OPT-IN:Target and Beam-Target Spin Asymmetries in exclusive Ï+ and Ïâ electroproduction with 1.6 to 5.7 GeV electrons

[Volker Burkert]

Dear lead authors,

The paper represents an incredible amount of information and it is essential that 
it is presented in all its details.  

Before I will dig into the details of the paper I want to ask for the following clarification:  

I found some of the kinematics used in Fig. 9 through Fig. 24 rather peculiar. 
The charged pion cms angles range from cos(theta)=+0.92 to cos(theta)=-0.73.  
I pick just one arbitrary kinematic point in Fig. 19.  W=2.08GeV, Q2 = 0.53 GeV2 and 
cos(theta)^cm = - 0.35. The cos(theta)=-0.35 translates into a cms angle of 110deg. 
A transformation into the lab system gives a value for the pion lab angle of 
approximately 60deg. for this W and Q^2.  Note that this is an angular range where 
the polarized target has no geometrical acceptance, which basically ends at 45deg 
(with the exception of a small angle range around 90deg. That means the pion would have to 
travel through massive amounts of coil material to be detected in the drift chambers. 
Even if the pion makes it through that material the vertex information is likely much 
distorted due to large energy losses and multiple scattering effects. Since the paper 
contains no simulations that would account for all the massive coil and cryostat material 
I believe the results in those regions are cannot be trusted.  

One can find dozens of pion kinematics, where there is no geometrical acceptance 
for the pion (basically from lab angles 45 to 83deg., and 97 to 135deg. and beyond 135deg.), 
and I like to see arguments presented why such kinematics should even be considered, 
or tell me why I am wrong.

Volker