791 research outputs found
The Isovector Quadrupole-Quadrupole Interaction Used in Shell Model Calculations
An interaction is used
in a shell model calculation for . Whereas for the state
is two-fold degenerate, introducing a negative causes an `isovector'
state to come down to zero energy at and an triplet
() to come down to zero energy at . These are
undesirable properties, but a large negative is apparently needed to fit
the energy of the isovector giant quadrupole resonance.Comment: 12 pages, revtex, 2 figures (available on request
Systematics of the Quadrupole-Quadrupole Interaction and Convergence Properties
Our main concern in this work is to show how higher shell admixtures affect
the spectrum of a Q.Q interaction. We first review how, in the valence space,
the familiar SU(3) result for the energy spectrum can be obtained using a
coordinate space Q.Q interaction rather than the Elliott one which is symmetric
in r and p. We then reemphasize that the Elliott spectrum goes as L(L+1) where
L is the orbital angular momentum. While in many cases this is compatible with
the rotational formula which involves I(I+1), where I is the total angular
momentum, there are cases, e.g. odd-odd nuclei, where there is disagreement.
Finally, we consider higher shell admixtures and devise a scheme so as to
obtain results, with the Q.Q interaction, which converge as the model spaces
are increased. We consider not only ground state rotational bands but also
those that involve intruder states.Comment: 13 pages, Revtex, to appear in Annals of Physic
Ricci-flat deformation of orbifolds and localized tachyonic modes
We study Ricci-flat deformations of orbifolds in type II theory. We obtain a
simple formula for mass corrections to the twisted modes due to the
deformations, and apply it to originally tachyonic and massless states in
several examples. In the case of supersymmetric orbifolds, we find that
tachyonic states appear when the deformation breaks all the supersymmetries. We
also study nonsupersymmetric orbifolds C^2/Z_{2N(2N+1)}, which is T-dual to N
type 0 NS5-branes. For N>=2, we compute mass corrections for states, which have
string scale tachyonic masses. We find that the corrected masses coincide to
ones obtained by solving the wave equation for the tachyon field in the smeared
type 0 NS5-brane background geometry. For N=1, we show that the unstable mode
representing the bubble creation is the unique tachyonic mode.Comment: 20 pages, minor collection
Closed-String Tachyons and the Hagedorn Transition in AdS Space
We discuss some aspects of the behaviour of a string gas at the Hagedorn
temperature from a Euclidean point of view. Using AdS space as an infrared
regulator, the Hagedorn tachyon can be effectively quasi-localized and its
dynamics controled by a finite energetic balance. We propose that the off-shell
RG flow matches to an Euclidean AdS black hole geometry in a generalization of
the string/black-hole correspondence principle. The final stage of the RG flow
can be interpreted semiclassically as the growth of a cool black hole in a
hotter radiation bath. The end-point of the condensation is the large Euclidean
AdS black hole, and the part of spacetime behind the horizon has been removed.
