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From [email protected] Sat Jun 28 10:33:22 2003
Date: Wed, 18 Jun 2003 19:06:53 UT
From: Physical Review C <[email protected]>
To: [email protected]
Subject: Your manuscript CD8947 Back
Re: CD8947
Production of $phi$ mesons in Au+Au collisions at 11.7A GeV/c
by B.B. Back, R.R. Betts, J. Chang, W.C. Chang, C.Y. Chi, et al.
Dr. Wen-Chen Chang
Inst. of Physics
Academia Sinica
Taipei 11529
TAIWAN
Dear Dr. Chang,
The above manuscript has been reviewed by one of our referees.
Comments from the report are enclosed for your consideration.
When you resubmit your manuscript, please include a summary of the
changes made, and a brief response to all recommendations or
criticisms. We regret the delay in obtaining this report.
Yours sincerely,
Debbie Brodbar
Assistant Editor
Physical Review C
Email: [email protected]
Fax: 631-591-4141
http://prc.aps.org/
Figure problems with manuscript:
In reviewing the figures of your paper, we noted that the following
changes would be needed in order for your figures to conform to the style
of the Physical Review. Minor changes may be made by cutting and pasting.
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) [3]:
Change bracketing sequence in the axis label to follow the standard:
{ [ ( ) ] }.
Reply: The y axis label of Fig. 4 has been changed accordingly.
Figure(s) [9-13]:
Complete radical roof on factor s (preferred style).
Reply: Add radical roof for $\sqrt{s}$ as the x axis label of Fig.10-14.
Figure(s) [2]:
Insert appropriate labels in figure captions: '(Color online)'
Reply: (Color online) is added at beginning of the figure caption.
---------------------------------------------------------------------------Report of the Referee -- CD8947/Back
----------------------------------------------------------------------------
Referee's report on MS CD8947, Back. et.al., "Production of \phi Mesons
in Au+Au Collisions at 11.7A GeV/c"
--------------------------------------------------------------Executive summary: The paper describes measurement of phi meson
production through the K+K- decay channel in A+A collisions at the AGS.
The technique seems reasonable, modulo a few questions as mentioned
below. The spectra are characterized in a reasonably complete and
unbiased fashion and various fit parameters are tabulated. The
interpretation of the results is minimal; only a few empirical trends
are noted, but no conclusions as to the phi production mechanism, or its
significance, are drawn (with one mild exception). The paper is quite
clean as simply a presentation of the data, and has sufficient archival
value, even in the absence of physics conclusions.
Recommendation: I would recommend revisions. I would encourage the
authors to consider adding more and deeper interpretation of their
results, which would make for a more valuable and interesting
publication.
Reply – We have expanded the discussion and possible interpretation of our results
with a more comprehensive introduction of theoretical models/scenarios.
Specific comments: These are divided into General, Introduction (Sec.
I), Method (ie Sec.s II and III), Discussion (Sec. IV), and
Minor/Typography. In the location notation, "P i Par j" indicates the
j^th full or partial paragrph on page i of the review copy. The list is
long, but after General the comments are nearly all basically minor and
can be addressed quite straightforwardly.
General
------------------As a systematic presentation of phi production results from this
colliding system & energy the paper is generally quite successful. It
is useful that the discussion is relatively "un-biased", by which I mean
that the analysis is not focussed on proving or disproving any one
particular model or scenario; what is shown here is sufficiently general
and broad that it will certainly be useful for future comparisons to
theories not yet imagined.
However, this does not mean it is a virtue to avoid
all discussion of which models/scenarios/pictures are now favored or
disfavored as a result of these results -- which are certainly a great
experimental achievement -- and the current draft offers very little
along these lines.
There is some discussion on P17, picking up from an intro
at the bottom of P13, to the effect that re-scattering models, which
have been widely embraced for explaining strangeness enhancement, would
generally predict an increasing phi/K ratio with centrality; and that
this is not seen here, bringing these widely-used models into question.
This is a very good point! to be sure; but we could use more of these.
