From Michal Sumbera [5 Aug 2009]:

Generally I found the article hard to read at some places but this can be
improved if we work on it a bit. My feeling is that this has something to
do with a lot of material and analysis techniques employed. You refer
quite often to your own publications wheer those techniques were either
introduced or further developed. I do not thing that's what the reader of
this (STAR collaboration) paper is expected to do. So I would recommend at
some places to be more definite. Thus for instance in spherical
decomposition part you should explain which l and m values are
permissible, what is the physical meaning of diffrent coefficient and what is the
reason for their non-monotic behavior (e.g. of A_{22}). You can do it in a
sketchy way but the reader must get the feeling he understands what are
you doing.

The paper has been largely rewritten, and hopefully is more clear.  Around Equation (4), we have put some text on what "l" and "m" values are allowed by symmetry, and a little of what they correspond to.  Please see if this improves things.

Now specific comments:

1) section 1, paragraph 1, line 13:  [6,9,10]. Please add reference to one
of the Richard's papers where the term femtoscopy was introduced. Maybe
even better is his QM2005 talk: R. Lednicky, Nucl. Phys. A 774 (2006) 189.

The same for 6 lines below.

Okay, good idea.  It actually appears in the second paragraph, in the present version.

2) section 1, paragraph 2: ... ,in which new phenomenon is not found
replace with: ... ,in which new phenomenon is not expected

Yes, this wording was certainly awkward.  The same message, hopefully in a smoother way, is now in the 4th paragraph.

3) section 1, paragraph 3: ..."apples-to-apples"....
Being not a native perhaps I should not complain too much. But I recommend
not to (mis)use this phrase (idiom) too much in the paper. Perhaps you can
use some other word for it, e.g. fair comparison.

Okay.  By popular demand, the paper is now fruit-free.

4) section 2, eq. (2): Correlation function defined as well as measured is
a function of 2 varibles - either p_1, p_2 or what is obtained via
orthogonal rotaion (e.i. does not change the Jacobian) q = p_1 - p_2 and k
= p_1 + p_2.
So either you change the definition eq.(1) accordigly or mention that in
eq.(2) you have integrated over k. Otherwise this is not
"apples-to-apples"
relation and you denote with the same letter two differnt objects.

This is a bit of a quibble, but okay in principle it is a good point, thanks.  We now explicitly include the subscript for the total pair momentum in Equation (2), to maintain the six-dimensional character of the correlation function, and then say that, following convention, we drop the explicit reference to P from now on.

5) page 2, left column, top: "It has been suggested [19–21] to
construct the correlation..."

I suggest: It has been suggested [19–21] to express the correlation...

Well, the correlation is constructed in those variables.  It may then be expressed in spherical harmonics.  So, we think "construct" is more appropriate than "express" at this point.

6) page 2, left column, just before the subsection A. Femtoscopic
correlations:

"However, it was experimentally observed ..., clearly seen at large q,..."

What do you mean by large? You can say e.g. q >> 1/R, where R is
characteristic dimension of the source.

We have now quantified large as > 400 MeV/c.

7) The next sentence: "Usually this structure is parameterized
... that contributes in addition to the femtoscopic component C_F(q)"

I suggest: Usually this structure is parameterized
... that modifies  in addition to the femtoscopic component C_F(q) at
large q.

Well, these forms contribute everywhere, not just at large q.  They may grow with q, but part of the point is that they affect things everywhere.  We don't think specifying "at large q" is appropriate.  The surrounding wording (near Equation (5)) now is hopefully more smoth.

8) page 2, right column, bottom: "In hadron-hadron or e+e collisions"
-> In hadron-hadron or lepton-lepton collisions

or at least -> "In hadron-hadron or e^+e^- collisions"

Okay thanks.  In the introduction (see line 51), we use the term lepton.  Elsewhere (e.g. line 197), we have changed all to e^+-e^-.

9) page 3 just before eq.(14): " Another form assumes that non-femtoscopic
correlations contribute only to the two higher moments of the spherical
harmonic"

Which values do you mean. In eq.(14) you are using explicit values of the
spherical harmonics. It seems to me that you use "only to the two lowest
moments" or I'm wrong?

Yes, thanks.  It assumes constant contribution to l=2 components only.  The text reflects this now.

10) Eq.(16): please put a blank space between M_3 and M_4

okey-dokey

11) page 3, right column, between eq. (18) and (19):
"where m∗ is the mass of a typical particle in the system (for
our pion-dominated system, m∗ ≈m_{\pi}),"

What about the rho meson? Isn't that a typical particle in the system?

Have you tried to see what comes if you plugg in the rho mass?

