Nuclear modification of J/psi production in Pb-Pb collisions at sqrt(s_NN)=2.76 TeV

ALICE is the dedicated heavy-ion experiment at the LHC. Due to the unique particle identification capabilities of the central barrel detectors (|eta|<0.9), J/psi can be measured in the di-electron channel in the very demanding environment of central Pb-Pb collisions at the LHC. In addition J/psi are measured at forward rapidity (2.5<y<4) with a dedicated muon spectrometer. ALICE is the only LHC experiment with an acceptance for J/psi that reaches down to p_T=0 at both, mid- and forward rapidity. Preliminary results on the nuclear modification factor of the inclusive J/psi production at mid- and forward rapidity in Pb-Pb collisions at sqrt(s_NN)=2.76 TeV are presented.Comment: 4 pages, 7 figures, proceedings for the 5th international conference on hard and electromagnetic probes of high-energy nuclear collisions (Hard Probes 2012), Cagliari, Ital

Measurement of the J/psi inclusive production cross-section in pp collisions at sqrt(s)=7TeV with ALICE at the LHC

ALICE measures the J/psi production at mid-rapidity (|y| < 0.9) in the di-electron decay channel as well as at forward rapidity (2.5 < y < 4.0) in the di-muon decay channel. In both channels the acceptance goes down to zero transverse momentum. We present the rapidity dependence of the inclusive J/psi production cross-section and transverse momentum spectra.Comment: Proceedings: Rencontres de Moriond QCD and High Energy Interactions 201

