Production of charged pions, kaons and protons at large transverse momenta in pp and Pb–Pb collisions at s

Transverse momentum spectra of pi(+/-), K-+/- and p((p) over bar) up to p(T) = 20 GeV/c at mid-rapidity in pp, peripheral (60-80%) and central (0-5%) Pb-Pb collisions at v root s(NN) = 2.76 TeV have been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pionratios both show a distinct peak at p(T) approximate to 3 GeV/c in central Pb-Pb collisions. Below the peak, p(T) 10 GeV/c particle ratios in pp and Pb-Pb collisions are in agreement and the nuclear modification factors for pi(+/-), K-+/- and p((p) over bar) 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

Production of charged pions, kaons and protons at large transverse momenta in pp and Pb–Pb collisions at sNN=2.76\sqrt{s_{NN}}=2.76 TeV

Transverse momentum spectra of pi(+/-), K-+/- and p((p) over bar) up to p(T) = 20 GeV/c at mid-rapidity in pp, peripheral (60-80%) and central (0-5%) Pb-Pb collisions at v root s(NN) = 2.76 TeV have been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pionratios both show a distinct peak at p(T) approximate to 3 GeV/c in central Pb-Pb collisions. Below the peak, p(T) 10 GeV/c particle ratios in pp and Pb-Pb collisions are in agreement and the nuclear modification factors for pi(+/-), K-+/- and p((p) over bar) 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

Production of charged pions, kaons and protons at large transverse momenta in pp and Pb–Pb collisions at sNN=2.76\sqrt{s_{NN}}=2.76 TeV

Transverse momentum spectra of pi(+/-), K-+/- and p((p) over bar) up to p(T) = 20 GeV/c at mid-rapidity in pp, peripheral (60-80%) and central (0-5%) Pb-Pb collisions at v root s(NN) = 2.76 TeV have been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pionratios both show a distinct peak at p(T) approximate to 3 GeV/c in central Pb-Pb collisions. Below the peak, p(T) 10 GeV/c particle ratios in pp and Pb-Pb collisions are in agreement and the nuclear modification factors for pi(+/-), K-+/- and p((p) over bar) 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

First proton-proton collisions at the LHC as observed with the ALICE detector: Measurement of the charged-particle pseudorapidity density at √s = 900 GeV

On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider (LHC), two counter-rotating proton bunches were circulated for the first time concurrently in the machine, at the LHC injection energy of 450 GeV per beam. Although the proton intensity was very low, with only one pilot bunch per beam, and no systematic attempt was made to optimize the collision optics, all LHC experiments reported a number of collision candidates. In the ALICE experiment, the collision region was centred very well in both the longitudinal and transverse directions and 284 events were recorded in coincidence with the two passing proton bunches. The events were immediately reconstructed and analyzed both online and offline. We have used these events to measure the pseudorapidity density of charged primary particles in the central region. In the range |η|<0.5, we obtain dNch/dη=3. 10±0. 13(stat.)±0. 22(syst.) for all inelastic interactions, and dNch/dη=3.51±0. 15(stat.)±0. 25(syst.) for non-single diffractive interactions. These results are consistent with previous measurements in proton-antiproton interactions at the same centre-of-mass energy at the CERN SppS̄ collider. They also illustrate the excellent functioning and rapid progress of the LHC accelerator, and of both the hardware and software of the ALICE experiment, in this early start-up phase

Multiplicity dependence of pion, kaon, proton and lambda production in p–Pb collisions at √sNN = 5.02 TeV

Inthis Letter, comprehensive results on π±,K±,K0S, p(pbar) and Λ(Λbar) 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

Centrality dependence of the pseudorapidity density distribution for charged particles in Pb\u2013Pb collisions at 1asNN = 2.76 TeV

We present the first wide-range measurement of the charged-particle pseudorapidity density distribution, for different centralities (the 0\u20135%, 5\u201310%, 10\u201320%, and 20\u201330% most central events) in Pb\u2013Pb collisions at 1asNN = 2.76 TeV at the LHC. The measurement is performed using the full coverage of the ALICE detectors, 125.0 < \u3b7 < 5.5, and employing a special analysis technique based on collisions arising from LHC \u2018satellite\u2019 bunches. We present the pseudorapidity density as a function of the number of participating nucleons as well as an extrapolation to the total number of produced charged particles (Nch = 17 165 \ub1 772 for the 0\u20135% most central collisions). From the measured dNch/d\u3b7 distribution we derive the rapidity density distribution, dNch/dy, under simple assumptions. The rapidity density distribution is found to be significantly wider than the predictions of the Landau model. We assess the validity of longitudinal scaling by comparing to lower energy results from RHIC. Finally the mechanisms of the underlying particle production are discussed based on a comparison with various theoretical models

Exclusive J/psi Photoproduction off Protons in Ultraperipheral p-Pb Collisions at root s(NN)=5.02 TeV

We present the first measurement at the LHC of exclusive J/psi photoproduction off protons, in ultraperipheral proton-lead collisions at root s(NN) = 5.02 TeV. Events are selected with a dimuon pair produced either in the rapidity interval, in the laboratory frame, 2.5 J/psi + p) are 33.2 +/- 2.2(stat) +/- 3.2(syst) +/- 0.7(theor) nb in p-Pb and 284 +/- 36(stat)(-32)(+27)(syst) +/- 26(theor) nb in Pb-p collisions. We measure this process up to about 700 GeV in the gamma p center of mass, which is a factor of two larger than the highest energy studied at HERA. The data are consistent with a power law dependence of the J/psi photoproduction cross section in gamma p energies from about 20 to 700 GeV, or equivalently, from Bjorken x scaling variable between similar to 2 x 10(-2) and similar to 2 x 10(-5), thus indicating no significant change in the gluon density behavior of the proton between HERA and LHC energies

