Subject: Your Submission From: "Donald Geesaman" Date: 4/4/17, 08:47 To: srklein@lbl.gov CC: geesaman@anl.gov, glover@phy.anl.gov Ms. Ref. No.: PLB-D-17-00300 Title: Coherent diffractive photoproduction of $\rho^{0}$ mesons on gold nuclei at RHIC Physics Letters B Dear Dr. Spencer Klein, Reviewers' comments on your work have now been received. You will see that one has major concerns and is advising that you revise your manuscript. I also must agree that the length and 9 figures are not appropriate for a Letter except in very special cases. The guidance for PLB is 6 pages. If you feel you can address the issues raised and you decide to revise your manuscript, please submit a list of changes and a rebuttal against each point which is being raised when you submit the revised manuscript. To submit a revision, please go to https://ees.elsevier.com/plb/ and login as an Author. 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Reviewer #1: I have concerns with this article in some respects: 1- The article is too long for a letter. It describes many details of the experimental analysis that make it very extensive. I suspect that it was first prepared for a publication as a regular article and the authors then decided to submit is as a letter first. It is not proper to the audience in PLB. 2- Some experimental analysis are disturbing. The authors speak of a 'classical Glauber calculation' versus a 'quantum Glauber calculation'. This does not make sense. All Glauber calculations are quantum in nature. Glauber calculations are based on the eikonal wavefunctions, which as the name says are 'wave functions'. The transverse part of this wave function may be interpreted as an impact parameter, and therefore can be used to understand results with a classical perspective. However, the calculations are quantum in nature. The authors do not clarify in their text what they mean by classical Glauber. The authors are probably wrong with their experimental analysis because they do not understand the very basics of what Glauber methods mean. In fact, it is very annoying to read that 'quantum Glauber calculations' give a better fit to the data than 'classical Glauber calculations' As if there were any choice. 3- The authors mystify some physics interpretation without need. Because one cannot explain some results with the use of a better theory, it does not disqualify the better theory. It is against the scientific method to diminish the value of the quantum theory because it does not explain a particular data. One needs to be humble also as an experimentalist and accept that experimental analysis can also be wrong. The existence of shadowing processes is well known in nuclear scattering theory. The authors fail to explain why one should use a theory without shadowing effects and give a solid explanation of why it does not work in their case. Here an experiment cannot beat a solid knowledge base. 4- On page 4 and elsewhere the authors write: 'A unitarization process is employed to account for the possibility of multiple photons exciting a single nucleus.' How would the reader ever understand what this mean? Please explain it, because this is an important part of the errors in the experimental analysis. 5- The authors lack experience on the literature. As one example, their reference [18] would be much better represented by Aumann, Bertulani and Suemmerer, Phys. Rev. C 51, 416 (1986) where the issue of multiple neutron emission in relativistic EM excitation is discussed. They should read it carefully, as well as previous methods describing multiple excitation of giant resonances, as e.g., Bertulani and Ponomarev, Phys. Reports 321, 139 (1999). 6- I think the authors try to overemphasize the role of the code STARLIGHT as a guide for their experimental analysis. As far as my experience tells, this code suffers from major physics inconsistencies and should not be taken seriously. The authors tend to question other models that do not reproduce the data, when little is questioned about their preferred code, mentioned above. In my opinion, this code is a disfavor to physics progress in this field. The experimental efficiencies and data are already being poisoned by the use of the code as a simulation tool. The poison adds a lot to the systematic errors. 7- One of the main findings in this work is that there are diffractive minima (see figure 8) in dsigma/dt. Evidently, there is nothing new here. Quantum mechanics works. I did not look carefully at all the numbers quoted in the article and several phenomenological fits. There are many. They could be right, but I have the impression that the article was heavily edited by mostly only one of the authors. It made the article very biased in the sense that the author has made many typical handwaving assertions. But he/she took the numbers seriously at the 3 significant figure level. I find this troubling. I would not recommend this work for publication without some heavy editing, with in style as well as in its physics content. Reviewer #2: I have read with interest the manuscript entitled: "Coherent diffractive photoproduction of $\rho^0$ mesons on gold nuclei at RHIC" by L. Adamczyk et al. (STAR collaboration). The manuscript presents results of the STAR collaboration for production of $\pi^+ \pi^-$ pairs (mostly $\rho^0$ meson) in UPC of Au+Au at RHIC. The collaboration presents not only $d \sigma / dM_{\pi \pi}$ but also $d \sigma/dt$. Two minima in very interesting $t$ distribution are clearly visible. In addition, a fit including $\rho^0$, $\omega$ and S\"oding contributions is presented. These are all photoproduction mechanisms related to rescattering of photon fluctuations in the second nucleus. In addition to these mechanisms there are also $\gamma \gamma \to \pi^+ \pi^-$ mechanisms not mentioned here, see e.g. \item M. K{\l}usek-Gawenda and A. Szczurek, ``Exclusive production of large invariant mass pion pairs in ultrarelativistic heavy ion collisions'', Phys. Lett. {\bf B700} (2011) 322 . \item M. K{\l}usek-Gawenda and A. Szczurek, ``$\pi^+ \pi^-$ and $\pi^0 \pi^0$ pair production in photon-photon and in ultraperipheral ultrarelativistic heavy ion collisions'', Phys. Rev. {\bf C87} (2013) 054908. The paper is very well written. The authors show results for different numbers of forward/backward neutrons. Interesting discussion related to $t$-distributions are given. A comparison to similar ALICE results is made. The paper presents high quality experimental results. I recommend the paper to be published in Phys. Lett. B. 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