Global Observables for Pb+Pb Collisions from the ATLAS Experiment

Global properties of heavy ion collisions at LHC, will provide insight into dynamics of the hot and dense medium formed at collision energies thirty times larger than energy available at RHIC. In addition to the jet studies, the ATLAS detector at LHC has a great potential to probe global observables like charged particle pseudo-rapidity densities, elliptic flow or total transverse energy production as a function of collision centrality and within almost a full solid angle. We report on the detector capabilities to measure the global observables in Pb+Pb collisions at the center of mass energy of 5.5 TeV/n. The focus of this talk will be on the importance of these measurements in the context of RHIC results

Recent Heavy Ion Results with the ATLAS Detector at the LHC

Results are presented from the ATLAS collaboration from the 2010 LHC heavy ion run, during which nearly 10 inverse microbarns of luminosity were delivered. Soft physics results include charged particle multiplicities and collective flow. The charged particle multiplicity, which tracks initial state entropy production, increases by a factor of two relative to the top RHIC energy, with a centrality dependence very similar to that already measured at RHIC. Measurements of elliptic flow out to large transverse momentum also show similar results to what was measured at RHIC, but no significant pseudorapidity dependence. Extensions of these measurements to higher harmonics have also been made, and can be used to explain structures in the two-particle correlation functions that had long been attributed to jet-medium interactions. New hard probe measurements include single muons, jets and high $p_T$ hadrons. Single muons at high momentum are used to extract the yield of $W^{\pm}$ bosons and are found to be consistent within statistical uncertainties with binary collision scaling. Conversely, jets are found to be suppressed in central events by a factor of two relative to peripheral events, with no significant dependence on the jet energy. Fragmentation functions are also found to be the same in central and peripheral events. Finally, charged hadrons have been measured out to 30 GeV, and their centrality dependence relative to peripheral events is similar to that found for jets.Comment: 9 pages, 9 figures, proceedings for Quark Matter 2011, Annecy, France, May 23-28, 201

Single-cell multi-omics analysis reveals IFN-driven alterations in T lymphocytes and natural killer cells in systemic lupus erythematosus

Background: The characterisation of the peripheral immune system in the autoimmune disease systemic lupus erythematosus (SLE) at the single-cell level has been limited by the reduced sensitivity of current whole-transcriptomic technologies. Here we employ a targeted single-cell multi-omics approach, combining protein and mRNA quantification, to generate a high-resolution map of the T lymphocyte and natural killer (NK) cell populations in blood from SLE patients. Methods: We designed a custom panel to quantify the transcription of 534 genes in parallel with the expression of 51 surface protein targets using the BD Rhapsody AbSeq single-cell system. We applied this technology to profile 20,656 T and NK cells isolated from peripheral blood from an SLE patient with a type I interferon (IFN)-induced gene expression signature (IFNhi), and an age- and sex- matched IFNlow SLE patient and healthy donor. Results: We confirmed the presence of a rare cytotoxic CD4+ T cell (CTL) subset, which was exclusively present in the IFNhi patient. Furthermore, we identified additional alterations consistent with increased immune activation in this patient, most notably a shift towards terminally differentiated CD57+ CD8+ T cell and CD16+ NKdim phenotypes, and the presence of a subset of recently-activated naïve CD4+ T cells. Conclusions: Our results identify IFN-driven changes in the composition and phenotype of T and NK cells that are consistent with a systemic immune activation within the IFNhi patient, and underscore the added resolving power of this multi-omics approach to identify rare immune subsets. Consequently, we were able to find evidence for novel cellular peripheral biomarkers of SLE disease activity, including a subpopulation of CD57+ CD4+ CTLs

The Importance of Correlations and Fluctuations on the Initial Source Eccentricity in High-Energy Nucleus-Nucleus Collisions

In this paper, we investigate various ways of defining the initial source eccentricity using the Monte Carlo Glauber (MCG) approach. In particular, we examine the participant eccentricity, which quantifies the eccentricity of the initial source shape by the major axes of the ellipse formed by the interaction points of the participating nucleons. We show that reasonable variation of the density parameters in the Glauber calculation, as well as variations in how matter production is modeled, do not significantly modify the already established behavior of the participant eccentricity as a function of collision centrality. Focusing on event-by-event fluctuations and correlations of the distributions of participating nucleons we demonstrate that, depending on the achieved event-plane resolution, fluctuations in the elliptic flow magnitude $v_2$ lead to most measurements being sensitive to the root-mean-square, rather than the mean of the $v_2$ distribution. Neglecting correlations among participants, we derive analytical expressions for the participant eccentricity cumulants as a function of the number of participating nucleons, \Npart,keeping non-negligible contributions up to \ordof{1/\Npart^3}. We find that the derived expressions yield the same results as obtained from mixed-event MCG calculations which remove the correlations stemming from the nuclear collision process. Most importantly, we conclude from the comparison with MCG calculations that the fourth order participant eccentricity cumulant does not approach the spatial anisotropy obtained assuming a smooth nuclear matter distribution. In particular, for the Cu+Cu system, these quantities deviate from each other by almost a factor of two over a wide range in centrality.Comment: 18 pages, 10 figures, submitted to PR

