APS News

MACHOs, Unity Session Mark 1995 April Meeting

Approximately 1,500 physicists assembled in Washington, DC for the 1995 Joint April Meeting of the APS and the American Association of Physics Teachers (AAPT), 18-21 April. The most varied of APS meetings because of the number of APS divisions represented in the program, the April Meeting explored current topics in particle physics, astrophysics, fluids, particle beams, physics of beams, nuclear physics, applications, and atomic, molecular and optical physics.

Topics of technical sessions included the first significant detection of dark matter in the halo of the Milky Way via its gravitational microlensing signature (see page 5), the development of a new MRI imaging technique (see page 3), using plasmas to monitor and manage waste disposal (see page 4), and new conflicting measurements of Newton's gravitational constant (see page 11). Nontechnical sessions included a panel discussion on physics and the media (see page 4), radiation and health, a centennial session on the discovery of x rays, and memorial sessions for Eugene Wigner and Julian Schwinger. In addition, the AAPT organized several sessions devoted to issues in education, some in conjunction with APS committees or units.

Another prominent feature of the meeting was the fifth annual Unity session (see page 2), a special plenary session intended to provide an opportunity for attendees to hear about the broader scope of physics as an alternative to the more specialized technical sessions. First established by the APS in 1990 to counter the increasing specialization of physics, the symposium proved successful enough to become an annual event.

The traditional ceremonial banquet for the bestowal of prizes and awards was held Wednesday evening, preceded by a reception hosted by APS President C. Kumar N. Patel (University of California, Los Angeles). Fourteen prizes and awards were presented, and the winners gave lectures on their respective award-winning topics at various sessions throughout the week. Citations and brief biographies of the recipients appeared in the April 1995 issue of APS NEWS.

Impact of Top Quark Discovery. Scientists from Fermilab discussed the significance of the recent discovery of the top quark - the last of the set of particles which, according to theorists, comprise the basic building blocks of matter - for future physics research during a Wednesday morning session. "Now that we have discovered it, the emphasis shifts toward precisely defining properties of the top quark, such as its mass," said William Carithers, Jr., a spokesman for the CDF collaboration at Fermilab. "Larger data samples will also allow us to begin exploring the possibility of unexpected effects in the production and decay of top quarks."

To get larger samples, Fermilab recently completed a massive upgrade to the Tevatron. The accelerator is now delivering data to the CDF and D0 detectors at more than twice the prior average rate, and Fermilab Director John Peoples expects to double the number of top quark events observed before the current run ends in 1995. The laboratory has also begun construction of the Main Injector accelerator, slated for completion in 1999. It is expected to increase the Tevatron's production of top quarks from roughly 100 per collider run to about 1,000 for each of the two collider experiments. Upgrades to the detectors are also planned.

The implications of the top quark for the future understanding of the basic laws of physics will most likely stem from its enormous mass: 176 GeV as measured by the CDF detector, and 199 GeV for the D0 collaboration. For instance, it may reveal the presence of new particles and new forces in the patterns of its production and decay, and may also shed light on the Higgs boson, a theoretical entity believed to be responsible for the origin of the masses of all particles in nature. "We know absolutely nothing experimentally about this key ingredient of our theory," said Fermilab theorist Chris Hill. "But we do know that the heaviest particles are most strongly interacting with the Higgs."

Future of High Energy Physics. The Large Hadron Collider (LHC) at CERN will pack less energy than had been proposed for the Superconducting Super Collider, but it will be the only machine in the foreseeable future capable of achieving some of the same scientific goals. Thus, U.S. particle physicists are drawing up plans (subject to governmental support) whereby they would be involved in the construction phase of the LHC project and in subsequent experiments. At a Wednesday morning session, several speakers described these plans for doing research at CERN, where an American presence has been substantial for years; indeed U.S. scientists have regularly constituted the largest single national contingent at CERN.

Some 38 U.S. institutions, including 300 physicists, will participate in the CMS detector experiment (about 25 percent of the whole). For ATLAS, the other large detector collaboration, 230 physicists at 28 institutions, are expected to participate. The U.S. contribution to the construction phase might consist of building the focusing magnets for the accelerator. Sid Drell of SLAC headed a subpanel of experts that advised the Department of Energy; the subpanel recommended last year that the U.S. scientific community participate in the LHC project. They proposed yearly support levels rising from a few million dollars in the early years to tens of millions at the height of construction.

