Coherent psi (2S) photo-production in ultra-peripheral Pb-Pb collisions at root s(NN)=2.76TeV

We have performed the first measurement of the coherent psi(2S) photo-production cross section in ultraperipheral Pb-Pb collisions at the LHC. This charmonium excited state is reconstructed via the psi(2S) -> l(+)l(-) and ->(2S) -> J/psi pi(+)pi(-) decays, where the J/psi decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 mu b(-1). The cross section for coherent psi(2S) production in the rapidity interval -0.9 <y <0.9is d sigma(coh)(psi(2S))/dy = 0.83 +/- 0.19 (stat+syst) mb. The psi(2S) to J/psi coherent cross section ratio is 0.34(-0.07)(+0.08)(stat+syst). The obtained results are compared to predictions from theoretical models. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe

Measurement of charged jet production cross sections and nuclear modification in p-Pb collisions at root s(NN)=5.02 TeV

Charged jet production cross sections in p-Pb collisions at root s(NN) = 5.02 TeV measured with the ALICE detector at the LHC are presented. Using the anti-k(T) algorithm, jets have been reconstructed in the central rapidity region from charged particles with resolution parameters R = 0.2 and R = 0.4. The reconstructed jets have been corrected for detector effects and the underlying event background. To calculate the nuclear modification factor, R-pPb, of charged jets in p-Pb collisions, a pp reference was constructed by scaling previously measured charged jet spectra at root s = 7 TeV. In the transverse momentum range 20Peer reviewe

Systematic studies of correlations between different order flow harmonics in Pb-Pb collisions at root s(NN)=2.76 TeV

The correlations between event-by-event fluctuations of anisotropic flow harmonic amplitudes have been measured in Pb-Pb collisions at root s(NN) = 2.76 TeV with the ALICE detector at the Large Hadron Collider. The results are reported in terms of multiparticle correlation observables dubbed symmetric cumulants. These observables are robust against biases originating from nonflow effects. The centrality dependence of correlations between the higher order harmonics (the quadrangular v(4) and pentagonal v(5) flow) and the lower order harmonics (the elliptic v(2) and triangular v(3) flow) is presented. The transverse momentum dependences of correlations between v(3) and v(2) and between v(4) and v(2) are also reported. The results are compared to calculations from viscous hydrodynamics and a multiphase transport (AMPT) model calculations. The comparisons to viscous hydrodynamic models demonstrate that the different order harmonic correlations respond differently to the initial conditions and the temperature dependence of the ratio of shear viscosity to entropy density (eta/s). Asmall average value of eta/s is favored independent of the specific choice of initial conditions in the models. The calculations with the AMPT initial conditions yield results closest to the measurements. Correlations among the magnitudes of v(2), v(3), and v(4) show moderate p(T) dependence in midcentral collisions. This might be an indication of possible viscous corrections to the equilibrium distribution at hadronic freeze-out, which might help to understand the possible contribution of bulk viscosity in the hadronic phase of the system. Together with existing measurements of individual flow harmonics, the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions

Two-particle differential transverse momentum and number density correlations in p-Pb collisions at 5.02 TeV and Pb-Pb collisions at 2.76 TeV at the CERN Large Hadron Collider

We present measurements of two-particle differential number correlation functions R2 and transverse momentum correlation functions P2, obtained from p-Pb collisions at 5.02 TeV and Pb-Pb collisions at 2.76 TeV. The results are obtained by using charged particles in the pseudorapidity range |\u3b7|<1.0 and transverse momentum range 0.2<2.0 GeV/c as a function of pair separation in pseudorapidity, |\u394\u3b7|, azimuthal angle \u394\u3c6, and for several charged-particle multiplicity classes. Measurements are carried out for like-sign and unlike-sign charged-particle pairs separately and combined to obtain charge-independent and charge-dependent correlation functions. We study the evolution of the width of the near-side peak of these correlation functions with collision centrality. Additionally, we study Fourier decompositions of the correlators in \u394\u3c6 as a function of pair separation |\u394\u3b7|. Significant differences in the dependence of their harmonic coefficients on multiplicity classes are found. These differences can be exploited, in theoretical models, to obtain further insight into charged-particle production and transport in heavy-ion collisions. Moreover, an upper limit of nonflow contributions to flow coefficients vn measured in Pb-Pb collisions based on the relative strength of Fourier coefficients measured in p-Pb interactions is estimated

Measurement of electrons from semileptonic heavy-flavor hadron decays in p

The p(T)-differential production cross section of electrons from semileptonic decays of heavy-flavor hadrons has been measured at midrapidity in proton-proton collisions at root s = 2.76 TeV in the transverse momentum range 0.5 < p(T) < 12 GeV/c with the ALICE detector at the LHC. The analysis was performed using minimum bias events and events triggered by the electromagnetic calorimeter. Predictions from perturbative QCD calculations agree with the data within the theoretical and experimental uncertainties

