Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at sNN=2.76\sqrt{s_{\rm NN}} = 2.76 TeV

Inclusive transverse momentum spectra of primary charged particles in Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}}$ = 2.76 TeV have been measured by the ALICE Collaboration at the LHC. The data are presented for central and peripheral collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross section. The measured charged particle spectra in $|\eta|<0.8$ and $0.3 < p_T < 20$ GeV/$c$ are compared to the expectation in pp collisions at the same $\sqrt{s_{\rm NN}}$, scaled by the number of underlying nucleon-nucleon collisions. The comparison is expressed in terms of the nuclear modification factor $R_{\rm AA}$. The result indicates only weak medium effects ($R_{\rm AA} \approx$ 0.7) in peripheral collisions. In central collisions, $R_{\rm AA}$ reaches a minimum of about 0.14 at $p_{\rm T}=6$-7GeV/$c$ and increases significantly at larger $p_{\rm T}$. The measured suppression of high-$p_{\rm T}$ particles is stronger than that observed at lower collision energies, indicating that a very dense medium is formed in central Pb-Pb collisions at the LHC.Comment: 15 pages, 5 captioned figures, 3 tables, authors from page 10, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/98

J/psi Production as a Function of Charged Particle Multiplicity in pp Collisions at sqrt{s} = 7 TeV

The ALICE collaboration reports the measurement of the inclusive J/psi yield as a function of charged particle pseudorapidity density dN_{ch}/deta in pp collisions at sqrt{s} = 7 TeV at the LHC. J/psi particles are detected for p_t > 0, in the rapidity interval |y| < 0.9 via decay into e+e-, and in the interval 2.5 < y < 4.0 via decay into mu+mu- pairs. An approximately linear increase of the J/psi yields normalized to their event average (dN_{J/psi}/dy)/ with (dN_{ch}/deta)/ is observed in both rapidity ranges, where dN_{ch}/deta is measured within |eta| < 1 and p_t > 0. In the highest multiplicity interval with = 24.1, corresponding to four times the minimum bias multiplicity density, an enhancement relative to the minimum bias J/psi yield by a factor of about 5 at 2.5 < y < 4 (8 at |y| < 0.9) is observed.Comment: Submitted to Phys. Lett.

Elliptic flow of charged particles in Pb-Pb collisions at √s nn=2.76TeV

We report the first measurement of charged particle elliptic flow in Pb-Pb collisions at √sNN=2.76TeV with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed in the central pseudorapidity region (|η|&amp;lt;0.8) and transverse momentum range 0.2&amp;lt;pt&amp;lt;5.0GeV/c. The elliptic flow signal v2, measured using the 4-particle correlation method, averaged over transverse momentum and pseudorapidity is 0.087±0.002(stat)±0.003(syst) in the 40%-50% centrality class. The differential elliptic flow v 2(pt) reaches a maximum of 0.2 near pt=3GeV/c. Compared to RHIC Au-Au collisions at √sNN=200GeV, the elliptic flow increases by about 30%. Some hydrodynamic model predictions which include viscous corrections are in agreement with the observed increase. © 2010 American Physical Society

Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at sNN=2.76 TeV

Inclusive transverse momentum spectra of primary charged particles in Pb-Pb collisions at sNN=2.76 TeV have been measured by the ALICE Collaboration at the LHC. The data are presented for central and peripheral collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross section. The measured charged particle spectra in |n|&lt;0.8 and 0.3&lt;pT&lt;20 GeV/c are compared to the expectation in pp collisions at the same sNN, scaled by the number of underlying nucleon-nucleon collisions. The comparison is expressed in terms of the nuclear modification factor RAA. The result indicates only weak medium effects (RAA≈0.7) in peripheral collisions. In central collisions, RAA reaches a minimum of about 0.14 at pT=6-7 GeV/c and increases significantly at larger pT. The measured suppression of high- pT particles is stronger than that observed at lower collision energies, indicating that a very dense medium is formed in central Pb-Pb collisions at the LHC. © 2010 CERN

Strange particle production in proton–proton collisions at √s = 0.9 TeV with ALICE at the LHC

The production of mesons containing strange quarks (K0 S, ϕ) and both singly and doubly strange baryons (⋀, ⊼, and Ξ− +Ξ+) are measured at mid-rapidity in pp collisions at √s = 0.9 TeV with the ALICE experiment at the LHC. The results are obtained from the analysis of about 250 k minimum bias events recorded in 2009. Measurements of yields (dN/dy) and transverse momentum spectra at mid-rapidity for inelastic pp collisions are presented. For mesons, we report yields (〈dN/dy〉) of 0.184 ± 0.002(stat.) ± 0.006(syst.) for K0 S and 0.021 ± 0.004(stat.) ± 0.003(syst.) for ϕ. For baryons, we find 〈dN/dy〉 = 0.048 ± 0.001(stat.) ± 0.004(syst.) for ⋀, 0.047 ± 0.002(stat.) ± 0.005(syst.) for ⊼ and 0.0101 ± 0.0020(stat.) ± 0.0009(syst.) for Ξ− +Ξ+. The results are also compared with predictions for identified particle spectra from QCD-inspired models and provide a baseline for comparisons with both future pp measurements at higher energies and heavy-ion collisions. © CERN for the benefit of the ALICE collaboration 2011

