Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe

Mittlerer Transversalimpuls in Proton-Proton-Kollisionen bei s = 900 GeV in ALICE

Diese Arbeit befasst sich mit der Analyse der ersten Proton-Proton-Kollisionen, die mit dem ALICE-Experiment am LHC gemessen wurden. Der Schwerpunkt der Arbeit liegt dabei auf der Analyse des mittleren Transversalimpulses, einer Messgröße, mit der sich die Transversalimpulsspektren geladener, nichtidentifizierter Teilchen charakterisieren lassen. ALICE misst bei sqrt(s) = 900 GeV einen mittleren Transversalimpuls von iINEL = 0;483 +/- 0;001(stat.) +/- 0;007(sys.) GeV=c (6.1) NSD = 0;489 +/- 0;001(stat.) +/- 0;007(sys.) GeV=c (6.2) Im Vergleich mit den Messungen anderer Experimente bei gleichem p s misst ALICE einen etwas höheren mittleren Transversalimpuls. Neben der Analyse des mittleren Transversalimpulses des inklusiven Spektrums wird auch die Analyse als Funktion der Multiplizität erläutert. Mit der skalierten Multiplizität z = nacc= hnacci können die mit ALICE gemessenen Werte mit den Messungen von UA1 verglichen werden. Für z > 1 stimmen die Daten innerhalb der Fehler überein, für z < 1 divergieren die Daten von ALICE und UA1. Im weiteren Verlauf werden zwei Methoden vorgestellt, mit denen ein Übergang von der gemessenen Multiplizität zu einer korrigierten Multiplizität vorgenommen werden kann. Beide Methoden basieren auf einer mit PYTHIA generierten Korrelationsmatrix, die den Zusammenhang zwischen der generierten Multiplizitätsverteilung und der rekonstruierten Multiplizitätsverteilung enthält. Bei der einen Methode (A) werden die gemessenen Daten mit der Korrelationsmatrix gewichtet auf die generierte Multiplizität übertragen. Für die andere Methode (B) wird die Matrix zunächst entfaltet, um dann mit der entfalteten Multiplizitätsverteilung eine neue Matrix zu generieren. Mit dieser neuen Matrix wird dann jeder gemessen Multiplizitätsklasse eine wahrscheinlichste wahre Multiplizität zugewiesen. Die Ergebnisse beider Methoden sind vergleichbar. Für die weitere Analyse wird jedoch Methode A verwendet, da die Zuordnung in Methode B nicht eindeutig ist. Die mit dieser Methode analysierten Daten werden dann mit verschiedenen Simulationspaketen verglichen. Dabei stellt sich heraus, dass der PYTHIA-Tune Perugia0 die Daten am besten, jedoch nicht exakt, beschreibt. Mit den gemessenen Daten lassen sich die Modelle weiter optimieren, um eine Vorhersage bei höheren Strahlenergien machen zu können..

Production of K∗(892)0 and ϕ(1020) in p–Pb collisions at √sNN = 5.02 TeV

The production of K∗(892)0 and ϕ(1020) mesons has been measured in p–Pb collisions at √sNN = 5.02 TeV. K∗0 and ϕ are reconstructed via their decay into charged hadrons with the ALICE detector in the rapidity range - 0.5 < y < 0. The transverse momentum spectra, measured as a function of the multiplicity, have a pT range from 0 to 15 GeV/c for K∗0 and from 0.3 to 21 GeV/c for ϕ. Integrated yields, mean transverse momenta and particle ratios are reported and compared with results in pp collisions at √s = 7 TeV and Pb–Pb collisions at √sNN = 2.76 TeV. In Pb–Pb and p–Pb collisions, K∗0 and ϕ probe the hadronic phase of the system and contribute to the study of particle formation mechanisms by comparison with other identified hadrons. For this purpose, the mean transverse momenta and the differential proton-to-ϕ ratio are discussed as a function of the multiplicity of the event. The short-lived K∗0 is measured to investigate re-scattering effects, believed to be related to the size of the system and to the lifetime of the hadronic phase

Coherent J/ψ photoproduction in ultra-peripheral Pb–Pb collisions at √sNN=2.76 TeV

The ALICE Collaboration has made the first measurement at the LHC of J/ψ photoproduction in ultra-peripheral Pb–Pb collisions at sNN=2.76 TeV. The J/ψ is identified via its dimuon decay in the forward rapidity region with the muon spectrometer 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 55 μb−1. The cross section for coherent J/ψ production in the rapidity interval −3.6<y<−2.6 is measured to be dσJ/ψcoh/dy=1.00±0.18(stat)−0.26+0.24(syst) mb. The result is compared to theoretical models for coherent J/ψ production and found to be in good agreement with those models which include nuclear gluon shadowing

Freeze-out radii extracted from three-pion cumulants in pp, p–Pb and Pb–Pb collisions at the LHC

In high-energy collisions, the spatio-temporal size of the particle production region can be measured using the Bose-Einstein correlations of identical bosons at low relative momentum. The source radii are typically extracted using two-pion correlations, and characterize the system at the last stage of interaction, called kinetic freeze-out. In low-multiplicity collisions, unlike in high-multiplicity collisions, two-pion correlations are substantially altered by background correlations, e.g. mini-jets. Such correlations can be suppressed using three-pion cumulant correlations. We present the first measurements of the size of the system at freeze-out extracted from three-pion cumulant correlations in pp, p-Pb and Pb-Pb collisions at the LHC with ALICE. At similar multiplicity, the invariant radii extracted in p-Pb collisions are found to be 5-15% larger than those in pp, while those in Pb-Pb are 35-55% larger than those in p-Pb. Our measurements disfavor models which incorporate substantially stronger collective expansion in p-Pb as compared to pp collisions at similar multiplicity

Multi-strange baryon production in pp collisions at √s=7 TeV with ALICE

A measurement of the multi-strange Ξ− and Ω− baryons and their antiparticles by the ALICE experiment at the CERN Large Hadron Collider (LHC) is presented for inelastic proton–proton collisions at a centre-of-mass energy of 7 TeV. The transverse momentum (pT) distributions were studied at mid-rapidity (|y|6.0 GeV/c. We also illustrate the difference between the experimental data and model by comparing the corresponding ratios of (Ω−+Ω¯+)/(Ξ−+Ξ¯+) as a function of transverse mass

Measurement of charged jet suppression in Pb-Pb collisions at √sNN = 2.76 TeV

A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at sNN−−−√=2.76 TeV is reported. Jets are reconstructed from charged particles using the anti-kT jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudo-rapidity |η|<0.5. The transverse momentum pT of charged particles is measured down to 0.15 GeV/c which gives access to the low pT 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 pT 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.

Inclusive J/ψ production in pp collisions at √s=2.76 TeV

The ALICE Collaboration has measured inclusive J/ψ production in pp collisions at a center-of-mass energy √s=2.76 TeV at the LHC. The results presented in this Letter refer to the rapidity ranges |y|<0.9 and 2.5<y<4 and have been obtained by measuring the electron and muon pair decay channels, respectively. The integrated luminosities for the two channels are Linte=1.1 nb−1 and Lintμ=19.9 nb−1, and the corresponding signal statistics are NJ/ψe+e−=59±14 and NJ/ψμ+μ−=1364±53. We present dσJ/ψ/dy for the two rapidity regions under study and, for the forward-y range, d2σJ/ψ/dydpt in the transverse momentum domain 0<pt<8 GeV/c. The results are compared with previously published results at s=7 TeV and with theoretical calculations