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The ALICE Transition Radiation Detector: Construction, Operation, and Performance Lawrence Berkeley National Laboratory Recent Work Title The ALICE transition radiation detector: Construction, operation, and performance Permalink https://escholarship.org/uc/item/3j08944n Authors Acharya, S Adam, J Adamova, D et al. Publication Date 2018-02-11 DOI 10.1016/j.nima.2017.09.028 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Nuclear Inst. and Methods in Physics Research, A 881 (2018) 88–127 Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima The ALICE Transition Radiation Detector: Construction, operation, and performance ALICE Collaboration 1 article info a b s t r a c t Keywords: The Transition Radiation Detector (TRD) was designed and built to enhance the capabilities of the ALICE detector Transition radiation detector at the Large Hadron Collider (LHC). While aimed at providing electron identification and triggering, the TRD Multi-wire proportional drift chamber also contributes significantly to the track reconstruction and calibration in the central barrel of ALICE. In this Fibre/foam sandwich radiator paper the design, construction, operation, and performance of this detector are discussed. A pion rejection factor Xenon-based gas mixture of up to 410 is achieved at a momentum of 1 GeV/c in p–Pb collisions and the resolution at high transverse Tracking momentum improves by about 40% when including the TRD information in track reconstruction. The triggering Ionisation energy loss dE/dx capability is demonstrated both for jet, light nuclei, and electron selection. TR ' 2017 CERN for the benefit of the Authors. Published by Elsevier B.V. This is an open access article under the Electron-pion identification CC BY license (http://creativecommons.org/licenses/by/4.0/). Neural network Trigger 1. Introduction (훼 = 1_137), many boundaries are needed in detectors to increase the radiation yield [10]. The absorption of the emitted X-ray photons in A Large Ion Collider Experiment (ALICE) [1,2] is the dedicated high-Z gas detectors leads to a large energy deposition compared to the heavy-ion experiment at the Large Hadron Collider (LHC) at CERN. specific energy loss by ionisation of the traversing particle. In central high energy nucleus–nucleus collisions a high-density de- Since their development in the 1970s, transition radiation detectors confined state of strongly interacting matter, known as quark–gluon have proven to be powerful devices in cosmic-ray, astroparticle and plasma (QGP), is supposed to be created [3–5]. ALICE is designed to accelerator experiments [10–20]. The main purpose of the transition measure a large set of observables in order to study the properties of the radiation detectors in these experiments was the discrimination of QGP. Among the essential probes there are several involving electrons, electrons from hadrons via, e.g. cluster counting or total charge/energy which originate, e.g. from open heavy-flavour hadron decays, virtual analysis methods. In a few cases they provided charged-particle track- photons, and Drell–Yan production as well as from decays of the and ing. The transition radiation photons are in most cases detected either Υ families. The identification of these rare probes requires excellent elec- by straw tubes or by multiwire proportional chambers (MWPC). In tron identification, also in the high multiplicity environment of heavy- some experiments [10,13,16,21] and in test setups [22–25], short ion collisions. In addition, the rare probes need to be enhanced with drift chambers (usually about 1 cm) were employed for the detection. triggers, in order to accumulate the statistics necessary for differential Detailed reviews on the transition radiation phenomenon, detectors, and studies. The latter requirement concerns not only probes involving the their application to particle identification can be found in [10,26–28]. production of electrons, but also rare high transverse momentum probes such as jets (collimated sprays of particles) with and without heavy The ALICE TRD, which covers the full azimuth and the pseudorapid- flavour. The ALICE Transition Radiation Detector (TRD) fulfils these two ity range *0:84 < 휂 < 0:84 (see next section), is part of the ALICE central tasks and thus extends the physics reach of ALICE. barrel. The TRD consists of 522 chambers arranged in 6 layers at a Transition radiation (TR), predicted in 1946 by Ginzburg and radial distance from 2.90 m to 3.68 m from the beam axis. Each chamber Frank [6], occurs when a particle crosses the boundary between two comprises a foam/fibre radiator followed by a Xe-CO2-filled MWPC media with different dielectric constants. For highly relativistic particles preceded by a drift region of 3 cm. The extracted temporal information (훾 ≳ 1000), the emitted radiation extends into the X-ray domain for represents the depth in the drift volume at which the ionisation