Physics Colloquia Archives

Fall 2006

Thursday, September 7, 2006
Princeton Unversity

"Recent results from the Wilkinson Microwave Anisotropy Probe"

Cosmological observations based on a wide variety of methods are in good agreement on the contents and history of the universe.  The universe contains roughly 4% baryons, 23% of a new kind of matter we hope to understand with future particle accelerators, and 73% of a new kind of pressure or energy that has yet to find explanation in fundamental theories. In the standard model, quantum fluctuations in the early universe led to the fluctuations in gravitational potential that seeded the formation of cosmic structure.  The inflation model (and others) predict the spectrum of these fluctuations. WMAP's measurement of the CMB polarization allows us to break a number of cosmic parameter degeneracies, measure the spectrum of fluctuations, and begin to test specific models of inflation. In other words, through measurements of the CMB, we are now testing the physics of quantum fluctuations in the universe when it was less than 10^{-20} seconds old.

Thursday, September 14, 2006

"Quantum criticality beyond the Landau-Ginzburg-Wilson paradigm."

The theory of second order phase transitions is one of the foundations of modern statistical mechanics and condensed matter theory. A central concept is the observable `order parameter', whose non-zero average value characterizes one or more phases and usually breaks a symmetry of the Hamiltonian. At large distances and long times, fluctuations of the order parameter(s) are described by a continuum field theory, and these dominate the physics near such phase transitions.

This talk will review recent work that shows that near second order quantum phase transitions, subtle quantum interference effects can invalidate this paradigm.

The critical points discussed separate conventional phases characterized by Landau order parameters. Nevertheless, there is a Landau-forbidden second order phase transition with an unusual description in terms of fractional degrees  of freedom and an emergent gauge field. The potential of this development  to resolving a number of experimental puzzles in correlated electron systems will be emphasized.

Thursday, September 21, 2006
Columbia University

"Decoding Cosmic Cryptography in Search for the Holy Grail"

The Cosmic Microwave Background (CMB) consists of a bath of photons emitted when the universe was 380,000 years old. Encoded in small fluctuations in the temperature of this blackbody radiation background is a wealth of information about fundamental physics, and the nature of the universe on the largest scales and at the earliest times. I will present preliminary results from our newly commissioned Sunyaev-Zel'dovich Array (SZA), an instrument that allows us to study the largest collapsed structures in the universe. I will also address our search for signatures that may have been imprinted on the CMB when it was much less than a second old, yielding clues about the origin of the universe and the nature of spacetime.  In this context I will introduce our current CMB polarization experiments, the Q U Imaging ExperimenT(QUIET) and the E B EXperiment(EBEX).

Thursday, September 28, 2006
Argonne National Laboratory

"Paths of Nuclear Structure"

Trying to understand a lump of nuclear material can be a daunting problem, whether one finds it taking part in big bang nucleosynthesis, tailor-made in an accelerator, or on the crust of a neutron star.  The force between neutrons and protons is a complex and still not well understood realization of quantum chromodynamics. Even for a large nucleus such as Pb-208, the number of particles in the surface, where the density changes rapidly, is of order of 50% of the total. This leads to physics more related to that of metal clusters and quantum dots, rather than to that of bulk material. We now know that nuclei far from the valley of stability are significantly different from their stable cousins and the competing ideas why this is so promise significant discovery potential.  Over the past decade, important advances in theory, simulation, and experiments at both small and long distance scales have set the stage for a clear vision forward in nuclear structure.

Thursday, October 5, 2006

"Inflation and the String Theory Landscape"

After a quick review of how inflation works, I will discuss some of the key features of our universe that suggest that it emerged from a period of inflation: its uniformity, its near-critical mass density, and the spectrum of density perturbations that is now observed in the cosmic microwave background radiation.

I will then turn to the biggest outstanding mystery in cosmology: the value of the cosmological constant, or equivalently the energy density of the vacuum. Nobody understands why it is so small. One controversial explanation starts with the claim that string theory offers a colossal number of vacuum states, with varying energy densities. If inflation can populate all of these vacua, and life evolves only in vacua with small energy densities, then the mystery might be solved. I will argue that this explanation is logically sound, but that the issue is far from settled.

