The Computing Center of the Institute of Physics (CC IoP) of the Czech Academy of Sciences serves a broad spectrum of users with various computing needs. It runs WLCG Tier-2 center for the ALICE and ...the ATLAS experiments; the same group of services is used by astroparticle physics projects the Pierre Auger Observatory (PAO) and the Cherenkov Telescope Array (CTA). OSG stack is installed for the NOvA experiment. Other groups of users use directly local batch system. Storage capacity is distributed to several locations. DPM servers used by the ATLAS and the PAO are all in the same server room, but several xrootd servers for the ALICE experiment are operated in the Nuclear Physics Institute in Řež, about 10 km away. The storage capacity for the ATLAS and the PAO is extended by resources of the CESNET - the Czech National Grid Initiative representative. Those resources are in Plzen and Jihlava, more than 100 km away from the CC IoP. Both distant sites use a hierarchical storage solution based on disks and tapes. They installed one common dCache instance, which is published in the CC IoP BDII. ATLAS users can use these resources using the standard ATLAS tools in the same way as the local storage without noticing this geographical distribution. Computing clusters LUNA and EXMAG dedicated to users mostly from the Solid State Physics departments offer resources for parallel computing. They are part of the Czech NGI infrastructure MetaCentrum with distributed batch system based on torque with a custom scheduler. Clusters are installed remotely by the MetaCentrum team and a local contact helps only when needed. Users from IoP have exclusive access only to a part of these two clusters and take advantage of higher priorities on the rest (1500 cores in total), which can also be used by any user of the MetaCentrum. IoP researchers can also use distant resources located in several towns of the Czech Republic with a capacity of more than 12000 cores in total.
We report on first measurements of low-mass electron-positron pairs in Pb-Au collisions at the CERN SPS beam energy of 40 AGeV. The observed pair yield integrated over the range of invariant masses ...0.2<m</=1 GeV/c(2) is enhanced over the expectation from neutral meson decays by a factor of 5.9+/-1.5(stat)+/-1.2(syst data)+/-1.8(syst meson decays), somewhat larger than previously observed at the higher energy of 158 AGeV. The results are discussed with reference to model calculations based on pi(+)pi(-)-->e(+)e(-) annihilation with a modified rho propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the rho are more driven by baryon density than by temperature.
Midrapidity production of π±, K±, and (¯p)p measured by the ALICE experiment at the CERN Large Hadron Collider, in Pb-Pb and inelastic pp collisions at √sNN=5.02 TeV, is presented. The invariant ...yields are measured over a wide transverse momentum (pT) range from hundreds of MeV/c up to 20 GeV/c. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range 0–90%. The comparison of the pT-integrated particle ratios, i.e., proton-to-pion (p/π) and kaon-to-pion (K/π) ratios, with similar measurements in Pb-Pb collisions at √sNN=2.76 TeV show no significant energy dependence. Blast-wave fits of the pT spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios (p/π, K/π) as a function of pT show pronounced maxima at pT≈3GeV/c in central Pb-Pb collisions. At high pT, particle ratios at 5.02 TeV are similar to those measured in pp collisions at the same energy and in Pb-Pb collisions at √sNN=2.76 TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high pT and compatible with measurements at √sNN=2.76 TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily.
A study of multiplicity and pseudorapidity distributions of inclusive photons measured in pp and p-Pb collisions at a center-of-mass energy per nucleon-nucleon collision of Formula omitted TeV using ...the ALICE detector in the forward pseudorapidity region 2.3 Formula omitted 3.9 is presented. Measurements in p-Pb collisions are reported for two beam configurations in which the directions of the proton and lead ion beam were reversed. The pseudorapidity distributions in p-Pb collisions are obtained for seven centrality classes which are defined based on different event activity estimators, i.e., the charged-particle multiplicity measured at midrapidity as well as the energy deposited in a calorimeter at beam rapidity. The inclusive photon multiplicity distributions for both pp and p-Pb collisions are described by double negative binomial distributions. The pseudorapidity distributions of inclusive photons are compared to those of charged particles at midrapidity in pp collisions and for different centrality classes in p-Pb collisions. The results are compared to predictions from various Monte Carlo event generators. None of the generators considered in this paper reproduces the inclusive photon multiplicity distributions in the reported multiplicity range. The pseudorapidity distributions are, however, better described by the same generators.
