles are also used as probes to study the mechanisms of particle production. The shape of the ${p}_{\mathrm{T}}$ distribution of the $\ensuremath{\phi}(1020)$ meson, but not its yield, is reproduced fairly well by hydrodynamic models for central Pb-Pb collisions. In central Pb-Pb collisions at low and intermediate ${p}_{\mathrm{T}}$, the $p/\ensuremath{\phi}(1020)$ ratio is flat in ${p}_{\mathrm{T}}$, while the $p/\ensuremath{\pi}$ and $\ensuremath{\phi}(1020)/\ensuremath{\pi}$ ratios show a pronounced increase and have similar shapes to each other. These results indicate that the shapes of the ${p}_{\mathrm{T}}$ distributions of these particles in central Pb-Pb collisions are determined predominantly by the particle masses and radial flow. Finally, $\ensuremath{\phi}(1020)$ production in Pb-Pb collisions is enhanced, with respect to the yield in $pp$ collisions and the yield of charged pions, by an amount similar to the $\ensuremath{\Lambda}$ and $\ensuremath{\Xi}$.

The inclusive J/ψ nuclear modification factor (RAA) in Pb–Pb collisions at sNN=2.76TeV has been measured by ALICE as a function of centrality in the e+e− decay channel at mid-rapidity (|y|<0.8) and as a function of centrality, transverse momentum and rapidity in the μ+μ− decay channel at forward-rapidity (2.5

The inclusive production cross sections at forward rapidity of J/ψ , ψ (2S) , Υ (1S) and Υ (2S) are measured in pp collisions at √(s)=7 TeV with the ALICE detector at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 1.35 pb ^-1 . Quarkonia are reconstructed in the dimuon-decay channel and the signal yields are evaluated by fitting the μ ^+μ ^- invariant mass distributions. The differential production cross sections are measured as a function of the transverse momentum p_T and rapidity y , over the ranges 0<p_T<20 GeV/c for J/ψ , 0<p_T<12 GeV/c for all other resonances and for 2.5<y<4 . The measured cross sections integrated over p_T and y , and assuming unpolarized quarkonia, are: σ _J/ψ=6.69± 0.04± 0.63 b, σ _ψ (2S)=1.13± 0.07± 0.19 b, σ _Υ (1S)=54.2 ± 5.0± 6.7 nb and σ _Υ (2S)=18.4 ± 3.7 ± 2.9 nb, where the first uncertainty is statistical and the second one is systematic. The results are compared to measurements performed by other LHC experiments and to theoretical models.

Transverse momentum spectra of π±, K± and p(p¯) up to pT=20GeV/c at mid-rapidity in pp, peripheral (60–80%) and central (0–5%) Pb–Pb collisions at sNN=2.76TeV have been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pion ratios both show a distinct peak at pT≈3GeV/c in central Pb–Pb collisions. Below the peak, pT<3GeV/c, both ratios are in good agreement with hydrodynamical calculations, suggesting that the peak itself is dominantly the result of radial flow rather than anomalous hadronization processes. For pT>10GeV/c particle ratios in pp and Pb–Pb collisions are in agreement and the nuclear modification factors for π±, K± and p(p¯) indicate that, within the systematic and statistical uncertainties, the suppression is the same. This suggests that the chemical composition of leading particles from jets in the medium is similar to that of vacuum jets.

In this Letter, comprehensive results on pi(+/-), K-+/-, K-S(0), p((p) over bar) and A((A) over bar) production at mid-rapidity (0 < y(CMS) < 0.5) in p-Pb collisions at root s(NN) = 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 pr 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. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.

We present the measurements of particle pair yields per trigger particle obtained from di-hadron azimuthal correlations in pp collisions at $\sqrt{s} = 0.9$, $2.76$, and $7$ TeV recorded with the ALICE detector. The yields are studied as a function of the charged particle multiplicity. Taken together with the single particle yields the pair yields provide information about parton fragmentation at low transverse momenta, as well as on the contribution of multiple parton interactions to particle production. Data are compared to calculations using the PYTHIA6, PYTHIA8, and PHOJET event generators.

