Re: LU8319 Rapidity dependence of charged antiparticle-to-particle ratios in Au+Au collisions at $sqrt s N N = 200$ GeV by I.G. Bearden, D. Beavis, C. Besliu, Y. Blyakhman, et al. Dr. J.J. Gaardhoje Niels Bohr Institute Blegdamsvej 17 DK-2100 Copenhagen, DENMARK Dear Dr. Gaardhoje: The above manuscript has been reviewed by our referees. We ask you to consider the enclosed comments from the reports. While we cannot make a definite commitment, the probable course of action if you choose to resubmit is indicated below. ( ) Acceptance, if the editors can judge that all or most of the criticism has been met. (X) Return to the previous referee A for review if available. ( ) Submittal to new referee(s) for review. Please accompany any resubmittal by a summary of the changes made, and a brief response to all recommendations and criticisms. Yours sincerely, Jerome Malenfant Senior Assistant Editor Physical Review Letters Email: prl@aps.org Fax: 631-591-4141 http://prl.aps.org/ ---------------------------------------------------------------------------- Report of Referee A -- LU8319/Bearden ---------------------------------------------------------------------------- Whether this is suitable for PRL is a tough call. On the one (negative) hand, the y=0 measurements are confirmations of earlier measurements by PHOBOS (nucl-ex/0206012), submitted to PRC a month earlier. Also, the rapidity dependence for p-bar/p was already established at 130 GeV in a PRL, and an abrupt change wasn't really expected when moving to 200 GeV. The trends of all p_t, centrality, and rapidity dependences are expected. On the other (positive) hand, the high-quality, finely-binned rapidity dependence for p-bar/p and K-/K+ (Fig. 3) presented in this paper offers a formidable challenge to modelists investigating things away from midrapidity (this should be pointed out by you!). And the empirical relationship between mu_S and mu_B (Fig. 4), based on experiment, is a nice and concise result. Both of these figures were widely admired and discussed at Quark Matter 2002 conference two weeks ago, and provide the paper with a solid foundation. However, in its current form, the paper is being stretched too thin. For example, the 3rd and 4th paragraphs are redundant and should be removed, since the combination is a near-exact copy of the discussion on Figure 3 later in the text (paragraph beginning with "Figure 3 shows..."). Also, the observations of midrapidity boost-invariance and low net-baryon density (low mu_B) are stated several times in the paper. Even the detector discussion is probably more detailed than it needs to be. Therefore, I think that the paper in its current form needs more substance to warrant acceptance in PRL. In my opinion, some substance can be added near the end of the paper in the discussion on chemical potentials and the relationship between the ratios, which I'll explain here. The paragraph which starts with "Surprisingly..." needs the most revising. First, the "surprisingly" isn't really necessary; a monotonic relationship between mu_B and mu_S is expected from statistical models employing both baryon number and strangeness conservation (in fact you point this out yourself in the paragraph's 6th sentence). The fact that K-/K+ is not equal to 1 at these energies is due entirely to the non-zero mu_B. Put another way, even if many/most hadrons are produced as antiparticle-particle pairs, *any* stopping in the collision means that the p_bar/p ratio must be less than 1 or baryon number is not conserved. Then, since mu_b is nonzero, the existence of the other conservation laws (strangeness, charmness, isospin) leads directly to other finite chemical potentials and non-unity ratios. For example, a finite mu_B must lead to non-unity anti-Lambda/Lambda (etc.), which needs a non-unity K-/K+ to balance the resulting net strangeness (strangeness is still conserved even though mu_S is nonzero). Of course this is very simplified, but the point is that it's not a surprise that mu_s and mu_q are related. They *have* to be. See papers by Redlich, Becattini, Magestro, Rafelski, etc. I write all of this because I think the discussion in the pre-summary paragraph can be developed much further: * Why is 1/3 the expected value (not straightforward to everyone)? * Why might your result fit to a power law with exponent = 1/4 instead of the expected 1/3? * What role do strange baryons play in the relationship between proton and kaon ratios (hint hint)? * Is there a quark-counting reference or scenario which can be developed here? I understand that this is an experimental paper and you may not have wanted to use lots of space on this type of discussion, but it is useful to explain the result and to conjecture about the source of the 1/4 in Fig. 4. It would also be helpful to elaborate on what it means to assign mu_B to points away from midrapidity... is the system in chemical equilibrium at these places? Either way, I feel the paper in its current form needs revision and extension in order to be accepted as a Letter. I hope my comments are helpful to improving the content of the paper. Other points needing attention: ------------------------------ The introductory paragraph summarizes qualitatively 1/2 the results of the paper... a more general introductory paragraph would be better, saving the discussion of rapidity trends for a new paragraph with more development. Paragraphs 3 & 4 should be removed (explained above). In paragraph beginning "The data presented here...", are we to assume that the same number of events was recorded for the two magnetic field directions for each spectrometer configuration? In PHOBOS' recent paper there is a discussion of weighted averages when determining ratios based on changing field orientation that seems just as relevant here. The feeddown discussion (paragraph begins with "The ratios shown in Fig. 2...") is incomplete. In addition to STAR, PHENIX has measured /\'s systematically but is not cited. Also, the STAR paper referenced there does not contain lambda/proton~0.5... where does this come from? You give initials for Grand Canonical Ensemble (GCE) but never refer to it again. Figure 2's caption contains "antihadron/hadron" and Figure 3 contains "Antiparticle-to-particle"... please choose one and stick to it. In the summary paragraph, just call them charged kaons, not "charged singly-strange mesons". ---------------------------------------------------------------------------- Report of Referee B -- LU8319/Bearden ---------------------------------------------------------------------------- It is the opinion of the referee that the results on particle/anti-particle ratios obtained in Au-Au collisions at 200 A/GeV are of general interest as they provide more complete understanding of the dynamics of particle production in ultrarelativistic heavy ion collisions. In addition the scaling between K^+/K^- and \bar p/p ratios presented in this paper is very interesting. Thus, I consider that the paper contains a sufficient amount of new results that can be published in Phys. Rev. Lett. However, before publication I would suggest to make the following minor corrections: 1. The authors should define \mu_s as the strange-quark chemical potential 2. After the sentence on p 8 "..a value of 1/3." I would recommend to give a reference to P. Koch et al. Phys. Rep. 142 (1986) 167 and to J.Cleymans et al. Z. Phys. C57 (1993) 135. 3. The authors should indicate that the relation \mu_s=1/4x\mu_q is only approximate and is valid only if neglecting contributions of strange and multistrange baryonic resonances to K^+, p and their antiparticles. After the above changes have been made I would recommend publication of the above manuscript without further contact with this referee.