Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: $\cal CPT$-invariant two-fermion Dirac equation with extended hyperfine
  operator   For the S-states of muonium and positronium, the hyperfine shifts to the
order $\alpha^6$ of a recently derived two-fermion equation with explicit $\cal
CPT$-invariance are checked against the results of a nonrelativistic reduction,
and the leading $\alpha^8$ shifts are calculated. An additional hyperfine
operator is discovered which can milden the singularity for $r\to 0$ of the
Dirac hyperfine operator, such that the resulting extended operator can be used
nonperturbatively. The binding correction to magnetic moments is mentioned.

21
The categories of this paper are: adap-org
35
The categories of this paper are: mtrl-th
5
The categories of this paper are: gr-qc
1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
18
The categories of this paper are: econ
4
The categories of this paper are: cond-mat
2
The categories of this paper are: math
24
The categories of this paper are: dg-ga
12
The categories of this paper are: nucl-th
7
The categories of this paper are: hep-th
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
16
The categories of this paper are: stat
37
The categories of this paper are: supr-con