[PEPPo] Answer to referee

[Eric VOUTIER]

 Dear All,

 Please find below the answer s we are proposing to the Referee B of the PRL review.
 The corresponding new version of the paper is also in attachment. Please send us any 
 comment/suggestion by monday 5/2 end of working day at JLab.

 Best regards, Eric & Joe

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 Answer to Referee B - LP15122/Abbott
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 We thank the referee for his thorough reading and comments about our
 manuscript. We summarize here our responses.
 
  - Hysteresis effects : While we had made measurements of the hysteresis
    effects, we had not included them in our estimate of the uncertainties
    in the positron momenta. That has been corrected in the updated 
    manuscript. 
  
    The spectrometer magnet was operated with a unipolar power supply; 
    we employed a switch to reverse polarity (electrons or positrons) and 
    followed a strict protocol cycling the magnet power supply three times 
    between limits of 5-295 A and approaching the final set point systematically 
    in the same sense (top-down). Prior to the experiment, the three magnetic 
    field components (Bx,By,Bz) were mapped for both polarities for simulation
    purposes and to quantify magnetic hysteresis effects. At the time
    of the experiment we used well measured electron beam momenta 
    (dp/p~0.6 %) to calibrate the spectrometer, and also found very 
    good agreement with our simulation (1-2 % level). We finally chose 
    the more precise electron calibration to define the central acceptance 
    of the spectrometer in the analysis.

    Over the range of positron momenta of the experiment, the spectrometer
    power supply was operated from 81-164 A; the hysteresis effects
    associated with polarity reversal are determined from the magnetic
    measurements and constitute an additional uncertainty that varies
    from 0.67-0.32 %, expectedly larger at the lower momentum.

    The uncertainties quoted in Table 1 for the positron momentum now reflect 
    the quadratic combination of the contribution from the calibration in
    electron mode and the hysteresis effect. This part of the Table now reads
   
      P      dP
     3.07   0.02 (was 0.01)
     4.02   0.03 (was 0.02)
     5.34   0.03 (was 0.02)
     6.25   0.04 (was 0.03) 

   We also added the following sentence in the section dedicated to 
   the magnetic line calibration :
   
   "The uncertainties on the positron momenta (Tab. 1) arise from  
    the electron calibration procedure and hysteresis effects 
    under polarity reversal for positron collection."  
 
 - Energy loss effects : Our current understanding of high positron 
   polarization at energies lower than the initial electron beam is 
   that it is indeed an effect of ionization energy loss on both the
   electron beam and the produced positrons. This interaction has very 
   small effect on the spin polarization of particles while reducing 
   their energy.

   For a clearer discussion, we added the following sentence in the 
   paragraph after Fig. 4 dedicated to the discussion of experimental
   results :
   
   "As a consequence of ionization energy loss in the production target, 
   large polarization transfer (>80%) is observed down to positron 
   momenta as low as 60% of the initial beam momentum." 
 
 - Formal error : We thank the referee for catching our omission of the
   overstrike in the text below formula(1); this error has been corrected
   in the present new version.  
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