23,399 research outputs found
Uptake of branched-chain alpha-keto acids in \u3ci\u3eBacillus subtilis\u3c/i\u3e
Bacillus subtilis has a constitutive system for the uptake of alpha-keto-beta-methylvalerate, alpha-ketoisovalerate, and (probably) alpha-ketoisocaproate. A mutation, kauA1, which blocks the uptake of alpha-keto-beta-methylvalerate and alpha-ketoisovalerate, is located between metB and citK on the B. subtilis chromosome
Duration discrimination in younger and older adults
Ten normal hearing young adults and ten older adults were asked to identify the longer of two sequentially presented tones. The duration of the standard tones ranged from 1.5 ms to 1000 ms across blocks. Duration discrimination was not related to audiometric thresholds. These results show that older adults are much more disadvantaged than young adults when discriminating very short durations (i.e., below 40 ms) that are characteristic of speech sounds, and that this disadvantage cannot be accounted for by hearing levels
Antichain cutsets of strongly connected posets
Rival and Zaguia showed that the antichain cutsets of a finite Boolean
lattice are exactly the level sets. We show that a similar characterization of
antichain cutsets holds for any strongly connected poset of locally finite
height. As a corollary, we get such a characterization for semimodular
lattices, supersolvable lattices, Bruhat orders, locally shellable lattices,
and many more. We also consider a generalization to strongly connected
hypergraphs having finite edges.Comment: 12 pages; v2 contains minor fixes for publicatio
Nondestructive evaluation of sintered ceramics
Radiography and several acoustic and thermoacoustic microscopy techniques are investigated for application to structural ceramics for advanced heat engines. A comparison is made of the results obtained from the use of scanning acoustic microscopy (SAM), scanning laser acoustic microscopy (SLAM), and thermoacoustic microscopy (TAM). These techniques are evaluated on research samples of green and sintered monolithic silicon nitrides and silicon carbides in the form of modulus-of-rupture (MOR) bars containing deliberately introduced flaws. Strengths and limitations of the techniques are described, with the emphasis being on statistics of detectability of flaws that constitute potential fracture origins. Further, it is shown that radiographic evaluation and guidance helped develop uniform high-density Si3N4 MOR bars with improved four-point flexural strength (875, 544, and 462 MPa at room temperature, 1200 C, 1370 C, respectively) and reduced scatter in bend strength
Non-Perturbative U(1) Gauge Theory at Finite Temperature
For compact U(1) lattice gauge theory (LGT) we have performed a finite size
scaling analysis on lattices for fixed by
extrapolating spatial volumes of size to . Within the
numerical accuracy of the thus obtained fits we find for , 5 and~6
second order critical exponents, which exhibit no obvious
dependence. The exponents are consistent with 3d Gaussian values, but not with
either first order transitions or the universality class of the 3d XY model. As
the 3d Gaussian fixed point is known to be unstable, the scenario of a yet
unidentified non-trivial fixed point close to the 3d Gaussian emerges as one of
the possible explanations.Comment: Extended version after referee reports. 6 pages, 6 figure
Local Two-Photon Couplings and the J=0 Fixed Pole in Real and Virtual Compton Scattering
The local coupling of two photons to the fundamental quark currents of a
hadron gives an energy-independent contribution to the Compton amplitude
proportional to the charge squared of the struck quark, a contribution which
has no analog in hadron scattering reactions. We show that this local
contribution has a real phase and is universal, giving the same contribution
for real or virtual Compton scattering for any photon virtuality and skewness
at fixed momentum transfer squared t. The t-dependence of this J=0 fixed Regge
pole is parameterized by a yet unmeasured even charge-conjugation form factor
of the target nucleon. The t=0 limit gives an important constraint on the
dependence of the nucleon mass on the quark mass through the Weisberger
relation. We discuss how this 1/x form factor can be extracted from high energy
deeply virtual Compton scattering and examine predictions given by models of
the H generalized parton distribution.Comment: 20 pages, 15 figure
Hadron Spin Dynamics
Spin effects in exclusive and inclusive reactions provide an essential new
dimension for testing QCD and unraveling hadron structure. Remarkable new
experiments from SLAC, HERMES (DESY), and the Jefferson Laboratory present many
challenges to theory, including measurements at HERMES and SMC of the single
spin asymmetries in pion electroproduction, where the proton is polarized
normal to the scattering plane. This type of single spin asymmetry may be due
to the effects of rescattering of the outgoing quark on the spectators of the
target proton, an effect usually neglected in conventional QCD analyses. Many
aspects of spin, such as single-spin asymmetries and baryon magnetic moments
are sensitive to the dynamics of hadrons at the amplitude level, rather than
probability distributions. I illustrate the novel features of spin dynamics for
relativistic systems by examining the explicit form of the light-front
wavefunctions for the two-particle Fock state of the electron in QED, thus
connecting the Schwinger anomalous magnetic moment to the spin and orbital
momentum carried by its Fock state constituents and providing a transparent
basis for understanding the structure of relativistic composite systems and
their matrix elements in hadronic physics. I also present a survey of
outstanding spin puzzles in QCD, particularly the double transverse spin
asymmetry A_{NN} in elastic proton-proton scattering, the J/psi to rho-pi
puzzle, and J/psi polarization at the Tevatron.Comment: Concluding theory talk presented at SPIN2001, the Third
Circum-Pan-Pacific Symposium on High Energy Physics, October, 2001, Beijin
Four generated, squarefree, monomial ideals
Let be two squarefree monomial ideals of a polynomial algebra
over a field generated in degree , resp. . Suppose that
is either generated by three monomials of degrees and a set of monomials of
degrees , or by four special monomials of degrees . If the Stanley
depth of is then the usual depth of is too.Comment: to appear in "Bridging Algebra, Geometry, and Topology", Editors
Denis Ibadula, Willem Veys, Springer Proceed. in Math. and Statistics, 96,
201
Hadronic Spectra and Light-Front Wavefunctions in Holographic QCD
We show how the string amplitude defined on the fifth dimension in
AdS space can be precisely mapped to the light-front wavefunctions of
hadrons in physical spacetime. We find an exact correspondence between the
holographic variable and an impact variable , which represents the
measure of transverse separation of the constituents within the hadrons. In
addition, we derive effective four dimensional Schr\"odinger equations for the
bound states of massless quarks and gluons which exactly reproduce the AdS/CFT
results and give a realistic description of the light-quark meson and baryon
spectrum as well as the form factors for spacelike . Only one parameter
which sets the mass scale, , is introduced.Comment: 4 pages, REVTex4, 2 figures. References added. To appear in Phys.
Rev. Let
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