FYI Michael -----Original Message----- From: prc_at_aps.org [mailto:prc_at_aps.org] Sent: Fri 8/6/2010 12:48 PM To: Murray, Michael J Subject: Your_manuscript CS10219 Arsene Re: CS10219 Rapidity dependence of deuteron production in Au+Au collisions at sqrt s NN=200 GeV by I. Arsene, I. G. Bearden, D. Beavis, et al. Dear Dr. Murray, The above manuscript has been reviewed by one of our referees. Comments from the report appear below. These comments suggest that specific revisions of your manuscript are in order. When you resubmit your manuscript, please include a summary of the changes made and a succinct response to all recommendations or criticisms contained in the report. Yours sincerely, Bradley Rubin Senior Assistant Editor Physical Review C Email: prc_at_ridge.aps.org Fax: 631-591-4141 http://prc.aps.org/ Physics - spotlighting exceptional research: http://physics.aps.org/ PROBLEMS WITH MANUSCRIPT: In reviewing the figures of your paper, we note that the following changes would be needed in order for your figures to conform to the style of the Physical Review. Please check all figures for the following problems and make appropriate changes in the text of the paper itself wherever needed for consistency. Figure(s) [4] Please rearrange power of 10 in axis label for clarity: Either (i) place the power of 10 as a factor, without parentheses, in front of the axis label quantity, changing the sign of the power as needed; or (ii) incorporate the power of 10 in the topmost or rightmost number on the scale. Please refer to the URL http://forms.aps.org/author/h18graphaxislbls.pdf for a pictorial representation of the preferred forms for axis labels. 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However, any manual intervention carries the risk of inadvertently introducing mistakes. ---------------------------------------------------------------------- Report of the Referee -- CS10219/Arsene ---------------------------------------------------------------------- Executive Summary: ----------------------------------------- The paper is a straightforward, almost minimalist, presentation of a data set on proton and deuteron production in central Au+Au RHIC collisions over a wide range of rapidity, and antiproton and antideuteron production over a smaller range in rapidity. The data are interpreted in terms of a standard coalescence picture and an extracted phase space density; some trends are noted but no definite physics conclusions are drawn. As a simple data presentation exercise the paper is generally acceptable, though the physics impact of the data are significantly limited by being restricted to only central collisions in only one bin of centrality. Though I would not suggest it as a requirement for publication in the Physical Review, I would urge the authors to consider enlarging the paper to include data from a greater range of centrality classes -- based on the error bars shown in Fig 5 it certainly looks as though sufficient statistics would be available. Modulo that decision, the paper has a number of minor errors in the physics introduction which should be addressed, as detailed below. My basic recommendation, then, is that the paper will be suitable for publication with minor corrections. Concerns: -------------------------------------- (1) Centrality selection. The data presented in Fig 3 are for some particular event selection; but which? One needs to scan the paper in some detail (or use a computer text search) to find the lone sentence "We present ... AuAu collisions with a centrality range of 0-20%." in the first paragraph under Section II. Analysis. (Note that this sentence itself is not quite grammatically correct.) Physics: Why was this one, and only one, centrality range chosen for this analysis? There is no motivation mentioned in the paper at all, which is quite puzzling. The paper describes interpretation in terms of geometrical quantities such as the coherence volume implied by the B_2 measurement; it is only natural to ask, then, how these might change as the collision geometry/centrality is changed. To present data from only one centrality selection, with no explanation, seems quite odd and un-natural, and I would call it a glaring defect. There is of course a limit on available particle statistics, but it is far from clear (partly because there is so little detail provided on the error analysis) that no statement could be made for any other selections, even by breaking the current one into two. Analysis: The section on the data analysis should include a description of how the events were selected on centrality. Formatting: Assuming that the paper is to be published on just the central collision results, that fact should be made clear throughout the work: it should appear in the title, in the abstract, in the summary and in the captions of all the figures and tables. And, for that matter, the main data graphs such as Fig 3 should also state that these results are for the Au+Au collision system! (2) Error analysis. The presentation in Table I does a good basic job of explaining the uncertainties: statistical errors are in the table, systematics are characterized in the text. All of the figures and tables should rise to this same standard: what is shown by the errors that are displayed? and what are the sizes of the other sources? Elsewhere the treatment of uncertainties is uneven. The discussion of errors from the feed-down correction in Section II.D is quite quantitative; while the preceding section II.C on particle identification describes inefficiencies and contaminations, but does not quote anything quantitative for the residual uncertainties from these effects. At a minimum the reader should be able to appreciate the relative contributions of statistical versus systematic uncertainties, and what the dominant source is for