The collision of 15 GeV/nucleon sulfur and carbon or oxygen ions with various fixed targets such as carbon, sulfur, and gold is to be observed in the MPS magnetic spectrometer with a time projection chamber (TPC) module. This TPC will be placed just downstream of the target and be able to track approximately 50% of all charged particles. The design and construction of the TPC is proceeding, and prototypes of the TPC readout system have been tested and production devices are to be delivered. A TPC track reconstruction program has been developed. Monte Carlo events have been generated and tracked. The track reconstruction program consists of three parts: a local pattern recognition which associates contiguous readouts on adjacent readout wires to form a single hit; a subroutine which positions the hits into slices in the vertical plane; and the track reconstruction section. (LEW

Asoka-Kumar, P.P.V.

Etkin, A.

Foley, K.J.

Hackenburg, R.W.

Lindenbaum, S.J.

Longacre, R.S.

Love, W.A.

Morris, T.W.

Platner, E.D.

Saulys, A.C.

Lindenbaum, S. J.

Foley, K. J.

Hackenburg, R. W.

Longacre, R. S.

Love, W. A.

Morris, T. W.

Platner, E. D.

Saulys, A. C.

Asoka-Kumar, P. P. V.

UNT Digital Library

BNL 38229LARGE SOLID ANGLE TRACKING OF MONTE CARLO EVENTS OF HEAVY ION COLLISIONS INTPC MAGNETIC SPECTROMETERS*S.J. LindenbaumBrookhaven National Laboratory and City College of New YorkA. E t k i n , K.J. Foley, R.W. Hackenburg, R.S. Longacre, W.A. Love,T.W. Mor r i s , E.D. P la tner , A.C. SaulysBrookhaven National Laboratory _ „ . 38229P.P.V. Asoka-Kumar, C.S. Chan, M.A. KramerCity College of New YorkINTRODUCTIONThe BNL/CCNY co l l abo ra t i on has fo r some t ime had as i t s goal the develop-ment and use of » 4TT s o l i d angle magnetic spectrometer t r ack i ng of chargedp a r t i c l e s produced in heavy ion c o l l i s i o n experiments at AGS,1 and eventua l lyRHIC.2 This program has led t o the approved AGS Experiment 810 which inc ludesco l l abo ra to rs from LBL* and Johns Hopkins*''"'' We are also p a r t i c i p a t i n g inthe NA36 experiment at CERN.In Phase I of AGS experiment 810 (F ig . l a ) the c o l l i s i o n of 15 GeV/nucleons u l f u r and carbon or oxygen ions wi th various f i x e d ta rge ts such as carbon,su l f u r and gold w i l l be observed in the MPS magnetic spectrometer w i th a TPCchamber module based on the one being developed fo r the CERN NA36 experiment.However, mod i f i ca t i ons w i l l be incorporated as needed t o make i t su i tab le f o rthe AGS 810 environment. In the major arrangement of Phase I of AGS 810, t h i sTPC w i l l be placed j u s t downstream of the t a r g e t and be able t o t rack - 50% ofa l l chargea p a r t i c l e s . F igure 2 shows the Monte Carlo acceptance of t h i sarrangement vs . r a p i d i t y f o r reconstructed t r a c k s . The f a l l - o f f of e f f i c i e n c y* This research was supported by the U.S. Department of Energy under CorNos. DE-AC02-76CH00016 (BNL) and DE-AC02-83ER40107 (CCNY).t W. Ge is t , C. Gruhn, M. Heident t T. Hallman, L. Madansky DISTRIBUTION OF THIS DOCUMENT IS UNLIMITEDIONBEAM 31!5GeV/iAS/ J1 TGTCCD ARRAYAROUND TGT\\TPC~X \VzFigure l a : Phase I of AGS 810. The cross section of the TPC module w i l l beat least 50 cm x 50 cm but these dimensions may be increased by a factor- 1.5. The Monte Carlo calculations in the remainder of th is paper assumethe minimum cross section stated.IONBEAM1Figure l b : Phase I I of AGS 810. The dashed lines show the approximate crosssection of the TPC in the magnetic bending plane. The d r i f t space along themagnetic f i e l d wi l l be increased to approximately 1 meter.of track reconstruction at low rapidi t ies is due to tracks leaving the TPCchamber module. In Phase I I this is expected to occur only a small percentageof the t ime.