Measurement of the centrality and pseudorapidity dependence of the integrated elliptic flow in lead-lead collisions at √sNN=2.76 TeV with the ATLAS detector

Marco Agustoni, Antonio Ereditato, Stefania Antonia Stucci, Basil Schneider, Hans Peter Beck, Valentina Gallo, Tobias Kruker, Gianfranco Sciacca, Michael Weber, Lukas Marti, Sigve Haug, Federico Meloni (+2 others)
The integrated elliptic flow of charged particles produced in Pb+Pb collisions at √ s NN = 2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v 2 , was measured in the pseudorapidity range |η| ≤ 2.5 with the event-plane method. In order to include tracks with very low transverse momentum p T , thus reducing the uncertainty in v 2 integrated over p T , a 1 μb −1 data sample recorded without a magnetic field in the
more » ... ng detectors is used. The centrality dependence of the integrated v 2 is compared to other measurements obtained with higher p T thresholds. The integrated elliptic flow is weakly decreasing with |η|. The integrated v 2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data. source: | downloaded: 29.2.2020 2982 Page 2 of 25 Eur. Phys. J. C (2014) 74:2982 struction methods, applicable at higher p T , were exploited in cross-checks using "field-on" data taken with the solenoid's magnetic field. The ATLAS detector The ATLAS detector is a multi-purpose particle physics apparatus and is described in detail elsewhere [24]. This analysis uses the three-level trigger system to select Pb+Pb collision events, the forward calorimeters (FCal) to measure the collision centrality, and the inner detector (ID) to measure charged-particle tracks. The ID tracking system comprises silicon pixel and microstrip detectors and a transition radiation tracker. It provides complete azimuthal coverage and spans the pseudorapidity region |η| < 2.5. 2 The pixel detector consists of a three-layer barrel section and three discs in each of the forward regions. The semiconductor tracker has four double layers of microstrip sensors in its barrel section and nine discs in each of the forward regions. The ID is surrounded by a thin superconducting solenoid, which produces a 2 T axial magnetic field for the field-on data. The FCal measures both electromagnetic and hadronic energy, using copper-tungsten/liquid-argon technology, and provides complete azimuthal coverage for 3.2 < |η| < 4.9. The hardware-based Level-1 trigger selected minimum-bias Pb+Pb collisions by requiring either a coincidence of signals recorded in the zero-degree calorimeters (ZDC) located symmetrically at z = ±140 m (|η| > 8.3) or a signal in at least one side of the minimum-bias trigger scintillators (MBTS) at z = ±3.6 m (2.1 < |η| < 3.9). To suppress beam backgrounds, the Level-2 trigger demanded MBTS signals from opposite sides of the interaction region and imposed a timing requirement on them. With these trigger conditions, ATLAS recorded a sample of Pb+Pb collisions corresponding to an integrated luminosity of approximately 1 μb −1 taken with the field provided by the solenoid turned off. In addition, approximately 0.5 μb −1 of field-on data was used in studies of track reconstruction performance. Event selection and centrality definition The offline event selection required each event to have a vertex formed by at least three charged-particle tracks recon-2 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). acknowledge the support of ANPCyT,
doi:10.7892/boris.68702 fatcat:4tjx24yxnffklfs3bha22i3toy