the systematic, for every quantity quoted and plotted in the paper. (3) Abstract: "in contrast to lower energy data". This should be re-worded to make clear that it refers to data from collisions at lower collision energies, rather than lower secondary particle energies. (4) p1 col2: "surrounding ... medium ensure energy and momentum conservation". "Ensure" would be better as "allow" or "permit" or "enable". Conservation laws always "ensure" that they are followed; the role of the medium is to "allow" p + n -> d to proceed without the need to emit a photon. (5) p1 col2 "As deuterons are formed inside the expanding system" Is this really true? How do you know? One could make the simple argument that particle within the medium are colliding often enough, on the order of once per ~1-10 fm/c or faster in the time before freezeout, then the constituent protons and neutrons will on average be off their mass shell by ~200-20 MeV at any given moment; if this is true, then how can one even distinguish a deuteron bound state, whose binding energy is only 2 MeV? The bound/unbound distinction doesn't apply over such short time scales. The same misconception appears later in the same paragraph with the phrase "deuterons are most likely formed very near freeze-out". What, exactly, does "formed" mean here? In order for a p,n pair to be meaningfully distinguished as either bound or unbound, the degree to which they are off-mass-shell must be no larger than the bound-state binding energy. This implies that bound-state deuterons cannot even be _defined_ until ~100 fm/c have passed since the particles' last momentum transfer interaction, ie freezeout; this time frame cannot be described as "very near freezeout." It really makes no sense to say that deuterons are "formed" on a timescale faster than they can even be distinguished or defined, whether inside the colliding system or following freezeout, so this whole passage of the introduction is really misconceived. The reason that sudden-approximation coalescence models can work is more subtle, quantum mechanically. After the last momentum exchanges, ie freezeout, all the protons and neutrons will be in wavefunctions which span a range of masses around their free-particle on-shell rest mass. The off-shell combinations of momentum and energy will decay away with time, ie the amplitude of those parts of the wavefunctions will diminish and only the on-shell states will have significant amplitudes. So, looking into the future at the time of freezeout it is reasonable to count only the on-shell states for future accounting purposes; but the off-shell states are still present in the wavefunction at that time. For this reason, one can get reasonable answers from a coalescence model while neglecting these off-shell subtleties. But, by the same token, there is no excuse at this point in the field for leaving a sloppy definition of "formed" in the introduction to a paper like this, and the section should be rewritten without this misconception. (6) p1 col2: "Coalescence models assume that the distribution of clusters..."; "density" or "phase-space density" would be better than "distribution" (7) p1 col2 and Eq. 1: The references [1-3] quoted to introduce the notation of the coalescence picture in Eq. 1 are incomplete; they date from the Bevelac era (or earlier), when the notation C_2 was used for the proportionality similar to that in Eq. 1 but with cross sections rather than per-event densities. The B_2 notation used here was originated during the AGS fixed-target heavy-ion program, starting with E858 and then E864 and E878; and it is a rather glaring omission that not all of these experiments are referenced in this paper, which should be corrected. (8) p1 col2: Mention is made of the n/p ratio in lower-energy collision data, but the meaning and significance are not clear at all. What system was this for? Does this n/p ratio correspond to that of the incoming nuclei, or not? Is the implication that Eq. 1 should be modified, or assigned a 20% systematic error? If not, why not? The bare inclusion of this observation, without any details, explanation or implication is simply confusing and not helpful to the reader. (9) p2 col1: "B_2 carries information about the cluster" What do you mean by "cluster"? Is it the deuteron itself, as implied on the previous page with the phrase "the distribution of clusters"? Or does "cluster" mean the system as a whole? Neither really makes sense; B_2 doesn't really tell you anything about the deuteron itself per se; and it's strange, as well as inconsistent with the previous page, to refer to refer to the whole system as a "cluster". (10) p2 col1: "B_2 ... is consistent with measurements of the deuteron wave-function." This is a very strange and jarring statement to read at this point in the paper, since there has been no discussion up to this point on how the deuteron's spatial properties figure into the value of B_2 through the coalescence process or otherwise. (11) Also, this statement is referenced to Ref 5, but only vague mention is made of what collision systems or what collision energies are being referred to; this makes it somewhat strange to read in the very next sentence that >= 4.9 GeV is the threshold of "high energy". It would be clearer to state the energy ranges, and at least whether heavy or light nuclei are involved, for all the data being referred to (this remark applies in several other places throughout the current paper, as already mentioned in point 8 above). (12) p2 col1: "assuming the region where the coalescence occurs has also a Gaussian shape" See point 5 above; the region which sources/radiates the nucleons is not the same thing as the region where coalescence occurs. Note also that "spatial profile" would be better than "shape" in this sentence. (13) p2 col1: "this ansatz ... facilitates comparison to interferometry radii". A traditional point; but, how does the comparison work in this case? First, should the R_G from the coalescence framework analysis be compared directly to any of the R parameters from HBT analysis? or is there a factor of 2, or pi, etc between them? Is this paper going to actually make the comparison? if not, then you should provide a reference to how the comparison should be done. (14) p2 col1: "However, it has been suggested that..." This certainly needs a reference, or several, at this point. (15) p2 col1: The middle paragraph discusses the results of deuteron production following a quark coalescence picture, which is a subject of considerable current interest. However, the logic is not laid out clearly here. Isn't it true, for example, that if protons follow the quark coalescence picture and deuterons follow the nucleon coalescence picture, then deuterons automatically/necessarily follow quark coalescence as well? ie isn't quark coalescence for deuterons redundant with nucleon coalescence? and so not really an independent piece of information. Alternatively, is the statement that deuterons follow quark coalescence equivalent to B_2 being constant with pT? But here B_2 is not constant with pT, as we see in Fig 4; doesn't that have immediate implications for quark coalescence interpretation of the present data? In general this section should be written so as to make it clear to the reader what are, and are not, redundant versus independent pieces of information. (16) p2 col1: The start of the discussion of discussion of phase-space densities in the last paragraph should have at least a few references right at the beginning, especially as to the motivations. Why is this an interesting quantity? The text mentions (i) an indicator or measure of the degree of equilibrium, which is directly connected to entropy, and (ii) "information about ... symmetrization efects". Reasonable enough; but what's the upshot? What do the results shown in Fig 5 demonstrate? Trends are noted in Section III, but what about the basic magnitude of the measured quantity? is it high, is it low? (17) p2 col2: The result of Eq. 5 is correct only for a true global equilibrium, with one universal temperature and where the particles have equal access to the entire relevant volume. The text acknowledges this, but only in a roundabout way with the later remark "we are ignoring the collective motion of the particles", as collective motion is a departure from true global equilibrium (also, "neglecting the possibility of" is more accurate than "ignoring" here). But since collective motion probably is the case in RHIC Au+Au collisions, from the B_2 results shown here as well as a host of other evidence, it is left unclear to what extent Eq.'s 5 - 9 should still be considered relevant. For example, in the statement farther down, attributed to Ref 23, that strong longitudinal flow could significantly reduce pion phase space densities, it is not clear if this is a true result about the actual density or an artifact of Eq. 5 and its corollaries being invalid in the extraction of the measured phase-space density. In general the utility of a framework which assumes global equilibrium to a system which probably exhibits only local equilibrium needs to be explained more carefully; if T is a function of space, then is Eq. 5 still true at a point? In the case of collective motion being significant, is Eq. 3 defined over some relevant coherence volume? etc. (18) p2 col2: "the maximum space averaged phase-space density, which is at the center of the Gaussian source." Sorry, but this makes no sense at al: a spatial average is not defined for different points within the space. The confusion over the relevant spatial volumes mentioned in point 17 above is clearly causing serious trouble here. (19) p2 col2: In Eq. 9, is R_G a function of particle momentum, particularly pT? Presumably so, and if so then it would be useful to write the dependence explicitly here. (20) Minor formatting: the text following Eq. 5 should probably be left-justified rather than indented. It is not clear whether the text following Eq.'s 4 and 6 should or should not start new paragraphs, either. (21) The presentation of Eq. 6 in terms of chemical potential is formally correct, but at the same time obscure -- in the dilute limit, what is the value of these chemical potentials? Is it just the particle mass, which would make Eq. (6) equivalent to exp[-mT/T] at all rapidities? or does it vary significantly with the net baryon density, which changes considerably with rapidity (as BRAHMS has made clear in other measurements)? (22) p5 col2: "expect the ratio of the proton and antiproton phase densities to be flat"; is this as a function of rapidity? or pT? or both? (23) Summary, but also relevant to several sections of the paper: There is a reasonable interpretation here that the B_2 and phase space density measurements have information about the existence of collective flow, particularly what is called radial flow. This is certainly valuable and worth publishing. However, the current paper doesn't say much about whether these implications are or are not consistent with the great body of work that has now been done on modeling RHIC A+A collisions with hydrodynamics. It is not necessarily the responsibility of this paper to make a full-blown analysis, but the reader deserves some basic orientation: are the results shown here generally consistent with existing hydrodynamical models? or is there some kind of surprise or contradiction brewing? _______________________________________________ Brahms-dev-l mailing list Brahms-dev-l_at_lists.bnl.gov https://lists.bnl.gov/mailman/listinfo/brahms-dev-lReceived on Fri Aug 06 2010 - 17:21:23 EDT
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