09/22/03

Creating Novel Forms of Hadronic Matter at the Alternating Gradient Synchrotron (AGS) And Relativistic Heavy Ion Collider (RHIC)

Fall 2002

            The field of relativistic heavy ion physics seeks to better understand non-perturbative Quantum Chromodynamics (QCD) through the study of hadronic matter under extreme conditions.  The goal of this new field of nuclear physics is to observe and study the transition of hadronic matter to a deconfined phase – the Quark Gluon Plasma (QGP) – where quarks are no longer bound in color singlet hadrons. I have been involved in the study of heavy ion and nucleon-nucleus collisions both at the AGS (in Experiments E864 and E941) and RHIC (as a member of the PHENIX collaboration). 

            Experiment E864 was designed to study heavy ion collisions by searching for rare objects that may be created Au+Pb collisions, from nuggets of strange quark matter (“strangelets”) to antimatter clusters such as the antideuteron.  These highly sensitive searches were made possible by a mass-based trigger designed and built at Iowa State.  Results from E864 have set the best strangelet limits available at AGS energies, measured light nucleus production by coalescence out to ⁶He, and provided new insight into antimatter production in these baryon rich collisions.  Our studies of antiprotons seem to indicate that antihyperons are preferentially produced over non-strange antibaryons in these collisions, a surprising and unanticipated result.  This work has culminated in key publications on antimatter production[1], on which I was the primary author.  In Experiment E941 we have studied antiproton and leading neutral particle production in p+A collisions as a complement to the heavy ion work.  These new measurements of antiproton production have allowed us to better interpret the Au+Pb results from E864 by measuring the in-medium annihilation cross section for antiprotons, while leading neutral particle studies are directly applicable to predictions for baryon stopping in higher energy nucleus-nucleus collisions at RHIC.

                 As part of the PHENIX collaboration, we are now studying collisions of heavy nuclei in an energy regime where we have strong theoretical evidence that a QGP will be created.  Detecting the presence of the QGP will require the correlation of many observables, since there is no one unambiguous signal for QGP formation. PHENIX is well suited for the study of processes that probe the QGP as directly as possible, via a combination of leptonic, electromagnetic and hadronic probes. Over the past several years I have been heavily involved in the design and construction on the Level-1 trigger for PHENIX, which has been essential for the experiment to make on online selection of interesting events for further study and allow PHENIX to collect high statistics samples of very rare processes.  The culmination of this work has been the development of the Muon Identifier Local Level-1 (MuID LL1) trigger system. The requirements for this trigger system are extreme – it must be able to process data from the detector at a rate of ~74Gbit/s while limiting the overall Level-1 accept rate to 12.5 kHz. These requirements dictated that we embrace cutting-edge technology throughout the design. Over the past two years I have developed the conceptual and engineering designs and prototyped the hardware for the MuID LL1 system, and the MuID LL1 system will be a key component of the PHENIX physics program for the upcoming RHIC Run-3.

            My physics interests at RHIC energies center around the study of hard scattering and jet formation in these collisions.  At RHIC energies in hadron-hadron collisions events are observed corresponding to large momentum transfer scattering between partons (quarks or gluons) in the incident hadrons.  These scattered partons are forced to “dress” themselves into color singlet hadrons by QCD confinement and what we observe is a highly collimated “jet” of particles in the final state.  Estimates for RHIC suggest that a substantial fraction of the energy transfer in the collision could be due to hard scattering in the early stage of the collision.  However, in a nucleus-nucleus collision a scattered parton must traverse a dense region of nuclear matter before it can be observed as a jet or a high-p_T hadron in the experiment.  Predictions for energy loss of a leading parton in confined and deconfined environments suggest that the number of jet events at RHIC may be reduced substantially for a QGP, since the parton will lose energy rapidly in traversing a plasma.  Therefore, a systematic study of  high-p_T particle prioduction in nucleus-nucleus collisions at RHIC over a variety of target and energy combinations may allow us to identify the transition to a deconfined state (by a dramatic decrease in the jet production cross section) as well as indirectly measure the density of nuclear matter in both the confined and deconfined states. 

            Preliminary studies using data from the first year of RHIC running do seem to indicate that there is a suppression of hadrons with large transverse momenta in central (“head-on”) collisions of Au nuclei at a center of mass energy of 130 GeV /nucleon[2], and I was a member of the PHENIX Internal Review Committee for this important paper. This suppression is apparent both in the comparison with data from nucleon-nucleon collisions scaled by the number of expected binary collisions in Au+Au, as well as in the comparison of central to peripheral Au+Au collisions.  This new data is very suggestive of exactly the effect we are looking for, but there are several known nuclear effects in the production of high transverse momentum particles (nuclear shadowing of the parton distribution functions and the Cronin effect) that are not well understood.  In order to extract quantitative information from the Au+Au data, additional data from p+p and light-ion collisions are required.

            Correlations between particles have been used to study heavy ion collisions in the hope that they can be used to distinguish between the anisotropic distribution in momentum space of particles produced in the exploding “fireball” and anisotropies at high momentum due to the production of particles in jets.  First results from PHENIX on the measured anisotropies suggest that at high momentum the anisotropy in central collisions does not scale with the initial overlap of the two nuclei[3]. (I served as chairman of the PHENIX Internal Review Committee for this paper.) 

            During the RHIC Year-2 physics run the collider operated at full energy of 200 GeV/nucleon and the luminosity of the machine was significantly higher than during the previous year.   The PHENIX collaboration collected high-statistics samples of Au+Au and  p+p collisions, providing a complete set of data that can be used to further or understanding of the interesting effects seen in the first year’s data. I contributed to this effort through the development of both hardware (Level-1) and software (Level-2) triggers for the PHENIX experiment that will enable to collection of high statistics samples of rare events (high-p_T hadrons,  decays, etc.)  In particular, I started the effort to develop a high-p_T charged particle Level-2 trigger that developed into a collaborative effort with my student, Paul Constantin, as well as Tom Hemmick and Jiangyong Jia of Stony Brook.  This Level-2 trigger allowed the experiment to collect high-p_T charged particles out to ~15 GeV/c in transverse momentum, greatly expanding the momentum range that can be used for studying jet suppression in Year-2 data.

I have been also involved in data reconstruction and continue to work on development of an integrated momentum reconstruction package.  This software uses detector hit information and knowledge of the inhomogeneous magnetic field in the PHENIX spectrometer to determine the momenta of particles detected in PHENIX using a Kalman filter method. Currently, the momentum of charged tracks in the PHENIX apparatus is determined using only the particle track in the central drift chamber. I have shown that by combining all detectors hits in a rigorous way, and using the full magnetic field map, the momentum resolution of the apparatus can be improved by a factor of three at a momentum of 20 GeV/c.  In addition, improved determination of the track residuals and a statistically proper c² for the tracks are also provided by this process.  This method will enhance the ability of PHENIX to measure charged particles at high p_T and I am working to incorporate the software into the PHENIX reconstruction chain.

            At the present time I am preparing for the third run of the RHIC collider, during which time the PHENIX experiment will collect data on d+A collisions.  These important baseline measurements will be critical to interpreting the heavy-ion data that we have already collected, as well as future measurement. With a wealth of exciting data coming from RHIC over the next several years, this promises to be a very exciting time in experimental heavy ion physics.  New data will permit us to explore the high temperature, low density phase of QCD in ways that were never before been possible and in a regime where connections can be made to theoretical predictions. This will allow substantial progress in our understanding of fundamental QCD over the next decade.