Well, since it is the restricted phasespace of the  final state that we are characterizing, pions are really more appropriate, and the rho we treat rather as an enhancement in the pi-pi cross section.  Using a larger mass increases the characteristic "N," so that around the rho mass one would have N~120 or so.

12) page 5, right column, bottom: "that is the energy and momentum
conservation projected on the twoparticle phase-space."

-> that is overall (total) energy and momentum conservation projected on
the twoparticle phase-space.

Yes, we agree that that entire section was awkward, so it has been fully reworded.  The message contained there is now explicitly addressed in the new subsection "Parameter Counting" that starts on line 287.

13) page 8, FIG.9 is obsolete. It jsut contains two pairs of values. Why
don't you mention them in a text?

Well, not sure what you mean by "obsolete," but we agree that there are only two datapoints, so that, especially relative to the other crowded plots, there is lots of white space.  We are open to removing the figure altogether, but see no harm in keeping it as is.  We leave it for now, but if you or others feel strongly, we will remove the figure and just put the numbers in the text.

14) page 11, secon paragraph: "In particular, we observe a significant
positive correlation between the one- and three-dimensional and the
multiplicity of the collision."

-> In particular, we observe a significant positive correlation
between the one- and three-dimensional radii and the multiplicity
of the collision.

D'oh!  Thanks.

 

 

---

---

 

From Dave Underwood [21 July 2009]:

    Here are my comments on the femtoscopy paper (ver 2, July 14) so far:

section 1, paragraph 1, line 6  after RHIC[1-4]  it looks like a line is missing in the middle of a sentence and I don't know what it was supposed to say.

Thanks yes, this is hopefully much improved in the present version.  Take a look at the first paragraph now.

section 1, paragraph 3, line 5   would read better as
     "problematic for two reasons"

We  actually mention the two reasons now in the third paragraph of section 1; that paragraph is considerably shorter now and hopefully reads more smoothly.  Then, in Section V.B., where we discuss details of comparing to high energy experiments, we get more specific.

section 1, paragraph 4    It may not be  obvious to all readers on first reading that "hadronic collisions" refers to no nuclei.

Okay, in lines 51-52, we say "hadron (e.g. p+p) and lepton (e.g. e+e-)..."  It looks ok I guess, but maybe there is yet a better way?

section III  paragraph 1
    the PID technique used in this analysis allows one to reconstruct........

We have reworded that part (see around line 324) to be more specific.  In particular "Pions could be identified up to a momentum of 800 MeV/c by correlating..."

section IV.   part A
       There should be some description of exactly what is being fitted in figures 1 and 2.
The fits of non-femtoscopic correlations in figure 1 look so good that there is no need for femtoscopic fits. Then the femtoscopic fits in figure 2 look very good, but one does not know whether the data selection is different from figure 1 or there is just a subtraction or what.

Oooh, here I guess maybe there is a real confusion.  The data and the fits in Figure 1 and 4 (I changed 2-->4, see below) are THE SAME data and fits.  It is just a different representation.  I have put in an explicit paragraph about this-- see lines 379-390.  Please see what you think.

Furthermore, ALL of the fits include femtoscopic correlations.  This was not mentioned in the caption to Figure 1, which is probably the root of some confusion.  I have added explicitly to Figure 1 "Femtoscopic correlations are parameterized with the form in Eq. 11; different curves represent various parameterizations of non-femtoscopic correlations used in the fit and described in detail in Sec.II.B"

     It might also be nice to give non-experts some crude idea of the extent to which kinematic boundaries affect the correlations without their reading all the references.  For example, the actual kinematic constraints (energy available per pion)  must be a function  of multiplicity.  How far are these momenta from the limits?

Well, actually the energy available per pion probably does NOT change much with multiplicity!  We make this argument in our spectra paper, but one can as well point to the fact that the average pT of pions changes only very little with multiplicity in heavy ion or pp collisions.  Nevertheless, the *effect on spectra shape* (and the strength of correlations induced by conservation laws) _does_ vary with multiplicity.  Maybe a good example is seen in Figure 12 in our spectra paper http://arxiv.org/abs/0807.3569

An idea of how close any given pion is to the edge of phasespace is probably given by N*<E> from our EMCICs fit.  This is 14.3*(0.68 GeV) = 9.7 GeV.  Since our midrapidity pions have ~0.5 GeV energy, they are not right at the edge.  Do you think it makes sense to discuss this around line 423?  I haven't done so, yet, since I'm not sure whether maybe there is a better way.  Let's discuss that.

section 4, part B,  paragraph 5   bottom of left column
 "expected to be approximately twice as large as the radius from..."

