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A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at sNN−−−√=2.76 TeV is reported. Jets are reconstructed from charged particles using the anti-kT jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudo-rapidity |η|<0.5. The transverse momentum pT of charged particles is measured down to 0.15 GeV/c which gives access to the low pT fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter R=0.3 considered in the analysis. The fragmentation bias introduced by selecting jets with a high pT leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with R=0.2 and R=0.3 is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with R<0.3.
The ALICE Collaboration at the LHC reports measurement of the inclusive production cross section of electrons from semi-leptonic decays of beauty hadrons with rapidity |y| < 0.8 and transverse momentum 1 < pT < 10 GeV/c, in pp collisions at √s = 2.76 TeV. Electrons not originating from semi-electronic decay of beauty hadrons are suppressed using the impact parameter of the corresponding tracks. The production cross section of beauty decay electrons is compared to the result obtained with an alternative method which uses the distribution of the azimuthal angle between heavy-flavour decay electrons and charged hadrons. Perturbative QCD predictions agree with the measured cross section within the experimental and theoretical uncertainties. The integrated visible cross section, σb→e = 3.47 ± 0.40(stat) +1.12 −1.33(sys) ± 0.07(norm) μb, was extrapolated to full phase space using Fixed Order plus Next-to-Leading Log (FONLL) calculations to obtain the total bb production ¯ cross section, σbb¯ = 130 ± 15.1(stat) +42.1 −49.8(sys) +3.4 −3.1(extr) ± 2.5(norm) ± 4.4(BR) μb.
Transverse momentum spectra of π±, K± and p(p¯) up to pT = 20 GeV/c at mid-rapidity in pp, peripheral (60–80%) and central (0–5%) Pb–Pb collisions at √sNN = 2.76 TeV have been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pion ratios both show a distinct peak at pT ≈ 3 GeV/c in central Pb–Pb collisions. Below the peak, pT < 3 GeV/c, both ratios are in good agreement with hydrodynamical calculations, suggesting that the peak itself is dominantly the result of radial flow rather than anomalous hadronization processes. For pT > 10 GeV/c particle ratios in pp and Pb–Pb collisions are in agreement and the nuclear modification factors for π±, K± and p(p¯) indicate that, within the systematic and statistical uncertainties, the suppression is the same. This suggests that the chemical composition of leading particles from jets in the medium is similar to that of vacuum jets.
Freeze-out radii extracted from three-pion cumulants in pp, p–Pb and Pb–Pb collisions at the LHC
(2014)
In high-energy collisions, the spatio-temporal size of the particle production region can be measured using the Bose–Einstein correlations of identical bosons at low relative momentum. The source radii are typically extracted using two-pion correlations, and characterize the system at the last stage of interaction, called kinetic freeze-out. In low-multiplicity collisions, unlike in high-multiplicity collisions, two-pion correlations are substantially altered by background correlations, e.g. mini-jets. Such correlations can be suppressed using three-pion cumulant correlations. We present the first measurements of the size of the system at freeze-out extracted from three-pion cumulant correlations in pp, p–Pb and Pb–Pb collisions at the LHC with ALICE. At similar multiplicity, the invariant radii extracted in p–Pb collisions are found to be 5–15% larger than those in pp, while those in Pb–Pb are 35–55% larger than those in p–Pb. Our measurements disfavor models which incorporate substantially stronger collective expansion in p–Pb as compared to pp collisions at similar multiplicity.
We report on the measurement of the inclusive Υ (1S) production in Pb–Pb collisions at √sNN = 2.76 TeV carried out at forward rapidity (2.5 < y < 4) and down to zero transverse momentum using its μ+μ−decay channel with the ALICE detector at the Large Hadron Collider. A strong suppression of the inclusive Υ (1S) yield is observed with respect to pp collisions scaled by the number of independent nucleon–nucleon collisions. The nuclear modification factor, for events in the 0–90% centrality range, amounts to 0.30 ± 0.05(stat) ± 0.04(syst). The observed Υ (1S) suppression tends to increase with the centrality of the collision and seems more pronounced than in corresponding mid-rapidity measurements. Our results are compared with model calculations, which are found to underestimate the measured suppression and fail to reproduce its rapidity dependence.
