Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Spin Structure of the qq Interaction and the Mass Spectra of Bound
  q\bar q Systems: Different Versions of the 3-Dimensional Reductions of the
  Bethe-Salpeter Equation   Bound q\bar q -systems are considered in the framework of three different
versions of the 3-dimensional reduction of the Bethe-Salpeter equation, all
having the correct one-body limit when one of the constituent quark masses
tends to infinity, and in the framework of the Salpeter equation. The spin
structure of confining qq interaction potential is taken in the form
x\gamma_{1}^{0}\gamma_{2}^{0}+(1-x)I_{1}I_{2}, with 0 \leq x \leq 1. The
problem of existence (nonexistence) of stable solutions of 3-dimensional
relativistic equations for bound q\bar q systems is studied for different
values of x from this interval. Some other aspects of this problem are
discussed.

22
The categories of this paper are: chao-dyn
5
The categories of this paper are: gr-qc
12
The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin
34
The categories of this paper are: chem-ph
35
The categories of this paper are: mtrl-th
3
The categories of this paper are: physics
18
The categories of this paper are: econ
27
The categories of this paper are: comp-gas
4
The categories of this paper are: cond-mat
13
The categories of this paper are: quant-ph
14
The categories of this paper are: math-ph