3,493 research outputs found
Transcription and the Pitch Angle of DNA
The question of the value of the pitch angle of DNA is visited from the
perspective of a geometrical analysis of transcription. It is suggested that
for transcription to be possible, the pitch angle of B-DNA must be smaller than
the angle of zero-twist. At the zero-twist angle the double helix is maximally
rotated and its strain-twist coupling vanishes. A numerical estimate of the
pitch angle for B-DNA based on differential geometry is compared with numbers
obtained from existing empirical data. The crystallographic studies shows that
the pitch angle is approximately 38 deg., less than the corresponding
zero-twist angle of 41.8 deg., which is consistent with the suggested principle
for transcription.Comment: 7 pages, 3 figures; v2: minor modifications; v3: major modifications
compared to v2. Added discussion about transcription, and reference
Color symmetrical superconductivity in a schematic nuclear quark model
In this note, a novel BCS-type formalism is constructed in the framework of a
schematic QCD inspired quark model, having in mind the description of color
symmetrical superconducting states. The physical properties of the BCS vacuum
(average numbers of quarks of different colors) remain unchanged under an
arbitrary color rotation. In the usual approach to color superconductivity, the
pairing correlations affect only the quasi-particle states of two colors, the
single particle states of the third color remaining unaffected by the pairing
correlations. In the theory of color symmetrical superconductivity here
proposed, the pairing correlations affect symmetrically the quasi-particle
states of the three colors and vanishing net color-charge is automatically
insured. It is found that the groundstate energy of the color symmetrical
sector of the Bonn model is well approximated by the average energy of the
color symmetrical superconducting state proposed here
Cycles, submanifolds, and structures on normal bundles
We give explicit examples of degree 3 cohomology classes not Poincare dual to
submanifolds, and discuss the realisability of homology classes by submanifolds
with Spin-C normal bundles.Comment: Several changes including an improvement of Theorem 1, our new
examples have torsion-free homolog
Electromagnetic Transition Strengths in Heavy Nuclei
We calculate reduced B(E2) and B(M1) electromagnetic transition strengths
within and between K-bands in support of a recently proposed model for the
structure of heavy nuclei. Previously, only spectra and a rough indication of
the largest B(E2) strengths were reported. The present more detailed
calculations should aid the experimental identification of the predicted ,
and bands and, in particular, act to confirm or refute the
suggestion that the model and bands correspond to the well known
and widespread beta and gamma bands. Furthermore they pinpoint transitions
which can indicate the presence of a so far elusive band by feeding
relatively strongly into or out of it. Some of these transitions may already
have been measured in Th, Th and U.Comment: 10 pages, 1 Figure, submitted to Physical Review
Quantum Monte Carlo calculations of excited states in A = 6--8 nuclei
A variational Monte Carlo method is used to generate sets of orthogonal trial
functions, Psi_T(J^pi,T), for given quantum numbers in various light p-shell
nuclei. These Psi_T are then used as input to Green's function Monte Carlo
calculations of first, second, and higher excited (J^pi,T) states. Realistic
two- and three-nucleon interactions are used. We find that if the physical
excited state is reasonably narrow, the GFMC energy converges to a stable
result. With the combined Argonne v_18 two-nucleon and Illinois-2 three-nucleon
interactions, the results for many second and higher states in A = 6--8 nuclei
are close to the experimental values.Comment: Revised version with minor changes as accepted by Phys. Rev. C. 11
page
Electron correlations in two-dimensional small quantum dots
We consider circular and elliptic quantum dots with parabolic external
confinement, containing 0 - 22 electrons and with values of r_s in the range 0
< r_s < 3. We perform restricted and unrestricted Hartree-Fock calculations,
and further take into account electron correlations using second-order
perturbation theory. We demonstrate that in many cases correlations
qualitatively change the spin structure of the ground state from that obtained
under Hartree-Fock and spin-density-functional calculations. In some cases the
correlation effects destroy Hund's rule. We also demonstrate that the
correlations destroy static spin-density waves observed in Hartree-Fock and
spin-density-functional calculations.Comment: 11 pages, 9 figures. This replacement contains new content. Results
have been recalculated for dots with zero effective thickness (true 2D). For
6 electrons, results have been compared with configuration interaction
results from the literatur
Similarities of gauge and gravity amplitudes
We review recent progress in computations of amplitudes in gauge theory and
gravity. We compare the perturbative expansion of amplitudes in N=4 super
Yang-Mills and N=8 supergravity and discuss surprising similarities.Comment: Talk presented by Harald Ita at "Continuous Advances in QCD 2006", 7
page
Quantum Monte Carlo calculations of electroweak transition matrix elements in A = 6,7 nuclei
Green's function Monte Carlo calculations of magnetic dipole, electric
quadrupole, Fermi, and Gamow-Teller transition matrix elements are reported for
A=6,7 nuclei. The matrix elements are extrapolated from mixed estimates that
bracket the relevant electroweak operator between variational Monte Carlo and
GFMC propagated wave functions. Because they are off-diagonal terms, two mixed
estimates are required for each transition, with a VMC initial (final) state
paired with a GFMC final (initial) state. The realistic Argonne v18 two-nucleon
and Illinois-2 three-nucleon interactions are used to generate the nuclear
states. In most cases we find good agreement with experimental data.Comment: v2: minor corrections to text and figure
Perturbative Gravity and Twistor Space
The recent progress in computing gauge theory amplitudes can be extended, in
many cases, to theories incorporating gravity. This has improved our
understanding of the perturbative expansion of N=8 supergravity supporting the
``no-triangle hypothesis'' that N=8 one-loop amplitudes may be expressed in
terms of scalar box integral functions.Comment: Talk presented by N. E. J. Bjerrum-Bohr at Loop and Legs 2006, 5
page
Relationships between a roller and a dynamic pressure distribution in circular hydraulic jumps
We investigated numerically the relation between a roller and the pressure
distribution to clarify the dynamics of the roller in circular hydraulic jumps.
We found that a roller which characterizes a type II jump is associated with
two high pressure regions after the jump, while a type I jump (without the
roller) is associated with only one high pressure region. Our numerical results
show that building up an appropriate pressure field is essential for a roller.Comment: 10 pages, 7 PS files. To appear in PR
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