* In Phase I I of th is program we intend to f i l l the MPS withTPC modules and thus have an » 4TT detector.The design and construction of the NA36 TPC chamber is proceeding in CERNunder the primary responsibi l i ty of the LBL group and wil x- described byChuck Gruhn in his paper at th is conference.3 The BNL/CCNY group is pr imari lyresponsible for designing and constructing the TPC readout system and the dataacquisit ion systems for NA36. The TPC readout system for NA36 is that deve-loped for AGS 810, and is based on MPS I I - s t y l e electronics.A data acquisit ion system which can be used in NA36 and AGS 810 experimentsis described in a paper by Eisen.an et a O The FASTBUS-based system w i l l beVcapable of recording > 50 events per second from a variety of detectorsincluding a TPC. Phase I of AGS experiment 810 is characterized by eventsthat t yp ica l l y involve tracking of - 100 or more charged par t ic les .Prototypes of the TPC readout system have been successfully tested andproduction devices are scheduled to be delivered soon.Very slow short tracks w i l l in general not be reconstructable.Monte Cario Positive Tracks in E810 Rill TPC0.4020.0 »-as 3.5Rapidity (Y)Figure 2a: Acceptance vs. rapidity for positive tracks in Phase I arrangementMonte Carlo Negative Tracks in E810 FUll TPC-0.5 1.5 ZSRapidity (Y)asFigure 2b: Acceptance vs« rapidity for negative tracks in Phase I arrangementPATTERN RECOGNITION OF TRACKSWe have also for AGS 810 developed a TPC track reconstruction program. Theprogram was written by Tom Morris and is based in part on algorithms deve-loped by Michael Mermikides for the ALEPH TPC. Bill Love and Ron Longacregenerated the Monte Carlo events using HIJET. They were tracked through theapparatus by using GEANT-3. The track reconstruction program consists ofthree parts: a local pattern recognition which associates contiguous readoutson adjacent readout wires to form a single hit; a subroutine which positionsthe hits into slices in the yz-view (the vertical plane containing the beamand the vertical magnetic f ie ld) and the track reconstruction section.The readout system consists of rows of wires parallel to the beam 1 cm longand separated by 1.27 mm. Thera are 40 such rows along the beam. The wiresare assumed sensitive only in the middle 7 mm. The simulation apportioned thetrack length to the wires lying over the track as i t passed through the 7 mmsensitive width. The track segment subtended by each wire then generated anamount of charge randomly distributed according to the Landau formula for thatlength. The charge under each wire was diffused in the y-direction with anrms half width of 4 mm for 1 meter of d r i f t . Sinusoidal pickup noise corres-ponding to half the threshold signal was added. Five percent of the readoutwires were considered dead and randomly distributed. Random hits equal to 20%of the track hits were distributed according to a 1/R algorithm where R isdistance from the track, in order to attempt to simulate experimental opera-ting conditions.Figure 3 shows the tracks in the TPC from a HIJET generated central c o l l i -sion 15 GeV/nucleon sulphur ion incident on a gold target, when projected on ahorizontal plane containing the beam and being perpendicular to the magneticf ie ld .The more than 100 tracks clearly leads to great confusion in any projectivegeoretry pattern recognition approach and is clearly not practical.However the three dimensional point detection feature of the TPC leads to aquite simplified pattern recognition problem. As shown in Figs. 3b and 3c(the xz view), when the TPC is partitioned into slices in the yz projectionwhose vertices are at the target, the pattern recognition appears quite•••*I1111It1II••9 • * *- I• * t• • m• • ! •; " . •• * # *j : ! :• 1 * •i !1; • •«! 1 ' *• j i *i • • •i ; J J• . i X'. ' !• :- ; .I : •. ' "• 1• . .* * i *' \ : iIII ij j j iilii! 111i • i *• § • *« : ; i1 I "j •. i. 1 • •• '» ' *" : ! ;. . ' • ' •:-;;i1it*•>111:• i •- * -j i jj j jiij •• . •i• i• !• ! *" i !: ]!* • *. * . ** * : • • ! !, I i i : i1 ! i " . ; s; j • • j 1 !| j i \;;;; i : 1 :: !1 | i ' ' i jI I "i • ! ! ! ; •• • " . 