Research at MIT
The Laboratory for Nuclear Science
What is LNS?
The MIT Laboratory for Nuclear Science (LNS) was established in 1946, and now, nearly 60 years later, it is still thriving, although its name has become somewhat anachronistic. The word "Laboratory" is in fact too narrow, as LNS includes the Center for Theoretical Physics. The word "Science" is in fact too broad; whereas originally LNS included researchers in Nuclear Chemistry, at present we are all members of the Physics Department. As for the word "Nuclear," back in 1946 the field of Elementary Particle Physics did not exist as a separate discipline, but was just called "High Energy Nuclear Physics."
Although the history and evolution of LNS would be an interesting tale to tell, that is not the purpose of this article. I will attempt to summarize the current research activities of what could be more properly termed the "Laboratory for Nuclear and Particle Physics."
My theorist colleagues, by the way, have assured me that they have no objection to working in a "Laboratory," as long as they're not expected to manipulate any equipment, or take night shifts on experiments!
Where is LNS?
Some readers might be wondering which of MIT's numbered buildings houses the 40 faculty, 35 research staff, and over 70 graduate students associated with LNS. Well, the administrative headquarters of LNS are located in the Karl Taylor Compton Laboratories (a.k.a. Building 26); several of the research groups and their students also reside there. Other groups can be found in Building 6 (the Center for Theoretical Physics), Building 24, and Building 44. LNS also includes the Bates Linear Accelerator Center, about which there was an article in a previous Faculty Newsletter [April/May 2003], located in Middleton, MA, 20 miles from campus. Moreover, many of the experimental researchers spend considerable time at off-campus accelerator facilities, as I will describe later. So, in a sense, LNS might be thought of as a "virtual laboratory," with much of the "real work" being done away from its actual premises.
Physics in LNS
The mission of LNS remains unchanged since its inception: to investigate the properties and interactions of the fundamental constituents of matter. Many of you may now ask "What are the fundamental constituents of matter?" and I will try to answer this question. We believe that all ordinary matter is composed of two types of fundamental particles: leptons and quarks . These are fundamental in that they are presumed to have no internal substructure. We have peeled away all the layers of the onion, from visible objects to molecules to atoms to nuclei to nucleons (the protons and neutrons that make up the nuclei) to quarks, and we have reached the end of the line.
The most familiar lepton is the electron. In numbers between one and about 100, electrons form the outer shell of atoms, and carry the current in all the electrical devices we use every day. Although single electrons can flow freely through wires and through space and can readily be observed in experiments, quarks cannot be observed singly but only in pairs or triplets. The proton, which is the nucleus of the Hydrogen atom, consists of three quarks, as does its uncharged partner, the neutron, which is needed to make up all of the other atoms. For each quark and lepton there is a corresponding anti-quark and anti-lepton, with opposite electric charge. There is an additional set of particles that transmit the forces (electromagnetic, and so-called "weak" and "strong") among the quarks and leptons. This picture is so successful in explaining the sub-atomic world that it has come to be known as the "Standard Model."
It is the task of Elementary Particle Physics to study the properties and interactions of the quarks and leptons. Quarks are bound into hadrons (a word of Greek origin signifying "strongly interacting particle") of which there are two types: baryons - heavy particles which include the proton and the neutron, and mesons - intermediate mass particles which are composed of quark-anti-quark pairs. Nuclear Physics deals with the interactions of protons, neutrons, and mesons to form the nucleus of the atom. (Atomic Physics takes over "after" the electrons are attached to the nucleus.)
The field of Elementary Particle Physics is often called High Energy Physics, because one needs to accelerate probing particle beams to very high energies in order to "see inside" the hadrons. As an everyday analogy, although most matter is opaque to visible light, it becomes transparent to x-rays, which are simply a higher energy (shorter wavelength) type of light.
The boundary between Particle and Nuclear Physics has become blurred during the past several decades: a new field called, not surprisingly, Intermediate Energy Physics has emerged, in which one works at the interface between High (i.e., particle) and Low (i.e., nuclear) energies. One studies the interplay between the quark structure of the hadrons and the interactions of these hadrons to form the simplest nuclei: Hydrogen (one proton), Deuterium (one proton and one neutron), Tritium (one proton and two neutrons), Helium-3 (two protons and one neutron), Helium-4 (two protons and two neutrons).