These few sentences on P17 are almost the only physics deductions in the
entire paper.
Generally it would be a service to the reader to go into
more detail on: what are the general classes of phi production
mechanisms that have been imagined? a few are mentioned, but only
briefly and not in a systematic way. The constant phi/K ratio is an
important result; but what do we learn from the phi/pi ratio? Is it
consistent with some kind of thermal or re-combination production? What
about the apparent <N_coll> scaling? does that confirm or support the
strange-quark-sea picture for strangeness production mentioned on P14?
Etc., etc. The reader is left with almost no guidance as to how these
data might affect our view of nuclear collisions.
I will mention as a separate point one specific lack
of interpretation that seems particularly striking. The authors have
made the very interesting pair of observations that (i) the total phi
production scales proprotionately to <N_coll> both vs centrality (Fig 8)
and beam energy (Fig 9 top); while at the same time (ii) the total phi
production per binary collision is _lower_ in nuclear collisions than in
p+p collisions (Fig 9 bottom). Typically, strange hadron production is
enhanced in A+A collisions relative to p+p collisions, as defined by a
broad range of measures; but here the authors have found one measure by
which strangeness seems to be _suppressed_ in A+A relative to p+p, very
loosely speaking. What can this be telling us? On general grounds this
seems like a noteworthy result! and yet the authors have not highlighted
it at all here (it's not even mentioned in the conclusion).
Reply: We have added more strong and clear statements and interpretation of our
results in the section of discussion following the suggestions of the referee. For
example, the point of “the total phi production per binary collision is _lower_ in
nuclear collisions than in p+p collisions ”has been emphasized and is interpreted
as hints of phi absorption by nucleons in A+A collisions.
Introduction
------------------P3 Par 2: "...E859 at the AGS. The ratio of the total phi yield...to be
10%." Was this for min-bias Si+Au? or for some specific centrality class?
Reply: “for the upper most 7% of the charged particle multiplicity distribution”
is added.
P3 Par 2: What are the beam energies for the collisions measured by
NA49?
Reply: We add “The beam energy ($E_{beam}$) was 158 AGeV and the rapidity range of
measurement $3 < y < 3.8$.”
P3 Par 2: For the trend quoted from NA50, what kind of collisions were
these? presumably Pb+Pb?
Reply: We add “, in \coll{Pb}{Pb} collisions within $0 < y < 1$ and $1.5 < m_t <
3.2 ~\GeVcc\$.”
Method
------------------P4 Par 3: It would be useful to be reminded at the start of this
paragraph what sqrt{s_NN} correspond to these beam energies.
Reply: We specify the corresponding values of sqrt{s_NN} instead of beam kinetic
energies.
P4 Par 3: The typical momentum resolution is quoted as 1%; is this
really true independent of momentum? How bad does it get for the
momentum range relevant here?
Reply: We provide more details on our momentum resolution. “The momentum ($p$)
resolution of the spectrometer, $\delta p/p$, was about 1\% for particles whose
momenta are greater than $1$ GeV/c and rises up to 2\% at $p=0.6$ GeV/c due to the
effect of multiple-scattering. The momentum distribution of kaons in the \Pphi\
reconstruction mostly lies in the range of $1.0 < p < 2.0$ GeV/c.”
P4 Par 4: The inefficiency of the LVL-2 trigger is quoted as less than
1% on average. "Average" over what, exactly? time? momentum? Also, a
trigger efficiency of over 99% is sort of hard to believe, depending on
how it is defined; for instance, were there no non-working elements in
any of the arrays? or is this figure only for particles in some kind of
nominal acceptance? Generally it would be good to insert reference here
to something which describes the LVL-2 trigger in detail, even if it
repeats one of the references in the previous paragraph.
Even with a reference, though, it would be good to
include a sentence or two here describing schematically how the LVL-2
trigger makes its decision; does it involve the TOF system, for
instance?