Thursday, October 12, 2006
University of California - Irvine

"Cosmology: Man's Place in the Universe"

Human beings come in the middle. We are middle sized, middle ages, and middle weight - half way (geometrically) between the small-scale phenomena of nuclear and atomic physics and the large scale ones of astronomy and cosmology.

This means that we can hope to learn about both the atom and the universe by looking at how we came to be here. Life on earth is part of the present epoch of a long history, beginning with (indeed very possibly before) the Big Bang, the early, hot dense stage of which we see several relics. Then came the emergence of structures - first, probably, very massive stars, and rather wimpish (in several senses) galaxies; then stars more like our own and larger galaxies and clusters; and planets, life, and (arguably) intelligence. Changing even one of the fundamental constants of physics or cosmology would prevent one or more of these vital stages from happening.

Thus the very fact that we are here to observe the universe and ask questions about it guarantees that the universe must be more or less the way it is. The implications of this will be explored.

Thursday, October 19, 2006
University of California - Berkeley

"Celestial Engineering: Resonance and the Dynamical Architecture of Planetary Systems"

The music of the spheres can be heard in resonant astrophysical systems. The fixed gaze of the Moon, the commensurate orbital periods of the Galilean satellites, and the arms of grand-design spiral galaxies testify to how dynamical resonances organize the universe. We discuss how resonances have ordered our view of one of the newest frontiers of planetary science: the Kuiper Belt, that great expanse extending beyond the orbit of the last known planet in our solar system. This space is strewn with icy, rocky bodies---Kuiper belt objects (KBOs)---of which Pluto is merely one (and not the largest) member. These objects occupy orbits of a formerly unimagined variety and inform our understanding of how planetary systems form and evolve. Their size spectrum preserves a record, unweathered by erosive collisions, of the process by which large bodies assemble from small bodies. Moreover, KBOs serve as test particles whose trajectories testify to how the giant planets---and perhaps even planets that once resided within our system but have since been evicted---had their orbits sculpted. We recount the formation history of our planetary system as told through KBOs recently discovered to be locked in orbital resonance with Neptune, highlighting unsolved problems. Connections are drawn between the Kuiper belt and extra-solar examples of nascent planetary systems. Time permitting, we illustrate the power of resonance in shaping systems on Galactic scales.

Thursday, October 26, 2006

"The Quark-Gluon Liquid at RHIC"

Heavy-ion collisions at Brookhaven Labs Relativistic Heavy Ion Collider (RHIC) produce a strongly interacting QCD medium at energy densities far exceeding those of normal nuclear matter. Under these conditions, which resemble the early universe shortly after the Big Bang, the produced matter behaves like an ideal liquid undergoing a rapid three-dimensional expansion. I will present a critical review of the most striking observations made at RHIC and discuss their connection to the expected quark-gluon plasma state of QCD. I will show how future experiments at RHIC and the CERN Large Hadron Collider can help confirm or disprove our current understanding of QCD matter at the highest energy densities.

Thursday, November 2, 2006

"Generation and Quantum Storage of Single Photons"

Single photons are ideal carriers of quantum information, but are neither easily generated nor stored. We discuss how a large material system (a sample of laser-cooled atoms) can be made to emit on demand a single photon into a well-defined direction, or store an unknown photon polarization state without measuring it. The system can also operate as a high-brightness source of identical-photon pairs, entangled-photon pairs, or photon-atom entanglement.

Thursday, November 9, 2006
Columbia University

"The Nus from MiniBooNE"

One of the great breakthroughs in physics of the past decade has been the evidence for neutrino mass from neutrino oscillation experiments. Recent oscillation data from solar and atmospheric neutrino sources can be accommodated in straightforward extensions to the Standard Model. If the oscillation result from the accelerator-based LSND experiment at Los Alamos is confirmed, then more exotic extensions, such as the introduction of non-interacting "sterile" neutrinos, must be incorporated into the theory. The goal of the MiniBooNE Experiment at Fermilab is to confirm or rule out the LSND result. This talk will report on the latest news from MiniBooNE.