Thanks.  Please see lines 480-484.

section VI  discussion #3 on last page
      "halo" not "hallo"     ??
 

Thanks.  Fixed.  Line 642.

 

---

---

 

From Debasish Das [4 Sept 2009]

 Dear PA and GPC,
                interesting analysis!

The abstract re-written:

The two-pion correlation functions from p+p collisions at sqrt_s = 200 GeV is
measured using the STAR detector at RHIC.
The measured correlation lengths (radii) extracted from this analysis in
p+p is
comapared with world dataset of femtoscopic
measurements done with hadron-hadron and e+-e- collisions.

What effects do you think are "surprizingly similar"? Not clear for the
reader.

We think the re-write that you suggest is too short, and does neglect the important issue of non-femtoscopic correlations.  However, you suggest to neglect the "surprisingly similar" statement (unless we go into more detail on it).  As we think about it, your suggestion makes sense, as this point is something that might unnecessarily cause a sticking point within STAR.  This is now removed.

In paper "HBT" and "femtoscopy" are both used at regular intervals. Its
misleading
so needs to have either one of the acronym.

Well, we think both need to be used, for (at least) the following reason.  We talk about non-femtoscopic correlations, so we do need to talk about "femtoscopic" correlations and it's natural to refer to femtoscopy then.  But the radii we are extracting are traditionally called "HBT radii."  It's a somewhat unfortunate term, but it's there, and a point about this paper is that we are making contact with previous analyses, in which the term "HBT radii" and "HBT" are used; there is also the so-called "HBT puzzle," to which we make passing reference.  But you are right that we hadn't really made the connection between them explicitly.  We have added a statement at lines 25-26, which we hope helps.

Section-I page1

line-6 "and that physics..." sentence incomplete.

Thanks.  It is fixed.

Next line: The flow is ---> Flow can be explored in .....

Yes, this part was awkward and actually too brief.  It is hopefully better worded in lines 14-21.

Next sentence: "However....." needs to be restructured, now difficult to
understand.

Please see lines 28-35 for a better wording.

So understanding flow using femtoscopy is one of the main if not the only
purpose of
this paper?

No.  The purpose is to make a connection between HBT measurements in hadronic ("particle") collision experiments, and in heavy ion collisions.  Obviously, we are *presenting* hadronic collisions.  The point to mentioning flow etc is to motivate and emphasize the importance of connecting to heavy ions.  In particular, we learn more about heavy ions when we understand the pp reference (hence we mention energy loss at high pT).  Also, HBT is particularly interesting (i.e. not only energy loss is interesting) because we think it tells us a lot about the underlying bulk dynamics of heavy ion collisions, AND because it has proven itself a sensitive observable in terms of the physics in the models. 

Hopefully, this case is made in lines 28-47 and 68-78.

Since the next sentence asserts that "The most direct probe of collective
flow is
...femtoscopy...."??

Well, that statement is true.  We word it hopefully more clearly now in lines 28-39.

2nd para, page1:

The motivation now has changed again. It states that we need pp for
baseline only
for Heavy Ions!
Mentions "jets" but none of plots have any "simulation" to show its effect in
femtoscopy?
Why do we need this line at all? Its confusing.

As mentioned above, it was meant to make an analogy to high-pT studies, in which one better understands hard probes (jets, leading particles) in AA when one has a well-understood reference (pp).  But you are right, it was worded and placed in a confusing way.  The message is now put in a less confusing way around line 75.

3rd para line6:

"....mostly within the particle physics community.....apples- to- apples"

such phrases maybe too informal and needs to be restructured.

Okay.  By popular demand, the paper is now fruit-free (no apples).

Next sentence: "Secondly...."---> Hard to understand what it means. Needs
to be
restructured.

Agree.  Please see lines 56-60, which are hopefully clearer.

Section-II

Eq1. If these are 4-vectors probably good to use bold. Or else needs to
have vector
sign.
Eq2. Shifts from 4 to 3 vectors, but its confusing. Needs to be explained.

Okay, that makes sense, you are right.  We make all three-vectors.

Part A (femtoscopic correlations) page2:

While discussing Lambda we state "mis-nomer" But in next line "we do not
discuss it
further" ? Why?

Well, lambda as you know, usually encodes the percentage of (uncorrelated) contamination from particle mis-id, etc.  It is not the point of the paper, and does not affect the HBT radii.  So we don't discuss it in this paper. (As we don't on any of STAR's HBT papers, actually.  Although soon I think Michal's group will produce a paper in which lambda is important.)

After Eq.9, Page2, right hand column,

"Another femtoscopic effect that affects...." -> difficult to understand,
needs to
be restructured

Right.  This was in introduction to Coulomb suppression.  Now it appears near line 203, hopefully more smoothly.

