ALICE Measurements in p-Pb Collisions: Charged Particle Multiplicity, Centrality Determination and implications for Binary Scaling

Measurements of particle production in proton-nucleus collisions provide a reference to disentangle final state effects, i.e. signatures of the formation of a deconfined hot medium, from initial state effects, already present in cold nuclear matter. Since many initial state effects are expected to vary as a function of the number of collisions suffered by the incoming proton, it is crucial to estimate the centrality of the collision. In p-Pb collisions categorization of events into different centrality classes using a particle multiplicity distribution is complicated by the low particle multiplicities and the large multiplicity fluctuations. We present ALICE measurements of particle production in p-Pb collisions at sqrt(sNN) = 5.02$ TeV, including the pseudo-rapidity and transverse momentum dependence, we discuss the event classification in centrality classes and its implications for the measurements of nuclear modification factors.Comment: 8 pages, 4 figures, to be published in NPA. Proceedings of IS201

Particle production in p-Pb collisions with ALICE at the LHC

Measurements of the transverse momentum spectra of light flavor particles at intermediate and high pT are an important tool for QCD studies. In pp collisions they provide a baseline for perturbative QCD, while in Pb-Pb they are used to investigate the suppression caused by the surrounding medium. In p-Pb collisions, such measurements provide a reference to disentangle final from initial state effects and thus play an important role in the search for signatures of the formation of a deconfined hot medium. While the comparison of the p-Pb and Pb-Pb data indicates that initial state effects do not play a role in the suppression of hadron production observed at high pT in heavy ion collisions, several measurements of particle production in the low and intermediate pT region indicate the presence of collective effects.Comment: Proceedings of Nucleus-Nucleus 2015, 8 pages, 6 (multiple) figure

Centrality Dependence of Particle Production in p--A collisions measured by ALICE

We present the centrality dependence of particle production in p-A collisions at sqrt(s_NN) = 5.02 TeV measured by the ALICE experiment, including the pseudo-rapidity and transverse momentum spectra, with a special emphasis on the event classification in centrality classes and its implications for the interpretation of the nuclear effects.Comment: Proceedings of Quark Matter 2014 4 pages, 3 figure

Measurement of low mass dielectron continuum in sqrt(s_NN)=200GeV Au+Au collisions with the PHENIX experiment at RHIC

Electromagnetic probes are ideally suited to investigate hot and dense matter produced in high energy heavy ion collisions. They do not undergo strong interactions and thus probe the time evolution of the collision. The dielectron continuum is rich in physics. Dalitz decays of light hadrons and direct decays of vector mesons, which might be modified in the medium, and correlated charm decays contribute to the spectrum. Furthermore, exotic states, e.g quasi-particles predicted to exist in the medium only beyond the phase transition, might leave their footprint in the continuum. Although correlated e+e- pairs are rare, the 0.24 nb^-1 collected by PHENIX for Au+Au collisions at sqrt(s_NN)= 200 GeV in 2004 provides a significant sample to investigate the dilepton continuum. The continuum and its resonances are separated from the combinatorial background via an event mixing technique. Mass spectra are presented and compared with the expectations from decays of hadronic sources.Comment: 4 pages, 5 figures, conference proceedin

e+e- Pairs: a clock and a thermometer of heavy ion collisions

Recently, there is growing evidence that a new state of matter is formed in sqrt(s_NN)= 200 GeV Au+Au collisions at RHIC: a strongly coupled Quark Gluon Plasma of partonic degrees of freedom which develops a collective motion. Dilepton spectra are not affected by strong interaction and can therefore probe the whole time evolution of the collision. Thus they may be sensitive to onset of deconfinement, chiral symmetry restoration, as well as the production of thermal photons. The PHENIX experiment measured the production of e+e- pairs in p+p and Au+Au collisions at sqrt(s_NN)= 200 GeV. An enhanced dilepton yield in the mass range 150<m_ee<750 MeV/c^2 is measured. The excess increases faster with centrality than the number of participating nucleons and is concentrated at p_T<1GeV/c. At higher p_T the excess below 300 MeV/c^2 has been related to an enhanced production of direct photons possibly of thermal origin.Comment: Proceedings of Quark Matter 2008, 8 pages, 7 figure

