1,794 research outputs found
On the energy-shell contributions of the three-particle~-~ three-hole excitations
The response functions for the extended second and third random phase
approximation are compared. A second order perturbation calculation shows that
the first-order amplitude for the direct excitation from the ground
state cancels with those that are engendered by the - coupling. As
a consequence nonvanishing effects to the response involve off
energy shell renormalization only. On shell processes are absent.Comment: 12 pages text (LaTex) and 1 figure included, to be published in Phys.
Rev.
Momentum Distribution in Nuclear matter within a Perturbation Approximation
It is shown that the norm corrections, introduced to avoid the violation of
the constraints on the depletion of the hole states in the standard
perturbative 2p2h approach, leads in nuclear matter to a dependence of the
momentum distribution with the total nucleon number. This unphysical behavior,
which in turn makes the depletion to be non-extensive, arises from
contributions of disconnected diagrams contained in the norm. It is found that
the extensivity is again recovered when the 4p4h excitations in the ground
state are included, and a reasonable value for the total number of nucleons
promoted above the Fermi level is obtained.Comment: 11 pages, LaTeX, 5 figures, figures 1 to 3 included in the latex
file, postscript files of figures 4 and 5 available from the Authors.
Accepted for publication in Phys. Rev.
Ultracold collisions between two light indistinguishable diatomic molecules: elastic and rotational energy transfer in HD+HD
A close coupling quantum-mechanical calculation is performed for rotational
energy transfer in a HD+HD collision at very low energy, down to the ultracold
temperatures: K. A global six-dimensional H-H
potential energy surface is adopted from a previous work [Boothroyd {\it et
al.}, J. Chem. Phys., {\bf 116}, 666 (2002).] State-resolved integral cross
sections of different
quantum-mechanical rotational transitions in the HD
molecules and corresponding state-resolved thermal rate coefficients
have been computed. Additionally, for comparison,
H+H calculations for a few selected rotational transitions have also
been performed. The hydrogen and deuterated hydrogen molecules are treated as
rigid rotors in this work. A pronounced isotope effect is identified in the
cross sections of these collisions at low and ultracold temperatures.Comment: 9 pages, 9 figures. Accepted for publication in Physical Review
Analysis of exchange terms in a projected ERPA Theory applied to the quasi-elastic (e,e') reaction
A systematic study of the influence of exchange terms in the longitudinal and
transverse nuclear response to quasi-elastic (e,e') reactions is presented. The
study is performed within the framework of the extended random phase
approximation (ERPA), which in conjuction with a projection method permits a
separation of various contributions tied to different physical processes. The
calculations are performed in nuclear matter up to second order in the residual
interaction for which we take a (pi+rho)-model with the addition of the
Landau-Migdal g'-parameter. Exchange terms are found to be important only for
the RPA-type contributions around the quasielastic peak.Comment: 29 pages, 6 figs (3 in postscript, 3 faxed on request), epsf.st
On the Connection between N=2 Minimal String and (1,n) Bosonic Minimal String
We study the scattering amplitudes in the N=2 minimal string or equivalently
in the N=4 topological string on ALE spaces. We find an interesting connection
between the tree level amplitudes of the N=2 minimal string and those of the
(1,n) minimal bosonic string. In particular we show that the four and
five-point functions of the N=2 string can be directly rewritten in terms of
those of the latter theory. This relation offers a map of physical states
between these two string theories. Finally we propose a possible matrix model
dual for the N=2 minimal string in the light of this connection.Comment: 40 pages, one figure, harvmac, minor corrections, references adde
The Penrose limit of AdS*S space and holography
In the Penrose limit, AdS*S space turns into a Cahen-Wallach (CW) space whose
Killing vectors satisfy a Heisenberg algebra. This algebra is mapped onto the
holographic screen on the boundary of AdS. I show that the Heisenberg algebra
on the boundary of AdS may be obtained directly from the CW space by
appropriately constraining the states defined on it. The transformations
generated by the constraint are similar to gauge transformations. The
``holographic screen'' on the CW space is thus obtained as a ``gauge-fixing''
condition.Comment: 12 pages, improved discussion, to appear in Mod. Phys. Lett.
Ghost D-branes
We define a ghost D-brane in superstring theories as an object that cancels
the effects of an ordinary D-brane. The supergroups U(N|M) and OSp(N|M) arise
as gauge symmetries in the supersymmetric world-volume theory of D-branes and
ghost D-branes. A system with a pair of D-brane and ghost D-brane located at
the same location is physically equivalent to the closed string vacuum. When
they are separated, the system becomes a new brane configuration. We generalize
the type I/heterotic duality by including n ghost D9-branes on the type I side
and by considering the heterotic string whose gauge group is OSp(32+2n|2n).
Motivated by the type IIB S-duality applied to D9- and ghost D9-branes, we also
find type II-like closed superstrings with U(n|n) gauge symmetry.Comment: 49 pages, 6 figures, harvmac. v2: references and acknowledgements
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