Inbetriebnahme und Kalibrierung der ALICE-TPC

ALICE (A Large Ion Collider Experiment), is the dedicated heavy-ion experiment at the Large Hadron Collider (LHC) at CERN. It is optimised to reconstruct and identify the particles created in a lead-lead collision with a centre of mass energy of 5.5TeV. The main tracking detector is a large-volume time-projection chamber (TPC). With an active volume of about 88m^3 and a total readout area of 32.5m^2 it is the most challenging TPC ever build. A central electrode divides the 5m long detector into two drift regions. Each readout side is subdivided into 18 inner and 18 outer multi-wire proportional read-out chambers. The readout area is subdivide into 557568 pads, where each pad is read out by and electronics chanin. A complex calibration is needed in order to reach the design position-resolution of the reconstructed particle tracks of about 200um. One part of the calibration lies in understanding the electronic-response. The work at hand presents results of the pedestal and noise behaviour of the front-end electronics (FEE), measurements of the pulse-shaping properties of the FEE using results obtained with a calibration pulser and measurements performed with the laser-calibration system. The data concerned were taken during two phases of the TPC commissioning. First measurements were performed in the clean room where the TPC was built. After the TPC was moved underground and built into the experiment, a second round of commissioning took place. Noise measurements in the clean room revealed a very large fraction of pads with noise values larger than the design specifications. The unexpected high noise values could be explained by the 'ground bounce' effect. Two modifications helped to reduce this effect: A desynchronisation in the the start of the readout of groups of channels and a modification in the grounding scheme of the FEE. Further noise measurements were carried out after the TPC has been moved to the experimental area underground. Here even a larger fraction of channels showed too large noise values. This could be traced back to a common mode current injected by the electronics power supplies. To study the shaping properties of the FEE a calibration pulser was used. To generate signals in the FEE a pulse is injected to the cathode wires of the read-out chambers. Due to manufacturing tolerances slight channel-by-channel variations of the shaping properties are expected. This effects the determination of the arrival time as well as the measured integral signal of the induced charge and has to be corrected. The measured arrival time variations follow a Gaussian distribution with a width (sigma) of 6.2ns. This corresponds to an error of the cluster position of about 170um. The charge variations are on the level of 2.8%. In order to reach the intrinsic resolution on the measurement of the specific energy loss of the particles (6%) those variations have to be taken into account. The photons of the laser-calibration system are energetic enough to emit photo electrons off metallic surfaces. Most interesting for the detector calibration are photo electrons from the central electrode. The laser light is intense enough to get a signal in all readout channels of the TPC. Since the central electrode is a smooth surface, differences in the arrival time between sectors reveal mechanical displacements of the readout sectors and can be used to correct for this effect. In addition the measurements can be used to determine the electron drift velocity in the TPC gas. The drift velocity measurements have shown a vertical as well as a radial gradient. The first can be explained by the temperature gradient, which naturally builds up in the 5m high detector. The second gradient is most probably caused by a relative conical deformation of the readout plane and the central electrode.ALICE (A Large Ion Collider Experiment), eines der Experimente am LHC (Large Hadron Collider) des europäischen Kernforschungszentrums CERN, ist darauf optimiert, die geladenen primären und sekundären Teilchen, die in einer Blei-Blei-Kollision bei einer Schwerkpunktenergie von 5.5TeV entstehen, einzeln zu rekonstruieren und zu identifizieren. Der Hauptdetektor für die Spurrekonstruktion der in der Kollision entstehenden Teilchen ist mit einem aktiven Volumen von ca. 88m^3 die größte bis jetzt gebaute Spurendriftkammer (Time-Projection Chamber - TPC). Eine zentrale Elektrode teilt den 5m langen Detektor in zwei Ausleseseiten, deren Enden mit jeweils 18 inneren und 18 äußeren Vieldraht-Proportionalkammern bestückt sind. Die insgesamt 32.5m^2 umfassende Auslesefläche der Kammern ist dabei in 557568 einzelne Ausleseeinheiten (Pads) segmentiert. Um die vorgesehene Präzision der Rekonstruierten Spuren von ca. 200um zu erreichen, ist eine komplexe Kalibrierung notwendig. Ein Teil dieser Kalibrierung umfasst eine genaue Kenntnis der von der Elektronik erzeugten Signale. Die vorliegende Arbeit beschäftigt sich mit der Bestimmung der Pedestalwerte und der Analyse des Elektronikrauschens der verwendeten 'Front-End Elektronik' (FEE). Zur Bestimmung des Pulsformverhaltens jedes einzelnen Auslesekanals wurden Messungen mit einem Kalibrations-Pulser durchgeführt und ausgewertet. Des Weiteren wurden Daten analysiert, die mit dem Laser-Kalibrations-System der TPC erzeugt wurden. Die analysierten Daten wurden während zwei Phasen der Inbetriebnahme genommen. Die ersten Messungen fanden in einem Reinraum, in dem der Detektor auch bereits montiert wurde. Weitere Messungen wurden durchgeführt nachdem der Detektor in der unterirdischen Kaverne im ALICE-Experiment eingebaut worden ist. Bei den Messungen im Reinraum wurde festgestellt, dass ein sehr großer Anteil der Auslesekanäle ein zu hohes Elektronikrauschen aufweist. Das unerwartet hohe Rauschen konnte durch den 'ground bounce' Effekt erklärt werden. Zwei Maßnahmen konnten diesen Effekt wesentlich verringern. Zum einen bietet die Elektronik die Möglichkeit die Auslese einzelner Gruppen von Kanälen zu einem verschiedenen Zeitpunkt zu starten, um den momentan fließenden Strom zu verringern. Zum anderen konnte das Rauschen weiter reduziert werden, indem die Erdung der FEE verbessert wurde. Durch diese beiden Änderungen wurde das Rauschen auf den erforderlich Wert gesenkt. Nachdem die TPC im Experiment eingebaut wurde ergaben die Messungen des Elektronikrauschens abermals zu hohe Werte. Dies konnte auf die Verwendeten Netzgeräte zurück geführt werden, die eine Gleichtaktstörung außerhalb der Spezifikationen aufwies. Mit Hilfe eines Kalibrations-Pulsers, der ein Signal auf die Kathodendrahtebene der Vieldraht-Proportionalkammern einspeist, wurde das Pulsformverhalten der Elektronik untersucht. Bedingt durch den Herstellungsprozess variiert dieses von Kanal zu Kanal. Das wirkt sich sowohl auf die Bestimmung der Ankunftszeit, als auch der gemessenen Ladung aus. Mit den Kalibrations-Pulser-Messungen lassen sich die Daten auf diese beiden Effekte hin korrigieren. Die gemessenen Variationen der Ankunftszeit folgen einer Normalverteilung mit einer Breite (Sigma) von 6.2ns. In der Ortsauflösung der Elektronen-Cluster entlang der Teilchenspur entsteht dadurch ein Fehler von etwa 170um. Für die gemessene relative Ladungsverteilung erhält man ebenfalls eine Normalverteilung. Ihre Breite entspricht einer relativen Abweichung von 2.8%. Um die intrinsische Auflösung des spezifischen Energieverlustes von ca. 6% zu erreichen, müssen die Variationen berücksichtigt werden. Die Photonen des Laser-Kalibrations-Systems sind energetisch genug, um beim Auftreffen auf metallische Oberflächen Photoelektronen zu erzeugen. Interessant für die Detektorkalibration sind Photoelektronen von der Zentralelektrode. Die ausgelösten Elektronen sind so zahlreich, dass in jedem Auslesepad ein Signal gemessen werden kann. Da die Zentralelektrode eine stetige Fläche ist, können Sprünge in der Ankunftzeitmessung als Fehljustage der Auslesekammern interpretiert und anschließend darauf korrigiert werden. Des Weiteren ist es möglich mit Hilfe der Daten der Zentralelektrode die mittlere Elektronen-Driftgeschwindigkeit sowie Driftgeschwindigkeitsgradienten zu messen. Die Messungen zeigten einen vertikalen Driftgeschwindigkeitsgradienten, sowie eine radiale Abhängigkeit in der Ankunftszeitmessung. Der erste Effekt lässt sich durch die Temperaturabhängigkeit der Driftgeschwindigkeit erklären: Auf Grund der Höhe der TPC bildete sich ein Temperaturgradient. Der zweite Effekt wird sehr wahrscheinlich von einer relativen konische Verformung der Ausleseebene zur Zentralelektrode verursacht

