BPA NEWS Board on Physics and Astronomy • the National Academies • Washington, D.C
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BPA NEWS Board on Physics and Astronomy • The National Academies • Washington, D.C. • 202-334-3520 • national-academies.org/bpa • Winter 2004 EPP 2010: An Assessment of and Outlook for Elementary Particle Physics s the first project in the Board on field telescopes will be needed to map dark collaborations at the European laboratory Physics and Astronomy’s decadal matter and study the acceleration of the CERN on the Large Hadron Collider (LHC), Asurvey of physics, the Elementary expansion of the universe, which appears to a facility currently under construction that Particle Physics 2010 Committee began its be caused by a mysterious dark energy, the will go into operation late in this decade. work in autumn 2004 with the ambitious nature of which may be elucidated by Beyond the LHC, the international commu- task of preparing a 15-year plan for the elementary particle physics. Other ques- nity is discussing the possibility of construct- future of elementary particle physics. tions can be addressed by observations that ing a very high energy electron-positron Fifty years of progress in understanding do not employ accelerators, including collider (the “Linear Collider”) as a possible the physics of matter, energy, space, and neutrino physics, proton decay, cosmic ray multinational project. Other accelerator- time have led the field to a new frontier of physics, the nature of dark matter particles, based experiments are under discussion that unprecedented breadth. The frontier is and various aspects of astrophysics. These address scientific problems that cannot be defined by deep questions. Are there more experiments also require a new generation addressed by the LHC or the Linear than the four dimensions defined by of observational apparatus and laboratories. Collider. Long-baseline experiments to Einstein in his general theory of relativity? Is To directly attack the questions of the search for charge-parity (CP) violation in there a unification of particles and forces? fundamental nature of space and time and the neutrino sector may require accelera- What is the nature of the dark matter and the nature of the particles and forces that tors with beam powers of megawatts. dark energy that seem to permeate the give rise to dark matter and energy will Studies of B-meson physics may require universe and account for most of its matter require a new generation of accelerator machines with luminosities of 10 to 100 and energy? These questions are linked experiments. Some experiments are now times those now attainable. together in complex ways and they make slated to be carried out in international See “EPP 2010” on page 8 contact with various other fields of research, including nuclear physics and astrophysics. Great progress has been made through new Bose-Einstein Condensation of Excitons at High theoretical ideas that promise deeper understanding of experimental observa- Magnetic Field tions. Such ideas have helped to frame these questions and will play a key role in James P. Eisenstein, California Institute of Technology understanding the results of future experi- ments. Professor James P. Eisenstein is a return- objects made up of an even number of Space-borne and ground-based wide- ing member of the BPA’s Solid State Sciences fermions can also often be regarded as Committee. He prepared this article based bosons. The prototypical example of such on an invited lecture he gave at the fall 2004 a composite boson is a 4He atom. In this issue: meeting of the Board on Physics and As- Bosons can undergo Bose-Einstein • EPP 2010. Page 1 tronomy. condensation (BEC), in which a macro- scopic number of particles begin to oc- • Excitonic Condensates. Page 1 lementary particles in nature are cupy the lowest energy quantum state divided into two classes: fermions available as the temperature is reduced • Highlights of the Fall BPA and bosons. Fermions are particles below a critical value. In the condensed Meeting. Page 2 E with spin angular momentum 1/2, 3/2, 5/2, state the many-particle quantum etc., times Planck’s constant h. Electrons, wavefunction of the system is coherent • High Magnetic Field Science. protons, and neutrons are all fermions. across macroscopic distances. At its root, Page 4 Fermions obey the Pauli exclusion prin- BEC is a purely statistical mechanical • Beckman Symposium. Page 5 ciple: no more than one fermion can phenomenon; no interactions between the occupy a single quantum state. Bosons bosons are required for the condensation however, have integer spin: 0, 1, 2, etc. to occur. The transition occurs when the • Physical and Life Sciences. Page π 7 times h/2 . Any number of bosons can thermal de Broglie wavelength of the occupy the same state. Elementary particles becomes comparable to their • Astronomy Before the Congress. bosons include photons, mesons, and separation. Page 10 gravitons. More importantly, bound See “Excitons” on page 12 2 BPA News • Winter 2004 Highlights of the Fall Meeting of the Board on Board on Physics and Astronomy Physics and Astronomy Burton Richter, Chair he Board on Physics and As- (Caltech) discussed his work with Stanford University tronomy met for its annual au- excitons to achieve Bose-Einstein con- Anneila Sargent, Vice Chair tumn meeting on November 6-7, densates. He pointed out that a key California Institute of Technology T 2004, at the Arnold and Mabel Beckman element of the research program of Elihu Abrahams Center of the National Academies in modern condensed-matter and materi- Rutgers University Irvine, California. Chair Burton Richter als physics is to understand how and Jonathan Bagger brought the meeting to order and intro- why “the whole is more than sum of its The Johns Hopkins University duced the new members of the Board parts.” The collective behavior of Gordon A. Baym University of Illinois at Urbana-Champaign (Ron Davidson, Andrea Ghez, Marc groups of interacting objectives, called Kastner, and Chris McKee). Dr. Richter emergent phenomena, is a crosscutting Ronald C. Davidson Princeton University briefly outlined the agenda for the meet- theme throughout much of physics, he William Eaton ing and thanked everyone for making said. Consider the neuron, he sug- 11 National Institutes of Health the trip to southern California. Board gested: you put 10 of them together Raymond Fonck director Don Shapero introduced David and suddenly you have a brain that University of Wisconsin at Madison B. Lang, who joined the BPA in Septem- exhibits very sophisticated properties Andrea M. Ghez ber 2004 as a research assistant. and behavior. And you cannot predict University of California at Los Angeles Deborah Jin (JILA) presented a any of them! He described his research Laura H. Greene discussion of her recent groundbreaking with doped bilayer semiconductors and University of Illinois at Urbana-Champaign observations of the condensation of magnetic fields to form novel excitons. Frances Hellman fermionic atom pairs in the BCS-BEC (See Dr. Eisenstein’s article elsewhere in University of California at San Diego crossover regime. With the use of a gas this issue for more details.) Erich P. Ippen of potassium-40 atoms at low tempera- BPA staff member Timothy Meyer Massachusetts Institute of Technology ture, her research has explored the then presented a short report on activi- Marc A. Kastner interactions of the fermionic atoms and ties related to the Board’s discussion of Massachusetts Institute of Technology the boson condensate simultaneously. opportunities at the intersection of the Christopher F. McKee Employing a 200-G magnetic field, the physical and life sciences at its meeting University of California at Berkeley molecular dimers of potassium atoms in autumn 2003. Dr. Meyer reported Julia M. Phillips can be formed such that the average that there had been significant develop- Sandia National Laboratories molecular size is much larger than the ments on the federal front, ranging Thomas N. Theis IBM Thomas J. Watson Research Center average spacing between atoms. Making from congressional report language to a condensates with fermions is well under- joint NSF/NIH workshop exploring the C. Megan Urry Yale University stood in the Bardeen-Cooper-Schreiffer possibilities for and challenges to Carl E. Wieman (BCS) theory of superconductivity, “bridging the sciences.” He also de- JILA/University of Colorado where fermions pair in momentum scribed a small meeting of experts space. Tuning the magnetic field sets the planned for December 2004 that would NRC Staff energy scale of the pairing energy per evaluate the case for and scope of a Dr. Donald C. Shapero, Director molecular dimer, allowing access to a potential NRC study in this area. (See Dr. Timothy I. Meyer, Program Officer Mr. Brian D. Dewhurst, Sr. Program Associate wide variety of regions on the overall article elsewhere in this issue for more Mr. David B. Lang, Research Assistant phase diagram. In her now-celebrated details.) BPA members endorsed the Ms. Pamela Lewis, Program Associate experiment, Dr. Jin used a gas of 500,000 concept and suggested that the full Ms. Van An, Financial Associate potassium atoms that was cooled to breadth of the physical sciences be ❧ temperatures below 50 nK and then included in the discussion, ranging from subject to a applied magnetic field. The chemistry and physics to materials The Board on Physics and Astronomy is a field coaxed the fermionic atoms to sciences and astrobiology. continuing interdisciplinary body with expertise spanning the various subfields of physics, match up into pairs, akin to the pairs of Ronald Davidson (Princeton Plasma astronomy, and astrophysics. It serves as a focal electrons that produce superconductiv- Physics Laboratory) presented a sum- point in the National Research Council for issues connected with these fields. The activities of the ity, the phenomenon in which electricity mary of the final report of the National Board are supported by funds from the National flows with no resistance.