Bulk Properties of Pb-Pb collisions at sqrt(sNN) = 2.76 TeV measured by ALICE

Global variables, such as the charged particle multiplicity and the transverse energy are important observables to characterize Relativistic Heavy Ion collisions and to constrain model calculations. The charged particle multiplicity dNch/deta and transverse energy dET/deta are measured at sqrt(sNN) = 2.76 TeV in Pb-Pb collisions as a function of centrality and in pp collisions. The fraction of inelastic cross section seen by the ALICE detector is calculated either using a Glauber model or the data corrected by simulations of nuclear and electromagnetic processes, or data collected with a minimum bias interaction trigger. The centrality, defined by the number of nucleons participating in the collision, is obtained, via the Glauber model, by relating the multiplicity distributions of various detectors in the ALICE Central Barrel and their correlation with the spectator energy measured by the Zero-Degree Calorimeters. The results are compared to corresponding results obtained at the significantly lower energies of the BNL AGS, the CERN SPS, and the BNL RHIC, and with models based on different mechanisms for particle production in nuclear collisions. Particular emphasis will be given to a discussion on systematic studies of the dependence of the centrality determination on the details of the Glauber model, and the validity of the Glauber model at unprecedented collision energies.Comment: Proceedings of the XXII International Conference on Ultrarelativistic Nucleus-Nucleus Collisions Quark Matter 2011, submitted to J. Phys. G: Nucl. Part. Phys. 8 pages, 7 (multi)figure

Study of exclusive one-pion and one-eta production using hadron and dielectron channels in pp reactions at kinetic beam energies of 1.25 GeV and 2.2 GeV with HADES

We present measurements of exclusive ensuremathπ+,0 and η production in pp reactions at 1.25GeV and 2.2GeV beam kinetic energy in hadron and dielectron channels. In the case of π+ and π0 , high-statistics invariant-mass and angular distributions are obtained within the HADES acceptance as well as acceptance-corrected distributions, which are compared to a resonance model. The sensitivity of the data to the yield and production angular distribution of Δ (1232) and higher-lying baryon resonances is shown, and an improved parameterization is proposed. The extracted cross-sections are of special interest in the case of pp → pp η , since controversial data exist at 2.0GeV; we find \ensuremathσ=0.142±0.022 mb. Using the dielectron channels, the π0 and η Dalitz decay signals are reconstructed with yields fully consistent with the hadronic channels. The electron invariant masses and acceptance-corrected helicity angle distributions are found in good agreement with model predictions

Energy dependence of ϕ meson production at forward rapidity in pp collisions at the LHC

The production of $\phi$ mesons has been studied in pp collisions at LHC energies with the ALICE detector via the dimuon decay channel in the rapidity region $2.5< y < 4$. Measurements of the differential cross section $\mathrm{d}^2\sigma /\mathrm{d}y \mathrm{d}p_{\mathrm {T}}$ are presented as a function of the transverse momentum ($p_{\mathrm {T}}$) at the center-of-mass energies $\sqrt{s}=5.02$, 8 and 13 TeV and compared with the ALICE results at midrapidity. The differential cross sections at $\sqrt{s}=5.02$ and 13 TeV are also studied in several rapidity intervals as a function of $p_{\mathrm {T}}$, and as a function of rapidity in three $p_{\mathrm {T}}$ intervals. A hardening of the $p_{\mathrm {T}}$-differential cross section with the collision energy is observed, while, for a given energy, $p_{\mathrm {T}}$ spectra soften with increasing rapidity and, conversely, rapidity distributions get slightly narrower at increasing $p_{\mathrm {T}}$. The new results, complementing the published measurements at $\sqrt{s}=2.76$ and 7 TeV, allow one to establish the energy dependence of $\phi$ meson production and to compare the measured cross sections with phenomenological models. None of the considered models manages to describe the evolution of the cross section with $p_{\mathrm {T}}$ and rapidity at all the energies.publishedVersio