1\ * r • * *• •-;**• * " * ! •; : • • * . i. • • ) • ** * . • • i *i ' . " • • t• • * •: • : ' i : •. * - 1 "* 1 1 i \ j• 1 j I ! ' .• 1 ! " • •' ! • • ' • ' •. * • I »• \ : • B • . .• ! - '* * • * " • • !; * * * • •1 • • " * t• * i I * • •• * • • • * *• : . I ; t • ;• • ! » " ! " •• • ! • - " • •" • , • • • • :t • " • * , *• • I ! " •I . " " * L* • " .! i 51• ' • .• • r •. • :' . ;i i • !• • «* • • •- • •• • •Li 1 ft' 11 ' !: i ;• * t• * •i • \i | j* * i• • .• • i• . •! •I• : .Figure 3a: HIJET 15 GeV * A sulphur-on Au target central collisions pro-jected on a horizontal plane con-taining the beam.Figure 3b: Tracks in sl ice in yzprojection 0 < yL < 2a«. aAm-•-••*•-•-- •--*•--*-*••*-•A 1 :• -•**•*-*••a,*•aaa•ft«aAa.-Figure 3c: Tracks in a slice in yzprojection where 2 < yi_ < 4manageable. The slices we are using are approximately 1 cm high near y = 0(- the beam region) and approximately 5 cm high at the top and bottom. Thehits in the readout pads are indexed by s l ice number in each wire row and thetrack recognition program normally searches hits only within a single sl icethereby greatly speeding up the track f ind ing.The event generating program is interfaced to the CERN GEANT-3 simulationprogram which at present propagates HIJET-generated tracks through the appara-tus and generates readouts at each TPC wire row. A front end for Monte Carlodata input has been wri t ten which simulates the effect of electron d i f fus ion ,charge mult ip l icat ion and the response of the TPC readout electronics,,The track recognition starts at the downstream end of the TPC and worksupstream. Only those hits within the working slice are searched; however ifthe track approaches a slice boundary, the hits in the adjacent slice areadded to the search list. A track in the process of reconstruction is calleda chain. There are forty rows of readout along the beam. When three hits onadjacent rows can be spacially associated in three dimensions, a chain isinitialized.The chain is developed upstream by local association from row to row inthree dimensions. Absence of hits in two consecutive rows or two or more hitcandidates in the same row terminates this stage of chain development.In the next stage circle (linear) least square fits are performed in thex,z (y,z) views.If the fits are successful, the fitted orbit is extended over missing wirer^ws and consistent hits are added to the chain. If the fitting fails thereis provision for seeking out and discarding bad hits. The event shown inFig. 3 had 206 charged tracks initially generated. Of these 113 chargedtracks had > 10 hits in the TPC. 98 of the 113 were reconstructed. If werequired >_ 20 hits in the TPC, only 85 tracks of those generated met thatrequirement. 82 of these were reconstructed. 74 of these 82 had a percentagemomentum error Ap/p < 10%.Preliminary results with Monte Carlo data simulating 15 GeV sulphur on goldevents indicate that » 93% of hits are assigned to tracks. Of these - 96% areassigned correctly. Approximately 95% of tracks which span at least 20 wirerows (i.e. half the chamber length) are reconstructed. Of these « 10% hadreconstructed momentum that differed from the generated numbers by more than10%. The reconstruction time on a VAX 8600 as a function of the number oftracks is shown in Fig. 4. Thus the reconstruction time appears to beapproximately linear with track numbers.The acceptance of the reconstructed tracks versus rapidity is shown inFigs. 