Reply: We insert a brief description of LVL2 trigger and cite a reference. “The
hardware LVL2 trigger looped over the combinations of drift chamber and TOF hits,
and formed combinations consistent with tracks corresponding to particles of various
momenta, charge, and particle type using a look-up table~\cite{LVL2}. In Au+Au
collisions with a field of 4KG and the spectrometers at $14^\circ$ and $19^\circ$,
typical rejection factors for a $K^+/K^-/\bar{p}$ trigger were $4.8$ and $8.5$,
respectively. Examining data that was taken with LVL1 trigger and recording the
decision of LVL2, monitored the inefficiency of LVL2 trigger, which was found to be
less than 1\% for events fully inside the acceptance. Most of the LVL2 triggered data
was background since the trigger was optimized to reject only events that were clearly
not of the correct type to keep the efficiency near 100\%.”
P5 Par 1: The text implies that there is a sudden jump at 1.75 GeV/c for
pi-K separation, that below that most kaons are included but above that
only a tiny fraction of kaons (outside 3 sigma one side is about 0.1%)
are used. Is this the impression the authors want to give?
Also, is there an estimate for the amount of
pion contamination in the kaon signal? Generally, since only genuine
kaons will contribute to the phi invariant mass peak, stringent PID is
not strictly necessary; typically the K-pi separation would be optimized
for overall S^2/B in the phi peak itself.
How the K PID was designed could be better explained.
Reply: Kaons are accepted if they lie in the regions of 3-sigma kaon band but outsides
of 3-sigma pion bands. The region is asymmetric around the expected TOF of kaons.
The accepted region only becomes small at large momentum around 2 GeV/c. We rewrite
the description of PID part as follows:
“A time-of-flight (TOF) system with a typical resolution of 130~ps served to identify
pions and kaons up to a momentum of 1.75~\GeVc\ with 3 standard deviations of TOF
resolution. Above this momentum, the $3\sigma$ contours in the ($p$, TOF) plane began
to overlap. Within this region, particles were identified as kaons only if their TOF
was inside the kaon region and outside of the pion region, since 10--30\% of the tracks
in the overlap region were from pions. The kaon identification inefficiency from the
exclusion of pion bands in this region of high momenta was corrected by extra
weighting.”
P5 Par 2: "The data were weighted to correct...." Are these pair data
that are being weighted? or single-track data?
It is a bit odd to see corrections for spectrometer
acceptance and momentum cuts described as "losses"; just a question of
terminology.
"The value of the overall weight..." Is this to
imply that the final correction was done via simulation including all of
the above-mentioned effects at the same time? Were the effects ever
studied individually?
Reply:
a. “losses” -> “experimental inefficiency”.
b. The effects were studied individually and the weights of compensation were applied
individually on a track-by-track basis except for those of two-track opening angle
cut and branching ratio.
P5 Par 3: The use of the E_Zcal to measure N_pp is a nice, well-defined
measurement that takes good advantage of the fixed-target setup. But,
how good is it? is there some nominal resolution on N_pp? at the most
central or most peripheral ends? These are sort of implied in the
errors shown in Table 1, though they will have other sources of
uncertainty as well.
The E_Zcal technique has been in use in E917 and
in its the pre-cursor experiments for some time; is there a reference
which describes its performance in detail?
Reply: The nominal resolution of ZCAL was sigma(E)/sqrt(E)=0.73 GeV^(1/2). This will
give the nominal resolution on N_pp to be about 1 at the most central and 3 at the
most peripheral ends. However, there are other sources of uncertainty from the ZCAL
calibration.
We were not clear in pointing out that the N_pp used in the centrally index for
our analysis are not the N_pp(ZCAL) calculated from the relationship with E(ZCAL)
but N_pp(Glauber) obtained by Glauber-model calculation with centrality cuts. The
reason for this is because of an equal basis of comparison on the study of the scaling
behavior and the comparison with other experiments. We have updated table 1 and make
this point clear in the text.
In Table 1 caption: since the actual E_ZCal energies are mentioned, it
would be good to remind us of E_Beam.
Reply: We add “The total beam kinetic energy in the collision is about 2132 GeV.”
in the caption of table 1.
