, b g0 ! f' II • o ° PROGRESS REPORT U.S. DEPARTMENT OF ENERGY Relativistic Heavy Ion Physics DE-FG02-92ER40692 11/15/92-11/14/93 John C. Hill Fred K. Wohn Department of Physics and Astronomy Iowa State University Ames, IA 50011 DISCLAIMER This reportwas preparedasanaccount ofworksponsored byanagencyof the UnitedStates Go',ernment. Neither theUnited States Governmentnoranyagencythereof,noranyoftheir employees,makesanywarranty,expressorimplied,orassumesanylegalliabilityorresponsi- bilityforthe accuracy,completeness,or usefulnessof any information,apparatus,product,or processdisclosed,or representsthat itsusewould not infringeprivatelyownedrights.Refer- ence hereinto anyspecificcommercialproduct,process,or servicebytradename,trademark, manufacturer,or otherwise does notnecessarilyconstitute orimply itsendorsement,recom- mendation,or favoring by the United States Governmentor any agencythereof.The views and opinions of authors expressedherein do not necessarily state or reflect those of the United States Governmentoranyagencythereof. DISTRIBU'iiU_40;: _i-siSDUCUMi_i',,It 16UNLIMITF=O,_,_ I ¢"" "I ." TABLE OF CONTENTS Page ABSTRACT 1 PURPOSE AND TRENDS 2 PHYSICS RESEARCH PROGRESS 4 A. Participation in RHIC Detector Development 4 B. Participation in E864 at the AGS accelerator 8 C. Studies of Electromagnetic Dissociation (ED) 9 D. Completion of Nuclear Structure Studies 11 PUBLICATIONS 13 APPENDIX ABSTRACT In 1991 a proposal by the Iowa State University experimental nuclear physics group entitled "Relativistic Heavy Ion Physics" was funded by the U.S. Department of Energy, Office of Energy Research, for a three-year period beginning November 15, 1991. This is a progress report for the period May 1992 through April 1993. In the first section, entitled "Purpose and Trends", we give some background on the recent trends in our research program and its evolution from an emphasis on nuclear structure physics to its present emphasis on relativistic heavy ion and RHIC physics. The next section, entitled "Physics Research Progress", is divided into four parts. First, we discuss our participation in the program to develop a large detector named PHENIX for the RHIC accelerator. Second, we discuss joining E864 at the AGS accelerator and our role in that experiment. Third, we outline progress made in the study of electromagnetic dissociation (ED). A highlight of this endeavor is an experiment carried out with the t97Au beam from the AGS accelerator in April 1992. Fourth, we discuss progress in completion of our nuclear structure studies. In the final section a list of publications, invited talks and contributed talks is given. i I 2 PURPOSE AND TRENDS The primary emphasis of the Iowa State University experimental nuclear physics program is the study of nuclear matter under extreme conditions of temperature and density by means of relativistic heavy ion collisions. This new emphasis began in July 1990 when we joined a collaboration to build a di-muon detector for the RHIC accelerator at Brookhaven. Later, as detector planning evolved, we joined together with others to build a large detector named PHENIX for RHIC. PHENIX will emphasize the study of photons and di-leptons produced in relativistic hea,,y ion collisions with a major motivation being to produce and detect signals from the quark-gluon plasma (QGP). The major effort of our group will be to design and implement the first-level trigger for PHENIX in collaboration with the Engineering Services Group of the USDOE Ames Laboratory. That group has extensive relevant experience since they designed the data acquisition system for the Hadro_ Projection Chamber on the DELPHI detector at the LEP collider at CERN. We also have participated in the RD 10/45 R&D experiment at the AGS accelerator whose purpose is to measure the transport properties of hadrons and muons in various absorbers. We also will be involved in the R&D program at the AGS test beam in Summer 1993. A second aspect of our program involves the search for strange matter and antinuclei with the 11 GeV/nucleon Au beam from the AGS accelerator. This is a new initiative on our part and also provides us with the opportunity to be involved in a large scale relativistic heavy ion experiment and "do some physics" before RHIC comes on line near the end of the decade. In March 1993 we were invited to join the E864 collaboration to participate in an experiment entitled "Measurements of Rare Composite Objects and High Sensitivity Searches for Novel Forms of Matter Produced in