====== TTP99-40 CPT-invariant two-fermion Dirac equation with extended hyperfine operator ======
 
 
   <hidden TTP99-40  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.
 </hidden>
|**V. Hund and H. Pilkuhn**  | 
|**  J. Phys.  B33 1617 2000  **  |
| {{preprints:1999:ttp99-40.pdf|PDF}} {{preprints:1999:ttp99-40.ps|PostScript}} [[http://arxiv.org/abs/hep-ph/9910281|arXiv]]   |
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