P7: Is the RBW(m) function normalized to an integral of 1.0?
Reply: No, we did not put in this normalization factor.
P7 Par 1 & 2 and Fig 2: The fitting procedure used is not clear. The
text describes a five-parameter fit to the full (same-event) invariant
mass distribution, with the "b" parameter multiplying the background
shape, and the explanation through the end of the paragraph containing
Eq.'s 1-5 seems clear.
However, the figure caption seems to state that the
background-subtracted distribution is fitted, since it refers to "the
phi distribution," not otherwise defined, right after mentioning the
subtracted distribution.
Also, it is never described explicitly how the background-subtracted
distribution is constructed. (The reader might presume that the “b BG(m)”
distribution is subtracted, after b is fixed from the fit; but this is not the only
possibility.) Is the background-subtracted distribution, as shown in Fig 2, ever
actually used for anything? or is it just for show?
It is worth going into even more detail here, just to be absolutely clear.
Reply: The background-subtracted distribution shown in Fig. 2, is just for
demonstration purpose and not used for any analysis of the yield later. We have
rewritten this section to make this point clear.
P8, more questions on the fits, since they are the heart of the measurement: How good
are the 5-parameter fits? what are the chi-squareds? as a function of m_t and
centrality? Are the best-fit peak positions stable vs m_t? what about the widths?
Do the Monte Carlo studies predict that the experimental mass resolution should change
with m_t? (this would usually be true) -- does the fitted parameter follow the
predicted behavior?
Frankly, even the one fit that is shown explicitly is
a little worriesome: in Fig. 2 it looks as though the fitted function
lies systematically below the data for nearly every mass bin below the
phi peak. This is exactly the sort of problem that usually indicates,
for example, that the mixed-event background distribution is not a
perfect approximation to the combinatorial background. The reader
deserves more assurance that these kinds of problems are under control
here.
Since everything that follows depends on these fits,
and there's plenty of room in a PRC paper, at a _minimum_ the authors
should show the fitted peak positions and widths vs m_t and centrality,
not just once for min-bias, to demonstrate that the measurement is
reasonable.
Even more detail, such as goodness-of-fit measures, would
be entirely appropriate for an archival paper on phi reconstruction.
Reply:
a. The way of obtaining the yield is not by the integrated fitting but the counting
of events in a defined signal window and estimating the signal-to-total ratio.
b. The chi-2/dof is 0.87. We have added this information.
c. The transverse mass spectra were not obtained by the integral fitting but instead
of a counting of events within defined signal window and an estimate of background.
d. We have no good explanation and no extra treatment about the residue of signal
in the lower side of mass peak. However, normally the background count is
over-estimated if there is extra correlation presented in the background
distribution.
P8 Par 1: Is there a reference in which the study of the lambda decay
can be seen in more detail?
Reply: Yes, but unfortunately they are the unpublished Ph.D. thesis. Reference is
inserted.
P9 Par 1: Similar trouble here. The authors have defined the signal
range, and the normalized background distribution. They then extract
S/(S+B), but how is not stated. Presumably the integral of the
same-event pairs is (S+B) and the integral of the background is B; one
way to get S/(S+B) is then to say S/(S+B) = 1 - B/(S+B), but it's not
the only way. What, exactly, was done here?
Along the same lines, what are typical values for S/(S+B)?
How is the uncertainty assigned? Can we see a plot of S/(S+B) vs m_t?
Reply: We have added one new plot showing the S/(S+B) vs m_t in two
spectrometer-angle settings and five centrality bins. The explanation of determining
the yields and errors is made clear.
P9 Par 2-3: This procedure of dividing the m_t bins into two definitely
seems, well, a bit odd. First, the spectra are nearly featureless
plotted agains (m_t - m_0); does it really help to have 0.1 GeV/c^1 bins
each with (roughly) half the statistics of half as many 0.2 GeV/c^2
bins?