signal a typical choice of radiator [7–9]. The radiation is extremely forward was produced and thus allows the contributions of the TR photon and peaked relative to the particle direction [7]. As the TR photon yield the specific ionisation energy loss of the charged particle dE_dx to be per boundary crossing is of the order of the fine structure constant separated. The former is preferentially absorbed at the entrance of the 1 See Appendix for the list of collaboration members. http://dx.doi.org/10.1016/j.nima.2017.09.028 Received 11 September 2017; Accepted 14 September 2017 Available online 21 September 2017 0168-9002/' 2017 CERN for the benefit of the Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Nuclear Inst. and Methods in Physics Research, A 881 (2018) 88–127 chamber and the latter distributed uniformly along the track. Electrons the High Momentum Particle Identification Detector (HMPID) [35] are can be distinguished from other charged particles by producing TR and used for electron, jet, photon and hadron identification. Their azimuthal having a higher dE_dx due to the relativistic rise of the ionisation coverage is shown in Fig. 1. Not visible in the figure are the V0 and T0 energy loss. The usage of the temporal information further enhances detectors [36,37], as well as the Zero Degree Calorimeters (ZDC) [38], the electron–hadron separation power. Due to the fast read-out and which are placed at small angles on both sides of the interaction online reconstruction of its signals, the TRD has also been successfully region. These detectors can be employed, e.g. to define a minimum-bias used to trigger on electrons with high transverse momenta and jets trigger, to determine the event time, the centrality and event plane of a (3 or more high-pT tracks). Last but not least, the TRD improves the collision [2,39,40]. Likewise, the muon spectrometer [41,42] is outside overall momentum resolution of the ALICE central barrel by providing the view on one side of the experiment, only, covering *4 < 휂 < *2.5. additional space points at large radii for tracking, and tracks anchored Fig. 1 also shows the definition of the global ALICE coordinate by the TRD will be a key element to correct space charge distortions system, which is a Cartesian system with its point of origin at the expected in the ALICE TPC in LHC Run 3 [29]. A first version of the nominal interaction point (xlab, ylab, zlab = 0); the xlab-axis pointing correction algorithm is already in use for Run 2. inwards radially to the centre of the LHC ring and the zlab-axis coinciding In this article the design, construction, operation, and performance of with the direction of one beam and pointing in direction opposite to the the ALICE TRD is described. Section2 gives an overview of the detector muon spectrometer. According to the (anti-)clock-wise beam directions, and its construction. The gas system is detailed in Section3. The services the muon spectrometer side is also called C-side, the opposite side required for the detector are outlined in Section4. In Section5 the A-side. read-out of the detector is discussed and the Detector Control System The design of the TRD is a result of the requirements and constraints (DCS) used for reliable operation and monitoring of the detector is discussed in the Technical Design Report [44]. It has a modular structure presented in Section6. The detector commissioning and its operation and its basic component is a multiwire proportional chamber (MWPC). are discussed in Section7. Tracking, alignment, and calibration are Each chamber is preceded by a drift region to allow for the recon- described in detail in Sections8–10, while various methods for charged struction of a local track segment, which is required for matching of hadron and electron identification are presented in Section 11. The use TRD information with tracks reconstructed with ITS and TPC at high of the TRD trigger system for jets, electrons, heavy-nuclei, and cosmic- multiplicities. TR photons are produced in a radiator mounted in front ray muons is described in Section 12. of the drift section and then absorbed in a xenon-based gas mixture. A schematic cross-section of a chamber and its radiator is shown in 2. Detector overview Fig. 2. The shown local coordinate system is a right-handed orthogonal Cartesian system, similar to the global coordinate system, rotated such A cross-section of the central part of the ALICE detector [1,2], that the x-axis is perpendicular to the chamber. Six layers of chambers installed at Interaction Point 2 (IP2) of the LHC, is shown in Fig. 1. are installed to enhance the pion rejection power. An eighteen-fold The central barrel detectors cover the pseudorapidity range |휂| ¿ 0:9 segmentation in azimuth ('), with each segment called `sector', was and are located inside a solenoid magnet, which produces a magnetic chosen to match that of the TPC read-out chambers.
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