High Energy Heavy Ion Collisions". This experiment is scheduled to start running at the AGS in 1994. Our role in the experiment is to design and build the late-energy trigger which must be operational for taking data in 1995. A third aspect of our program is the study of electromagnetic dissociation (ED). ED is a process in relativistic heavy ion reactions in which intense electromagnetic fields excite the nucleus through the exchange of virtual photons. ED is predicted to strongly dominate ultrarelativistic heavy ion reactions with heavy projectiles (due to the very intense electromagnetic pulse produced by ultrarelativistic heavy ions), thus it has important implications for the design of experiments and the next generation of heavy ion colliders. A proposal, AGS experiment E862, by our group to extend our ED studies using the 11 GeV/nucleon 197Aubeam from the AGS-booster accelerator, was approved in June 1990 and initial running was carried our at the AGS in April 1992. In April 1993 a proposal by us to measure ED at the CERN-SPS was approved as experiment NA53 and will run in 1995 L'sing the 160 GeV/nucleon 2°Spbbeam. In view of our new commitment to relativistic heavy ion physics with future experiments at RHIC as the centerpiece of our program, we are phasing out our efforts in nuclear structure physics. Our final effort in that regard is toward completion of the dissertation of Mr. Vo by the end of 1993. In this report we outline progress made from May 1992 through April 1993. The grant covers the three-year period from November 15, 1991 to November 14, 1994. Progress expected in the next year and our plans for the future are outlined in a proposal for funding of the third year (November 15, 1993 to November 14, 1994) of our three-year grant. PHYSICS RESEARCH PROGRESS A. Participation in RHIC Detector Development The participation of the Iowa State experimental nuclear physics group with RHIC began in July 1990 when we joined the collaboration to build a di-muon detector. Along with many of our colleagues from the di-muon collaboration, we subsequently joined the larger collaboration to design and build the PHENIX detector which will emphasize the study of di- leptons and photons. We also joined the PHOBOS detector collaboration, but in view of our major responsibilities in the PHENIX project, we withdrew from PHOBOS in February 1993. The Iowa State group has the major responsibility for designing and implementing the first- level trigger for the PHENIX detector. The concept for such a role began in October 1990 when Glenn Young (ORNL) visited Ames to join us in discussions with Harold Skank and Bill Thomas of the Ames Laboratory Engineering Services Group (ESG). The ESG is uniquely suited to work on electronics for PHENIX due to their experience in developing the data acquisition system for the hadron projection chamber (HPC) for the DELPHI detector at the LEP collider at CERN. In April 1992 Glenn Young and Leo Paffrath (BNL) visited Ames for two days to discuss the role of our group in electronics and triggering for PHENIX. In March 1993 we hosted at Ames a meeting of the PHENIX Online group for discussions of trigger and data acquisition issues. The Iowa State group joined the RD-45 experiment which was funded through R&D funds for RHIC. The goal of this experiment was to measure leakage of hadrons through a series of filters upstream from a muon detector. The experiments were carried out using test beams from the AGS accelerator of particles (p,'n',l_,e) in the momentum range from 1 to 10 GeV/c. Hill, Wohn, and our graduate student Ewell participated in both the preparations for and running of the RD-45 experiment in Summer 1992. 2. Progress in the PHENIX Level-1 Trigger Design The major contribution to the PHENIX detector effort by the Iowa State Group will be the level-1 trigger. We have formed the Ames Trigger Group, which consists at present of 4 physicists (2 Principal Investigators, 1 postdoe, and 1 grad student) and 4 electronics engineers (3 Ames Lab staff and 1 grad student). The Ames Trigger Group will cover all aspects of the Level-1 Trigger for PHENIX - design, prototype testing, production, assembly and final testing at RHIC. Our group, the level-2 trigger group, and the data acquisition group are the three major components of the PHENIX Collaboration's Online Group, which is headed by Glenn Young (ORNL). The level-1 trigger is crucial to the experimental program at PHENIX. Only events that pass one (or more) of the various level-1 