Analysis of longitudinal bunching inan FEL driven two-beam accelerator

Recent experiments [1] have explored the use of a free-electron laser (FEL) as a buncher for a microwave two-beam accelerator, and the subsequent driving of a standing-wave rf output cavity. Here we present a deeper analysis of the longitudinal dynamics of the electron bunches as they are transported from the end of the FEL and through the output cavity. In particular, we examine the effect of the transport region and cavity aperture to filter the bunched portion of the beam. [1] T. Lefevre, et. al., Phys. Rev. Lett. 84 (2000), 1188.Comment: 3 pages, 8 figures. Submitted to XX Int'l LINAC Conferenc

Transverse momentum dependence of the angular distribution of the Drell-Yan process

We calculate the transverse momentum Q_{\perp} dependence of the helicity structure functions for the hadroproduction of a massive pair of leptons with pair invariant mass Q. These structure functions determine the angular distribution of the leptons in the pair rest frame. Unphysical behavior in the region Q_{\perp} --> 0 is seen in the results of calculations done at fixed-order in QCD perturbation theory. We use current conservation to demonstrate that the unphysical inverse-power and \ln(Q/Q_{\perp}) logarithmic divergences in three of the four independent helicity structure functions share the same origin as the divergent terms in fixed-order calculations of the angular-integrated cross section. We show that the resummation of these divergences to all orders in the strong coupling strength \alpha_s can be reduced to the solved problem of the resummation of the divergences in the angular-integrated cross section, resulting in well-behaved predictions in the small Q_{\perp} region. Among other results, we show the resummed part of the helicity structure functions preserves the Lam-Tung relation between the longitudinal and double spin-flip structure functions as a function of Q_{\perp} to all orders in \alpha_s.Comment: 18 pages, 4 figures; typos corrected, references updated, a few clarifications recommended by the referee. Paper accepted for publication in Physical Review

Above- and belowground herbivory jointly impact defense and seed dispersal traits in Taraxacum officinale

Plants are able to cope with herbivores by inducing defensive traits or growth responses that allow them to reduce or avoid the impact of herbivores. Since above- and belowground herbivores differ substantially in life-history traits, for example feeding types, and their spatial distribution, it is likely that they induce different responses in plants. Moreover, strong interactive effects on defense and plant growth are expected when above- and belowground herbivores are jointly present. The strengths and directions of these responses have been scarcely addressed in the literature. Using Taraxacum officinale, the root-feeding nematode Meloidogyne hapla and the locust Schistocerca gregaria as a model species, we examined to what degree above- and belowground herbivory affect (1) plant growth responses, (2) the induction of plant defensive traits, that is, leaf trichomes, and (3) changes in dispersal-related seed traits and seed germination. We compared the performance of plants originating from different populations to address whether plant responses are conserved across putative different genotypes. Overall, aboveground herbivory resulted in increased plant biomass. Root herbivory had no effect on plant growth. Plants exposed to the two herbivores showed fewer leaf trichomes than plants challenged only by one herbivore and consequently experienced greater aboveground herbivory. In addition, herbivory had effects that reached beyond the individual plant by modifying seed morphology, producing seeds with longer pappus, and germination success

Axial anomaly and magnetism of nuclear and quark matter

We consider the response of the QCD ground state at finite baryon density to a strong magnetic field B. We point out the dominant role played by the coupling of neutral Goldstone bosons, such as pi^0, to the magnetic field via the axial triangle anomaly. We show that, in vacuum, above a value of B ~ m_pi^2/e, a metastable object appears - the pi^0 domain wall. Because of the axial anomaly, the wall carries a baryon number surface density proportional to B. As a result, for B ~ 10^{19} G a stack of parallel pi^0 domain walls is energetically more favorable than nuclear matter at the same density. Similarly, at higher densities, somewhat weaker magnetic fields of order B ~ 10^{17}-10^{18} G transform the color-superconducting ground state of QCD into new phases containing stacks of axial isoscalar (eta or eta') domain walls. We also show that a quark-matter state known as ``Goldstone current state,'' in which a gradient of a Goldstone field is spontaneously generated, is ferromagnetic due to the axial anomaly. We estimate the size of the fields created by such a state in a typical neutron star to be of order 10^{14}-10^{15} G.Comment: 18 pages, v2: added a discussion of the energy cost of neutralizing the domain wall charg

