In this talk an overview of the ALICE TRD is presented. The ALICE TRD consists of 540 individual detector modules with a total of 1.2 million readout channels. It allows electron identification above a momentum of 1 GeV/c and is capable of providing a very fast and efficient trigger on electrons with large transverse momentum pt. It will operate in a very high multiplicity environment. The rapidity density of charged particles in collisions of Pb nuclei at ps = 5:5 ATeV is expected to be as high as dN/dy = 8000

Wessels, J P

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsThe ALICE Transition Radiation DetectorPublicationALICE reference numberALICE-PUB-2001-55 version 1.0Institute reference numberDate of last change2001-12-03Author:J.P. Wesselsfor the ALICE CollaborationFrascati Physics Series Vol. nnn (2001), pp. 000-000Title Conference - Conference Town, Date dd mm, 2001THE ALICE TRANSITION RADIATION DETECTORJohannes P. WesselsPhysikalisches Institut, Universitat Heidelberg, Germanyfor the ALICE TRD collaborationABSTRACTIn this talk an overview of the ALICE TRD1is presented. The ALICE TRDconsists of 540 individual detector modules with a total of 1.2 million readoutchannels. It allows electron identication above a momentum of 1 GeV/c andis capable of providing a very fast and eÆcient trigger on electrons with largetransverse momentum pt. It will operate in a very high multiplicity environ-ment. The rapidity density of charged particles in collisions of Pb nuclei atps = 5:5 ATeV is expected to be as high as dN=dy = 8000.1 IntroductionThe main goal of ultra-relativistic heavy ion physics is the investigation of theproperties of the so-called quark gluon plasma (QGP). At the Large Hadron1a list of the members1)of the ALICE TRD2)working group can be foundat the end of the manuscriptCollider (LHC) this plasma of deconned quarks and gluons is produced incollisions of Pb nuclei at 5.5 ATeV. This is an unprecedented energy regimefor the study of heavy ion collisions. ALICE3)is the dedicated heavy ionexperiment at LHC to study theses collisions. The anticipated initial energydensity reached there is as high as  ' 1000 GeV/fm3(about 6000 times theenergy density of a Pb nucleus in its ground state). The lifetime of such aplasma until freeze-out may be as long as f' 70 fm/c and the volume atfreeze-out may reach V ' 105fm3(about 65 times the volume of a Pb nucleus).These collisions may produce a charged particle density of up to dN=dy = 8000.Most of these particles are pions and are subject to strong nal state interaction.They will be detected in the central barrel of the ALICE experiment in arapidity interval jj < 0:9. Leptons are an excellent tool to study the earlystage of the collision, i.e., the QGP, since they do not interact strongly. Theinvestigation of electron-positron pairs from the decay of Quarkonia (J/ , 0,),from semi-leptonic decays of B- and D-mesons, from the decay of low-massvector mesons, and from continuum radiation in the central barrel of ALICEare accessible with the TRD. It will have to identify electrons with momentaabove 1 GeV/c in a large background of pions. Also, because of the smallproduction cross sections of  that decay into electron-positron pairs, it will inaddition serve as a fast trigger detector for electrons with transverse momentapt> 3 GeV/c.In the following an overview of this detector will be presented. Specicissues of the trigger capabilities4), the custom front-end electronics5), andresults from test beam measurements6)are presented in separate contributionsto this conference.2 The ALICE TRD in NumbersThe individual detector elements (a cross section - not to scale - is shown inFigure 2) of the ALICE TRD consist of a radiator, a drift and conversionregion with Xe/CO2(85:15), and a multi-wire proportional section with padreadout. The largest chamber is 120 cm wide, 159 cm long, and 13 cm highincluding electronics and cooling. The readout and trigger electronics of theapproximately 1.2 million pads is directly connected to the backside of the padplane. The signals induced on the pads are sampled 15 times for the durationof the drift time leading to a total of roughly 17 million readout pixels.Figure 1: View of the ALICE space frame with the TPC and TRD installed.The central detectors of ALICE are attached to the so-called space frameshown in Figure 1. The TRD is located just outside the TPC starting at aradius of 2.9 m and extending to 3.7 m. The overall length of the detector is7 m. It covers the central rapidity region jj < 0:9. The 540 individual detectormodules of the TRD are grouped into 18 supermodules in azimuthal directionwhich are subdivided into 5 sections along the barrel and consist of 6 layerswith a total active area of 736 m2and a total gas volume of about 27 m3.For the successful measurement of electrons in this environment the designof the detectors has to minimize any material in its active region. The totalanticipated thickness of the 6 detector layers is X=X0= 14:3%.In the nominal magnetic eld of the ALICE magnet (B=0.4 T) the posi-tion resolution in r-direction of 400(600) m respectively for low(high) multi-24681012141 23 45 6 780255075100Pad numberTime