The RNC Program conducts experiments studying the collision of heavy ions in four energy regimes: (i) the Bevalac, where nuclear matter is compressed sufficiently to study its equation of state; (ii) the AGS at BNL, extending the studies of the Bevalac to an energy range where the maximum pressure from the baryons is likely to occur; (iii) the SPS at CERN, where the energy density of the nucleons in the collision of very heavy nuclei may be sufficient to produce a phase transition to a plasma of free quarks and gluons; and (iv) RHIC, where the energy density of the produced particles will be sufficiently high that production of the quark-gluon plasma is expected to occur. The fundamental interest is understanding the reaction dynamics in these energy regimes and the nuclear matter equation of state.
The major efforts at the Bevalac have been the Dilepton Spectrometer (DLS) and the EOS Time Projection Chamber (TPC). Because of the shutdown of the Bevalac in January 1993, the extensive local activities are now in the analysis stage. The EOS TPC has been moved to the AGS to continue its studies at higher energies as experiment E895. The various experiments at CERN with LBL participants finished data-taking with 32^S beams, and have been consolidated into one experiment, NA49, for the Pb beams program which started in November 1994. However, the main focus of the future high energy heavy-ion research program at LBL is the STAR experiment at RHIC, which will begin data taking in 1999.
CERN/RHIC Physics
At CERN, two large acceptance experiments measuring hadrons were carried out. The main goal of the NA36 TPC experiment was to determine if the trends in strange particle production indicate a signature of the quark gluon plasma, while the NA35 experiment covered a wide region of phase space with both a streamer chamber and a TPC, addressing several experimental topics.
The collisions of the heaviest nuclei at the highest energies (lead ions at the SPS, gold ions at RHIC) are expected to create systems whose space-time dynamics are qualitatively different from those of the colliding light ions studied up to now. The heavy systems have significantly higher energy densities over longer time scales. The extremely large number of produced hadrons in such collisions (several thousand in a central gold-gold event at RHIC) presents a real technical challenge and a unique opportunity: nontrivial, statistically significant signals can be extracted from single events, a technique known as event-by-event analysis. The correlation of extreme values of several observables sensitive to the quark-gluon plasma phase transition in a single event is a powerful tool for selecting ensembles of interesting events for detailed study. An event-by-event measurement of the produced particles provides the opportunity to select events with extreme values of temperature (particle spectrum), flavor (strangeness content), shape of the flow (particle momenta), and size (two-particle correlations). This technique requires a large acceptance detector that can determine the momentum and identify a large fraction of the particles emitted in the collision.
NA49 is a fixed target experiment at the SPS designed to study lead-lead collisions at 160 GeV/nucleon (square root (s_nn) = 17 GeV). Its goal is to simultaneously measure many hadronic signals that are thought to be sensitive to the quark-gluon plasma. In order to perform event-by-event analysis, it will measure and identify almost all charged particles in the forward half of phase space, and will carry out detailed ensemble measurements of all the single event observables, as well as strange particle decays, two-particle correlation functions, and other hadronic observables.
STAR is a collider experiment at RHIC designed to study gold-gold collisions at square root (s_nn) = 200 GeV. Its goal is similar to NA49's, to simultaneously measure many hadronic signals. To perform event-by-event analysis, it will measure and identify almost all charged particles over two units of rapidity, centered at midrapidity. At RHIC there is a high rate of hard processes. Hard-scattered partons (the precursors of high pt particles and jets) are predicted to be sensitive to the medium through which they propagate and are directly calculable in perturbative quantum chromodynamics. The study of high pt particles and jets as a function of energy and mass of the colliding system may also be an attractive experimental approach to identify the presence of quark matter, and STAR is being planned with this capability.
Bevalac/AGS Physics
Due to the relatively weak interaction of dileptons with matter, they provide a unique tool for probing the early phase of the hot, condensed system created in central A-A collisions. Theoretical calculations indicate that the yield of dileptons is sensitive to the density and temperature of this early phase of the collision, providing information on the nuclear matter equation of state. This formed the basis of the experimental program of the DLS.