The ALICE measurement of K^0_S and {\Lambda} production at mid-rapidity in Pb-Pb collisions at sqrt(sNN) = 2.76 TeV is presented. The transverse momentum (pT) spectra are shown for several collision centrality intervals and in the pT range from 0.4 GeV/c (0.6 GeV/c for {\Lambda}) to 12 GeV/c. The pT dependence of the {\Lambda}/K^0_S ratios exhibits maxima in the vicinity of 3 GeV/c, and the positions of the maxima shift towards higher pT with increasing collision centrality. The magnitude of these maxima increases by almost a factor of three between most peripheral and most central Pb-Pb collisions. This baryon excess at intermediate pT is not observed in pp interactions at sqrt(s) = 0.9 TeV and at sqrt(s) = 7 TeV. Qualitatively, the baryon enhancement in heavy-ion collisions is expected from radial flow. However, the measured pT spectra above 2 GeV/c progressively decouple from hydrodynamical-model calculations. For higher values of pT, models that incorporate the influence of the medium on the fragmentation and hadronization processes describe qualitatively the pT dependence of the {\Lambda}/K^0_S ratio.

The average transverse momentum (p(T)) versus the charged-particle multiplicity N-ch was measured in p-Pb collisions at a collision energy per nucleon-nucleon root S-NN = 5.02 TeV and in pp collisions at collision energies of root s = 0.9, 2.76, and 7 TeV in the kinematic range 0.15 < p(T) < 10.0 GeV/c and vertical bar eta vertical bar < 0.3 with the ALICE apparatus at the LHC. These data are compared to results in Pb-Pb collisions at root S-NN = 2.76 TeV at similar charged-particle multiplicities. In pp and p-Pb collisions, a strong increase of (p(T)) with N-ch is observed, which is much stronger than that measured in Pb-Pb collisions. For pp collisions, this could be attributed, within a model of hadronizing strings, to multiple-parton interactions and to a final-state color reconnection mechanism. The data in p-Pb and Pb-Pb collisions cannot be described by an incoherent superposition of nucleon-nucleon collisions and pose a challenge to most of the event generators. (C) 2013 CERN. Published by Elsevier B.V. All rights reserved.

This publication describes the methods used to measure the centrality of inelastic Pb-Pb collisions at a center-of-mass energy of 2.76 TeV per colliding nucleon pair with ALICE. The centrality is a key parameter in the study of the properties of QCD matter at extreme temperature and energy density, because it is directly related to the initial overlap region of the colliding nuclei. Geometrical properties of the collision, such as the number of participating nucleons and the number of binary nucleon-nucleon collisions, are deduced from a Glauber model with a sharp impact parameter selection and shown to be consistent with those extracted from the data. The centrality determination provides a tool to compare ALICE measurements with those of other experiments and with theoretical calculations.

The ratios of yields of anti-baryons to baryons probes the mechanisms of baryon-number transport. Results for p/p , Λ/Λ , Ξ^+/Ξ ^- and Ω^+/Ω^- in pp collisions at √(s) = 0.9, 2.767 , measured with the ALICE detector at the LHC, are reported. Within the experimental uncertainties and ranges covered by our measurement, these ratios are independent of rapidity, transverse momentum and multiplicity for all measured energies. The results are compared to expectations from event generators, such as PYTHIA and HIJING/B, that are used to model the particle production in pp collisions. The energy dependence of p/p , Λ/Λ , Ξ^+/Ξ^- and Ω^+/Ω^- , reaching values compatible with unity for √(s) = 7 , complement the earlier p/p measurement of ALICE. These dependencies can be described by exchanges with the Regge-trajectory intercept of α_J≈0.5 , which are suppressed with increasing rapidity interval Δ y . Any significant contribution of an exchange not suppressed at large Δ y (reached at LHC energies) is disfavoured.