binSignalcathode padsregiondriftRadiatorDriftChamber54electron3anodewirescathodewireselectronxyregionamplificationFigure 2: Schematic illustration of the ALICE TRD principle (for actual di-mensions refer to the text). A projection of the chamber in the x   y plane,the bending plane of the particles, is shown. In the inset the pulse height foradjacent pads as a function of drift time is plotted for the depicted electrontrack with a large energy deposition from the conversion of a TR photon at theentrance of the chamber.plicity will yield a momentum resolution of Æp=p = 2:5%L0:5%(0:8%)=GeV=c.The pion suppression at 90% electron eÆciency for pt> 3 GeV/c will be betterthan 100.3 Principle of OperationFigure 2 shows a cross section through one of the six layers of the detector. Be-fore entering the drift and conversion section of the readout chambers particlespass through the radiator. The radiator has been optimized to provide a goodcompromise between TR yield, radiation thickness, and mechanical stability.Therefore, a large variety of radiators has been tested. The current designforesees a sandwich of 48 mm thickness consisting of carbon ber reinforcedROHACELL HF71 (polymethacrylimide) and 40 mm of polypropylene bermats (17 m ber diameter). The drift electrode will be directly glued to theradiator. The radiator is followed by a drift and conversion region of 30 mmwith a drift eld of 700 V/cm. For the chosen gas mixture this correspondsto a drift velocity of 2 cm/s. The secondary electrons are amplied in themulti-wire proportional counter with a gas gain of around 6000.The wires run in r-direction where the best position resolution has tobe achieved for an accurate momentum determination. The readout pads arerectangular with an average area of about 6 cm2. The induced signal on thepads is recorded in 15 time samples. The inset in the drawing shows thetime sequence of pulse heights for the depicted electron track. The ratios ofcharges recorded on adjacent pads for each time sample allows local positiondetermination along the track. From the inclination of the track the momentumcan be inferred.4 Electronics OverviewA block diagram of the main components of the readout electronics of the TRDis shown in Figure 3. All components apart from the Global Tracking Unit(GTU) are implemented in two custom ASICs and are directly mounted on thedetector in order to optimize latency for the trigger decision. The analog ASICcontains the pre-amplier, shaper, and output driver. The other ASIC is amixed analog/digital design containing the ADC, tracklet pre-processor, eventbuer, and tracklet processor. Both chips are mounted on a so-called multi-chipmodule (MCM) serving 18 pads. The pre-amplier and shaper circuit has aconversion gain of 6.1 mV/fC, a shaping time of 120 ns, an input dynamic rangeof 164 fC, and provides a maximum dierential output of 1 V. The anticipatedmaximum equivalent input noise is 1000 e in system. The power consumptionper channel is 10 mW.The baseline for the ADC relies on a 0.25 m design of a 10 bit 10 MHzdierential ADC with a maximum power consumption of 20 mW. It is followedby a digital lter. This is necessary in order to compensate the long tails ofthe pulses due to slow ion drift and tails from the electronics, i.e., the timeresponse function. Without such a lter signicant distortions in the positionGTUADCPASAeventbufferADC10 Bit10 MHzcharge sensitivepreamplifiershaperreadouttree216 GB/sL1 trigger infoto CTPTRD L1 triggerregions of interestship data at L2Astore raw datauntil L1Afit trackletzero suppr.ship raw dataat L1A120 MHzvia DDLto HLT& DAQTRDassemble datatail cancellationsubtract pedestalfor tracklet calc.CPUTPPProcessorTracklet PreProcessorTrackletMIMDFigure 3: Basic components of the TRD front-end electronics. Everything butthe Global Tracking Unit (GTU) is mounted on the detector itself. The ADC,digital lter, tracklet pre-processor, tracklet processor, and event buer are in-corporated in a single chip. (L1A, L2A refer to the dierent trigger levels ofthe Central Trigger Processor (CTP) of ALICE.)measurement would be incurred depending on the history of pulse heights.These position measurements are used to reconstruct the short track pieces(tracklets) inside the drift region.The output of the ADC is directly fed into the tracklet pre-processor.Here, all relevant sums are calculated which are subsequently used by the track-let processor in order to calculate the position and inclination of tracklets, i.e.,the track pieces contained in the drift region. All tracklets are shipped to theGTU, where tracklets from the individual layers are combined and matchingcuts as well as momentum cuts and more complicated cuts (such as cuts on theinvariant mass of pairs) are applied.In Figure 4 the timing diagram for the individual steps in the processing ofthe tracklets are shown. All computations are nished and the trigger decisionof the TRD is sent to the Central Trigger Processor (CTP) 6.1 s after theinteraction. Also shown at the bottom of the Figure is the frequency at whichthe digital circuitry needs to run at the various stages of processing after anFigure 4: Timing of the TRD trigger. The time axis