Experimental Research in LNS
LNS researchers are active in various facets of the physics described broadly above. I will first discuss Intermediate Energy Physics. The Bates electron accelerator laboratory was built in the early 1970s to pioneer this then new field, and continues to perform research at its forefront. The detector known as BLAST (Bates Large Acceptance Spectrometer Toroid) that was described in the earlier Newsletter article, has begun taking data of unprecedented precision and will provide PhD thesis material for at least a dozen graduate students. One of the most challenging measurements will be of the internal structure of the neutron: although electrically neutral overall, its charged-quark composition implies regions of positive and negative charge inside the neutron.
Although Bates has had many successes (including the education of over 100 graduate students, many of whom are now leaders in the field), its "reach" is limited by the maximum energy of the accelerator. There are some experiments that require higher energy electron beams in order to probe more deeply into the substructure of nucleons and nuclei, and such beams exist at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia . Several LNS researchers have made major contributions to the construction of the experimental apparatus at this laboratory and maintain extensive research programs there.
High Energy Physics requires particle accelerators that are generally of such large scale that they are situated at National Laboratories rather than on university campuses. The Fermi National Accelerator Laboratory, located near Chicago, is one of these. The very highest energies can be achieved by using the same machine to accelerate two separate particle beams and allowing the beams to collide head-on. Fermilab currently has the highest energy colliding (proton - anti-proton) beams in the world, but in a few years the "energy frontier" will be ceded to the European laboratory CERN in Geneva, Switzerland, at which the Large Hadron Collider is under construction.
The primary motivation for performing experiments at ever higher energies is to test the predictions of the Standard Model and search for new physics beyond the Standard Model. There is a large group of LNS researchers currently working on these problems at Fermilab, many of whom will move their activities to CERN within a few years.
The other national high-energy accelerator facility is the Stanford Linear Accelerator Center, located in California . This is an electron - anti-electron collider, with the energies of the colliding beams tuned precisely to optimize the production of so-called "B" mesons. These particles and their anti-particles, the so-called "B-bars," are studied in a special purpose detector, elephantine in size, named BaBar. An LNS group plays a central role in this research, which has the goal of studying fundamental symmetries between matter and anti-matter.
Not all particle physics experiments require a man-made accelerator. "Cosmic rays," primarily high-energy protons accelerated by intergalactic electromagnetic fields, interact with the nuclei of atoms and molecules in the earth's atmosphere (e.g., carbon and nitrogen). These interactions can produce the nearly-massless leptons known as neutrinos. LNS researchers are studying these elusive particles, along with neutrinos produced by nuclear reactions in the Sun, using a very large underground detector in Japan .
Just as in astronomy, where there are advantages to be gained by placing telescopes above the earth's atmosphere, enhanced sensitivity to cosmic rays could be achieved by putting a particle detector in space. This was done for the first time by an LNS research group; a massive detector incorporating a large permanent magnet flew on a space shuttle mission in 1998. An improved version of this instrument, using a superconducting electromagnet, is scheduled for a five-year residency on the International Space Station.
The detector, along with the liquid helium needed to keep the magnet cold, will be transported to the space station on a shuttle flight in 2007. The principal goal of this experiment is to search for anti-matter in the cosmos. We do not understand why we observe a Universe comprised predominantly of matter, when it is presumed that equal amounts of matter and anti-matter were created in the Big Bang. "Where is all the anti-matter?"
As does the terrestrial neutrino detector, this space-based instrument will look for particles produced by "the great accelerator in the sky." Although the "beams" are far weaker than those from man-made accelerators, at least they do not have to rely on federal funding for their operation!
Although, as I said previously, single quarks do not exist in ordinary matter and free quarks cannot be observed in the laboratory, we believe that just after the Big Bang temperatures were sufficiently high for matter to consist of a "quark soup" that had not yet condensed into hadrons. Is there any possibility of observing this state of matter without time-traveling back to the Big Bang? The answer is "yes," if we could reproduce the required high temperatures in the laboratory.
This has now been accomplished at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory on Long Island . Instead of accelerating electrons or protons, one takes atoms of a heavy element, such as Gold, strips them of their electrons (hence, "heavy ion"), accelerates them to very high (i.e., "relativistic") energies, and collides them with another high-energy Gold beam. One of four experiments at this facility is led by an LNS group, and recent data indicate that the conditions for creation of the "quark soup" have indeed been met. The "energy frontier" for heavy ion physics currently resides at Brookhaven, but in a few years it will also move to CERN, and the LNS researchers will follow.
Theoretical Research in LNS
As stated previously, LNS includes the Center for Theoretical Physics (CTP), in which the mathematical underpinnings of the physics discussed above are investigated. The activities in the CTP actually go beyond the theoretical study of nuclei and particles, to encompass, for example, cosmology, field theory, string theory, quantum gravity, and quantum computation. This far-ranging research program will be the subject of a future Newsletter article.