In the flat-space limit, holography is manifest by the system creating its own
transverse screen at infinity. This leads to an argument, based on the
energetics of the system, explaining why the non-supersymmetric type 0A string
theory decays into the supersymmetric type IIB vacuum. We also suggest a notion
of `boundary entropy', the value of which decreases along the line of flow.Comment: 24 pages, Harvmac. 2 Figures. Typos corrected and reference adde
Dynamics of Coronal Bright Points as seen by Sun Watcher using Active Pixel System detector and Image Processing (SWAP), Atmospheric Imaging Assembly AIA), and Helioseismic and Magnetic Imager (HMI)
The \textit{Sun Watcher using Active Pixel system detector and Image
Processing}(SWAP) on board the \textit{PRoject for OnBoard Autonomy\todash 2}
(PROBA\todash 2) spacecraft provides images of the solar corona in EUV channel
centered at 174 \AA. These data, together with \textit{Atmospheric Imaging
Assembly} (AIA) and the \textit{Helioseismic and Magnetic Imager} (HMI) on
board \textit{Solar Dynamics Observatory} (SDO), are used to study the dynamics
of coronal bright points. The evolution of the magnetic polarities and
associated changes in morphology are studied using magnetograms and
multi-wavelength imaging. The morphology of the bright points seen in
low-resolution SWAP images and high-resolution AIA images show different
structures, whereas the intensity variations with time show similar trends in
both SWAP 174 and AIA 171 channels. We observe that bright points are seen in
EUV channels corresponding to a magnetic-flux of the order of Mx. We
find that there exists a good correlation between total emission from the
bright point in several UV\todash EUV channels and total unsigned photospheric
magnetic flux above certain thresholds. The bright points also show periodic
brightenings and we have attempted to find the oscillation periods in bright
points and their connection to magnetic flux changes. The observed periods are
generally long (10\todash 25 minutes) and there is an indication that the
intensity oscillations may be generated by repeated magnetic reconnection
Leptogenesis and neutrino parameters
We calculate the baryonic asymmetry of the universe in the
baryogenesis-via-leptogenesis framework, assuming first a quark-lepton symmetry
and then a charged-neutral lepton symmetry. We match the results with the
experimentally favoured range. In the first case all the oscillation solutions
to the solar neutrino problem, except the large mixing matter solution, can
lead to the allowed range, but with fine tuning of the parameters. In the
second case the general result is quite similar. Some related theoretical hints
are discussed.Comment: RevTex, 21 pages with 8 figure
Dualities, Twists, and Gauge Theories with Non-Constant Non-Commutativity
We study the world volume theory of D3-branes wrapping the Melvin universe
supported by background NSNS B-field. In the appropriate decoupling limit, the
open string dynamics is that of non-commutative guage field theory with
non-constant non-commutativity. We identify this model as a simple Melvin twist
of flat D3 branes. Along similar lines, one recognizes the model of Hashimoto
and Sethi as being the Melvin null twist, and the model of Dolan and Nappi as
being the null Melvin twist, of the flat D3-brane. This construction therefore
offers a unified perspective on most of the known explicit constructions of
non-commutative gauge theories as a decoupled theory of D-branes in a B-field
background. We also describe the world volume theory on the D3-brane in Melvin
universe which is decaying via the nucleation of monopole anti-monopole pair.Comment: 18 pages, 1 figure, References added, typo correcte
Closed String Tachyon Condensation on Twisted Circles
We study IIA/B string theory compactified on twisted circles. These models
possess closed string tachyons and reduce to type 0B/A theory in a special
limit. Using methods of gauged linear sigma models and mirror symmetry we
construct a conformal field theory which interpolates between these models and
flat space via an auxiliary Liouville direction. Interpreting motion in the
Liouville direction as renormalization group flow, we argue that the end point
of tachyon condensation in all these models (including 0B/A theory) is
supersymmetric type II theory. We also find a zero-slope limit of these models
which is best described in a T-dual picture as a type II NS-NS fluxbrane. In
this limit tachyon condensation is an interesting and well posed problem in
supergravity. We explicitly determine the tachyon as a fluctuation of
supergravity fields, and perform a rudimentary numerical analysis of the
relevant flows.Comment: 21 pages plus appendices (12 pages), harvmac, 1 fig, v2: minor
changes and references added, v3: minor changes version published in JHE
Seesaw mechanism, baryon asymmetry and neutrinoless double beta decay
A simplified but very instructive analysis of the seesaw mechanism is here
performed. Assuming a nearly diagonal Dirac neutrino mass matrix, we study the
forms of the Majorana mass matrix of right-handed neutrinos, which reproduce
the effective mass matrix of left-handed neutrinos. As a further step, the
important effect of a non diagonal Dirac neutrino mass matrix is explored. The
corresponding implications for the baryogenesis via leptogenesis and for the
neutrinoless double beta decay are reviewed. We propose two distinct models
where the baryon asymmetry is enhanced.Comment: 21 pages, RevTex. Revise
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