is calibrated in units of theLHC clock period. Each tick corresponds to four clock cycles or about 100 ns.initial pre-trigger signal that wakes up the TRD electronics has been received.The width of the boxes indicates the power consumption. It is worthwhilenoting that the integral bandwidth to the GTU that is handled by the readouttree on the detector is 216 GB/s. The bandwidth for data to the DAQ or theHigh Level Trigger via the DDL is 1.8 GB/s.5 Physics PerformanceResults from our test beam measurements6)have been used to adjust parame-ters in the simulation of the detector. In particular data from measurements ofthe TR performance of the sandwich radiator have been parameterized. A cor-rect description of the energy deposition, Lorentz angle, TR absorption crosssection, pad response function, and the response of the electronics were alsoincluded in the simulations. They were carried out within the object-orientedframework AliRoot7)developed for ALICE.The response of the detector has been tested with events of varying mul-tiplicity up to the highest anticipated multiplicity density of primary chargedparticles of dN=dy = 8000. To enhance statistics additional electron tracks 10-310-20.8 0.85 0.9 0.95eeffπeff11500300060008000Data, p=2 GeV/cFigure 5: Pion eÆciency as a function of the electron eÆciency for dierentevent multiplicities (1-8000) and tracks of p = 2GeV=c total momentum.have been embedded into these events in order to evaluate the pion detectioneÆciency as a function of electron detection eÆciency for various event mul-tiplicities as shown in Figure 5. At 2 GeV/c electron momentum and for thehighest multiplicity, a pion rejection of a factor of 50 can still be achieved, whilefor lower multiplicities rejection factors well above 100 are possible.The mass resolution for the  (m = 9:46 GeV/c2) that can be achieved bycombining electrons and positrons with pt> 3 GeV/c as provided by the Level 1trigger is 245 MeV/c2. It has a tail towards lower masses due to radiative lossesof the electrons in material in front of the TRD. A global track t includingmeasurements from the Inner Tracking System (ITS) and the Time ProjectionChamber (TPC) improves the resolution to around 100 MeV/c2at the nominalmagnetic eld of B=0.4 T.The importance of the dedicated trigger becomes obvious when lookingat the yield of high ptJ= and . ALICE runs for about one month per year.Hence all estimates are based on a total of 106s running at a luminosity ofL = 5  1026cm 2s 1. Without a trigger on high ptelectrons and positronsabout 2500 J= (pt> 5:5 GeV/c) and about 160  can be recorded independentof centrality. With the trigger (pt> 3 GeV/c) the number of high ptJ= isquadrupled and about 2300  will be collected.6 ConclusionsThe ALICE TRD is amongst other things well suited to study quarkonia pro-duction in the high multiplicity environment of ultra-relativistic Pb collisionsat the LHC. In conjunction with the other tracking detectors of the ALICEcentral barrel it will be possible to measure the  mesons with a mass resolu-tion of 1%. With an active area of 736 m2and 1.2 million readout channels itwill be one of the largest TRDs. The integration of all readout and trigger elec-tronics directly on the detector will greatly reduce electronics costs comparedto conventional readout schemes.Currently, it is foreseen to install 50% of the detector in time for therst LHC beam at the end of 2005. Provided additional funds can be madeavailable, the second half of the detector can be installed during the wintershutdown at the beginning of 2007.7 ReferencesReferences1. A. Andronic2, V. Angelov4, H. Appelshauser3, C. Blume2, P. Braun-Munzinger2, D. Bucher5, O. Busch2, A. Castillo-Ramirez2, V. Catanescu1,M. Ciobanu1, S. Chernenko6, V. Chepurnov6, A. Devismes2, O. Fateev6,Ch. Finck2, P. Foka2, C. Garabatos2, R. Glasow5, M. Guteisch4,N. Herrmann3, M. Ivanov2, F. Lesser4, V. Lindenstruth4, T. Lister5,T. Mahmoud3, A. Marin2, D. Miskowiec2, Yu. Panebratsev6,T. Peitzmann5, V. Petracek3, M. Petrovici1, C. Reichling4, A. Reischl3,K. Reygers5, A. Sandoval2, R. Santo5, R. Schicker3, R. Schneider4,M. Schulz4, S. Sedykh2, S. Shimanski6, R.S. Simon2, L. Smykov6,H.K. Soltveit3, H.J. Specht3, J. Stachel3, H. Stelzer2, H. Tilsner3,G. Tsiledakis2, I. Rusanov3, B. Vulpescu3, I. Weimann3, S. Wende3,J.P. Wessels3, B. Windelband3, O. Winkelmann5, C. Xu3, V. Yurevich6,Yu. Zanevsky6, O. Zaudtke5;1IPNE, Bucharest, Romania;2GSI,Darmstadt, Germany;3Physikalisches Institut, Heidelberg, Germany;4Kirchho Institut, Heidelberg, Germany,5Institut fur Kernphysik,Munster, Germany,6JINR, Dubna, Russia.2. ALICE TRD, Technical Design Report, CERN/LHCC 2001-021,http://www.gsi.de/~alice.3. ALICE, Technical Proposal, CERN/LHCC 95-71,http://alice.web.cern.ch/Alice/.4. B. Vulpescu, Physics with the ALICE TRD, contribution to this conference.5. V. Angelov, ALICE TRD Trigger Architecture, contribution to this con-ference.6. A. Andronic, Results from prototype tests for the ALICE TRD, contribu-tion to this conference; A. Andronic et al., IEEE Trans. Nucl. Sci. 48, 1259(2001).7. R.D. Apppuhn et al., Nucl. Instr. Meth. A263, 309 (1988).

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