Secondly, it is not immediately clear that the overall
procedure doesn't hurt more than it helps. If S/(S+B) is better derived
from the larger m_t bins, this implies that the uncertainty on S/(S+B)
is statistics-limited; otherwise, why not just use S/(S+B) in the small
bins? Deriving S/(S+B) from the large bins is potentially an
improvement _if_ the added information that S/(S+B) is slowly-varying is
incorporated, as the authors have done. But that assumption is not, of
course, perfect and so will inevitably add some error. No quantitative
estimate is offered to justify that this procedure actually yields
better estimates for the small bins than just taking (S+B) - B for each
bin, for example.
The authors describe here two checks intended to verify
the assumption that S/(S+B) is slowly-varying within one
0.2 GeV/c^2 mass bin. Both of these are somewhat problematical: the
differences between the two halves averaging to zero is not necessarily
significant if the bins include ranges both where the quantity is
increasing and decreasing vs m_t; and the behavior of S/(S+B) is
probably quite strong with centrality, so it is not clear that a larger
centrality bin should have the same answer as a smaller one. Also, both
of these conclusions are described only qualitatively.
More to the point, it seems strange that the authors have
avoided, or at least not presented, the most obvious analysis: plot
S/(S+B) versus m_t, and then see what the derivative is within any
particular bin. This, in turn, brings up the obvious question: if
S/(S+B) really is slowly-varying within one large bin, why not fit it
across several bins, or with a smooth function, to get a better S/(S+B)
at each point? As it is written it is very hard to understand why the
author's procedure is in any way optimal, and there is basically _no_
explanation of how the errors on the end product are determined.
On the whole the signal extraction procedure really needs
to be explained in much more detail; especially how the errors on
S/(S+B) and on the final yield counts are determined. What's in the
draft now is really not sufficient for an archival paper.
Reply: We have redone the analysis and determined the mt spectra by using the S/(S+B)
in the same bin size of 0.2 GeV/c^2. The way of determining errors of S/(S+B) and
of the final yield counts are described in more clear way.
P9 Par 3: It is not clear why the uncertainties in the determination of
centrality should propagate any uncertainty into the differential yield
per event. After all, as long as you define a class of events you can
count the number of events and the number of pairs, and so determine
1/N_event dN/d(something). Am I just being thick here? what might I be
missing?
Reply: Due to the radiation damage of the plastic scintillator materials of ZCAL and
the resulted decrease in signal, a run-dependence calibration of ZCAL was necessary.
We did the calibrations in a few separate periods of runtime and estimated the
systematic uncertainty of run-independent ZCAL cuts in defining a fixed centrality
range. Systematic change of cross section was studied by varying ZCAL cuts within
the uncertainty range.
Discussion
------------------P12 Par 2: Is the phi-nucleon cross section for elastic collisions only?
This line of reasoning is good to bring up, but isn't it true that the
bulk of the particles involved in a hydrodynamic flow will be pions or
resonances? Their cross sections on a phi are (presumably) unknown, but
is there some reason to believe that the phi's interactions with
nucleons will be the dominant factor deciding if it feels the flow?
Reply: Indeed we missed this point that phi-nucleon might not completely determine
how phi mesons feel the transverse flow since the pions and resonances are also
important components of the nuclear fireball at AGS. We have added “The total $\Pphi
\pi$ cross section is similarly speculated to be small, on the order of
1~\mb~\cite{SHOR}. \Pphi's might therefore be expected to decouple rather early from
the nuclear fireball, which is composed of nucleon resonances and pions.”
The two complicating mechanisms mentioned in this
paragraph could certainly both be relevant. However, if these effects
were present, would an exponential m_t spectrum still be expected? Does
the fact that spectrum is so nicely fit with the exponential rule out,
or limit, the presence of these effects?
Reply: The hadronic development of transverse flow induces a Boltzmann like shape
to the transverse spectra for heavier mass particles. Such an effect would show up
at mt values below our acceptance.
Also, it would be useful to the reader to be reminded
of the phi lifetime, or c-tau, to better be able to picture the scenario
of K daughter rescattering.
