Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Relativistic two fermion treatment of hyperfine transitions   A system of two fermions with different masses and interacting by the Coulomb
potential is presented in a completely covariant framework. The spin-spin
interaction, including the anomalous magnetic moments of the two fermions, is
added by means of a Breit term. We solve the resulting fourth order
differential system by evaluating the spectrum and the eigenfunctions. The
interaction vertex with an external electromagnetic field is then determined.
The relativistic eigenfunctions are used to study the photon emission from a
hyperfine transition and are checked for the calculation of the Lamb shift due
to the electron vacuum polarization in the muonic Hydrogen.

12
The categories of this paper are: nucl-th
15
The categories of this paper are: nucl-ex
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics
31
The categories of this paper are: ao-sci
22
The categories of this paper are: chao-dyn
10
The categories of this paper are: q-bio
13
The categories of this paper are: quant-ph
7
The categories of this paper are: hep-th
28
The categories of this paper are: alg-geom
33
The categories of this paper are: plasm-ph
18
The categories of this paper are: econ
24
The categories of this paper are: dg-ga
30
The categories of this paper are: q-alg
16
The categories of this paper are: stat
19
The categories of this paper are: eess