[Clascomment] OPT-IN:Di erential cross sections and spin density matrix elements for the reaction gamma p --> phi p

[Volker Burkert]

Dear CMU group,
 This is a very well written paper with potentially important impact on N* physics. 
I trust that other folks caught the typos and editorial aspects already. I only want t
o comment on the physics discussion and interpretation. 

1) Fig. 13 and Fig.14 show the structure in the sqrt(s)-dependence. One should 
point out that the peak is present at nearly all angle bin but appears to be moving
 from ~2.16GeV at forward angle to ~2.08GeV at backward angles.  

2)One of the important topics is the possible effect of the K-L1520 channel on the 
phi-p channel as they both have the same final states in the charged decay mode 
of the phi. There is a long discussion on the cuts designed to reduce the impact of 
that interference effects, from no mass cuts on L1520 up to a +/- 15 MeV cut that 
removes nearly all direct K-L1520 events in the sample.  Finally a "hard" cut is 
chosen (+/-15MeV) and the diff. x-section and and SDME are given by that. What is 
not shown is the effect of no cut vs soft cuts vs hard cut on the angle distributions 
and the  SDME. Nevertheless in the physics discussion section XI. it is speculated that 
"around  the structure at sqrt(s)~2.1GeV, the results show show a distinctly large 
deviation from B~3GeV^(-2), possibly due to effects from the L1520."  The observed 
structure is in fact visible at nearly all polar angles and could have something to do 
with either s-channel nucleon resonances or with some coupled-channel effect. 
To be able to make some more definitive statement it would be good to add some 
graph(s) showing how this structure is affected by the L1520-mass cut. For example 
one could produce the same kind of  Fig.25 and Fig.26 but without the hard cut. 
Even better, if the differential cross section could be shown with and without that 
hard cut, at least for the forward most angle where the structure is most visible. 
Also, how are the SDME affected by the cuts?  Having this additional information 
would allow a more definitive discussion on a possible coupling between the two 
final states. Here one could also make the case for studying the neutral decay channel 
of the phi where there should not be a similarly strong coupling as the two final states 
are different, although there could probably still be re-scattering or loop diagrams 
that contribute.

 3) In the discussion section one should discuss, or at least reference, the recent paper 
by H.-Y. Ryan, A. Titov, A. Hosaka, H.-C. Kim, arXiv: 1212.6075 [hep-ph], that 
discusses phi-photoproduction with coupled-channel effects. They study the impact 
of hadronic box diagrams on phi photoproduction.

I realize that this would require additional work but I think it is worthwhile to 
extend the physics discussion somewhat.   

Volker