Measurement of charged jet suppression in Pb-Pb collisions at sNN\sqrt{s_{NN}}=2.76TeV

A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76TeV is reported. Jets are reconstructed from charged particles using the anti-$k_T$ jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudo-rapidity |$\eta$|<0.5. The transverse momentum p_T of charged particles is measured down to 0.15 GeV/c which gives access to the low p_T fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter R=0.3 considered in the analysis. The fragmentation bias introduced by selecting jets with a high p_T leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with R=0.2 and R=0.3 is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with R<0.3.A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV is reported. Jets are reconstructed from charged particles using the anti-$k_{\rm T}$ jet algorithm with jet resolution parameters $R$ of $0.2$ and $0.3$ in pseudo-rapidity $|\eta|<0.5$. The transverse momentum $p_{\rm T}$ of charged particles is measured down to $0.15$ GeV/$c$ which gives access to the low $p_{\rm T}$ fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter $R=0.3$ considered in the analysis. The fragmentation bias introduced by selecting jets with a high $p_{\rm T}$ leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with $R=0.2$ and $R=0.3$ is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with $R<0.3$

D Meson Elliptic Flow in Noncentral Pb-Pb Collisions at root(S)(NN)=2.76 TeV

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Azimuthally anisotropic distributions of D-0, D+, and D*+ mesons were studied in the central rapidity region (vertical bar y vertical bar < 0.8) in Pb-Pb collisions at a center-of-mass energy root(S)(NN) = 2.76 TeV per nucleon-nucleon collision, with the ALICE detector at the LHC. The second Fourier coefficient upsilon(2) (commonly denoted elliptic flow) was measured in the centrality class 30%-50% as a function of the D meson transverse momentum p(T), in the range 2-16 GeV/c. The measured upsilon(2) of D mesons is comparable in magnitude to that of light-flavor hadrons. It is positive in the range 2 < p(T) < 6 GeV/c with 5.7 sigma significance, based on the combination of statistical and systematic uncertainties.11110State Committee of ScienceWorld Federation of Scientists (WFS)Swiss Fonds Kidagan, ArmeniaConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Financiadora de Estudos e Projetos (FINEP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)National Natural Science Foundation of China (NSFC)Chinese Ministry of Education (CMOE)Ministry of Science and Technology of China (MSTC)Ministry of Education and Youth of the Czech RepublicDanish Natural Science Research CouncilCarlsberg FoundationDanish National Research FoundationEuropean Research Council under the European CommunityHelsinki Institute of PhysicsAcademy of FinlandFrench CNRS-IN2P3, FranceRegion Pays de Loire, FranceRegion Alsace, FranceRegion Auvergne, FranceCEA, FranceGerman BMBFHelmholtz AssociationGeneral Secretariat for Research and Technology, Ministry of Development, GreeceHungarian OTKANational Office for Research and Technology (NKTH)Department of Atomic EnergyDepartment of Science and Technology of the Government of IndiaIstituto Nazionale di Fisica Nucleare (INFN)Centro Fermi-Museo Storico della Fisica e Centro Studi e Ricerche 'Enrico Fermi,' ItalyMEXT, JapanJoint Institute for Nuclear Research, DubnaNational Research Foundation of Korea (NRF)CONACYT, MexicoDGAPA, MexicoALFA-ECEPLANET Program (European Particle Physics Latin American Network) Stichting voor Fundamenteel Onderzoek der Materie (FOM), NetherlandsNederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), NetherlandsResearch Council of Norway (NFR)Polish Ministry of Science and Higher EducationNational Authority for Scientific Research-NASR (Autoritatea Nationala pentru Cercetare Stiintifica-ANCS)Ministry of Education and Science of Russian FederationRussian Academy of SciencesRussian Federal Agency of Atomic EnergyRussian Federal Agency for Science and InnovationsRussian Foundation for Basic ResearchMinistry of Education of SlovakiaDepartment of Science and Technology, South AfricaCIEMATEELAMinisterio de Economia y Competitividad (MINECO) of SpainXunta de Galicia (Conselleria de Educacion)CEADENCubaenergiaCubaIAEA (International Atomic Energy Agency)Swedish Research Council (VR)Knut & Alice Wallenberg Foundation (KAW)Ukraine Ministry of Education and ScienceUnited Kingdom Science and Technology Facilities Council (STFC)United States Department of EnergyUnited States National Science FoundationState of TexasState of OhioConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Measurement of electrons from semileptonic heavy-flavor hadron decays in pp collisions at root s=2.76 TeV

The p(T)-differential production cross section of electrons from semileptonic decays of heavy-flavor hadrons has been measured at midrapidity in proton-proton collisions at root s = 2.76 TeV in the transverse momentum range 0.5 < p(T) < 12 GeV/c with the ALICE detector at the LHC. The analysis was performed using minimum bias events and events triggered by the electromagnetic calorimeter. Predictions from perturbative QCD calculations agree with the data within the theoretical and experimental uncertainties