Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Positronium collapse in hypercritical magnetic field and restructuring
  of the vacuum in QED   \\The Bethe-Salpeter equation in a strong magnetic field is studied for
positronium atom in an ultra-relativistic regime, and a (hypercritical) value
for the magnetic field is determined, which provides the full compensation of
the positronium rest mass by the binding energy in the maximum symmetry state.
The compensation becomes possible owing to the falling to the center
phenomenon. The relativistic form in two-dimensional Minkowsky space is derived
for the four-dimensional Bethe-Salpeter equation in the limit of an infinitely
strong magnetic field, and used for finding the above hypercritical value. Once
the positronium rest mass is compensated by the mass defect the energy barrier
separating the electron-positron system from the vacuum disappears. We thus
describe the structure of the vacuum in terms of strongly localized states of
tightly mutually bound (or confined) pairs. Their delocalization for still
higher magnetic field, capable of screening its further growth, is discussed.

24
The categories of this paper are: dg-ga
2
The categories of this paper are: math
16
The categories of this paper are: stat
19
The categories of this paper are: eess
36
The categories of this paper are: cmp-lg
33
The categories of this paper are: plasm-ph
3
The categories of this paper are: physics
10
The categories of this paper are: q-bio
35
The categories of this paper are: mtrl-th
31
The categories of this paper are: ao-sci
26
The categories of this paper are: bayes-an
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex
30
The categories of this paper are: q-alg
25
The categories of this paper are: patt-sol