triggers will be retained for further analysis by the level-2 trigger. Therefore the level-1 trigger philosophy is to accept as many events as possible that can be classified as candidates for interesting physics. Rejection of some of the level-1 candidates by the level-2 trigger (or by higher levels) can be done on the basis on tighter restrictions and/or different algorithms than those used at level 1. However, the level-1 trigger must be able to control the rate of accepted events in order to allow the entire system, from level 1 until the final output to data tapes, to digest the incoming data stream without suffering deadtime losses. This is accomplished for the PHENIX detector by selecting a pipeline architecture. The level-1 trigger follows a "clock-based" pipeline, where eacb clock tick is the ll2-ns beam crossing rate. The depth of the level-1 pipeline will be 32 ticks, or 3.58 _s. Subsequently, the level-2 trigger follows an "event-based" pipeline, where only events accepted at level 1 are allowed to enter the event pipeline. Six of the detectors that make up the PHENIX detector will be used in the level-I trigger. The six detectors are Beam-Beam, Silicon MVD (Multiplicity Vertex Detector), RICH (Ring Imaging Cherenkov), TOF (Time of Flight), EMCal (Electromagnetic Calorimeter) and Muon ID (Identifier) detectors. None of the tracking detectors (drift and pad chambers) will be used at level 1 because of the inherently slow speed of such detectors. For charged particles, this means that no momentum information can be used at level 1, since determining momenta requires tracking information. However, the main objective of the level-1 trigger is to identify the events that have candidates of interest, and for this tracking is not necessary. The phrase "candidates of interest" means the combinations of particles for physics of interest, thus combinations such as electron pairs, muon pairs, hadrons, and photons. The level-1 trigger will use the TOF trigger to select hadron candidates, with additional information from the Beam-Beam detector about whether there is a valid start time (from Beam-Beam) to permit an accurate time-of-flight measurement using the TOF stop time. Photon candidates will be identified by the EMCal, and muon candidates by the Muon ID. Both the RICH and the EMCal will be used to identify electron candidates. Since the most interesting events are central-collisions that have high multiplicity, the level-1 trigger will generate multiplicity information from both the Silicon MVD and the EMCal. A sum over the MVD of charged particle hits will give one, and the other will be a sum, over the four EMCAL walls, of the total energy (which, due to the rapidity covered by the EMCal ismainly the transverse energy Et). They give two alternatives for determining the "centrality" of a collision. Control of the data rate will be accomplished in the level-1 trigger by using, in conjunction with the combinations of particle candidates, different prescale factors for different multiplicity ranges. To pick a simple example, there will be a trigger called Photon-1 that accepts single photons above a relatively high energy threshold, such as 2-3 GeV. That is, if any one of the 7000 EMCal trigger units (4x4 arrays of PMT towers) were to have an energy above the threshold, then that event would be kept, provided it would also pass the prescale requirement. With binary prescale factors 2N, only 1 of each 2Nevents passes the prescale requirement. Thus, to select every photon means the factor N must be 0. However, if this were to be done for the Photon-1 trigger, the rate of accepted events would be too high t" ! without imposing multiplicity prescaling. By using prescale factors on "global" parameters such as multiplicity, we expect to be able to "flatten out" the number of events accepted for the selected multiplicity ranges. Our plan for using prescalers for each of the level-1 triggers permits us to obtain "sampling mode" data of various types. This means that the number of triggers will be in the range of 100-200, in order to permit several values of multiplicity-prescale combinations for each of the particle combinations of interest. Normalization-type triggers with less restrictions would be included, but with high prescale factors. (For example, "Min Bias" events could be kept at a very large prescale factor such as 22°.) In addition to the obvious particle candidate types, such as 2 (or more) electrons in the RICH or 2 (or more) muons in the Muon ID, there