New results from ATLAS at Quark Matter 2014

Results use Run 1 lead-lead and proton-lead data

30 May 2014 | By

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Figure 1: The modification of jet yields, quantified by the nuclear modification factor RAA , as a function of jet pT for three centrality bins: 0-10% (red circles), 10-20% (blue squares) and 40-50% (green diamonds). (Image: ATLAS Experiment © 2014 CERN)

ATLAS has prepared a variety of new results for the Quark Matter 2014 conference using lead-lead (Pb+Pb) and proton-lead (p+Pb) data collected during Run1. The Pb+Pb results include new measurements of event-by-event correlation and fluctuations of collective flow, high-statistics measurements of photon and W production, and studies of jet quenching using charged particles, single jets, nearby jet pairs, and jet fragmentation functions. The results from p+Pb data include precision measurements of long-range pseudorapidity correlation and associated azimuthal structures, and high-pT production of charged particles, Z's and jets.

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Figure 2: The correlation of (a) v2 in two different pT ranges, (b) v3 and v2 in the same pT range and (c) v4 and v2 in the same pT range. The data points in each centrality interval correspond to the fourteen event classes with different ellipticity selected via an event-shape engineering technique. The data are overlaid with the centrality dependence without event-shape selection (think line). The thin solid straight lines in the left panel represent a linear fit of the data in each centrality, and error bars represent the statistical uncertainties. (Image: ATLAS Experiment © 2014 CERN)

Since the first observation of highly asymmetric dijets in Pb+Pb collisions, the study of jet quenching has been an essential part of the heavy ion physics program at the LHC. Jets produced in heavy ion collisions can interact with the medium produced in these collisions and lose energy through the phenomenon of jet quenching. This energy loss suppresses the rate of jets produced in these collisions, where significant quenching is expected, relative to proton-proton collisions, where no such effects are present. Using the high-statistic proton-proton dataset from 2013 as the new reference, ATLAS has presented the most precise measurement of jet suppression. In central collisions, the jet yields are found to be suppressed by more than a factor of two below pT≈&150 GeV, but the suppression is reduced at higher pT.

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Figure 3: The modification of jet yields in 0-10% central p+Pb collisions relative to 60-90% to peripheral p+Pb collisions, quantified by the nuclear modification factor RCP. Jets are reconstructed by anti-kT algorithm with R=0.4 and plotted as a function of pT cosh(y*), which reflects the Bjorken x in the proton (left) and nucleus (right) of the hard-scattering process. Vertical error bars represent the statistical uncertainty while the boxes represent the systematic uncertainties. (Image: ATLAS Experiment © 2014 CERN)

Pb+Pb event–shape engineering
ATLAS presented first results on direct correlations between the elliptic flow coefficient, ν2, and higher-order flow harmonics, ν3, ν4 and ν5 in Pb+Pb collisions. This correlation is obtained via an event-shape engineering technique, in which events within the same centrality interval are divided into different classes according to the observed ellipticity in the forward pseudorapidity. The correlation of v2 between two different pT ranges shows non-trivial centrality dependence while they are found to be always linear within a narrow centrality interval. This linearity indicates that viscous effects are controlled by the system size not its overall shape. The v3-v2 correlations reveal a surprising anti-correlation between the ellipticity and trangularity of the initial geometry, which is not accessible via the traditional measurements. The v4-v2 and v5- v2 correlations provide the most direct and detailed picture of the interplay between the linear and non-linear collective dynamics in the final state of the Pb+Pb.

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Figure 4: The azimuthal harmonics of the ridge, v2(pT), v3(pT) v4(pT) and v5(pT) obtained for |Δ η|>2 and the reference pT range of 1-3 GeV. The error bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Results in 220≤Nchrec<260 are compared to the CMS data obtained by subtracting the peripheral events (the number of offline tracks Ntrkoff<20), shown by the solid and dashed lines. (Image: ATLAS Experiment © 2014 CERN)

Jets in proton-lead
The large data sample of p+Pb collisions at √sNN = 5.02 TeV collected in 2013 allows the ATLAS collaboration to measure the jet production over the widest kinematic range ever probed in proton-nucleus collisions. This measurement has the potential to reveal how the hard partonic content inside matter is modified deep inside the high-density nucleus and explores the interplay between hard processes and collision geometry, which is a major topic at this year’s Quark Matter. When considering collisions of all impact parameters, the rate of jets was found to be slightly above that which should be expected just from the proton colliding with individual nucleons in the lead nucleus. This slight excess is generally consistent with models of the modified parton densities in nuclei. However, in p+Pb collisions selected by "centrality", unexpected effects are seen. The rate of jets is strongly suppressed in apparently central (small impact parameter) and enhanced in apparently peripheral (large impact parameter) events. Furthermore, the modifications of the jet rate have a striking pattern as a function of the energy and rapidity, implying that the origin of the effect may actually originate in the proton, rather than the nuclear, wave-function.

Proton-lead ridges
Using the same p+Pb dataset, ATLAS also performed a detailed study of the long-range pseudorapidity correlation and its azimuthal structure as characterized by the first five Fourier harmonics v1-v5. The study extended the previous LHC measurements of v2 and v3 to higher pT and events with higher charged particle multiplicities. Moreover, the measurements of v1, v4 and v5 were first among the heavy ion experiments. The pT dependences of vn are found to be similar to Pb+Pb collisions with comparable multiplicities, suggesting that the collective flow, the main attribute of the dense system created in Pb+Pb collisions, may also be present in p+Pb interactions.