[en] The ALICE High-Level Trigger (HLT) has a large high-performance computing cluster at CERN whose main objective is to perform real-time analysis on the data generated by the ALICE experiment and scale it down to at-most 4GB/sec - which is the current maximum mass-storage bandwidth available. Data-flow in this cluster is controlled by a custom designed software framework. It consists of a set of components which can communicate with each other via a common control interface. The software framework also supports the creation of different configurations based on the detectors participating in the HLT. These configurations define a logical data processing “chain” of detector data-analysis components. Data flows through this software chain in a pipelined fashion so that several events can be processed at the same time. An instance of such a chain can run and manage a few thousand physics analysis and data-flow components. The HLT software and the configuration scheme used in the 2011 heavy-ion runs of ALICE, has been discussed in this contribution.

[en] The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair √(s_NN)=2.76 TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is 1584±4(stat)±76(syst), which corresponds to 8.3±0.4(syst) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at √(s_NN)=0.2 TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies.

[en] The High Level Trigger (HLT) system of the ALICE experiment is an online event filter and trigger system designed for input bandwidths of up to 25 GB/s at event rates of up to 1 kHz. The system is designed as a scalable PC cluster, implementing several hundred nodes. The transport of data in the system is handled by an object-oriented data flow framework operating on the basis of the publisher-subscriber principle, being designed fully pipelined with lowest processing overhead and communication latency in the cluster. In this paper, we report the latest measurements where this framework has been operated on five different sites over a global north-south link extending more than 10,000 km, processing a 'real-time' data flow. (authors)

[en] We report the first measurement of charged particle elliptic flow in Pb-Pb collisions at √(s_NN)=2.76 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed in the central pseudorapidity region (|η|<0.8) and transverse momentum range 0.2< p_t<5.0 GeV/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 v₂(p_t) reaches a maximum of 0.2 near p_t=3 GeV/c. Compared to RHIC Au-Au collisions at √(s_NN)=200 GeV, the elliptic flow increases by about 30%. Some hydrodynamic model predictions which include viscous corrections are in agreement with the observed increase.

[en] The centrality dependence of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at √(s_NN)=2.76 TeV is presented. The charged-particle density normalized per participating nucleon pair increases by about a factor of 2 from peripheral (70%-80%) to central (0%-5%) collisions. The centrality dependence is found to be similar to that observed at lower collision energies. The data are compared with models based on different mechanisms for particle production in nuclear collisions.

[en] We report on the high statistics two-pion correlation functions from pp collisions at √(s)=0.9 TeV and √(s)=7 TeV, measured by the ALICE experiment at the Large Hadron Collider. The correlation functions as well as the extracted source radii scale with event multiplicity and pair momentum. When analyzed in the same multiplicity and pair transverse momentum range, the correlation is similar at the two collision energies. A three-dimensional femtoscopic analysis shows an increase of the emission zone with increasing event multiplicity as well as decreasing homogeneity lengths with increasing transverse momentum. The latter trend gets more pronounced as multiplicity increases. This suggests the development of space-momentum correlations, at least for collisions producing a high multiplicity of particles. We consider these trends in the context of previous femtoscopic studies in high-energy hadron and heavy-ion collisions and discuss possible underlying physics mechanisms. Detailed analysis of the correlation reveals an exponential shape in the outward and longitudinal directions, while the sideward remains a Gaussian. This is interpreted as a result of a significant contribution of strongly decaying resonances to the emission region shape. Significant nonfemtoscopic correlations are observed, and are argued to be the consequence of ''mini-jet''-like structures extending to low p_t. They are well reproduced by the Monte-Carlo generators and seen also in π⁺π^- correlations.

[en] We present the first measurements of the pseudorapidity distribution of primary charged particles in Cu+Cu collisions as a function of collision centrality and energy, √(s_NN)=22.4, 62.4, and 200 GeV, over a wide range of pseudorapidity, using the PHOBOS detector. A comparison of Cu+Cu and Au+Au results shows that the total number of produced charged particles and the rough shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants. More detailed studies reveal that a more precise matching of the shape of the Cu+Cu and Au+Au pseudorapidity distributions over the full range of pseudorapidity occurs for the same N_part/2A rather than the same N_part. In other words, it is the collision geometry rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence at RHIC energies

[en] A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity (Δη) and azimuthal angle (Δφ) correlations between high transverse momentum charged hadrons (p_T>2.5 GeV/c) and all associated charged particles is presented at both short-range (small Δη) and long-range (large Δη) over a continuous detector acceptance covering -4<Δη<2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au + Au collisions at √(s_NN)=200 GeV. The results provide new information about the longitudinal (Δη) extent of the near-side 'ridge' structure, first observed by the STAR Collaboration over a narrower η range. In central Au+Au collisions the ridge structure extends to at least Δη=4, and its strength completely diminishes as collisions become more peripheral. (orig.)