Million-atom molecular dynamics simulation by order-N electronic structure theory and parallel computation

Parallelism of tight-binding molecular dynamics simulations is presented by means of the order-N electronic structure theory with the Wannier states, recently developed (J. Phys. Soc. Jpn. 69,3773 (2000)). An application is tested for silicon nanocrystals of more than millions atoms with the transferable tight-binding Hamiltonian. The efficiency of parallelism is perfect, 98.8 %, and the method is the most suitable to parallel computation. The elapse time for a system of $2\times 10^6$ atoms is 3.0 minutes by a computer system of 64 processors of SGI Origin 3800. The calculated results are in good agreement with the results of the exact diagonalization, with an error of 2 % for the lattice constant and errors less than 10 % for elastic constants.Comment: 5 pages, 3 figure

Mott-Superfluid transition in bosonic ladders

We show that in a commensurate bosonic ladder, a quantum phase transition occurs between a Mott insulator and a superfluid when interchain hopping increases. We analyse the properties of such a transition as well as the physical properties of the two phases. We discuss the physical consequences for experimental systems such as Josephson Junction arrays.Comment: 4 pages, 2 figures, revtex

Tight-binding study of high-pressure phase transitions in titanium: alpha to omega and beyond

We use a tight-binding total energy method, with parameters determined from a fit to first-principles calculations, to examine the newly discovered gamma phase of titanium. Our parameters were adjusted to accurately describe the alpha Ti-omega Ti phase transition, which is misplaced by density functional calculations. We find a transition from omega Ti to gamma Ti at 102 GPa, in good agreement with the experimental value of 116 GPa. Our results suggest that current density functional calculations will not reproduce the omega Ti-gamma Ti phase transition, but will instead predict a transition from omega Ti to the bcc beta Ti phase.Comment: 3 encapsulated Postscript figures, submitted to Phyical Review Letter

Effect of pressure on the Raman modes of antimony

The effect of pressure on the zone-center optical phonon modes of antimony in the A7 structure has been investigated by Raman spectroscopy. The A_g and E_g frequencies exhibit a pronounced softening with increasing pressure, the effect being related to a gradual suppression of the Peierls-like distortion of the A7 phase relative to a cubic primitive lattice. Also, both Raman modes broaden significantly under pressure. Spectra taken at low temperature indicate that the broadening is at least partly caused by phonon-phonon interactions. We also report results of ab initio frozen-phonon calculations of the A_g and E_g mode frequencies. Presence of strong anharmonicity is clearly apparent in calculated total energy versus atom displacement relations. Pronounced nonlinearities in the force versus displacement relations are observed. Structural instabilities of the Sb-A7 phase are briefly addressed in the Appendix.Comment: 10 pages, 8 figure

Evaluating Exposure of Northern Fur Seals, Callorhinus Ursinus, to Microplastic Pollution Through Fecal Analysis

Environmental microplastics are widely documented in marine life and bioaccumulation may present risks to marine predators. Investigations of microplastics in marine mammals are increasing, though none have examined animals routinely consumed by humans. Here, we investigate microplastic exposure in the northern fur seal (Callorhinus ursinus), a species consumed by humans, using fecal material. We examined 44 feces (scat) at sites encompassing the seals\u27 eastern Pacific range. Multiple contamination control measures were implemented, including field and laboratory controls. Fragments were the most common microplastic recovered, in 55% (24/44) of scat and no controls (range 1 to 86 fragments/scat, mean 16.6, sd 19.1). Microplastic fibers were recovered from 41% of scats (18/44), though some controls contained fibers confounding fiber results. Fecal analysis documented northern fur seal exposure to microplastics throughout their eastern Pacific range. © 201

Intrinsic transverse momentum and the polarized Drell-Yan process

In this paper we study the cross section at leading order in $1/Q$ for polarized Drell-Yan scattering at measured lepton-pair transverse momentum $Q_T$. We find that for a hadron with spin $1/2$ the quark content at leading order is described by six distribution functions for each flavor, which depend on both the lightcone momentum fraction $x$, and the quark transverse momentum \bbox{k}_T^2. These functions are illustrated for a free-quark ensemble. The cross sections for both longitudinal and transverse polarizations are expressed in terms of convolution integrals over the distribution functions.Comment: 25 pages, REVTEX 3.0 (3 figures included in separate LATEX file using feynman.tex), NIKHEF-94-P1 (Revised version