MIT Reports to the President 1997-98
The Laboratory for Nuclear Science (LNS) provides support for research by faculty and research staff members in the fields of high energy and nuclear physics. These activities include those at the Bates Linear Accelerator Center and in the Center for Theoretical Physics. Almost half of the faculty in the Department of Physics conduct their research through LNS. During fiscal year 1998, the Department of Energy is expected to provide LNS a total of $29,663,000 in research funding.
EXPERIMENTAL HIGH ENERGY PHYSICS
LNS researchers in experimental high energy physics are active at a number of laboratories around the world, including CERN (Switzerland), SLAC (California), Fermilab (Illinois), and Brookhaven (New York). The overall objective of current research in high energy physics is to test as precisely as possible the Standard Model, which has been very successful in describing a wide variety of phenomena, and to look explicitly for physics beyond the Standard Model. LNS researchers are playing leading roles in much of this research, as described below.
The L3 experiment at CERN is the largest of four detectors at the Large Electron Positron (LEP) Collider, which is the highest energy such collider in the world. The aim of the experiment is to deepen our knowledge of the Standard Model by measuring with high precision the properties of the intermediate vector bosons, Z and W, their couplings to other particles and, perhaps, the mechanism of spontaneous symmetry breaking. One of course always keeps open the possibility of finding new phenomena beyond the Standard Model. This project has been led from the beginning by an LNS group, and broke new ground in bringing together a large number of scientists from many countries into a highly successful collaboration. Important recent L3 tests of the Standard Model include precise measurements of the properties of the Z0 particle (the carrier of the neutral electroweak force); demonstration, by two independent methods, that there are only three types of light neutrinos in the Universe; limits on the possible mass of the Higgs boson; and the measurement of the strong coupling constant a. After a number of years of operation at the maximal Z0 production energy, LEP is now running at energies high enough to produce large numbers of the W+/- particle, the carrier of the charged electroweak force. This next stage of the L3 experiment is testing the Standard Model in an even more stringent fashion. So far no disagreements with the Standard Model have been observed. It is possible that the higher LEP energies will even reveal the existence of the Higgs boson.
LNS researchers are playing a leading role in exploiting the unique properties of the SLD detector at SLAC. With micron size beams, very high resolution vertex detection, excellent particle identification and calorimetry, and a polarized electron beam, SLD is making important contributions to the precise determination of Standard Model parameters and to our understanding of heavy quark physics. Measurements of the left-right cross section asymmetry, ALR, for Z0 boson production using polarized electrons have yielded a determination of the effective weak mixing angle which is even more precise than the individual LEP results.
The Collider Detector Facility (CDF) Experiment at Fermilab is designed to study the Standard Model and its possible extensions at the highest energy accelerator in the world, the Tevatron collider. A highlight of the project is the discovery of the top (t) quark, by far the most massive elementary particle ever seen. The MIT group played an important role in the data acquisition and analysis which led to this result. As the measured mass of the t quark has become more precise, this information combined with other results has begun to provide significant constraints on the mass of the undiscovered Higgs boson. Current objectives of CDF include studies of the b quark, the low mass partner of the t quark; precision measurement of the mass of the W; and the search for possible quark sub-structure. With the anticipated completions of a major upgrade of CDF and of the new Main Injector at Fermilab, important new data will soon be available.
An experiment to search for the axion, a particle predicted to exist as a minimal extension of the theory of strong interactions as well as a possible solution to the "dark matter" problem in cosmology, is now providing a precise scan of possible axion energies. This experiment is the first to search for the axion in a physically interesting region with sufficient sensitivity to mean a discovery is plausible.
LNS is involved in both large detector initiatives at the Large Hadron Collider (LHC) project at CERN, viz., the CMS and ATLAS detectors. In CMS, LNS scientists are engaged in the development of the data acquisition system; in ATLAS the effort is mainly in the development of the muon detection systems. LNS scientists have considerable expertise in both data acquisition and muon detection systems and expect to be major participants in the U.S. projects at the LHC. Our efforts on the LHC experiments are now growing rapidly.
The Alpha Magnetic Spectrometer (AMS) experiment had a very successful first flight on the Space Shuttle Discovery in June 1998. AMS is an experiment designed to look for cosmic anti-matter and evidence for dark matter by operating a large magnetic spectrometer above the Earth's atmosphere. The international AMS collaboration is composed primarily of particle physicists and is led by an LNS group. The centerpiece of the AMS experiment is a large permanent magnet which takes advantage of significant recent improvements in permanent magnet technology. The recent 10-day mission on Discovery was designed to shake down important aspects of this challenging project and to take initial data. The mission accomplished all of its objectives, despite the fact that failure of a primary communications channel meant that not all of the data could be transferred to the ground during the mission. The detector operated well in all respects. The AMS experiment is scheduled for a 3-year data-taking period on the International Space Station starting in 2002.
EXPERIMENTAL NUCLEAR PHYSICS
Experimental nuclear physics at present has two main thrusts: medium-energy physics and heavy-ion physics. LNS has active, leading groups in both of these sub-fields.