The production cross section of electrons from semileptonic decays of beauty hadrons was measured at mid-rapidity (vertical bar y vertical bar < 0.8) in, the transverse momentum range 1 < p(T) < 8 GeV/c with the ALICE experiment at the CERN LHC in pp collisions at a center of mass energy root s = 7 TeV using an integrated luminosity of 2.2 nb(-1). Electrons from beauty hadron decays were selected based on the displacement of the decay vertex from the collision vertex. A perturbative QCD calculation agrees with the measurement within uncertainties. The data were extrapolated to the full phase space to determine the total cross section for the production of beauty quark-antiquark pairs. (C) 2013 CERN. Published by Elsevier B.V. All rights reserved.

Abelev, B; Adam, J; Adamová, D; Adare, A M; Aggarwal, M M; Aglieri Rinella, G; Agnello, M; Agocs, A G; Agostinelli, A; Ahammed, Z; Ahmad, N; Ahmad Masoodi, A; Ahmed, I; Ahn, S U; Ahn, S A; Aimo, I; Aiola, S; Ajaz, M; Akindinov, A; Aleksandrov, D; Alessandro, B; Alexandre, D; Alici, A; Alkin, A; Alme, J; Alt, T; Altini, V; Altinpinar, S; Altsybeev, I; Alves Garcia Prado, C; Andrei, C; Andronic, A; Anguelov, V; Anielski, J; Antičić, T; Antinori, F; Antonioli, P; Aphecetche, L; Appelshäuser, H; Arbor, N; Arcelli, S; Armesto, N; Arnaldi, R; Aronsson, T; Arsene, I C; Arslandok, M; Augustinus, A; Averbeck, R; Awes, T C; Azmi, M D

ALICE is an LHC experiment devoted to the study of strongly interacting matter in proton-proton, proton-nucleus and nucleus-nucleus collisions at ultra-relativistic energies. The ALICE VZERO system, made of two scintillator arrays at asymmetric positions, one on each side of the interaction point, plays a central role in ALICE. In addition to its core function as a trigger source, the VZERO system is used to monitor LHC beam conditions, to reject beam-induced backgrounds and to measure basic physics quantities such as luminosity, particle multiplicity, centrality and event plane direction in nucleus-nucleus collisions. After describing the VZERO system, this publication presents its performance over more than four years of operation at the LHC.

The ALICE Collaboration at the LHC has measured the J/ψ and ψ' photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at [Formula: see text]. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 μb-1. The cross section for coherent and incoherent J/ψ production in the rapidity interval -0.9<y<0.9, are [Formula: see text] and [Formula: see text], respectively. The results are compared to theoretical models for J/ψ production and the coherent cross section is found to be in good agreement with those models incorporating moderate nuclear gluon shadowing at Bjorken-x around 10-3, such as EPS09 parametrization. In addition the cross section for the process γγ→e+e- has been measured and found to be in agreement with models implementing QED at leading order.

In high-energy heavy-ion collisions, the correlations between the emitted particles can be used as a probe to gain insight into the charge creation mechanisms. In this Letter, we report the first results of such studies using the electric charge balance function in the relative pseudorapidity (Delta eta) and azimuthal angle (Delta phi) in Pb-Pb collisions at root s(NN) = 2.76 TeV with the ALICE detector at the Large Hadron Collider. The width of the balance function decreases with growing centrality (i.e. for more central collisions) in both projections. This centrality dependence is not reproduced by HIJING, while AMPT, a model which incorporates strings and parton rescattering, exhibits qualitative agreement with the measured correlations in Delta phi but fails to describe the correlations in Delta eta. A thermal blast-wave model incorporating local charge conservation and tuned to describe the p(T) spectra and v(2) measurements reported by ALICE, is used to fit the centrality dependence of the width of the balance function and to extract the average separation of balancing charges at freeze-out. The comparison of our results with measurements at lower energies reveals an ordering with root s(NN): the balance functions become narrower with increasing energy for all centralities. This is consistent with the effect of larger radial flow at the LHC energies but also with the late stage creation scenario of balancing charges. However, the relative decrease of the balance function widths in Delta eta and Delta phi, with centrality from the highest SPS to the LHC energy exhibits only small differences. This observation cannot be interpreted solely within the framework where the majority of the charge is produced at a later stage in the evolution of the heavy-ion collision. (C) 2013 CERN. Published by Elsevier B.V. All rights reserved.