Part B(Non-femtoscopic...)

Line3: What is a "standard fit" and why is it "standard"?

As the text says, "standard" fits are those that neglect non-femtoscopic correlations altogether (\Omega=1).    We use "standard" only as a term, that's all, to refer to such a fit.  (Since we have so many fit forms, we give them names.)  "Standard" is also handy, since it implies that this is what most people use (e.g. in STAR).

Last line, same page:
Phrases "ad-hoc" and "physically motivated" needs explanation. Difficult
otherwise.

Here, we actually think the wording is clear as it presently stands, but that's often true from a PA's point of view ;-)  Can you suggest an alternative?

Page4:

KT definition incorrect. Correct one here:
http://arxiv.org/abs/0903.1296(Phys. Rev. C 80, 024905 (2009))
Check vector sign-all over the text.

???? but it's the same as in that paper, or?  In particular, kT=0.5*|\vec{p}_{T,1}+\vec{p}_{T,2}|  Please look again, maybe you misread (or I'm just overlooking something).

Under Fig.1 "blue triangles" and in same page under Fig.2 "red stars" and
"violet
circles" can be misleading if one
take and reads from a print-out where all in black. Correct them for all
medium
online as well as offline(paper mode).

We have modified most figures to make sure that the symbols can be distinguished in black-and-white mode.  (i.e. there are no "red triangles" and "blue trianges" on the same plot).  There is no problem with saying "red triangles" and "blue squares."  The person in black-and-white mode will obviously look for the "squares," not the "blue squares."

But nuts, I now see that we didn't do it for all figures-- there are still ambiguities that would appear in black-and-white.  I am going to release the present version and correct this in the next one, okay?  I am modifying macros that Zibi gave me, and I don't want to rush; I am slowly implementing these changes now, and with other stuff going on, it probably takes a few days.

 

Page-8:

Fig9 and Fig10:

They look very different...

I am a little confused.  Figures 9 (multiplicity dependence of RG and Rinv) and 10 (mT dependence of Rout, Rside Rlong ratios) are different, so that's why they look that way.  Note that in the new draft 2.1, these are Figures 9 and 11.

 

...and Fig10 is "too loaded" for a non expert reader
to even
understand whats going on.

Make it in two parts.

Fig 10 (11 in the new draft) is the mT dependence of the 3d radii ratios.  I agree that there is plenty of data on there, but disagree that it is too loaded.  It allows to see the flatness of the ratios, at the same time as the systematics due to different treatment of non-femtoscopic correlations.

One thing I could probably agree is to remove the 50-80% Au+Au points.  Would this help, do you think?

 

 Also why is Cu+Cu missing here since in next Figure
we have
Cu+Cu added? Its
very confusing and making more difficult to understand for any reader.

No, come on.  Then it would be too loaded, with zero additional information content.

Why
only
Au+Au 200 GeV chosen and not Au+Au 62.4 and
Cu+Cu 200 and 62.4?

Because the point here is to compare p+p to Au+Au with all else constant.  So we don't want to change the energy.  After all, the first paper from STAR was 130 GeV, and we don't put that on there, either. ;-)

Page9:
Presented results show that the multiplicity is a scaling variable that
drives
geometric the femtoscopic radii at midrapidity.
Because Rout includes both space and time information
the simple scaling with the final state geometry is not expected.
Because of the finite intercepts of the linear scaling
[10, 42], results do not confirm predictions that freeze-out
takes place at the constant density [43].

What does this line mean? Does this not sound contrary to "universal mean
free path at freeze-out"?

We have removed this discussion, as it is besides the main point of the paper.

Page10:

The details mentioned from item1 to item5 are various previous analyses
and models.
How do we relate them to the presented pp data here?
Are they just for mentioning? If so just citation would have been enough,
rest the
references shall speak?

Those items are proposed (usually without numerical predictions) physics mechanisms which might be behind the pT-dependence of the HBT radii.  We said at the beginning motivation that the pT-dependence could be interesting, so we have measured it and presented it, and these items make sense for the discussion, as possible physics.  For each, we give reasons that they could or could not be plausible.

Page11:
Discussion:
"This is crucial, since in the later case, the pT systematics have
provided the most
compelling evidence for collective flow in heavy ion collisions"
Not clear, this is known,

Well, it's been pointed out many many times in the literature.

Nevertheless, we have removed this sentence, as it might cause hold-ups in the collaboration as a whole.  It might appear in a separate follow-up publication.

so what way is this new for pp and how to make
an inference?

We won't make an inference here.  That might show up in a separate follow-up publication.