Experiments with the first generation 4pi hadronic detectors at the Bevalac, namely the Plastic Ball and the Streamer Chamber, provided first insights into the dynamics of nuclear matter at high densities and temperatures through comparison of the experimental data with macroscopic and microscopic model calculations. However, still more precise and systematic data were necessary to determine the parameters of the nuclear equation of state. Such high quality data were accumulated by the EOS TPC at the Bevalac. It is expected that the source temperature can be inferred from the energy distributions, the pressure reached in the reaction zone from the collective flow, the entropy from the ratio of protons to composite particles and from pion production, the source radii at freeze-out from the correlations of identical particles, and the amount of stopping from the rapidity distributions. Another important aspect is the study of multifragmentation. E895 will utilize the EOS TPC to extend all these measurements to the higher energies of the AGS.
Experiments
The Dilepton Spectrometer (DLS)
The DLS collaboration from late 1986 until the closure of the Bevalac, carried out a systematic study of e+e- production as a function of beam energy and kinematics of the pair. The DLS results, about 30,000 pairs, represent the world's only e+e- data at Bevalac/SIS energies. Important results include: 1) existence of measurable dielectron yields, 2) observation of contributions from mesonic decays (pi^0, eta, rho/omega), bremsstrahlung and Delta/N* decays, 3) strong energy dependence of the pd/pp yield ratios signifying the presence of the eta-meson, 4) absolute value and shape of the mass spectrum at 5 GeV in p-p and p-d collisions shows need to modify existing N-N model calculations (pp vs. pn contributions, inelasticity), and 5) observation of high mass pairs (>500 MeV) in Ca-Ca collisions may be evidence for pionic annihilation. Analysis continues on the high statistics Ca-Ca studies and evolution of mass and pt-distributions with projectile/target mass.
EOS TPC at the Bevalac
EOS was designed to study heavy-ion reactions over the whole energy range of the Bevalac. The TPC enables the measurement of the production cross sections for protons, light composite particles and pions over a large dynamic range. The EOS collaboration performed an extensive series of measurements prior to the shutdown of the Bevalac. Excitation functions of four systems (Ni + Cu, Ni + Au, La + La, and Au + Au) were measured from 250 MeV per nucleon up to the highest energy. In addition, the systems Au + C, Kr + C, and La + C at 1 GeV per nucleon were measured.
The physics analysis of the data is being performed at LBL and other collaborating institutes. A complete excitation function for the directed flow in the Au + Au system has been measured. The new flow results show a striking scaling behavior that would be expected from non-viscous hydrodynamics. New correlation methods to study the nature of the directed flow have been developed. It can be shown that the flow is generated by particles being focused and having larger mean momenta in the flow direction. Systematic comparison of the data with model calculations are in progress. Preliminary results favor models with momentum dependent interactions and a soft equation of state. In addition, multifragmentation of the Au + C system at 1 GeV per nucleon has been analyzed as a critical phenomenon. A method to extract critical indices from the data has been developed. Preliminary results show that the critical indices extracted are compatible with the critical indices of the liquid-gas phase transition.
EOS TPC at the AGS (E895)
E895 will carry out a systematic and exclusive measurement of the energy and mass dependence of particle production, correlations, and collective effects in Au+Au collisions at the AGS. In addition, E895 will study the dynamics of dilute nuclear matter and explore the emergence of critical phenomena (liquid-vapor phase transition) by varying the energy deposited into the Au projectile nucleus. E895 will measure the four-momentum of light mass particles, projectile fragments, and anti-proton production. The experiment provides a large acceptance and thus many observables can be examined in fine detail at once.
NA36 at the SPS
NA36 was a TPC-based experiment designed to handle high charge multiplicity and high event rates. It measured strangeness production in relativistic heavy-ion collisions using 32^S beams at 200 GeV/nucleon. About 6 million events were recorded, mainly with a Pb target. The experiment had very wide acceptance in rapidity and transverse momentum, so that it overlapped the phase space of all other experiments. Neutral strange particles (Lambda, Lambda-bar, K-zero-short), which decay to pairs of charged particles (protons, antiprotons, pions), were identified by the topology of their decays. Data-taking was completed in 1990. The Lambda production increases linearly with the event multiplicity and, for the same multiplicity, does not depend on the size of the target nucleus.
