[Clascomment] OPT-IN: Observation of Beam Spin Asymmetries in the Process ep -> epi+pi-X with CLAS12

[Sebastian Kuhn]

Overall a nice paper. I am not sure I quite follow the insistence (twice) in the abstract that this provides "a first signal" or "a first observation" when in the text (line 65ff) you refer to an earlier result from CLAS. Line 181 appears a bit garbled? ("convolution of the TMD of the PDF..."). In line 175 you claim that e(x) COULD be extracted which begs the question (for a PRL) why you don't present that result (maybe because the difficulties alluded to in line 227 ff?). This should be made clearer (or left out). 

My biggest quibble though is the lack of experimental information (as opposed to lots of theoretical/phenomenological discussions). For the first (flagship) publication from a brand new spectrometer, having only 12 lines describing the latter is a bit meager. No spectra are shown, no cuts explained (on SF, PCal, ToF, ...). The only cuts you mention are on Q2, W, MX, xF and y - what about z1, z2, z, PT, RT etc.? Not even a phi_Rperp or Phi_h distribution. At least some indication of the kinematic coverage would be helpful in the main text. When you claim that systematic uncertainties due to particle mis-ID are "negligible", some more quantitative statement would be better; also, what is the most serious contamination? (Any of the 2 pi's could really be a K or an e, and the e could be a pi-). Finally, radiative effects are more than just "smearing" (line 224) - basically all variables (q-vector, phi's, etc.) could be shifted, and exclusive rho production could "leak in" from initial- a
 nd final electron radiation.