Online Calibration of the TPC Drift Time in the ALICE High Level Trigger

ALICE (A Large Ion Collider Experiment) is one of four major experiments at the Large Hadron Collider (LHC) at CERN. The High Level Trigger (HLT) is a compute cluster, which reconstructs collisions as recorded by the ALICE detector in real-time. It employs a custom online data-transport framework to distribute data and workload among the compute nodes. ALICE employs subdetectors sensitive to environmental conditions such as pressure and temperature, e.g. the Time Projection Chamber (TPC). A precise reconstruction of particle trajectories requires the calibration of these detectors. Performing the calibration in real time in the HLT improves the online reconstructions and renders certain offline calibration steps obsolete speeding up offline physics analysis. For LHC Run 3, starting in 2020 when data reduction will rely on reconstructed data, online calibration becomes a necessity. Reconstructed particle trajectories build the basis for the calibration making a fast online-tracking mandatory. The main detectors used for this purpose are the TPC and ITS (Inner Tracking System). Reconstructing the trajectories in the TPC is the most compute-intense step. We present several improvements to the ALICE High Level Trigger developed to facilitate online calibration. The main new development for online calibration is a wrapper that can run ALICE offline analysis and calibration tasks inside the HLT. On top of that, we have added asynchronous processing capabilities to support long-running calibration tasks in the HLT framework, which runs event-synchronously otherwise. In order to improve the resiliency, an isolated process performs the asynchronous operations such that even a fatal error does not disturb data taking. We have complemented the original loop-free HLT chain with ZeroMQ data-transfer components. [...]Comment: 8 pages, 10 figures, proceedings to 2016 IEEE-NPSS Real Time Conferenc

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Forward-central two-particle correlations in p-Pb collisions at root s(NN)=5.02 TeV

Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 2GeV/c. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B. V.Peer reviewe

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions at root(NN)-N-S=2.76 TeV

Peer reviewe

Elliptic flow of muons from heavy-flavour hadron decays at forward rapidity in Pb-Pb collisions at root s(NN)=2.76TeV

The elliptic flow, v(2), of muons from heavy-flavour hadron decays at forward rapidity (2.5 <y <4) is measured in Pb-Pb collisions at root s(NN)= 2.76TeVwith the ALICE detector at the LHC. The scalar product, two- and four-particle Q cumulants and Lee-Yang zeros methods are used. The dependence of the v(2) of muons from heavy-flavour hadron decays on the collision centrality, in the range 0-40%, and on transverse momentum, p(T), is studied in the interval 3 <p(T)<10 GeV/c. A positive v(2) is observed with the scalar product and two-particle Q cumulants in semi-central collisions (10-20% and 20-40% centrality classes) for the p(T) interval from 3 to about 5GeV/c with a significance larger than 3 sigma, based on the combination of statistical and systematic uncertainties. The v(2) magnitude tends to decrease towards more central collisions and with increasing pT. It becomes compatible with zero in the interval 6 <p(T)<10 GeV/c. The results are compared to models describing the interaction of heavy quarks and open heavy-flavour hadrons with the high-density medium formed in high-energy heavy-ion collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe