Oak Ridge National Laboratory
High-Energy Reactions Group

Experiments


PHENIX Experiment at RHIC

Work associated with the RHIC PHENIX experiment is a central part of this program and forms the focus of its efforts for the future (see for a full description) . PHENIX concentrates on detection of lepton pairs, photons, and identified hadrons emitted in heavy-ion collisions produced by RHIC. The central idea of the PHENIX experiment is to focus on electromagnetic and high Q2 probes of the system formed in a relativistic heavy-ion collision, since these probes should provide the best way to view these collisions' early-time behavior when quark deconfinement, large gluon density, and direct radiation from the ensemble of partons should be most visible. PHENIX addresses this by having a wide kinematical coverage (in rapidity, transverse momentum, and mass) combined with excellent particle identification, stressing excellent detection of electrons, muons, and photons in particular. The experiment had its first major data-taking run during June-August 2000. The muon arms will start operation during the next beam period, which is set for March-September 2001.

ORNL has concentrated its efforts upon development of the muon identifier, the lead-glass electromagnetic calorimeter, monolithic electronics for several PHENIX subsystems, and the off-line computing system. These efforts have included Monte Carlo simulation studies, detector hardware efforts, and electronics development work. The WA98 lead-glass photon detector was moved directly CERN to PHENIX and serves as one of two main detectors for direct photon physics in the PHENIX baseline design.

ORNL initiated and led the effort within WA80/93/98 to develop an event-mixing algorithm for use in analyzing photon data. The algorithm allows us to extract cross sections for p0 and h mesons, even in the presence of 100 times larger combinatoric background. The p0 and h cross sections are essential input to determining what fraction of the total photon cross section observed in heavy-ion collisions is of hadronic origin and, thus, not due to direct radiation from a deconfined system. This algorithm is a necessary development to analyze photon results without being overwhelmed by combinatoric background in the high multiplicity environment that is encountered at RHIC. The tools developed for this effort are also used to estimate signal/noise, counting rates, and detection limits for direct photons at RHIC. This effort culminated in the present design of the photon part of the PHENIX spectrometer.

Our major development effort for new spectrometer design and construction for PHENIX has been in the area of the PHENIX Muon Arm. The North and South MuID arrays were constructed and installed in 1998 at BNL. All the readout electronics for the MuID has been designed at ORNL and is in fabrication (August 2000). In support of this detector design and construction, we have determined signal rates and expected counts distributions for the muon arm and took lead responsibility for performing trigger selectivity simulations, signal-to-noise estimates, hadron absorber design, and parametric studies of various spectrometer designs. We also performed the mechanical engineering design for the muon identifier (MuID). We earlier carried out, between 1990 and 1994, an R&D program at the BNL AGS test beam to optimize hadron absorber and muon identifier design for the Muon Arm. This culminated in construction of a 1.2-meter-square by 8-interaction-length-deep mockup of a MuID sector instrumented with limited streamer tubes and scintillator arrays; this had final tests during the 1993 AGS run and has since been decommissioned. The data from it were used to size the PHENIX MuID. Data from it and a subsequent device at RIKEN have been used to prepare simulation code and fast trigger algorithms for the PHENIX MuID.

ORNL heads the on-line effort within PHENIX from inception through construction. This includes front-end electronics, trigger, data acquisition, and computing systems plus software. We carried out an R&D program on monolithic electronics development from 1989 to 1998 under the auspices of RHIC. The development in support of the CERN program described above has provided us with experience that was directly transportable to PHENIX and has informed the architecture, design, performance specifications, and costing efforts for the PHENIX on-line system. ORNL designed and constructed the monolithic front-end electronics for the silicon strip, pad chamber, ring-imaging Cerenkov EM calorimeters, and muon tracker for PHENIX (some six custom-mixed analog and digital ASICs in all). ORNL handled design and construction of the entire front-end chain for the EM calorimeters and muon identifier. ORNL also provided the system architecture and circuit board designs and manufacture for the entire signal processing chain for the silicon strip detector and the control board design and manufacture for the pad chamber. In addition, we contributed the system architecture for the RICH and MuTR subsystems. Recent efforts include designing trigger boards for use with the EM Calorimeter, RICH, and MuID system.

ORNL staff contribute to the management of PHENIX in various ways. G. R. Young is Deputy Spokesman and headed the on-line system from inception up until commissioning in 1999. F. Plasil has served on the Executive Council, drafted most of the governing agreements for PHENIX, and served as Period Coordinator during our first-ever major data-taking run. K. Read is the Detector Council member for the MuID, and T. V. Cianciolo heads the MuID front-end electronics effort. T. C. Awes is head of the lead-glass calorimeter effort, and P. W. Stankus is in charge of all electromagnetic calorimeter electronics. S. P. Sorensen headed the off-line effort during its initial years and remains head of the technical group overseeing the RHIC computing facility.

Experiment E910 at the BNL AGS

E910 was a fixed-target proton-nucleus (p-A) experiment performed at the AGS in the spring and summer of 1996. It was the first experiment to study pA collisions with the combination of high statistics, large acceptance and good particle identification. For a full description, go here

The experiment used a time projection chamber (the EOS TPC) housed in a dipole magnet for primary tracking. Particle identification (PID) in the TPC is done by calculating the ionization energy loss. For downstream tracking and PID, there were three drift chambers followed by a Cerenkov counter and a time-of-flight wall and finally two more drift chambers. Upstream of the target, there were six proportional wire chambers along the beamline, A1-A6. A1-A4 are used to measure the beam momentum, and A5 and A6 are used to measure the incident angle of the beam at the target plane. Data were collected with several targets (Be, Cu, Au, and U) at three different proton beam momenta, 6, 12 and 18\,GeV/c.

Only in p-A collisions, with a single projectile nucleon, can one study multiple collisions and re-interactions within the nucleus in detail. The primary goal of E910 is to investigate particle production as a function of the number of binary nucleon-nucleon collisions (nu) suffered by the incident proton during the reaction. This would help improve understanding of the physics in A-A collisions (where these processes occur simultaneously). In particular, the experiment was designed to:

  1. Search for H0 production.
  2. Elucidate strangeness production mechanisms.
  3. Measure rapidity loss (dy) of the incident proton as function of nu.
  4. Measure strange meson, vector meson and baryon production as function of dy and nu.
  5. Make high statistics measurements of anti-proton production and absorption as a function of both dy and nu.
E910 has published a paper on slow proton production and its utility as a measure of nu. E910 collected a very nice sample of strange and multi-strange particles and anti-protons, all of which are being studied as a function of nu. We have a paper under review on Lambda production that may prove to be important to the interpretation of strangeness enhancement in AA collisions (one of the pillars of the CERN announcement of consistency of data with QGP formation).

E866 Experiment at FNAL

Writeup to be provided by Paul.

ORLaND at SNS

Writeup to be provided by Frank.

WA98 Experiment at CERN

WA98 Experiment at CERN

The WA98 experiment (Bhubaneswar IOP, Calcutta VECC, Chandigarh, Darmstadt GSI, Dubna JINR, Geneva, Groningen, Jaipur, Jammu, Lund, Moscow RRC, MIT, Muenster, Nantes SUBATECH, Oak Ridge, Rez NPI, Tennessee, Utrecht, Warsaw) has studied hadron and photon production in the 158·A GeV Pb+Pb collisions with the goal to produce hot hadronic matter over a large volume and search for signatures of a transition to a Quark Gluon Plasma (QGP). Special emphasis of the experiment is on broad global characterization of the reaction to identify and select unusual event features or event classes. The space-time evolution of the reaction volume is studied via Bose-Einstein correlations of identified charged hadrons. The thermal history of the system is investigated by measurement of direct photons. There have been seven papers published in refereed journals, seven more submitted, and four are in preparation.

The WA98 experiment includes some 14 subsystems grouped to form charged-particle spectrometers, a photon spectrometer, and global event measurement devices at target, projectile, and midrapidity. The photon spectrometer consists of a highly segmented LEadglass photon Detector Array (LEDA) of 10,000 modules placed at a distance of 21.5 m from the target. The momentum spectra of neutral pions and etas are reconstructed from their two-photon decay over the rapidity region of 2.3 < y < 3. Careful determination of the photon yield from neutral pions and etas allows to search for excess direct thermal photons. The photon multiplicity measurement is extended over the range 2.8 < y < 4.1 with a 7 x 4 m2 preshower Photon Multiplicity Detector (PMD). Negative charged-particle tracking with time-of-flight measurement allows momentum measurement of identified pions, kaons, and anti-protons in the range 2 < y < 3. The tracking is achieved using a set of six 1.6 x 1.2 m2 multi-step avalanche counters with image-intensified CCD camera readout, located downstream of the Goliath magnet. Positive charged-particle tracking was obtained with a second set of detectors consisting of two avalanche counters and two streamer tube planes, all with pad readout and with time-of-flight measurement. Total charged-particle multiplicity is measured in the range 2.0 < y < 3.4 in the Silicon Drift Detector (SDD) and in the range 2.4 < y < 3.8 in the Silicon Pad Multiplicity Detector (SPMD). Pions and protons, deuterons, and tritons, emitted in the target fragmentation region are measured in the Plastic Ball detector. Finally, for centrality selection, the transverse energy and forward energy are measured in the MIRAC and Zero Degree Calorimeters, respectively.

