Physics with the ALICE experiment

ALICE experiment at LHC collects data in pp collisions at $\sqrt{s}$=0.9, 2.76 and 7 TeV and in PbPb collisions at 2.76 TeV. Highlights of the detector performance and an overview of experimental results measured with ALICE in pp and AA collisions are presented in this paper. Physics with proton-proton collisions is focused on hadron spectroscopy at low and moderate $p_T$. Measurements with lead-lead collisions are shown in comparison with those in pp collisions, and the properties of hot quark matter are discussed.Comment: Presented at the Conference of the Nuclear Physics Division of the Russian Academy of Science, 11-25.11.2011, ITEP, Moscow. 16 pages, 14 figure

Measurement of heavy-flavour production in proton-proton collisions at sqrt{s} = 7 TeV with ALICE

The measurement of the heavy-flavour production cross sections in pp collisions at the LHC will allow to test perturbative QCD calculations in a new energy domain. Moreover, within the physics program of the ALICE experiment, it will provide the reference for the study of medium effects in Pb-Pb collisions, where heavy quarks are regarded as sensitive probes of parton-medium interaction dynamics. We present the status and first preliminary results of charm and beauty production measurements with the ALICE experiment, using hadronic D meson decays and semi-leptonic D and B meson decays, including the first cross section measurement of muons from heavy flavour decays at forward rapidity. We also describe the preliminary cross section measurement for J/psi production, obtained using the di-electron decay channel at central rapidity and the di-muon decay channel at forward rapidity.Comment: 8 pages, 5 figures, proceedings of the conference HARD PROBES 2010, Eilat, October 201

Multi-strange baryon elliptic flow in Pb-Pb collisions at \sqrt{s_{NN}} = 2.76 TeV measured with the ALICE detector

We present the results on elliptic flow with multi-strange baryons produced in Pb-Pb collisions at \sqrt{s_{NN}} = 2.76 TeV. The analysis is performed with the ALICE detector at LHC. Multi-strange baryons are reconstructed via their decay topologies and the v_2 values are analyzed with the two-particle scalar product method. The p_T differential v_2 values are compared to the VISH2+1 model calculation and to the STAR measurements at 200 GeV in Au+Au collisions. We found that the model describes \Xi and \Omega v_2 measurements within experimental uncertainties. The differential flow of \Xi and \Omega is similar to the STAR measurements at 200 GeV in Au+Au collisions.Comment: Prepared for the Proceedings of the International Conference on "Critical Point and Onset of Deconfinement - CPOD 2011", Wuhan, November 7-11, 201

Femtoscopy of Pb-Pb and pp collisions at the LHC with the ALICE experiment

We report on the results of femtoscopic analysis of Pb-Pb collisions at sqrt(s_NN)=2.76 TeV and pp collisions at sqrt(s)=0.9, 2.76 and 7 TeV with identical pions and kaons. Detailed femtoscopy studies in heavy-ion collisions at SPS and RHIC have shown that emission region sizes ("HBT radii") decrease with increasing pair transverse momentum k_T, which is understood as a manifestation of the collective behavior of matter. The trend was predicted to persist at the LHC. The data from Pb-Pb collisions confirm the existence of a flowing medium and provide strict constraints on the dynamical models. Similar analysis is carried out for pp collisions for pions and kaons and qualitative similarities to heavy-ion data are seen, especially in collisions producing large number of particles. The observed trends give insight into the soft particle production mechanism in pp collisions. 3D radii were also found to universally scale with event multiplicity in heavy-ion collisions. We extend the range of multiplicities both upwards with the Pb-Pb data and downwards with the pp data to test the scaling in new areas. In particular the high multiplicity pp collisions reach particle densities comparable to the ones measured in peripheral Cu-Cu and Au-Au collisions at RHIC. This allows for the first time to directly compare freeze-out sizes for systems with very different initial states.Comment: 8 pages, 5 figures, Proceedings of the Quark Matter 2011 plenary tal

