Here are my general comments on the Ds+ - Ds- asymmetry paper:
1) The measurement of A_p assumes that the only source of asymmetry is the efficiency to detect the pion. The fact that the kaon momenta are not identical and that there are momentum dependent material effects is not addressed. We think that this issue should be addressed by a full MC simulation. A demonstration that a MC input asymmetry would be correctly reconstructed would solve this.
2) A specific difference between magnet up and magnet down data is that there is a different crossing angle of the colliding proton bunches. This could have a potential effect on the magnet up and magnet down asymmetry measurement. This is however not addressed in the paper.
3) The main subject of the paper is the determination of the pion efficiency. The pion detection efficiency asymmetry is derived in a beautiful way from the D* decays. However, the absolute pion detection efficiency, as can be derived from table 1 is about 45%. Clearly, this is not a pion detection efficiency for a particle inside the LHCb acceptance, but includes analysis specific cuts. Although the absolute value of the detection efficiency is specific to this analysis, it is assumed that the asymmetry is universal. Also here, we think that the argument would be more solid if one could correctly reconstruct an input asymmetry in a MC simulation. For this a signal-only MC would be sufficient.
4) The efficiency ratio for a given magnet field orientation has a strong azimuthal dependency. In the pi+/pi- average the dependency cancels within statistics (Fig 8). This implies that a positive pion in, say, magnet up configuration should have an identical azimuthal detection efficiency distribution as compared to a negative pion in magnet down configuration. Could this point perhaps be made more explicit?
5) To determine the efficiency per momentum bin the inferred momentum of the missing pion is required, as stated in the paper. However this momentum has a large uncertainty. How is the binning done? Or, more specifically, how large is the effect of applying the "unfolding matrix" (line 125)?
6) We miss text to justify the PDF's (appendices) used to describe the mass shapes. In particular the use of a Bifurcated Student t-function in connection with a mass shape is unexpected. Please motivate it. Also the shape of the WS mass distribution in K2pi distribution has a "dip" that is modelled with a Gaussian with negative fraction f_1. What could be the physics behind such a distribution?
Cheers, - marcel