The focus of LNS medium-energy activities is of course the Bates Linear Accelerator Center, which is operated by LNS for the Department of Energy as a national user facility. Bates has been a premier national and international resource for nuclear and particle physics studies for more than two decades. A major upgrade of its capabilities, the South Hall Ring, has recently been completed. This upgrade allows both external and internal-target experiments using the continuous (as opposed to pulsed) beam from the Ring. The continuous nature of the beam is critical for a number of experiments, such as those using coincidence techniques. The opportunity to pursue internal target experiments, especially those involving polarized beams and polarized targets, maintains a unique and important position for Bates in the international community. A new detector (BLAST) for internal target experiments is now under construction.
In addition to the new capabilities provided by the South Hall Ring, Bates has recently constructed several major new detectors. The Focal Plane Polarimeter is now being used very successfully for experiments which require measurement of outgoing proton polarization. The Out-of-Plane Spectrometer allows unique measurements of kinematic correlations of outgoing reaction products. These new experimental capabilities, coupled with ongoing improvements in accelerator operation, provide an unprecedented opportunity to address critical issues in medium-energy physics.
The SAMPLE experiment at Bates, designed to provide crucial information on the structure of the proton, is now in its main data-taking run. This experiment places very demanding requirements on beam quality, polarization, and stability, but represents a world-class physics contribution which uses the unique capabilities at Bates.
LNS nuclear physics researchers are also leading several important efforts at accelerator facilities other than Bates. These facilities include TJNAF (Virginia), LANSCE (New Mexico), DESY (Germany), and Mainz (Germany). The project at DESY is an experiment to study the spin structure of neutrons and protons, using among other targets a polarized 3He target constructed at MIT. The first few years of data-taking have been notably successful and a recent detector upgrade promises important new coincidence data. LNS researchers have also led the design and construction of detectors for experiments at other facilities, such as TJNAF. Our programs at TJNAF are now producing precise new data for a variety of reactions.
LNS has a major role in the field of heavy-ion physics. In recent years the emphasis has been on studies of relativistic interactions of heavy-ion projectiles, especially as they may shed light on the question of the existence and properties of the so-called "quark-gluon plasma". This new state of matter is predicted to exist at temperatures and densities higher than those present in normal nuclear matter, which may be present for a brief time in collisions of heavy ions. LNS researchers led the recent experimental efforts using heavy-ion beams at the Brookhaven AGS. The LNS group is also the leader of one (PHOBOS) of the few experiments for the Relativistic Heavy Ion Collider (RHIC) under construction at Brookhaven. This experimental project is now well underway and will be ready for RHIC startup in 1999. A complementary effort of the heavy-ion group is a recent search at CERN for the possible creation of a new excited state of the vacuum known as a disoriented chiral condensate.
THEORETICAL NUCLEAR AND PARTICLE PHYSICS
Research at the Center for Theoretical Physics (CTP) seeks to extend and unify our understanding of the fundamental constituents of matter and the theory that governs them. In addition, it uses our present knowledge of this theory to advance our understanding of a variety of subjects, including the structure and interactions of hadrons and nuclei, new forms of matter which may be created experimentally or observed astrophysically, and the behavior of the early universe. A few examples of recent work are mentioned below.
String theory aims to unite the strong, electroweak, and gravitational interactions and to explain the observed hierarchy of particles and interactions. An important contribution at MIT has been the development of a general field theory of closed strings. It has been shown that this theory is independent of the background field that is used in its construction. Recently an MIT theorist (with collaborators) showed that string theory leads to a successful microscopic description of low-energy Hawking radiation, reproducing the semiclassical results both for the emission rate and for the angular momentum distribution. We have recently significantly strengthened our efforts in the area of string theory.
Since string theory reduces to traditional quantum field theory at low energies, it can be used to analyze the properties of field theory. An MIT researcher (in collaboration) has uncovered strong evidence that some of the exciting dualities (i.e., exact equivalences between seemingly disparate theories) recently found in quantum field theories can be understood as low-energy consequences of string theory symmetries.
MIT theorists have also been pursuing the use of the early universe as a complement to accelerator experiments in testing particle theory ideas. Recently they have developed, and are pursuing, a new version of inflationary cosmology based on the underlying particle of supersymmetry breaking.
A major thrust in the CTP has been in the area of lattice gauge theory, which provides a unique tool to solve, rather than model, QCD. Recent lattice solutions have provided strong evidence that the structures of nucleons, pions, and other light hadrons are dominated by topological excitations of the gluon field. The recent donation of a
24-Gigaflops cluster of symmetric multiprocessors has greatly enhanced lattice calculations in LNS.
MIT has played a pioneering role in exploiting high energy scattering to determine the quark and gluon structure of nucleons and nuclei. Significant new developments have been the determination of the behavior of structure functions in the regime being studied at TJNAF, the discovery of new ways to measure spin-dependent structure functions, and the first successful theory of the fragmentation function for pions. Electroweak interactions are a continuing focus of research. The unique opportunities provided by the South Hall Ring at the Bates accelerator have motivated studies of reaction mechanisms, of new ways to use nuclei to test fundamental symmetries, and of spin and polarization observables.
Since its founding LNS has placed education at the forefront of its goals. At present approximately 75 graduate students are receiving their training through LNS research programs. A number of undergraduate students are also heavily involved in LNS research. Evidence shows that LNS educates a significant portion of the leaders of nuclear and high-energy physics in this country and abroad.
Robert P. Redwine
MIT Reports to the President 1997-98