Nuclear physicists from around the world converged on Newport News Virginia for the annual fall meeting of the APS Division of Nuclear Physics (DNP), held October 11-13. Among the highlights of the technical program were talks on the latest news from the Relativistic Heavy Ion Collider (RHIC), new insights into nucleosynthesis gleaned from observations of metal-poor stars, and the latest research on quark gluon plasmas, including potential insights to be gleaned from string theory.
Odd Coupling. Collisions of high-energy gold nuclei at Brookhaven’s Relativistic Heavy Ion Collider (RHIC) create exploding droplets of quark-gluon plasma (QGP), the stuff that filled the universe microseconds after the Big Bang. However, the QGP turns out to be close to an ideal liquid, and also attenuates high-energy quarks attempting to pass through it–both properties that standard QCD calculations have not been able to explain satisfactorily.
To help resolve this issue, theoretical physicists are turning to string theory (particularly the gauge-string duality), which has revealed a deep connection between quantum gravity and gauge theories similar to QCD, according to MIT’s Hong Liu. Along with several other speakers, he discussed examples where string theory techniques have been used to shed light on existing data from RHIC, and to make at least one prediction that can be experimentally tested in the near future.
Princeton University’s Steven Gubser has been finding interesting comparisons between QCD and string theory computations regarding thermalization time, energy loss by heavy quarks, and the formation of sonic booms. The string theory computations hinge on dynamics of black horizons in a fifth dimension, but Gubser argues that while such horizons “may appear fanciful, they in fact provide very practical and direct tools for computing dynamical properties of analogs of the QGP.” There are a few string theory predictions that are quite close to experimentally favored values, although he cautions that there are still significant barriers to making those predictions more precise.
Elemental Matters. According to NSCL’s Fernando Montes, recent observations of the abundances of metal-poor stars suggest that an additional mechanism besides the known r-process is responsible for the production of material within a specific region (nucleosynthesis). He finds that mixing the r-process pattern found in such stars with a light element primary process (LEPP) can explain these observations. He has used the LEPP abundance pattern based on those observations to explore the astrophysical conditions that would create it.
Why Stars Explode. Physicists continue to explore potential explosion mechanisms for core-collapse supernovae explosions, an area of research that spans four decades. While much progress has been made in understanding the basic physics and hydrodynamics, there is still no truly satisfactory explanation. According to Adam Burrows of the University of Arizona, an acoustic mechanism and one relying on magnetohydrodynamics jets are the newest candidates for the core collapse mechanism. In addition, a new class of energetic supernovae, called “hypernovae,” has been discovered. “As a result, the study of the supernova mechanism has assumed a far wider portfolio and a greater richness than ever in the past,” he said. It will require a synergistic interplay between nuclear physics and sophisticated numerical simulation to shed further light on this phenomenon.
The Future of Nuclear Theory. Nuclear theory has reinvented itself in the last 10 years, creating new paradigms for matter under extreme conditions, and developing better methods for investigating the structure and interaction of hadrons in few-and many-body systems. The renaissance is far from over, according to David Kaplan of the Institute for Nuclear Theory, who cited the advent of petascale computing as providing even more opportunities for theorists to solve complex open questions in the field.
For instance, over the next decade, there will be a number of experimental studies of neutrinos and fundamental symmetries, and nuclear theory will play a critical role in interpreting those results and their implications for the “New Standard Model” of fundamental interactions. Michael Ramsey-Musolf of the University of Wisconsin-Madison discussed a few of the biggest challenges for nuclear theory, including neutrino-less double beta-decay, electric dipole moments, and precision measurements of neutrino properties and electroweak processes.