MIT Reports to the President 1998-99

LABORATORY FOR NUCLEAR SCIENCE

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 1999, the Department of Energy is expected to provide LNS a total of $29,225,000 in research funding.

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. 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 Wą particle, the carrier of the charged electroweak force, and possibly to reveal the existence of the Higgs boson.

LNS researchers have played leading roles in exploiting the unique properties of the SLD detector at SLAC, which has now completed its data-taking. With micron size beams, very high resolution vertex detection, excellent particle identification and calorimetry, and a polarized electron beam, SLD has made 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 recent highlight of the project was 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. 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 completion of important upgrades and the imminent start of Run II, 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 10-day mission on Discovery in 1998 was designed to shake down important aspects of this challenging project. The mission accomplished all of its objectives, and provided intriguing new data on the energy spectra of cosmic rays as well. The AMS experiment is scheduled for a 3-year data-taking period on the International Space Station starting in 2003.

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 major new detectors. For example, 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 nuclear physics.

The SAMPLE experiment at Bates, designed to provide crucial information on the structure of the proton, is expected to complete data-taking this year. This experiment places very demanding requirements on beam quality and polarization, 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 later this year.

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. The CTP has recently expanded its efforts in string theory with the addition of two new faculty members in this field. Important work includes the study of extended objects known as "branes" that connect string theory with field theories, the exploration of matrix quantum mechanics which may be the fundamental structure that unifies various versions of string theory, and the study of tantalizing connections between string theories in anti-de-Sitter space and conformal quantum field theories.

String theories suggest patterns of supersymmetry breaking which may have implications for physics at the energy scales of the next accelerators. CTP researchers have been exploring these patterns. Also, string theory and quantum gravity suggest that space-time may have other dimensions, which influence physical phenomena only indirectly. Their effects would be manifest in the properties of the "superpartners" of ordinary particles, whose patterns and signatures have been explored by CTP researchers.

MIT theorists have been actively developing new calculational tools for the study of non-perturbative effects in quantum field theories. Variational methods, consistent with renormalization and adapted for easy numerical computation, have been developed and are being applied to problems that arise in the Standard Model.

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, quantum field theories beyond perturbation theory. Recent advances have centered on new algorithms for implementing non-Abelian gauge theories on a lattice, and for studying systems with finite density. A collaboration to develop a high speed computing facility has been initiated between the CTP, the Jefferson Lab, and several European universities. These efforts parallel a new thrust in the study of quantum chromodynamics (QCD) at finite density and pressure. CTP researchers have suggested novel effects, such as "color superconductivity", and explained how they may be observed in heavy ion collisions.

CTP researchers continue to lead the exploration of the spin and flavor structure of hadrons, as seen in experiments (many led by MIT faculty) at Bates, Jefferson Lab, SLAC and DESY.

Since its founding LNS has placed education at the forefront of its goals. At present approximately 95 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.

More information about LNS can be found on the World Wide Web at http://pierre.mit.edu/.

Robert P. Redwine

MIT Reports to the President 1998-99