Thursday, November 16, 2006
Universität Heidelberg

"Dark energy - a cosmic mystery"

Dark energy - a homogeneously distributed cosmic energy density - seems to explain various cosmological observations. The anisotropies in the microwave background radiation, the formation of structure and the late time acceleration of the Hubble expansion fit into a consistent picture.

The role and origin of dark energy are among the greatest mysteries in fundamental physics, touching the question of unification of gravity with the fundamental quantum interactions.

We discuss quintessence - a dynamical form of dark energy - and possible signatures distinctive from a cosmological constant. In particular, an "early dark energy " may influence the cosmological evolution and the growth of structures already at high redshift.

Quintessence could be related to a new "fundamental" macroscopic force and induce a small time variation of fundamental constants.

Thursday, November 30, 2006

"HERA and the Structure of Matter"

Our picture of the structure of matter at the smallest scales has been evolving continuously. The HERA accelerator, which has been colliding high energy electrons and protons for nearly fifteen years, has brought interesting new information on what matter looks like when probed at very small spatial and time scales. These data will be summarized, and put in the context of previous measurements. The current understanding of matter will be reviewed, leaving a list of unanswered questions. The outlook for future measurements will be discussed.

Thursday, December 7, 2006
University of Manitoba

"The Qweak Experiment at Jefferson Lab:   Probing Physics Beyond the Standard Model via Parity-Violating Electron-Proton Scattering"

A major new initiative is under underway at Jefferson Laboratory to measure the proton’s weak charge -- a basic property, like its electric charge and mass, which determines how a proton responds to the weak interaction.  The Standard Model makes a firm prediction of the proton’s weak charge , Qwp = 1 – 4 sin2qW, based on the running of the weak mixing angle sin2qW  from the Z0 pole down to low energies, corresponding to a 10s effect in our experiment.  The Qweak collaboration will carry out the first precision measurement of  Qwp by measuring the parity violating asymmetry in elastic electron-proton scattering at very low momentum transfer (Q2 = 0.03 (GeV/c)2).  Our ultimate goal is to determine the proton's weak charge with 4%  combined statistical and systematic errors which in turn leads to a 0.3% measurement of  sin2qW.   A longitudinally polarized electron beam, a liquid hydrogen target, a room temperature toroidal magnetic spectrometer, and a set of quartz Cerenkov detectors for the scattered electrons are the key elements of the experimental apparatus.  The experiment will benefit from technical advances in polarized source operation at JLab and from the results of weak hadronic form factor measurements that have been made in the laboratory's parity violation program and elsewhere. The experiment is currently under construction;  installation in Hall C at Jefferson Lab followed by data taking is planned for 2009.

Qweak collaboration:  http:/

Thursday, December 14, 2006

"Understanding the Adaptive Immune Response to Pathogens: A Crossroad of Physics and Biology"

Higher organisms, like humans, have an adaptive immune system which enables us to combat pathogens that have not been encountered before.  The adaptive immune system can also go awry, and many diseases are the direct consequence of the immune system failing to discriminate between “self” and “non-self”.  T lymphocytes (T cells) are the orchestrators of the adaptive immune response.  They interact with cells, called antigen presenting cells, which display molecular signatures of pathogens on their surface.  T cells detect the presence of these molecular signatures of pathogens with great sensitivity.  How T cells hunt for antigen, how they discriminate between “self” and “non-self” with extraordinary sensitivity, and how intracellular signaling leading to activation is regulated are central questions in fundamental biology.  I will discuss recent efforts where synergy between theory and computation (rooted in statistical physics) and genetic, biochemical, and imaging experiments have shed light on the molecular mechanisms underlying the ability of T cells to discriminate between molecular markers of “self” and “non-self”.  The importance of stochastic fluctuations and spatial organization of signaling components will be highlighted.