Doubly-Magic Nuclei. Beams of chilled heavy ions that collide with stationary targets continue to produce particles never before seen on Earth, such as "doubly magic" tin-100, the heaviest nucleus with equal amounts of protons and neutrons, and beryllium atoms with angel-like halos of protons. According to Hans Geissel, who spoke at a Tuesday morning session, the projectile fragment separator at Germany's GSI facility has proven to be a versatile magnetic spectrometer for experiments with heavy ions, and spatially separated relativistic exotic beams of all elements up to uranium can now be provided for experiments and applications, such as decay studies of highly charged nuclei.

Materials Processing at Accelerators. Up-and-coming accelerators, such as the Continuous Electron Beam Facility (CEBAF) in Virginia, boost electrons to extremely high energies in order to probe the deepest mysteries of nuclear structure, and the special challenges presented even at lower energies have led to such important breakthroughs as cavity design, manipulation of short bunches, special lattices, vacuum chamber design, and most importantly, superconducting rf technology. According to CEBAF spokesman Christopher Leemann, the facility has implemented superconducting rf technology - first proposed in the 1960s, with little initial success - on an unprecedented scale as the sole means of acceleration at CEBAF.

Completed within budget and on schedule, CEBAF has fostered key advances in that technology, as well as in 2K cryogenics and rf control. It has also provided a technology basis for other accelerator applications of the future. For instance, accelerators have the ability to drive high-power free electron lasers (FELs), which can be used to change a material's chemical properties. Leemann said that IBM, DuPont, and other companies are planning to use CEBAF's free electron lasers for materials processing without the use of hazardous chemicals. Other future possible applications include TESLA, a TeV-scale linear accelerator in preliminary development at DESY, and the use of intense proton beams for spallation neutron sources.

Highlights from the Hubble Telescope. Observations made with the rejuvenated Hubble Space Telescope are providing important information on supernovas, galaxies in the early universe, comet Shoemaker-Levy's collision with Jupiter, and the search for evidence of black holes in active galaxies, according to speakers at a Tuesday afternoon session. Thanks to improved imaging with kiloparsec-scale resolution of galaxies at any distance, "The Hubble telescope is serving as a powerful 'time machine,' giving us pictures of galaxies when the universe was young," said Mark Dickinson (Space Telescope Science Institute). "The faint, fuzzy lumps which previously were our only access to galaxies at such ancient eras have, for the first time, become distinct, sharply delineated individuals."

H.C. Ford of Johns Hopkins University reported on the recent observation of small disks of gas and dust in the centers of four bright radio galaxies, one of which seems to carry the gravitational signature of a black hole with a mass equivalent to two billion stars like our sun. And STSI's Mario Livio reported on recent Hubble images that reveal that many planetary nebulae produced by stellar explosions are remarkable similar in morphology, contrary to expectations. "In particular, it came as a big surprise that many appear to be axisymmetric, rather than spherical, exhibiting a variety of rings, bipolar outflows, and jets," he said.

Supernova Shocks. Ultra-strong shocks, such as those produced in a supernova and nuclear explosions, undergo unstable dynamics that create the interesting structure seen in astrophysical objects like the crab nebulae, according to Jacob Grun of the Naval Research Laboratory (NRL), who reported on laboratory simulations of such astrophysical phenomena using laser-generated high mach-number shocks during a Thursday morning session. Grun and colleagues C. Manka and B. Ripin focused a laser onto the surface of a foil placed in an ambient gas to heat the foil to a few hundred eVs, creating a powerful miniature explosion. "Depending on details, this explosion can initiate hundred-kilobar shocks in the ambient gas or launch laminar and turbulent flows with Mach numbers of a few hundred," he said.

Cosmic Rays. Cosmic rays constitute a constant stream of particles raining down on Earth. At energies up to 1011 eV, cosmic ray particles (mostly protons) are probably galactic in origin, almost certainly supernova remnants, according to Arnold Wolfendale (University of Durham, England), who spoke at a Tuesday morning session. At energies around 1018 eV, they seem to be heavy nuclei. While shock acceleration in our galaxy has been thought to be responsible for the bulk of cosmic rays up to about 1014 eV, at the highest energies above 1019 eV, the particles again appear to be protons and may be extragalactic in origin.

"How to get beyond this energy in our galaxy remains a challenge, since to accelerate cosmic rays to some maximum energy, by any mechanism, requires that at the maximum energy, particles can be contained for sufficient time in the acceleration region, and that the energy loss rate must be less than or equal to the rate of energy gain," said R.J. Rutheroe of the University of Adelaide in Australia. He added that propagation of the highest energy cosmic rays from extragalactic acceleration sites through the cosmic background radiation also results in predictable changes in composition, spectral shape, and anisotropy, as well as the production of accompanying diffuse gamma-ray and neutrino background that must not exceed those observed.