Correlations and fluctuations studied with ALICE

The measurement of particle correlations and event-by-event fluctuations of physical observables allows to study a large variety of properties of the matter produced in ultra relativistic heavy-ion collisions. We will present results for two-particle correlations, mean transverse momentum fluctuations, and net charge fluctuations in Pb-Pb collisions at 2.76 TeV.Comment: 7 pages, 5 figures, Proceedings submitted for the 28th Winter Workshop on Nuclear Dynamics, Puerto Rico, April 7-14, 2012; corrected typo

Correlations of Electrons from Heavy Flavor Decay with Hadrons in Au+Au and p+p Collisions

Measurements of electrons from the decay of open-heavy flavor mesons have shown that the yields are suppressed in Au+Au collisions compared to expectations from binary-scaled p+p collisions. These measurements indicate that charm and bottom quarks interact with the hot-dense matter produced in heavy-ion collisions much more than expected. Here we extend these studies to two-particle correlations where one particle is an electron from the decay of a heavy-flavor meson and the other is a charged hadron from either the decay of the heavy meson or from jet fragmentation. These measurements provide more detailed information about the interactions between heavy quarks and the matter, such as whether the modifcation of the away-side-jet shape seen in hadron-hadron correlations is present when the trigger particle is from heavy-meson decay and whether the overall level of away-side-jet suppression is consistent. We statistically subtract correlations of electrons arising from background sources from the inclusive electron-hadron correlations and obtain two-particle azimuthal correlations at $\sqrt{s_{NN}}$ =200 GeV between electrons from heavy-flavor decay with charged hadrons in p+p and also first results in Au+Au collisions. We find the away-side-jet shape and yield to be modified in Au+Au collisions compared to p+p collisions.Comment: talk given at Winter Workshop in Nuclear Dynamics 201

On QGP Formation in pp Collisions at 7 TeV

The possibility of QGP formation in central pp collisions at ultra-high collision energy is discussed. Centrality-dependent \pt-spectra and (pseudo)rapidity spectra of thermal photons (charged hadrons) from pp collisions at 7 TeV are presented (addressed). Minimal-bias \pt-spectrum of direct photons and charged hadrons is compared under the framework with and without hydrodynamical evolution process.Comment: 4pages, 5figs, submitted to the Proceedings of the 22nd International Conference on Ultra-relativistic Nucleus-Nucleus Collision (Quark Matter 2011), 23 - 28 May 2011, Annecy, Franc

Forward-Backward Correlations and Event Shapes as probes of Minimum-Bias Event Properties

Measurements of inclusive observables, such as particle multiplicities and momentum spectra, have already delivered important information on soft-inclusive ("minimum-bias") physics at the Large Hadron Collider. In order to gain a more complete understanding, however, it is necessary to include also observables that probe the structure of the studied events. We argue that forward-backward (FB) correlations and event-shape observables may be particulary useful first steps in this respect. We study the sensitivity of several different types of FB correlations and two event shape variables - transverse thrust and transverse thrust minor - to various sources of theoretical uncertainty: multiple parton interactions, parton showers, colour (re)connections, and hadronization. The power of each observable to furnish constraints on Monte Carlo models is illustrated by including comparisons between several recent, and qualitatively different, PYTHIA 6 tunes, for pp collisions at sqrt(s) = 900 GeV.Comment: 13 page

Baryon stopping and saturation physics in relativistic collisions

We investigate baryon transport in relativistic heavy-ion collisions at energies reached at the CERN Super Proton Synchrotron, BNL Relativistic Heavy-Ion Collider (RHIC), and CERN LHC in the model of saturation. An analytical scaling law is derived within the color glass condensate framework based on small-coupling QCD. Transverse momentum spectra, net-baryon rapidity distributions and their energy, mass and centrality dependences are well described. In a comparison with RHIC data in Au + Au collisions at sqrt (s_NN) = 62.4 GeV and 200 GeV, the gradual approach to the gluon saturation regime is investigated, and limits for the saturation-scale exponent are determined. Predictions for net-baryon rapidity spectra and the mean rapidity loss in central Pb + Pb collisions at LHC energies of sqrt (s_NN) = 5.52 TeV are made.Comment: 11 pages, 10 Figures; improved figure inscriptions, corrected typos, minor changes in text/titl