Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Isoscalar dipole coherence at low energies and forbidden E1 strength   In 16O and 40Ca an isoscalar, low-energy dipole transition (IS-LED)
exhausting approximately 4% of the isoscalar dipole (ISD) energy-weighted sum
rule is experimentally known, but conspicuously absent from recent theoretical
investigations of ISD strength. The IS-LED mode coincides with the so-called
isospin-forbidden E1 transition. We report that for N=Z nuclei up to 100Sn the
fully self-consistent Random-Phase-Approximation with finite-range forces,
phenomenological and realistic, yields a collective IS-LED mode, typically
overestimating its excitation energy, but correctly describing its IS strength
and electroexcitation form factor. The presence of E1 strength is solely due to
the Coulomb interaction between the protons and the resulting isospin-symmetry
breaking. The smallness of its value is related to the form of the transition
density, due to translational invariance. The calculated values of E1 and ISD
strength carried by the IS-LED depend on the effective interaction used.
Attention is drawn to the possibility that in N-not-equal-Z nuclei this
distinct mode of IS surface vibration can develop as such or mix strongly with
skin modes and thus influence the pygmy dipole strength as well as the ISD
strength function. In general, theoretical models currently in use may be unfit
to predict its precise position and strength, if at all its existence.

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The categories of this paper are: stat
5
The categories of this paper are: gr-qc
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The categories of this paper are: math-ph
25
The categories of this paper are: patt-sol
7
The categories of this paper are: hep-th
6
The categories of this paper are: astro-ph
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
33
The categories of this paper are: plasm-ph
4
The categories of this paper are: cond-mat
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The categories of this paper are: quant-ph
34
The categories of this paper are: chem-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: chao-dyn
31
The categories of this paper are: ao-sci
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The categories of this paper are: cs