Helium Fluids. Mixing helium-3 and helium-4 isotopes in networks of highly porous glasses known as aerogels producing new kinds of superfluid phenomena whose workings are just starting to be understood, according to Donald Candela (University of Massachusetts, Amherst), who spoke at a Thursday morning session. His research team at Amherst performed a series of nuclear magnetic resonance experiments on pure helium-3 filling the pores of silica aerogel, and observed several effects characteristic of nonlinear spin dynamics in highly polarized systems, including multiple spin echoes and a long-lived transient following the creation of a magnetic domain wall.

At the same session, Isaac Silvera of Harvard University reported on progress in ongoing efforts to produce a collection of helium atoms so cold and so dense as to produce a state known as Bose-Einstein condensation (BEC), in which the collection of atoms collapses into the same quantum state. Previous efforts relied on a static magnetic trap to avoid limiting wall recombinations of atoms. However, as temperatures were lowered and atoms approached BEC conditions, they reverted to a ground spin barrier state and were ejected from the trap. Silvera and his colleagues have now devised a hybrid approach, in which atoms are cooled in a static trap and then transferred to a microwave electromagnetic trap for further cooling, which should prevent atoms from reverting to the ground spin state and enable them to achieve BEC conditions.

One Hundred Years of X Rays. On Tuesday morning, the APS Forum on the History of Physics and the American Association of Physicists in Medicine sponsored a joint symposium marking the centennial of Wilhelm Roentgen's discovery of x rays, focusing on the discovery, early theories and applications of x rays. For instance, Roentgen supposed that the rays resulted from longitudinal vibrations in the ether, believed to exist together with transversal vibrations since the 1820s. Experimental efforts to discovery longitudinal ether waves failed, but the idea was never completely abandoned, and the dispute influenced the subsequent debate on the nature of light.

Although diagnostic applications of low-energy x rays were initiated shortly after their discovery, only superficial treatment was feasible, according to J.S. Laughlin (Memorial Sloan-Kettering Cancer Center). But over the next two decades optimum energies for diagnosis were achieved, as well as sufficient development of tube and equipment design to make diagnostic radiology practically feasible. Major advances include digital radiography and its application to procedures like angiography; computer tomography; and magnetic resonance imaging and spectroscopy. In addition, the development of the betatron and subsequent linear accelerators permitted delivery of x radiation energy to any region in the body.

Computation and Visualization in Physics Education. On Wednesday morning, the APS Forum on Education and the Division of Computational Physics sponsored a joint symposium on computation and visualization in physics education. For example, as an alternative to large lectures, a physics studio course at Rensselaer Polytechnic Institute incorporates a Comprehensive Unified Physics Learning Environment (CUPLE) in redesigned classrooms that facilitates student centered "hands-on" learning, using such advanced technologies as computer-based video, video data acquisition, microcomputer based laboratories, and powerful data analysis and visualization tools.

However, there are areas of physics, such as those that use field theories, in which it is difficult to provide "hands-on" activities and the mathematics is abstract and difficult to learn. To address this problem, R.W. Cole (University of California, Davis) developed a series of multi-media interactive computer lessons to help students visualize the patterns in fluid flow and electrodynamics, understand the basic concepts, and link the patterns to the traditional mathematical statements of the physical laws. "This linking of visualization, concepts and mathematics is key to developing an intuitive understanding for these topics in physics," he said.

1995 APRIL MEETING PROGRAM COMMITTEE: Chair: Noemie Benczer-Koller, Rutgers University; Vice-Chair: David Cassel, Cornell University; AAPT Program Chair: Robert Hillborn, Amherst College. Members included: Eric Adelberger, University of Washington (PMFCTG); Hassan Aref, University of Illinois (DFD); Dan Barnes, Los Alamos National Laboratory (DCOMP); Stephen Berry, University of Chicago (FBSTG); Claude Canizares, Massachusetts Institute of Technology (DAP); Charles Duke, Xerox (CAP); Elizabeth Garber, SUNY-Stony Brook (FHP); Ruth Howe (FED); Chun Lin, University of Wisconsin-Madison (DAMOP); Roberto Merlin, University of Michigan (FIP); Alvin Saperstein, Wayne State University (FPS); Robert Siemann, Stanford Linear Accelerator Center (DPB); John Walecka, CEBAF (DNP).


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Editor: Barrett H. Ripin