are some less obvious types that must also be included in the level-1 triggers. One is the electron-muon combination that is needed in order to determine the substantial contributions from "open charm" decays to the observed dilepton channels (e.e and I_+ls). It isworth noting here that, due to combinatorics (such as an electron from each of 2 different decays) we need to accept all 2-electron combinations (e+e . and ee as well as e+e and ee+). This is implicit in all level-1 triggers since there is no selection at level 1 on a particle's charge. Chapter 11 of the PHENIX CDR (Conceptual Design Report) provides a detailed discussion of the various level-1 triggers we have designed. The level-1 trigger discussion comprises CDR pages 11-41 to 11-66 and is provided in the Appendix to this report. The initial cost estimate for the level-1 trigger was clone by us in conjunctibn with PHENIX management, in particular with Leo Paffrath (Project Engineer) and Glenn Young (Deputy Project Director). A total construction cost for the level-1 trigger of $1.2M was estimated. 3. Progress with PHENIX Detector Simulations Dr. Athan Petridis was hired in July 1992 to initiate at ISU an involvement with the PHENIX detector simulations. Since our involvement as a trigger group would be with the entire detector, Dr. Petridis' first task was to develop the capability for running the PISA (PHENIX Integrated Simulation Application) detector simulation program (from Professor C. Maguire of Vanderbilt) and install a UNIX version of it on the network of UNIX workstations at ISU. Dr. Petridis found it necessary to develop modifications to the code to make it produce data suitable for the trigger algorithms we had developed. The modified PISA code has been used so far to examine the performance of two PHENIX detector subsystems- the Muon ID and the RICH. For the Muon ID, simulations were run to determine the minimal number of detection planes that need to be instrumented, and the optimal location of these planes. When the choice of 3 active planes was made by the collaboration, the simulations were used to determine the efficiency of muon detection as a function of muon energy. We found out that the muon acceptance of the muon detector begins to drop sharply as the muon energy falls below 2 GeV. The trigger level-1 algorithm for the RICH detector uses PMT modules (a module is a 5x5 array of PMTs) in a tiling scheme to detect the Cherenkov rings that are formed on the focal plane of the mirrors of the RICH detector. A modification was made to produce from PISA the data needed to test our RICH algorithm. Dr. Petridis then wrote a separate code to test the algorithm as a function of its threshold and the position of an electron ring on the RICH array. This required developing a code to generate an isotopic distribution of single electrons over an arm of the RICH detector. The original RICH algorithm was evaluated by use of these simulations, which suggested an extension to the original algorithm to provide a nearly perfect 100% counting of single electrons with essentially no overcounting. (Without the extension, overcounting of electrons was unacceptably large and could have misled the RICH level-1 trigger into thinking an event had two electrons when only one actually hit the RICH.) This significant improvement of the RICH algorithm could not have been readily deduced without the trigger simulation tests. Additional tests of the RICH algorithm are currently nearing completion. They involve testing the high transverse-momentum limit of the RICH level-1 trigger algorithm to resolve and count two closely spaced electrons. We need to know the maximum transverse momentum that a particle, such as a vector meson, can have such that its decay into an e*e pair is detected by the RICH detector. In this case, the high transverse-momentum limit implies a minimum opening-angle for the RICH level-1 algorithm. The tests, although as yet incomplete, indicate no difficulty until transverse momenta above about 20 GeV/c are reached for the J/_ meson. This is large enough not to be a significant factor in using the RICH to study the e+e decays of the vector mesons. The sensitivity of this result to magnetic effects and the event position in the interaction diamond will soon be evaluated. 