A newly developed observable for correlations between symmetry planes, which characterize the direction of the anisotropic emission of produced particles, is measured in Pb-Pb collisions at Formula ...omitted = 2.76 TeV with ALICE. This so-called Gaussian Estimator allows for the first time the study of these quantities without the influence of correlations between different flow amplitudes. The centrality dependence of various correlations between two, three and four symmetry planes is presented. The ordering of magnitude between these symmetry plane correlations is discussed and the results of the Gaussian Estimator are compared with measurements of previously used estimators. The results utilizing the new estimator lead to significantly smaller correlations than reported by studies using the Scalar Product method. Furthermore, the obtained symmetry plane correlations are compared to state-of-the-art hydrodynamic model calculations for the evolution of heavy-ion collisions. While the model predictions provide a qualitative description of the data, quantitative agreement is not always observed, particularly for correlators with significant non-linear response of the medium to initial state anisotropies of the collision system. As these results provide unique and independent information, their usage in future Bayesian analysis can further constrain our knowledge on the properties of the QCD matter produced in ultrarelativistic heavy-ion collisions.
Differential elliptic flow spectra v2(pT) of π−, KS0, p, Λ have been measured at sNN=17.3 GeV around midrapidity by the CERN-CERES/NA45 experiment in mid-central Pb+Au collisions (10% of σgeo). The ...pT range extends from about 0.1 GeV/c (0.55 GeV/c for Λ) to more than 2 GeV/c. Protons below 0.4 GeV/c are directly identified by dE/dx. At higher pT, proton elliptic flow is derived as a constituent, besides π+ and K+, of the elliptic flow of positive pion candidates. This retrieval requires additional inputs: (i) of the particle composition, and (ii) of v2(pT) of positive pions. For (i), particle ratios obtained by NA49 are adapted to CERES conditions; for (ii), the measured v2(pT) of negative pions is substituted, assuming π+ and π− elliptic flow magnitudes to be sufficiently close. The v2(pT) spectra are compared to ideal-hydrodynamics calculations. In synopsis of the series π−–KS0–p–Λ, flow magnitudes are seen to fall with decreasing pT progressively even below hydro calculations with early kinetic freeze-out (Tf=160 MeV) leaving not much time for hadronic evolution. The proton v2(pT) data show a downward swing towards low pT with excursions into negative v2 values. The pion-flow isospin asymmetry observed recently by STAR at RHIC, invalidating in principle our working assumption, is found in its impact on proton flow bracketed from above by the direct proton flow data, and not to alter any of our conclusions. Results are discussed in perspective of recent viscous hydrodynamics studies which focus on late hadronic stages.
Two-particle correlations with Formula omitted, Formula omitted/ Formula omitted, and charged hadrons as trigger particles in the transverse momentum range Formula omitted GeV/ Formula omitted, and ...associated charged particles within Formula omitted GeV/ Formula omitted, are studied at midrapidity in pp and central Pb-Pb collisions at a centre-of-mass energy per nucleon-nucleon collision Formula omitted TeV with the ALICE detector at the LHC. After subtracting the contributions of the flow background, the per-trigger yields are extracted on both the near and away sides, and the ratio in Pb-Pb collisions with respect to pp collisions ( Formula omitted) is computed. The per-trigger yield in Pb-Pb collisions on the away side is strongly suppressed to the level of Formula omitted Formula omitted for Formula omitted GeV/ Formula omitted as expected from strong in-medium energy loss, while an enhancement develops at low Formula omitted on both the near and away sides, reaching Formula omitted Formula omitted and 2.7 respectively. These findings are in good agreement with previous ALICE measurements from two-particle correlations triggered by neutral pions ( Formula omitted-h) and charged hadrons (h-h) in Pb-Pb collisions at Formula omitted TeV. Moreover, the correlations with Formula omitted mesons and Formula omitted/ Formula omitted baryons as trigger particles are compared to those of inclusive charged hadrons. The results are compared with the predictions of Monte Carlo models.
The transverse-momentum Formula omitted spectra of K Formula omittedand Formula omittedmeasured with the ALICE detector up to Formula omitted = 16 GeV/c in the rapidity range Formula omitted in p-Pb ...collisions at the center-of-mass energy per nucleon-nucleon collision Formula omitted TeV are presented as a function of charged particle multiplicity and rapidity. The measured Formula omitted distributions show a dependence on both multiplicity and rapidity at low Formula omitted whereas no significant dependence is observed at high Formula omitted. A rapidity dependence is observed in the Formula omitted-integrated yield (dN/dy), whereas the mean transverse momentum Formula omitted shows a flat behavior as a function of rapidity. The rapidity asymmetry ( Formula omitted) at low Formula omitted (< 5 GeV/c) is more significant for higher multiplicity classes. At high Formula omitted, no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K Formula omittedand Formula omittedshow similar Formula omitted. The nuclear modification factor Formula omitted as a function of Formula omitted shows a Cronin-like enhancement at intermediate Formula omitted, which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high Formula omitted (> 5 GeV/ Formula omitted), the Formula omitted values are greater than unity and no significant rapidity dependence is observed.
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