2a and 2b. The fall-off of the efficiency of track reconstruction atlow rapidities is due to tracks leaving the chamber module. In Phase II thisis expected to occur a small percentage of the time.In AGS 810 in addition to hardware triggers for central collisions we willhave a CCD array around the target in which further software selection ofdesired events can be made rapidly, and multiplicities and angular distribu-tions of the tracks emitted can be determined.The Physics objectives in AGS 810 and later at RHIC are to look on anf>vent-by-event basis for unusual events not expected from known processes.These events could be characterized by:TIME TO PROCESS I EVENTLdP 80 100 120 140 160TRACKSFigure 4:Global pattern recognition time as a function of the number of tracks. Toobtain the total processing time per event one should mult iply the above timesby « 1.5 to allow for unpacking and local pattern recognition times.1. Excessive local f luctuations (up or down) in pseudorapiditydensity ( i . e . , psuedorapidity bumps). In the case of negative particles whichare momentum analyzed we can assume they are pioris and look for rapiditybumps.2. Excessive f luctuations in m u l t i p l i c i t y .3. Excessive local or global enhancement of strangeness.4. Anomalous behavior in Pi (E j ) , or energy flow patterns.5. Hanbury-Brown and Twiss ef fects.6. Evidence for deflagrations (or detonations).7. Special effects that correspond to the increase in entropy thatmay occur in the baryon dense region,8. Something else which catches our eye.9. Most important - the correlations between these — For examplewe might f ind that pseudorapidity (or rapid i ty) bumps or dips or other anoma-lous behavior are associated with one or more of the above and may have simi-lar psaudorapidity (or rap id i t y ) .In summary the above i l l us t ra t i ons are to be taken, only as a guide. Theimportant point is that we are planning to see a great deal of the character-i s t i cs of each event and the i r correlations on an event-by-event basis andtherefore we shall see what i f anything is anomalous, in a most favorablesignal-to-background environment. We believe th is approach is also importantfor RHIC.This track recognition program has been translated into the tentative NA36arrangement in the 3 Tesla EHS magnet. An approximate simulation of the mag-net was used but E * B effects were neglected since their correction is aseparate matter.In the case of AGS 810, Phase I , the E x B effects are small enough so thatneglecting them does not s ign i f icant ly affect the results.Figure 5a shows a HIJET generated central co l l is ion event in the TPC moduleby a 200 GeV * A oxygen on Au event projected on the magnetic bending planecontaining the beam. Figure 5b shows how th is event looks in a sl ice near thebeam. When we require >_ 20 h i ts in the TPC, 77 of the or ig ina l ly generated279 charged tracks remain. Of these 76 were reconstructed. When we requireAp/p < 10% for a l l tracks, 67 were reconstructed.Figure 6 shows a 5 cm s l ice of a HIJET 100 GeV * A Gold + 100 GeV * A Goldcol l id ing beam central co l l i s ion in the SREL 5-meter-diameter magnet discussedin the RHIC Workshop (BM, April 1985).2 This event has - 4,000 tracks. Wehad argued at that workshop and in various committees etc. that one couldhandle any number of tracks i f the track h i ts /p ixe l were small enough.Secondly that the computer time would be approximately l inear with tracknumber i f one used a reasonable approach to the problem.I * * *li'i'ii'l!.1:!. i l l i l IS!!M i l - " . : ! ! :j ' : • • ' • J '• ' !j • . ; j i • ' . " .iniil-ji j jT i l. ' . ' . ' • •• • • - • .• • - i . . •: • : • : ; ; • ' . j i !'S i l l i i i J i i '• : j ; ; ' ! - - , •< ' 1 1 ! 5 i ' i! i i i ' ! ! I !•! 1ijiiiiliiii iFigure 5a: A HIJET 200 GeV * AOxygen ion on Au event in the tenta-t i ve NA36 arrangement projected onthe plane containing the beam and.the magnetic f i e l d .Figure 5b: A sl ice with 2 < y 4 inth is eventMAGNETIC FIELD 5.0 Y SLICE-0.0± 2.5cmGOLD COLLIDING BEAMSf M M/++++C^&1/101/101/100Zx EVENT 3. _ . Figure 6ThP i n n ^ n H ?/ ,nnRvC H I J E I 6 V e n t f 0 r 1 0° GeV x A Au on 100 GeV x A Gold.The 1/10 and 1/100 lines refer to track h i ts /p ixe l ra t io .10As you can see indicated in the diagram we had stated conservatively thati f h i ts /p ixe l were £ 1/100 the problem of track recognition would be re la t i ve -ly easy, This corresponds to a distance of « 30 cm from the beam.