Reply: The information of phi lifetime (c tau = 45 fm) is provided.
P13 Par 2: There seems to be some confusion between the mean transverse
mass <m_t> and the truncated mean transverse mass <m_t - m_0> or <m_t>
- m_0. Certainly the truncated mean is more comparable to the inverse
slope parameter, and this is what is plotted in Fig. 6. If this is the
relevant quantity, the text should also reflect that.
Reply: Use the truncated mean transverse mass <m_t - m_0> in the text.
P13 Par 2: What is the "Boltzmann (T_B) fit"? The only fit to the m_t
spectrum described here is the straight exponential; if another kind of
fit is going to be reported, its basic functional form should be
described. Is the T_B parameter redundant with the T parameter? if
not, which one is used to extract the <m_t-m_0> shown in Fig. 6? if so,
then the presentation here is only confusing and should either be
expanded or left out completely (having the reference is not enough to
make this relevant for the reader of this paper).
Reply: The Boltzmann function for the fit of transverse spectra is described. The
<mt-m0> of individual particle is determined by either T or T_B depending on which
one to be used in the parameterization of its transverse spectra in the references.
In Fig.6, the fit is done on <m_t-m_0> versus m_0, assuming an exponential transverse
spectra and a linear dependence of T on the mass.
P13 Par 3: The phrase "The mean transverse mass for the phi falls
slightly below the systematic trend" is not really quantitative, and may
be misleading: the value for the phi is perfectly consistent with the
trend to within the stated error, and so cannot be said to differ
significantly. The phi and lambda together might be said to differ, but
not the phi by itself.
It is not clear what "this observation" refers to
exactly: is it the phi being below the trend, or the phi being closer to
the trend than the lambda? The statement which follows is extremely
weak; the paper might be better without this paragraph altogether.
Reply: We have rewritten this paragraph according to the suggestions of referee.
P14 Par 1: It is not clear what "wounded projectile quarks" means; at
least, I think the phrase is not in very common use. Are these defined
in some kind of constituent quark picture, within wounded nucleons? or
are they more like interacted partons? It would be worth the space to
expand on this description in this paper, beyond just the reference.
Reply: In the paragraph "wounded projectile quarks" is changed into “interacting
projectile constituent quarks”. More descriptions are added.
Similarly, does "quark sea" refer exactly to
"sea quarks" as defined in parton/DIS language? If so, there are
measurements of the strange quark sea, and so its significance is not
entirely unknown.
Reply: The“quark sea”is replaced by “intrinsic ssbar component”.
P14 Par 1: The explanation of the quoted E910 results is confusing.
First, N_pp is defined earlier as strictly the number of projectile
participants; in a p+A collision, then, N_pp must either be 1 if the
proton is considered the projectile, or nu if the nucleus is considered
the projectile. So it is not clear what "scaling of p+p by the number
of participants (N_pp)" means.
Of course, the total number of participants in
p+A collisions is just 1+nu, and this may be what the authors mean by
number of participants in the sentence quoted. But then it is also
confusing to say that the yields increase faster than 1+nu but are still
bounded by a linear nu scaling.
Reply: We made a mistake here. The number of projectile nucleons (N_pp) is replaced
by the number of total participants (N_part).
Also, what does "produced per binary collision
involving an incident nucleon" mean? is this different than the usual
definition of binary collisions in a Glauber picture, ie the average
nuclear thickness function times the N+N cross section? or does
"incident nucleon" mean an un-wounded, or not-already-interacted
nucleon? or is it synonymous with "projectile nucleon" as mentioned on
P5?
This whole section needs to be re-written
with greater care, especially for the reader who may not be a
afficianado of wounded nucleon models.
Reply: This paragraph is rewritten according to the comments of the referee.
P16 Par 1: The text states that the ratio of total yields of phi and K
could vary with centrality by up to 50% given what is seen here for the
fiducial yields, whose ratio does not change with centrality.