Measurement of jet quenching with semi-inclusive hadron-jet distributions in central Pb-Pb collisions at s N N = 2.76 sNN=2.76 \sqrt{s_{\mathrm{NN}}}=2.76 TeV

We report the measurement of a new observable of jet quenching in central Pb-Pb collisions at root s(NN) = 2.76 TeV, based on the semi-inclusive rate of charged jets recoiling from a high transverse momentum (high-p T) charged hadron trigger. Jets are measured using collinear-safe jet reconstruction with infrared cutoff for jet constituents of 0.15 GeV, for jet resolution parameters R = 0.2, 0.4 and 0.5. Underlying event background is corrected at the event-ensemble level, without imposing bias on the jet population. Recoil jet spectra are reported in the range 20 < p(T,jet)(ch) < 100 GeV. Reference distributions for pp collisions at root s = 2.76TeV are calculated using Monte Carlo and NLO pQCD methods, which are validated by comparing with measurements in pp collisions at root s = 7TeV. The recoil jet yield in central Pb-Pb collisions is found to be suppressed relative to that in pp collisions. No significant medium-induced broadening of the intra-jet energy profile is observed within 0.5 radians relative to the recoil jet axis. The angular distribution of the recoil jet yield relative to the trigger axis is found to be similar in central Pb-Pb and pp collisions, with no significant medium-induced acoplanarity observed. Large-angle jet deflection, which may provide a direct probe of the nature of the quasi-particles in hot QCD matter, is explored

Inclusive J/ψ production in Xe–Xe collisions at sNN=5.44 TeV

Inclusive J/ψ production is studied in Xe–Xe interactions at a centre-of-mass energy per nucleon pair of sNN=5.44 TeV, using the ALICE detector at the CERN LHC. The J/ψ meson is reconstructed via its decay into a muon pair, in the centre-of-mass rapidity interval 2.5&lt;4 and down to zero transverse momentum. In this Letter, the nuclear modification factors RAA for inclusive J/ψ measured in the centrality range 0–90% as well as in the centrality intervals 0–20% and 20–90% are presented. The RAA values are compared to previously published results for Pb–Pb collisions at sNN=5.02 TeV and to the calculation of a transport model. A good agreement is found between Xe–Xe and Pb–Pb results as well as between data and the model

Charged-particle production as a function of multiplicity and transverse spherocity in pp collisions at root s=5.02 and 13 TeV

We present a study of the inclusive charged-particle transverse momentum (p(T)) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity, dNch/d., in pp collisions at root s = 5.02 and 13TeV covering the kinematic range vertical bar eta vertical bar &lt; 0.8 and 0.15 &lt; p(T) &lt; 20GeV/c. The results are presented for events with at least one charged particle in vertical bar eta vertical bar &lt; 1 (INEL&gt; 0). The p(T) spectra are reported for two multiplicity estimators covering different pseudorapidity regions. The p(T) spectra normalized to that for INEL&gt; 0 show little energy dependence. Moreover, the high-p(T) yields of charged particles increase faster than the charged-particle multiplicity density. The average p(T) as a function of multiplicity and transverse spherocity is reported for p(T) collisions at root s = 13TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the average pT is higher (smaller) than that measured in INEL&gt; 0 pp collisions. Within uncertainties, the functional form of &lt; p(T)&gt; (N-ch) is not affected by the spherocity selection. While EPOSLHCgives a good description ofmany features of data, PYTHIA overestimates the average p(T) in jet-like events

Real-time data processing in the ALICE High Level Trigger at the LHC

At the Large Hadron Collider at CERN in Geneva, Switzerland, atomic nuclei are collided at ultra-relativistic energies. Many final-state particles are produced in each collision and their properties are measured by the ALICE detector. The detector signals induced by the produced particles are digitized leading to data rates that are in excess of 48GB/s. The ALICE High Level Trigger (HLT) system pioneered the use of FPGA- and GPU-based algorithms to reconstruct charged-particle trajectories and reduce the data size in real time. The results of the reconstruction of the collision events, available online, are used for high level data quality and detector-performance monitoring and real-time time-dependent detector calibration. The online data compression techniques developed and used in the ALICE HLT have more than quadrupled the amount of data that can be stored for offline event processing

Coherent J/psi photoproduction at forward rapidity in ultra-peripheral Pb-Pb collisions at root s(NN)=5.02 TeV

The ALICE collaboration performed the first rapidity-differential measurement of coherent J/ψ photoproduction in ultra-peripheral Pb–Pb collisions at a center-of-mass energy sNN=5.02 TeV. The J/ψ is detected via its dimuon decay in the forward rapidity region (−4.0<y<−2.5) 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 750 μb−1. The cross section for coherent J/ψ production is presented in six rapidity bins. The results are compared with theoretical models for coherent J/ψ photoproduction. These comparisons indicate that gluon shadowing effects play a role in the photoproduction process. The ratio of ψ′ to J/ψ coherent photoproduction cross sections was measured and found to be consistent with that measured for photoproduction off protons