* We f e l tthat even i f hits per pixel approached » 1/10 at the upstream end one couldprobably s t i l l do track recognition i f one started from the downstream endwith h i ts /p ixe l 5 1/100. In the AGS 810 Monte Carlo simulations discussed wefind that the pixel size is « 10 mm2. This is approximately an order of mag-nitude less than other TPC's and is made possible by the compact AGS 810 read-out electronics.The track hi ts/p ixel rat ios vary from » 1/250 at the downstream end to«• 1/3.5 at the upstream end. The eff iciency for putting upstream hits en atrack when h i ts /p ixe l « 1/10 was found to be « 99%. When h i ts /p ixe l » 1/3.5the eff ic iency for putting upstream hits on track drops to « 63%.Therefore Gold on Gold events can also be tracked i f one merely increasesthe 25 cm distance of the upstream end of the chamber by a factor» /mu l t i p l i c i t y of Gold on Gold/mul t ip l ic i ty of sulphur on gold « 2.5. Inregard to the RHIC 100 GeV * A Au on Au events, preliminary investigationsshow that th is program w i l l perform track recognition sa t is fac tor i l y on thesetypes of events.I t should be noted that the readout geometry selected does introduce biasesfor tracks in certain di rect ions. For example in the paral lel row readoutperpendicular to the beam chosen for AGS 810, tracks more or less perpendicu-lar to the beam would be d i f f i c u l t to reconstruct. In the RHIC case, c ircularor e l l i p l i c a ! row readouts were primarily envisioned. Here again tracks moreor less paral le l to the readout system would be d i f f i c u l t to reconstruct.Combinations of directional arrays of readout geometries can of course beemployed to solve these problems. The same electronics can be arranged indifferent readout configurations depending upon needs.* For higher event mu l t i p l i c i t i es (n), the distance from the beam (r) re-quired to maintain the same hi ts/p ixel ra t io which corresponds to a dis-tance r0 and mu l t i p l i c i t y n 0 , scales approximately as rn » r0 /n /n0 due tothe approximate inverse square law reduction of track densities withdistance.11CONCLUSIONSHIJET + GEANT3 Monte Carlo simulations lead to the fo l lowing conclusions:1 . High e f f i c iency tracking of very high m u l t i p l i c i t y events in aTPC (or other three-dimensional pa r t i c le detectors) with good monentum reso lu -t i on for tracks generated in heavy ion co l l i s i ons is c lear ly feasible providedtrack h i t s / p i x e l « 1/10 to 1/100.2. I f h i t s /p i xe l « 1/100 the problem is r e l a t i ve l y s t ra ight forward,even at RHIC.3. The computer time for track reconstruction is « proportional tothe number of t racks.REFERENCES1. AGS E-810, A Search for Quark Matter (QGP) and Other New PhenomenaU t i l i z i n g Heavy Ion Col l is ions at the AGS.2. Schroeder, L. and Lindenbaum, S.J. Large Magnetic Spectrometer. RHICWorkshop: Experiments fo r a R e l a t i v i s t i c Heavy Ion Co l l i de r , Apri l 15-19,1985, P.E. Haustein and C.L. Woody, Edi tors , pp. 211-252 (BrookhavenNational Laboratory, Upton, New York, 1985). See Part I I .3. Charles Gruhn, these proceedings.4. Eiseman et a l . , A Very High Event Rate Data Acquisi t ion System in FASTBUS,IEEE Transactions on Nuclear Science NS-33, pp. 111-112 (1986).DISCLAIMERThis report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, or use-fulness of any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference herein to any spe-cific commercial product, process, or service by trade name, trademark, manufac-turer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof.The views and opinions of authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof.

Large solid angle tracking of Monte Carlo events of heavy ion collisions in TPC magnetic spectrometers

Large solid angle tracking of Monte Carlo events of heavy ion collisions in TPC magnetic spectrometers

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