NA35 at the SPS
NA35 was a large acceptance experiment, having a large volume Streamer Chamber within a 1.5T magnetic field and a TPC downstream of the magnet. It measured many hadronic signals, and has published results on particle spectra, two particle correlations, strange particle yields, and other observables. LBL produced 6000 channels of readout electronics for the TPC, significantly expanding its capability. Data taking was completed in 1992.
Analyses at LBL on the TPC data concentrated on charged kaon production, nuclear stopping, and pion interferometry. TPC results on K^- production indicate a strangeness enhancement at midrapidity in agreement with previous results backward of midrapidity from the Streamer Chamber. This enhancement is a factor two for S+S collisions with respect to an independent superposition of p+p or p+A interactions. Nuclear stopping, defined as the mean rapidity shift of projectile protons from beam rapidity, is estimated using TPC data in the forward direction by subtracting the number of negative hadrons from positive hadrons. For central S+Au collisions the rapidity shift measured indicates a significant amount of stopping (as large as at AGS energies). With the high statistics of the NA35 TPC data, a decrease of observed source size with transverse momentum was found.
NA49 at the SPS
NA49 is a large acceptance experiment based on a set of Time Projection Chambers. Particle identification is performed primarily by the measurement of dE/dx in the relativistic rise regime (leading to TPCs that are 3.6 m deep), supplemented by time-of-flight over a part of phase space. Event characterization for triggering is performed by forward calorimetry.
LBL's responsibility for NA49 hardware is the development and manufacture of electronics for the 182,000 TPC readout channels. This very large number of channels necessitated new developments in TPC front end integrated circuitry for cost, engineering, and reliability reasons. LBL developed a 16 channel integrated preamplifier/shaper-amplifier and a 16 channel Switched-Capacitor-Array/ADC. In addition, LBL produced the control-and-transfer boards and the readout boards. Lead beams were first delivered by the SPS in November 1994. Half of the TPCs and electronics were installed and the rest will be ready for the fall 1995 run.
STAR at RHIC
The STAR experiment will consist of a Time Projection Chamber (TPC) and Silicon Vertex Tracker located inside a 5.2 m diameter solenoidal magnet to provide tracking, momentum analysis and particle identification of charged particles using the dE/dx technique. The trigger detector systems include a central scintillator barrel around the TPC, vertex position detectors near the beam line just outside the magnet, and calorimeters located in the region of the beam insertion magnets to selectively veto events according to the number of spectators. Anticipated as upgrades are an electromagnetic calorimeter to trigger on transverse energy and measure jet cross sections, a time-of-flight system surrounding the TPC for particle identification at higher momenta, and external time projection chambers outside the magnet to extend the pseudorapidity coverage.
LBL's Relativistic Nuclear Collisions Program is providing a focus for these RHIC activities. With 40 physicists and engineers from LBL working on this experiment, the STAR collaboration now consists of 350 physicists and engineers from 34 institutions internationally. Within the STAR organization, RNC has primary responsibility for the TPC, TPC electronics, and overall detector integration. RNC also has significant responsibilities within the DAQ and software efforts in STAR. RNC physicists form the core of the software development which is focused on tracking and particle identification by dE/dx in the TPC. STAR will be ready to begin taking data in 1999 when RHIC begins operations.
Development
Micro-strip gas chambers for TPC readout: Traditionally, TPCs have been read out with multiwire proportional chambers located over a surface of pads that pick up the induced signal from avalanches on the wires. This technology sets a practical limit on the two-track resolution and the position resolution that can be obtained with a TPC. The new Microstrip Gas Chamber (MSGC) devices can overcome this limit and allow TPCs to be operated in much higher track density environments with improved position resolution. In the last year conductive coatings have been developed which stabilize the gas gain, reduce the leakage current, and make MSGC fabrication compatible with many substrates.
P-type silicon drift detectors: P-type silicon drift detectors have been designed and fabricated at LBL. Preliminary tests show that the detectors function well and that the signal transit time is linear with position for these devices. The energy and the two-track resolution are being measured. Work is also going on to integrate polycrystalline silicon voltage dividers directly on the detectors.
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