Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Note on restoring manifest rotational symmetry in hyperfine and fine
  structure in light-front QED   We study the part of the renormalized, cutoff QED light-front Hamiltonian
that does not change particle number. The Hamiltonian contains interactions
that must be treated in second-order bound state perturbation theory to obtain
hyperfine structure. We show that a simple unitary transformation leads
directly to the familiar Breit-Fermi spin-spin and tensor interactions, which
can be treated in degenerate first-order bound-state perturbation theory, thus
simplifying analytic light-front QED calculations. To the order in momenta we
need to consider, this transformation is equivalent to a Melosh rotation. We
also study how the similarity transformation affects spin-orbit interactions.

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The categories of this paper are: cmp-lg
6
The categories of this paper are: astro-ph
18
The categories of this paper are: econ
30
The categories of this paper are: q-alg
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The categories of this paper are: dg-ga
29
The categories of this paper are: funct-an
11
The categories of this paper are: hep-lat
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The categories of this paper are: comp-gas
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The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
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The categories of this paper are: chem-ph
2
The categories of this paper are: math
1
The categories of this paper are: cs
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The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph