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. Approximately half of the faculty in the Department of Physics conduct their research through LNS. During fiscal year 1996, the Department of Energy is expected to provide LNS a total of $29,795,000 in research funding.
LNS researchers in experimental high energy physics are active at a number of laboratories around the world, including CERN (Switzerland), Gran Sasso (Italy), and the US accelerator facilities 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 is of course always keeping 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. So far L3 has provided many important tests of the Standard Model. These 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. L3 has recently upgraded its experiment with the installation of a precise vertex detector and implementation of greater coverage for muon detection. After a number of years of operation at the maximal Z0 production energy, LEP is now running at higher energies to produce large numbers of the W+/- particle, the carrier of the charged electroweak force. This next stage of the L3 experiment should test the Standard Model in an even more stringent fashion.
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 been completed with high statistics and small systematic uncertainties. The measurements of ALR have yielded a determination of sin2([Theta]W)eff, 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 recent highlight was of course 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 development of the 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 constraints on the mass of the undiscovered Higgs boson. Future 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.
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 taking data at Livermore (California). This experiment is the first to search for the axion in a physically interesting region with sufficient sensitivity to mean a discovery is plausible.
The "strangelets" experiment at Brookhaven is now in full data production, and information on the possible existence of this exotic form of matter is expected soon.
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 and muon detection systems. 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. efforts at the LHC.
A new experimental project, the Alpha Magnetic Spectrometer (AMS) collaboration, has made great strides recently. AMS will look for anti-matter and dark-matter candidates above the Earth's atmosphere, first with a Space Shuttle mission in 1998 and then with a long mission on the International Space Station beginning in 2001. This remarkable collaboration, led by LNS, involves researchers from many different countries as well as coordination between DOE and NASA.
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 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 beam and polarized targets, maintains a unique and important position for Bates.
In addition to the new capabilities provided by the South Hall Ring, Bates is completing the construction of 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 is nearing completion and has already been used for experiments. These new experimental capabilities, coupled with ongoing improvements in accelerator operation, will provide an unprecedented opportunity to address critical issues in medium-energy physics.
The SAMPLE experiment at Bates, designed to provide crucial information on the quark structure of the proton, is now taking data. This experiment has placed very demanding requirements on the Bates beam quality, polarization, and stability; these requirements have been fulfilled.
LNS nuclear physics researchers are also leading several important efforts at accelerator facilities other than Bates. These facilities include TJNAF (Virginia), DESY (Germany), Mainz (Germany), and PSI (Switzerland). 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 run at DESY, in 1995, was a notable success and the 3He target performed excellently. LNS researchers are also leading the design and construction of detectors for experiments at other facilities, such as TJNAF.
LNS is a leader 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, but which may be present for a brief time in collisions of heavy ions. LNS researchers are leading the current experimental efforts using heavy-ion beams at Brookhaven. The LNS group is also the leader of one of the few experiments (called PHOBOS) approved for the Relativistic Heavy Ion Collider (RHIC) under construction at Brookhaven. This experimental project is now well underway and aiming for RHIC startup in 1999. A complementary effort of the group is a search for the possible creation of a new state of the vacuum, using high-energy Pb-Pb collisions at CERN.
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. Finite temperature effects have been incorporated into string theory, leading the way to the study of cosmology and radiation from black holes.
Topological terms in field theories, which were introduced by this group several years ago, are now widely studied in problems ranging from gravity to high temperature superconductivity. Recently, these terms were shown to play an important role in QCD at high temperature and used to understand the response function in the quark-gluon plasma.
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.
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 new regime to be 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.
CTP theorists have developed extensions of the Standard Model, including extended Technicolor and supersymmetric models, which agree with known data and make testable predictions. Developments in heavy quark physics have been used to find new ways to study CP violation in B meson mixing, and to determine weak matrix elements from B decays.
Electroweak interactions are a continuing focus of research. The unique opportunities provided by the new 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. The use of neutral current probes to study the strange quark content of the nucleon has been studied. Anti-neutrino and heavy-flavor neutrino production from the sun have been studied to obtain new information on the solar neutrino anomaly and neutrino mixing.
Since its founding LNS has placed education at the forefront of its goals. At present approximately 79 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.
Robert P. Redwine
MIT Reports to the President 1995-96