[Clascomment] OPT-IN:Suppression of neutral pion production in deep-inelastic scattering off nuclei with the CLAS detector

[Mikhail Osipenko]

Dear Authors of pi0 paper,

first let me congratulate you with this useful new set of data presented in this article.

Here are few thoughts coming to me when reading it:

1) "Since the background in the two-photon invariant mass spectrum was combinatorial" - was this demonstrated? I would guess that the background under the pi0 peak is mostly due to Dalitz decays of heavier mesons.

2) in abstract: "An enhancement of the multiplicity ratio at low-z and high p2T is observed suggesting an interconnection between these two variables", as well as in summary: "On the other hand, the low z regime pertains  to  the  opposite  behavior  which  leads  to  the  enhancement of transverse momenta".
All these are nice DIS effects, but applied to 5 GeV CLAS data seem to be inappropriate. Your low-z region is constraining p_T-range very strongly (see Fig.15 of https://arxiv.org/pdf/0809.1153.pdf), and at z=0.3 p_T=1 GeV is out of reach for proton target (see Fig.14 <0.3%). On nuclei, Fermi motion allows you to reach unphysical regions, but does it have anything to do with quark dynamics?
Perhaps, if you would apply a cut on missing mass: M_X>M_p (mandatory for SIDIS on proton) all the data at p_T->1 GeV will disappear.

3) "This enhancement is the largest for carbon and the smallest for lead. Such behavior is opposite to the published HERMES results". The same as above: HERMES had 27 GeV beam, and therefore much larger phase space. p_T->1 GeV is not a corner of phase space as for CLAS. 
Notice, that if instead of considering the absolute value of the ratio you take the difference between the ration at p_T^2=0.2 and p_T^2>0.8 - this difference will be almost the same in all nuclei.


Good Luck,
              Mikhail.