This is a
good point to make; but how, roughly, is the figure arrived at? What
assumptions does it depend on? It is stated on the next page that the
fiducial yield is only 20% of the total yield, so there would seem to be
plenty of room for variation in the unseen part of the spectrum.
Reply: The possible variation of 50% for the ratio of total yields of phi and K across
centrality is estimated by the 1_sigma error range of a linear fit on the right plot
of the figure. Indeed we did not take into account the fact that only 20% of the total
yield is within our measurement. We have restricted the statement to be valid about
the fiducial yields instead of total yields.
P16 Par 2: "consistent with scaling with <N_coll>" might sound better to
be more explicit: "consistent with scaling proportional to <N_coll>".
Reply: The statement is changed according to the suggestion.
P16 Par 3: Can you remind us: what was the coverage of E859 and NA49
results for phi's? and K's? how much of the total production did they
observe? (Also, just for curiosity: why should E859's coverage be much
different from E917's?)
Reply: The rapidity coverage of E859 and NA49 is provided in the introduction section.
NA49 , being a TPC experiment, had a very broad coverage, approximately 3 units of
rapidity spanning the central region. In the published E859 analysis, the data were
from 14 deg and 5 deg data sets, and the coverage of the measurement was 1.2 < y <
2.0. This is larger than the E917 coverage, which might be said to usefully cover
1.2 < y < 1.6. Fig. 1 makes it clear that when we go to small angle, it is much more
effective in covering rapidity while at large angle, and the coverage of mt is better.
In principle, E917 makes better measurements of the mt distributions (and in fact,
the individual dN/dy points) but has less coverage overall. Also, E859 had the
advantage of measuring the rapidity distribution forward of the peak (y_PCM = 1.25
for Si+Au), while E917 measured backward of the peak (y_PCM = y_NN ~ 1.7 for Au+Au).
Particles at a given mt and angle have much more momentum when forward of the peak,
which translated into an acceptance gain for E859 relative to E917. In addition,
the rapidity distribution itself seems to be significantly broader in Au+Au
collisions than in Si+Au collisions. Again, the fact that the kaon rapidity
distributions are significantly wider in Au+Au suggests this. Finally, even E859 was
able to extrapolate the rapidity distribution and get a value of 0.6 units for sigma_y
(with a coverage of 0.6 units) only when a value for y_0 was assumed (which is less
trivial in the case of Si+Au than it is in the case of Au+Au).
Generally it would be a service to the reader
to have a reminder in the Introduction of the context of existing phi
measurements in nuclear collisions: who measured phi's before? in which
systems? over what rapidity range? (and maybe even what were the
publication dates)
Reply: We have put the related information about these experiments in the section
of introduction.
P16 Par 3: The widths of the K distributions are quoted as 0.9 and 0.7.
Are these RMS's of the dN/dY shapes? or something else? it would be
useful to be exact.
Reply: It is the width of a Gaussion fit of dn/dy. The “width” is changed to be
“Gaussian width”.
P17 Par 2: It is not clear exactly what "the same analysis is performed
using E866 data..." means. Is this an analysis that E866 published? if
so, it would be good to put in an immediate reference. Or is this the
present authors re-examining the E866 data in a new light?
The phrase "a similar scaling of the yields is observed" is
ambiguous, since a number of different trends have been noted so far for
phi's and it is not clear exactly for which one the behavior of K's is
"similar". This could be spelled out more precisely.
Again, "excess kaon production" is unclear; excess over
what expectation?
Reply: We have done the revision of this paragraph according to what referee
suggested.
At the end of the paragraph, it is confusing to read the
phrase "that phi production should scale with N_pp faster than kaon
production". Previously in the paper the phrase "scale with N_pp" has
been taken to mean "scale proportional to N_pp"; but that clearly cannot
be the meaning here, since neither the phis or the kaons scale
proportional to N_pp. Do the authors mean something like "that phi
production should increase with centrality faster than kaon production"?
If so, it would be good to avoid using "with N_pp" as a generic
statement for "with centrality", especially for quantities which are not
seen to be proportional to N_pp itself. (Similar comments may apply
elsewhere in the paper.)
