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, while 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 [[alpha]]. L3 has recently upgraded its experiment with the installation of a precise vertex detector and implementation of greater coverage for muon detection. These upgrades are critical for the next stage of LEP operation, in which the energy will be roughly doubled to allow detailed study of the properties of the W+/- particle, the carrier of the charged electroweak force.
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 fully competitive with measurements elsewhere.
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. The most important recent result is confirmation of the existence 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. The experiment will acquire data through 1995, providing a dramatic increase in the size of the data sample. Other 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.
Smaller LNS efforts in experimental high energy physics include a search for the "axion", a particle predicted to exist by a minimal extension of the theory of strong interactions and as well a possible solution to the "dark matter" problem in cosmology. Another project at Brookhaven is designed to search for "strangelets," an exotic form of matter allowed by QCD.
Following termination of the SSC project, LNS is now 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 become major participants in the U.S. efforts at the LHC.
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, is now being commissioned. This upgrade will allow 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, will maintain 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 partially complete 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.
LNS nuclear physics researchers are also leading several important efforts at accelerator facilities other than Bates. These facilities include LAMPF (New Mexico), CEBAF (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 a polarized 3He target constructed at MIT. This research also includes experiments which use pion-nucleus reactions to study the nature of multi-nucleon interactions in the nucleus. LNS researchers are leading the design and construction of detectors for experiments at other facilities, such as CEBAF.
LNS is also 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 exist for a brief time in collisions of heavy ions. The LNS group is 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.
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 structure of nucleons, pions, and other light hadrons is 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 CEBAF, 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 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.
Robert Redwine
MIT Reports to the President 1994-95