WA98 completed its data-taking program with the fall 1996 run of the CERN SPS with Pb beams. The experiment is now in the data analysis phase with a number of  WA98 results  published.

Here one can obtain more information about WA98 or about CERN. There is also the February 10, 2000 CERN press release  which announced the creation of Quark Gluon matter based on the results of the CERN heavy-ion program.

WA80 and WA93 Experiments at CERN

The CERN program was initiated in early 1984 with the decision to join the PS190 collaboration, which was the precursor of the first CERN experiments with heavy ions at relativistic energies, WA80 and NA35. (At the time of this decision, the relativistic heavy-ion program at the AGS was not approved.) The ORNL group proceeded to play a major role in the WA80 collaboration. The primary initial effort was to design and build the calorimeters of WA80. The zero-degree sampling calorimeter (ZDC) was designed and constructed at ORNL, with ORNL/DOE resources. (It served as a model for a similar calorimeter constructed later for the AGS program.) The large midrapidity calorimeter (MIRAC) was a joint venture of GSI, U. of Lund, LBNL, and ORNL, with ORNL having the primary responsibility. The calorimeters played a crucial role in establishiing the global characteristics of nucleus-nucleus collisions at 60 and 200 GeV/nucleon.

The WA80 papers published on the basis of the calorimeter data were produced at ORNL. They constitute the most comprehensive set of global results from the entire CERN heavy-ion program, relating impact parameters to the number of participating nucleons and providing a sound basis for the use of calorimeters at both zero degrees and at midrapidity for accurate event characterization. Related entities extracted from the calorimeter data are estimates of attained energy densities and of the degree of nuclear stopping. It was concluded that the energy densities attained in 160- and 32S-induced reactions at 200 GeV/nucleon were close to the range required for QGP formation, and that there was a large (but not complete) degree of nuclear stopping, indicating the formation of a hot baryon-rich plasma and/or hadron gas. Complementary proton-nucleus data were also analyzed and compared to calculations. From a systematic comparison of nucleus-nucleus data with hadron-nucleus data, a consistent picture has emerged regarding the role of partcipating nucleons at CERN energies.

In the area of QGP signals, WA80 has concentrated on measurements of single photons that might be directly radiated from the plasma. Such measurements constitute a formidable task due to the presence of a very large number of photons from the decay of hadrons and due to the background in the photon detectors created by an equally large number of charged hadrons. Early data have only been able to provide results at the 15% accuracy level. In the second round of WA80 experiments, the photon measurement capability was greatly expanded, thanks in part to new collaborators (BNL and Kurchatov Institute). In addition, sophisticated data analysis techniques were implemented. While most of the photon data analysis has been performed elsewhere, ORNL provided much of the methodology and intellectual leadership of the new approaches. These efforts, coupled with the maximum exploitation of the "event-mixing" technique for the determination of backgrounds, enabled us to extract meaningful results at the level of 0.002 signal/noise ratio. Results on the production of p0 mesons were obtained in the pT range from 0.5 to 3 GeV. The postulated mT scaling was confirmed for the case of nucleus-nucleus reactions. The errors associated with the measurement of single photons were reduced to the 6-7% level in the pT range of 0.3 to 1.5 GeV. A slight but insignificant excess of photons (over the number accounted for by hadronic decays) was observed for central (and minimum bias) events in the pT range 0.5 to 2.0 GeV, but not for peripheral events. The attained upper limit on the excess photon pT distribution constrains the initial temperature of the hot, dense system to be less than about 250 MeV. This result indicates that the system has access to a large number of degrees of freedom, as for a QGP, or a hadron gas with an extensive spectrum of resonances. Overall, the WA80 experiment has been very successful, and numerous results have been published.

The WA93 experiment was the successor to WA80. The goals were to obtain higher-statistics photon data and to serve as a prototype setup for the later CERN-based WA98 experiment that used the SPS lead beams. A large magnet (GOLIATH) and multistep avalanche counters (MSACs) were added for the purpose of tracking charged particles over a wide angular range. A photon multiplicity detector (PMD), consisting of a lead converter backed by scintillating tiles, was also implemented. MIRAC and the ZDC were retained from WA80 and continued to be the responsibility of the ORNL group. Two WA93 runs took place, one each in 1991 and 1992.



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Rev:  Tuesday, 26-Sept-2000 16:00 EDT