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Erratum to ``Remarks on the phase space  for the  one-photon production 
in electron-proton scattering''   
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\centerline{Wei Lu} 

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   Centre de Physique Th\'eorique, Ecole Polytechnique, 
91128 Palaiseau  Cedex, France
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\begin{abstract}
 I   clarify  the mistakes in   (Remarks on the 
kinematics of  the  one-photon production in electron-proton scattering) 
about the phase space factor  for the  one-photon production in 
electron-proton scattering and  confirm that the earlier result 
given by Kroll, Sch\"urmann and Guichon is right. 
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In Ref. [1], I claimed that the the phase space factor for the  
one-photon production in electron-proton scattering presented by 
Kroll, Sch\"urmann and Guichon
[2] is incorrect.  This is wrong. 

Indeed, as demonstrated in Ref. [2],  there are  a lot of
advantages as one works in   the c.m. frame of the virtual 
Compton scattering, with the  $z$-axis specified to be in the 
traveling direction of the virtual photon.  However,  
derivation of  the correct phase space factor for the 
cross section formula  becomes  very tricky.  
The reason goes like this: As one sets 
the virtual photon momentum to be in the $z$-axis,  the degrees 
of freedom of the   final-state electron is reduced because of the 
momentum conservation  at the electron-photon  interaction vertex. 

 To derive the correct phase space factor, one should choose {\it  a 
fixed axis}, {\it rather than the  moving direction of the  virtual photon},
 to  specify  the azimuthal angles of the final-state  particles,  
 whatever the working frame  frame is.  A convenient choice is to work 
in the laboratory frame, with the $z$-axis in the beam direction, 
in which one can derive the phase space factor presented in Ref. [2]
with a little algebra.  Of course, one can also  derive the correct phase 
space factor  in the c.m. frame of the  virtual Compton scattering
process. As emphasized  above,  however, one has to choose 
a fixed  axis to specify the azimuthal angles of the final-state 
particles. 

   The phase space boundaries 
given in Ref. [1]  stands intact.  In particular,  Eq. (28) in Ref. [1] 
can be used to determine the experimentally  accessible region of 
the c.m.  energy squared $s$  for the virtual Compton scattering with 
fixed beam energy $E$ and  photon virtuality $Q$. 

   In  passing, there is a typographical error in  the second reference 
of  Ref. [1], which  goes like in Ref. [3]. 
 
   I thank Markus Diehl for pointing out the mistake. 


\begin{thebibliography}{150}
\bibitem{lu} 
W. Lu, .
\bibitem{kroll} 
P. Kroll, M. Sch\"urmann and P.A.M.  Guichon,  
Nucl. Phys. {\bf A598}, 435 (1996).
\bibitem{Rad}
A.V. Radyushkin,  Phys. Lett. {\bf 380B}, 417 (1996). 
\end{thebibliography}
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