Counting Form Factors of Twist-Two Operators

We present a simple method to count the number of hadronic form factors based on the partial wave formalism and crossing symmetry. In particular, we show that the number of independent nucleon form factors of spin-n, twist-2 operators (the vector current and energy-momentum tensor being special examples) is n+1. These generalized form factors define the generalized (off-forward) parton distributions that have been studied extensively in the recent literature. In proving this result, we also show how the J^{PC} rules for onium states arise in the helicity formalism.Comment: 7 pages, LaTeX (revtex

From actinides to zinc: Using the full abundance pattern of the brightest star in Reticulum II to distinguish between different r-process sites

The ultra-faint dwarf galaxy Reticulum II was enriched by a rare and prolific r-process event, such as a neutron star merger. To investigate the nature of this event, we present high-resolution Magellan/MIKE spectroscopy of the brightest star in this galaxy. The high signal-to-noise allows us to determine the abundances of 41 elements, including the radioactive actinide element Th and first ever detections of third r-process peak elements (Os and Ir) in a star outside the Milky Way. The observed neutron-capture element abundances closely match the solar r-process component, except for the first r-process peak which is significantly lower than solar but matches other r-process enhanced stars. The ratio of first peak to heavier r-process elements implies the r-process site produces roughly equal masses of high and low electron fraction ejecta, within a factor of 2. We compare the detailed abundance pattern to predictions from nucleosynthesis calculations of neutron star mergers and magneto-rotationally driven jet supernovae, finding that nuclear physics uncertainties dominate over astrophysical uncertainties. We measure \log\mbox{Th/Eu} = -0.84 \pm 0.06\,\text{(stat)} \pm 0.22\,\text{(sys)}, somewhat lower than all previous Th/Eu observations. The youngest age we derive from this ratio is $21.7 \pm 2.8\,\text{(stat)} \pm 10.3\,\text{(sys)}$ Gyr, indicating that current initial production ratios do not well describe the r-process event in Reticulum II. The abundance of light elements up to Zn are consistent with extremely metal-poor Milky Way halo stars. They may eventually provide a way to distinguish between neutron star mergers and magneto-rotationally driven jet supernovae, but this would require more detailed knowledge of the chemical evolution of Reticulum II.Comment: 23 pages, 7 figures, accepted to Ap

Manipulating Majorana fermions in one-dimensional spin-orbit coupled atomic Fermi gases

Majorana fermions are promising candidates for storing and processing information in topological quantum computation. The ability to control such individual information carriers in trapped ultracold atomic Fermi gases is a novel theme in quantum information science. However, fermionic atoms are neutral and thus are difficult to manipulate. Here, we theoretically investigate the control of emergent Majorana fermions in one-dimensional spin-orbit coupled atomic Fermi gases. We discuss (i) how to move Majorana fermions by increasing or decreasing an effective Zeeman field, which acts like a solid state control voltage gate; and (ii) how to create a pair of Majorana fermions by adding a magnetic impurity potential. We discuss the experimental realization of our control scheme in an ultracold Fermi gas of $^{40}$K atoms.Comment: 4 papges, 6 figure

Implications of Color Gauge Symmetry For Nucleon Spin Structure

We study the chromodynamical gauge symmetry in relation to the internal spin structure of the nucleon. We show that 1) even in the helicity eigenstates the gauge-dependent spin and orbital angular momentum operators do not have gauge-independent matrix element; 2) the evolution equations for the gluon spin take very different forms in the Feynman and axial gauges, but yield the same leading behavior in the asymptotic limit; 3) the complete evolution of the gauge-dependent orbital angular momenta appears intractable in the light-cone gauge. We define a new gluon orbital angular momentum distribution $L_g(x)$ which {\it is} an experimental observable and has a simple scale evolution. However, its physical interpretation makes sense only in the light-cone gauge just like the gluon helicity distribution $\Delta g(x)$y.Comment: Minor corrections are made in the tex

The Chemical Imprint of Silicate Dust on the Most Metal-Poor Stars

We investigate the impact of dust-induced gas fragmentation on the formation of the first low-mass, metal-poor stars (< 1Msun) in the early universe. Previous work has shown the existence of a critical dust-to-gas ratio, below which dust thermal cooling cannot cause gas fragmentation. Assuming the first dust is silicon-based, we compute critical dust-to-gas ratios and associated critical silicon abundances ([Si/H]crit). At the density and temperature associated with protostellar disks, we find that a standard Milky Way grain size distribution gives [Si/H]crit = -4.5 +/- 0.1, while smaller grain sizes created in a supernova reverse shock give [Si/H]crit = -5.3 +/- 0.1. Other environments are not dense enough to be influenced by dust cooling. We test the silicate dust cooling theory by comparing to silicon abundances observed in the most iron-poor stars ([Fe/H] < -4.0). Several stars have silicon abundances low enough to rule out dust-induced gas fragmentation with a standard grain size distribution. Moreover, two of these stars have such low silicon abundances that even dust with a shocked grain size distribution cannot explain their formation. Adding small amounts of carbon dust does not significantly change these conclusions. Additionally, we find that these stars exhibit either high carbon with low silicon abundances or the reverse. A silicate dust scenario thus suggests that the earliest low-mass star formation in the most metal-poor regime may have proceeded through two distinct cooling pathways: fine structure line cooling and dust cooling. This naturally explains both the carbon-rich and carbon-normal stars at extremely low [Fe/H].Comment: 14 pages, 8 figures; accepted to Ap

The Emerging Role of Ten-Eleven Translocation 1 in Epigenetic Responses to Environmental Exposures.

Mounting evidence from epidemiological studies and animal models has linked exposures to environmental factors to changes in epigenetic markers, especially in DNA methylation. These epigenetic changes may lead to dysregulation of molecular processes and functions and mediate the impact of environmental exposures in complex diseases. However, detailed molecular events that result in epigenetic changes following exposures remain unclear. Here, we review the emerging evidence supporting a critical role of ten-eleven translocation 1 (TET1) in mediating these processes. Targeting TET1 and its associated pathways may have therapeutic potential in alleviating negative impacts of environmental exposures, preventing and treating exposure-related diseases