Reply: The statement is changed into“that \Pphi\ production should increase faster
than kaon production with centrality.”
P18 Par 1: The word "facilitate" is odd. What kind of an error is being
made by not accounting for isospin? is there any idea how large this
might be? just citing a reference would be OK.
Reply: We assume (N K+)nn = (N K+)pp. Detailed consideration gives (N K+)NN = 1.24
(N K+)pp. The related references on the consideration of isospin effect are cited.
Fig 11: The fast rise of phi/K- at low root{s_NN} is quite striking;
presumably this is due to the fact that this is near the threshold for
K- production? But, isn't that threshold quite similar to that for phi
production? Is the parameterization used here for pi and K production
(ref 40) reliable at these low beam energies?
Reply: The fast rise is due to the threshold for K- production and the phi/K- ratio
becomes close to 1, as measured by DISTO which is mentioned in our paper.
The lowest sqrt(s) of data used in the parameterization of ref 40 for pi, K+ and Kare 3.0, 2.98 and 5.03 GeV. We believe that the parameterization is still valid for
the comparison at the lowest sqrt(s) in AA data which is 4.87 GeV. We limited the
range of the pp band to be above 4.8 GeV.
Presumably any un-reliability would be shown in the
vertical width of the hatched band for p+p results. But it is hard to appreciate
the vertical width of the hatched band when it is rising
steeply (though I have no good alternate graphic suggestion, except
possibly to consider using a logarithmic scale on the vertical).
Reply: Because the lowest sqrt(s) for pp band has been changed to be 4.8 GeV, the
vertical width is easy to discern now.
Minor/Typography
------------------P3 Par 1: The phrase "assuming a quark-gluon plasma (QGP) scenario"
might be better as simply "in a quark-gluon plasma (QGP) scenario".
Reply: “assuming” -> “in”.
P4 Par 2: The last sentence here, "This work should serve..." is
somewhat non-scientific. It certainly will be a benchmark; and while it
seeme likely to be important, the authors cannot be sure of this at the
present time.
Also, the phrase "extra degree of enhancement" is
unclear; extra compared to what? This sentence should either be
re-worded or removed.
Reply: This sentence is removed.
P4 Par 3: The sentence "Event centrality is characterized..." is in the
present tense, while most of the description of the technique is in the
past tense.
Reply: “Event centrality was characterized…”
P4 Par 4: In the very last sentence the word "uniquely" is redundant and
would better be removed.
Reply: “ … served to identify uniquely pions and kaons”, uniquely
is removed.
P5 Par 2: "the spectrometer acceptance" might be better without the word
"the"; same with "single-particle tracking inefficiency".
Reply: "the spectrometer acceptance" -> "spectrometer acceptance"; "the
single-particle tracking inefficiency" -> "single-particle tracking inefficiency".
P6 Par 1: The basic variable m_t is defined here as sqrt{m^2 + p_t^2}.
Throughout the paper, whenever the authors have quoted a value of m_t
they have been very careful to specify mass units GeV/c^2, spelling out
the factors of c explicitly. In the equations, though, the factors of c
have been left implicit.
For consistency, then, it would be better to spell them out there explicitly
as well; for instance, m_t would then be defined as sqrt{m^2 + p_t^2/c^2}. A similar
comment would apply, for example, to Equations 6, 7 and 8 which mix energy and mass
quantities.
Reply: m_t = sqrt{m^2 + p_t^2/c^2}. And m -> mc^2.
Figure 3: The squares and circles are hard to distinguish from one
other; could triangles be used?
Reply: square -> triangle.
Figures 9--12, incl. caption of Fig 9: Technically the heavy-ion
collision data are plotted against sqrt{s_NN}, not sqrt{s} as indicated.
The fact that one stands for the other is fairly clear from reading the
nearby text; but it is always good to have the figures and captions be
completely correct on their own.
Reply: sqrt{s} -> sqrt{s_NN} for the figures 9-12, incl. Caption of Fig. 9.