Particle-yield modification in jetlike azimuthal dihadron correlations in Pb-Pb collisions at √sNN=2.76TeV

The yield of charged particles associated with high-pt trigger particles (8&amp;lt;pt&amp;lt;15GeV/c) is measured with the ALICE detector in Pb-Pb collisions at √sNN=2.76TeV relative to proton-proton collisions at the same energy. The conditional per-trigger yields are extracted from the narrow jetlike correlation peaks in azimuthal dihadron correlations. In the 5% most central collisions, we observe that the yield of associated charged particles with transverse momenta pt&amp;gt;3GeV/c 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. © 2012 American Physical Society

Harmonic decomposition of two particle angular correlations in Pb-Pb collisions at sNN=2.76TeV

Angular correlations between unidentified charged trigger (t) and associated (a) particles are measured by the ALICE experiment in Pb-Pb collisions at sNN=2.76TeV for transverse momenta 0.25&amp;lt;pTt,a&amp;lt;15GeV/c, where pTt&amp;gt;pTa. The shapes of the pair correlation distributions are studied in a variety of collision centrality classes between 0 and 50% of the total hadronic cross section for particles in the pseudorapidity interval |η|&amp;lt;1.0. Distributions in relative azimuth δφ≡φt-φa are analyzed for |δη|≡|ηt-ηa|&amp;gt;0.8, and are referred to as &quot;long-range correlations&quot;. Fourier components Vnδ≡〈cos(nδφ)〉 are extracted from the long-range azimuthal correlation functions. If particle pairs are correlated to one another through their individual correlation to a common symmetry plane, then the pair anisotropy Vnδ(pTt,pTa) is fully described in terms of single-particle anisotropies vn(pT) as Vnδ(pTt,pTa)=vn(pTt)vn(pTa). This expectation is tested for 1≤n≤5 by applying a global fit of all Vnδ(pTt,pTa) to obtain the best values vn{GF}(pT). It is found that for 2≤n≤5, the fit agrees well with data up to pTa~3-4GeV/c, with a trend of increasing deviation as pTt and pTa are increased or as collisions become more peripheral. This suggests that no pair correlation harmonic can be described over the full 0.25&amp;lt;pT&amp;lt;15GeV/c range using a single vn(pT) curve; such a description is however approximately possible for 2≤n≤5 when pTa&amp;lt;4GeV/c. For the n=1 harmonic, however, a single v1(pT) curve is not obtained even within the reduced range pTa&amp;lt;4GeV/c. © 2012 CERN

J/ψ production as a function of charged particle multiplicity in pp collisions at s=7TeV

The ALICE Collaboration reports the measurement of the relative J/ψ yield as a function of charged particle pseudorapidity density dNch/dη in pp collisions at s=7TeV at the LHC. J/ψ particles are detected for pt&amp;gt;0, in the rapidity interval |y|&amp;lt;0.9 via decay into e+e-, and in the interval 2.5&amp;lt;y&amp;lt;4.0 via decay into μ+μ- pairs. An approximately linear increase of the J/ψ yields normalized to their event average (dNJ/ψ/dy)/〈 dNJ/ψ/dy〉 with (dNch/dη)/〈 dNch/dη〉 is observed in both rapidity ranges, where dNch/dη is measured within |η|&amp;lt;1 and pt&amp;gt;0. In the highest multiplicity interval with 〈 dNch/dη(bin)〉 =24.1, corresponding to four times the minimum bias multiplicity density, an enhancement relative to the minimum bias J/ψ yield by a factor of about 5 at 2.5&amp;lt;y&amp;lt;4 (8 at |y|&amp;lt;0.9) is observed. © 2012 CERN

Rapidity and transverse momentum dependence of inclusive J/Ψ production in pp collisions at s=7 TeV

The ALICE experiment at the LHC has studied inclusive J/Ψ production at central and forward rapidities in pp collisions at s=7 TeV. In this Letter, we report on the first results obtained detecting the J/Ψ through the dilepton decay into e+e- and μ+ μ- pairs in the rapidity ranges |y|&amp;lt;0.9 and 2.5&amp;lt;y&amp;lt;4, respectively, and with acceptance down to zero pT. In the dielectron channel the analysis was carried out on a data sample corresponding to an integrated luminosity Lint=5.6 nb- 1 and the number of signal events is NJ/Ψ=352±32(stat.)±28(syst.); the corresponding figures in the dimuon channel are Lint=15.6 nb-1 and NJ/Ψ=1924±77(stat.)±144(syst.). The measured production cross sections are σJ/Ψ(|y|&amp;lt;0.9)=10.7±1.0(stat.)±1.6(syst.)-2.3+1.6(syst.pol.)μb and σJ/Ψ(2.5&amp;lt;y&amp;lt;4)=6.31±0.25(stat.)±0.76(syst.)-1.96+0.95(syst.pol.)μb. The differential cross sections, in transverse momentum and rapidity, of the J/Ψ were also measured. © 2011 CERN

Centrality Dependence Of The Charged-particle Multiplicity Density At Midrapidity In Pb-pb Collisions At √snn=2.76tev

106