We report on the production of inclusive Υ (1S) and Υ (2S) in p–Pb collisions at √sNN = 5.02 TeV at the LHC. The measurement is performed with the ALICE detector at backward (−4.46 < ycms < −2.96) and forward (2.03 .< ycms < 3.53) rapidity down to zero transverse momentum. The production cross sections of the Υ (1S) and Υ (2S) are presented, as well as the nuclear modification factor and the ratio of the forward to backward yields of Υ (1S). A suppression of the inclusive Υ (1S) yield in p–Pb collisions with respect to the yield from pp collisions scaled by the number of binary nucleon–nucleon collisions is observed at forward rapidity but not at backward rapidity. The results are compared to theoretical model calculations including nuclear shadowing or partonic energy loss effect.
The transverse momentum (pT) spectrum and nuclear modification factor (RAA) of reconstructed jets in 0–10% and 10–30% central Pb–Pb collisions at √sNN = 2.76 TeV were measured. Jets were reconstructed using the anti-kT jet algorithm with a resolution parameter of R = 0.2 from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal). The jet pT spectra are reported in the pseudorapidity interval of |ηjet| < 0.5 for 40 < pT, jet < 120 GeV/c in 0–10% and for 30 < pT, jet < 100 GeV/c in 10–30% collisions. Reconstructed jets were required to contain a leading charged particle with pT > 5 GeV/c to suppress jets constructed from the combinatorial background in Pb–Pb collisions. The leading charged particle requirement applied to jet spectra both in pp and Pb–Pb collisions had a negligible effect on the RAA. The nuclear modification factor RAA was found to be 0.28 ± 0.04 in 0–10% and 0.35 ± 0.04 in 10–30% collisions, independent of pT, jet within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching.
We present STAR measurements of charged hadron production as a function of centrality in Au+Au collisions at sqrt[sNN ]=130 GeV . The measurements cover a phase space region of 0.2< pT <6.0 GeV/c in transverse momentum and -1< eta <1 in pseudorapidity. Inclusive transverse momentum distributions of charged hadrons in the pseudorapidity region 0.5< | eta | <1 are reported and compared to our previously published results for | eta | <0.5 . No significant difference is seen for inclusive pT distributions of charged hadrons in these two pseudorapidity bins. We measured dN/d eta distributions and truncated mean pT in a region of pT > pcutT , and studied the results in the framework of participant and binary scaling. No clear evidence is observed for participant scaling of charged hadron yield in the measured pT region. The relative importance of hard scattering processes is investigated through binary scaling fraction of particle production.
We present the charged-particle pseudorapidity density in Pb–Pb collisions at √sNN = 5.02 TeV in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from −3.5 to 5, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0–5%) collisions we find 21 400 ± 1 300, while for the most peripheral (80–90%) we find 230 ± 38. This corresponds to an increase of (27 ± 4)% over the results at √sNN = 2.76 TeV previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The chargedparticle pseudorapidity density of the most central collisions is compared to model calculations — none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.
We have performed the first measurement of the coherent ψ(2S) photo production cross section in ultraperipheral Pb–Pb collisions at the LHC. This charmonium excited state is reconstructed via the ψ(2S) → l +l − and ψ(2S) → J/ψπ+π− decays, where the J/ψ decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 μb−1. The cross section for coherent ψ(2S) production in the rapidity interval −0.9 < y < 0.9 is dσcoh ψ(2S)/dy = 0.83±0.19 stat+syst mb. The ψ(2S) to J/ψ coherent cross section ratio is 0.34+0.08 −0.07(stat + syst). The obtained results are compared to predictions from theoretical models.