MIT Reports to the President
1999–2000
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 2000, the
Department of Energy is expected to provide LNS a total of $27,608,000 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),
and Fermilab (Illinois). 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. Final
data analysis is underway. The BaBar experiment on the SLAC B-Factory is now
taking data, with the expectation that important insights into the nature of
charge symmetry/parity violation will soon be forthcoming.
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. CDF is now completing major upgrades (including
a time-of-flight system led by the LNS group) and Run II of the Tevatron will
begin in 2001.
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. A major upgrade of the axion search experiment
has recently been approved.
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
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. Also, the SAMPLE experiment has
used the unique capabilities at Bates to provide crucial information on the
structure of the proton.
LNS nuclear physics researchers are 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 is
providing 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. An LNS group leads the PHOBOS experiment on the Relativistic Heavy
Ion Collider (RHIC) at Brookhaven. First beam-beam collisions have recently
been observed, and this exciting experimental program is now underway.
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. 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, and DESY.
EDUCATION
Since its founding LNS has placed education at the forefront of its goals.
At present approximately 87 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
1999–2000