4. Progress in PHENIX Tracking We have participated in the studies of the PHENIX Tracking group, for which Ed O'Brien (BNL) is the spokesman. We participated in the AGS test beam run in June 1992 of the TEC (Time Expansion Chamber) which was run in the dE/dx mode and also as a TRD (Transition Radiation Detector) after being loaded with polypropylene foils. A TEC can detect electrons produced either by ionization (i.e., the usual dE/dx signal that is spread out over the entire path of the charged particle in the gas of the detector) or by the electron "burst" due to a TR photon. (A burst occurs because the photon of several keV' is absorbed at a specific point along the path of the charged particle in the detector.) The TEC used in the 1992 test beam run could not be optimized simultaneously for both ionization and TR detection, but provided useful data for designing a new, thicker TEC. Three new, thicker TECs will be ready for use in the June 1993 test beam run at the AGS. We (Hill and Wohn) plan to participate in this run by taking shifts during the run. In addition, Dr. Bruce Libby, who is our postdoc stationed at BNL, and Mr. Lars Ewell, a , graduate student who will be at BNL from April through July 1993, have been working with Ed O'Brien and the TEC chambers. Mr. Ewell has been assisting in assembling and preparing the new TEC chambers for the test beam run. d" ! Dr. Libby has assisted with TEC assembly but he has concentrated on simulating the performance of the chambers using the GARFIELD code, which is a CERN library program that calculates electron and positive-ion drift path, drift times, and the signals induced on the sense wires by a minimum ionizing particle. GARFIELD has been used to monitor changes in the drift path, drift times, and the sense-wire signals as the potential in both the drift and proportional regions is changed. In addition, the effects of changing the gas in the TEC, using a variety of gas mixtures based on Ar and Xe, were studied with GARFIELD. The signals calculated by GARFIELD can be convoluted with response functions to simulate the signals from preamplifiers and shapers. In this manner GARFIELD provides a reasonable approximation to the expected output of the TEC, which, when compared with the actual test beam data, will be an important tool to be exploited in the design of the final TEC for PHENIX. B. Participation in E864 at the AGS Accelerator In March 1993 the Iowa State group was invited to join E864, which is scheduled to begin taking data at the AGS in the summer of 1994. The purpose of this experiment is conveyed in the title "Measurement of Rare Composite Objects and High Sensitivity Searches for Novel Forms of Matter Produced in High Energy Heavy Ion Collisions". We are particularly interested in the search for "strangelets" or other forms of strange matter and antinuclei. This will also give us an opportunity to gain experience in a large-scale relativistic heavy ion experiment prior to running at RHIC and to do some interesting physics. Our role in E864 is already very well defined. We are responsible for designing and building the late-energy trigger. Our experience with the level-1 trigger at PHENIX and the availability of the expertise of the Ames Laboratory Engineering Services Group make this role a natural for our group. Also the late-energy trigger will not be implemented until 1995 which is convenient for us since we only recently joined the collaboration. Although our group has the lead responsibility for the late-energy trigger, we will be assisted by a group from Purdue University that also recently joined E864. In E864 it is not possible to reach the desired level of sensitivity (1 part in 10tl) for the rarest events without a very selective trigger. A very restrictive trigger condition is that an event isaccepted only if ithits the calorimeter at least 2.5 ns after a speed-of-light particle and if it deposits a minimum of 3 GeV in one tower of the calorimeter. Fortunately this condition should be met by the rarest events of interest such as stranglets, antideuterons, antitritons, and the heavier nuclear systems. A first design for the late-energy trigger has been proposed by our group and presented to the E864 collaboration at the collaboration meeting in May 1993. The preliminary design for the trigger is shown in Fig. 1. The trigger operates in the following manner. The output from each of 616 towers of the calorimeter is fed to an integrator that produces an energy signal. Each tower output also passes through a discriminator that is used as a stop for a TAC. The TAC is started by a signal from the multiplicity counter located near the target. The time information from each TAC and the energy information from each