Centrality dependence of charged antiparticle to particle ratios near mid-rapidity in d+Au collisions at sqrt(s_NN)=200 GeV

The ratios of the yields of charged antiparticles to particles have been obtained for pions, kaons, and protons near mid-rapidity for d+Au collisions at sqrt(s_NN) = 200 GeV as a function of centrality. The reported values represent the ratio of the yields averaged over the rapidity range of 0.1<y_pi<1.3 and 0<y_(K,p)<0.8, where positive rapidity is in the deuteron direction, and for transverse momenta 0.1<p_(T)^(pi,K)<1.0 GeV/c and 0.3<p_(T)^(p)<1.0 GeV/c. Within the uncertainties, a lack of centrality dependence is observed in all three ratios. The data are compared to results from other systems and model calculations.Comment: 6 pages, 4 figures, submitted to PR

Collision geometry scaling of Au+Au pseudorapidity density from sqrt(s_NN) = 19.6 to 200 GeV

The centrality dependence of the midrapidity charged particle multiplicity in Au+Au collisions at sqrt(s_NN) = 19.6 and 200 GeV is presented. Within a simple model, the fraction of hard (scaling with number of binary collisions) to soft (scaling with number of participant pairs) interactions is consistent with a value of x = 0.13 +/- 0.01(stat) +/- 0.05(syst) at both energies. The experimental results at both energies, scaled by inelastic p(pbar)+p collision data, agree within systematic errors. The ratio of the data was found not to depend on centrality over the studied range and yields a simple linear scale factor of R_(200/19.6) = 2.03 +/- 0.02(stat) +/- 0.05(syst).Comment: 5 pages, 4 figures, submitted to PRC-R

Centrality and pseudorapidity dependence of elliptic flow for charged hadrons in Au+Au collisions at sqrt(sNN) = 200 GeV

This paper describes the measurement of elliptic flow for charged particles in Au+Au collisions at sqrt(sNN)=200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). The measured azimuthal anisotropy is presented over a wide range of pseudorapidity for three broad collision centrality classes for the first time at this energy. Two distinct methods of extracting the flow signal were used in order to reduce systematic uncertainties. The elliptic flow falls sharply with increasing eta at 200 GeV for all the centralities studied, as observed for minimum-bias collisions at sqrt(sNN)=130 GeV.Comment: Final published version: the most substantive change to the paper is the inclusion of a complete description of how the errors from the hit-based and track-based analyses are merged to produce the 90% C.L. errors quoted for the combined results shown in Fig.

Charged-Particle Pseudorapidity Distributions in Au+Au Collisions at sqrt(s_NN)=62.4 GeV

The charged-particle pseudorapidity density for Au+Au collisions at sqrt(s_NN)=62.4 GeV has been measured over a wide range of impact parameters and compared to results obtained at other energies. As a function of collision energy, the pseudorapidity distribution grows systematically both in height and width. The mid-rapidity density is found to grow approximately logarithmically between AGS energies and the top RHIC energy. As a function of centrality, there is an approximate factorization of the centrality dependence of the mid-rapidity yields and the overall multiplicity scale. The new results at sqrt(s_NN)=62.4 GeV confirm the previously observed phenomenon of ``extended longitudinal scaling'' in the pseudorapidity distributions when viewed in the rest frame of one of the colliding nuclei. It is also found that the evolution of the shape of the distribution with centrality is energy independent, when viewed in this reference frame. As a function of centrality, the total charged particle multiplicity scales linearly with the number of participant pairs as it was observed at other energies.Comment: 6 pages, 7 figures, submitted to Phys. Rev. C - Rapid Communication

Non-flow correlations and elliptic flow fluctuations in gold-gold collisions at sqrt(s_NN)= 200 GeV

This paper presents results on event-by-event elliptic flow fluctuations in Au+Au collisions at sqrt(s_NN)=200Gev, where the contribution from non-flow correlations has been subtracted. An analysis method is introduced to measure non-flow correlations, relying on the assumption that non-flow correlations are most prominent at short ranges (Delta eta < 2). Assuming that non-flow correlations are of the order that is observed in p+p collisions for long range correlations (Delta eta > 2), relative elliptic flow fluctuations of approximately 30-40% are observed. These results are consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. It is found that the long range non-flow correlations in Au+Au collisions would have to be more than an order of magnitude stronger compared to the p+p data to lead to the observed azimuthal anisotropy fluctuations with no intrinsic elliptic flow fluctuations.Comment: 9 pages, 7 figures, Published in Phys. Rev.

Event-by-event fluctuations of azimuthal particle anisotropy in Au+Au collisions at sqrt(s_NN) = 200 GeV

This paper presents the first measurement of event-by-event fluctuations of the elliptic flow parameter v_2 in Au+Au collisions at sqrt(s_NN) = 200GeV as a function of collision centrality. The relative non-statistical fluctuations of the v_2 parameter are found to be approximately 40%. The results, including contributions from event-by-event elliptic flow fluctuations and from azimuthal correlations that are unrelated to the reaction plane (non-flow correlations), establish an upper limit on the magnitude of underlying elliptic flow fluctuations. This limit is consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. These results provide important constraints on models of the initial state and hydrodynamic evolution of relativistic heavy ion collisions.Comment: 5 pages, 2 figures, Published in Phys. Rev. Lett