STAR GPC #355

["Feng, Yicheng" <feng216 AT purdue.edu>]

Hi GPC, 

We just got the referee's report from Phys.Rev.C for the long paper of the isobar CME baseline study. 

I plan to also forward this to starpapers-l later today. 

BTW, the short and long papers were submitted jointly, but now they seem being reviewed separately... 

I am not sure about the review procedure for joint submissions, but I hope it will be fine. 

Sincerely,

Yicheng for the PAs

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From: prc AT aps.org <prc AT aps.org>
Sent: Thursday, December 7, 2023 2:09 PM
To: Feng, Yicheng <feng216 AT purdue.edu>
Subject: Your_manuscript CK10905 Collaboration

 

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Re: CK10905
    Estimate of background baseline and upper limit on the chiral magnetic
    effect in isobar collisions at  sqrt s NN=200 GeV at the Relativistic
    Heavy Ion Collider
    by The STAR Collaboration and Yicheng Feng

Dear Dr. Feng,

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,

Christopher Wesselborg, Sc.D. (he/him/his)
Managing Editor
Physical Review C
Email: prc AT aps.org
https://nam04.safelinks.protection.outlook.com/?url="https%3A%2F%2Fjournals.aps.org%2Fprc%2F&data=05%7C01%7Cfeng216%40purdue.edu%7C8e8cac4c78cf4e19c97f08dbf7581e35%7C4130bd397c53419cb1e58758d6d63f21%7C0%7C0%7C638375730061558298%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=3boIL4WC%2BNR9OBS6kMKsSOM1bB03FLb69mfIIzoqfeI%3D&reserved=0
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----------------------------------------------------------------------
Report of the Referee -- CK10905/Collaboration
----------------------------------------------------------------------

The manuscript concerns the search for the chiral magnetic effect
(CME), an important part of the scientific program of the Relativistic
Heavy Ion Collider. A flagship paper (Ref. [25]) used a blind analysis
technique which did not find evidence for the CME according to the
defined observable. However, post-blinding, it was pointed out that
this measurement did not take into account possible species-dependent
backgrounds in the two isobars. Thus this present study by STAR is a
step toward quantifying these backgrounds and thus potentially
re-interpreting the possible presence of a CME signal, although no
longer in a blinded way. The main conclusion is that, within some
model assumptions, after evaluating several aspects of the
backgrounds, the data does not necessarily exclude the presence of a
small signal.

I appreciate the diligent work by the many members of the STAR
Collaboration to treat these issues in a serious and rigorous way.
This paper represents substantial, although partial, progress on a
specific aspect of an open physics question, and thus PRC is overall a
good target journal for this kind of work.

I have some comments below that I hope the Collaboration can address.

Given the focus on documenting a set of quantitative, systematic
studies, I think the paper would be much improved with an additional
table that summarizes the key sources of systematic uncertainties and
their relative magnitudes, and particularly how they impact the major
terms in Eq. (15) (which Table I nicely does). I found myself skimming
back and forth through the text for mentions of systematic studies.

It would be good to expand the description of the long Eq. (18). Can
the authors enumerate all the free parameters (vs. which are
externally fixed by something else), what the different terms are
supposed to model, how many of the functional forms are just
empirically chosen vs. based on some physics guidance, etc.?

Section 3.2 discusses non-flow contamination in v2 measurements. This
is of course a topic of much interest and activity in the field, with
extensive studies by the other active RHIC experiment (PHENIX), by
experiments at the LHC, and by few-author model studies. It would be
good for the authors to reference some representative sample of these
many other papers here. Where does the knowledge gained by the STAR
study fit into the big picture of what has been studied elsewhere? In
general, it would be good to connect what is done here to the
published literature outside of STAR.

In Sec. 3.2, a potentially important source of uncertainty is related
to the longitudinal flow decorrelations in these systems. There is a
quantitative uncertainty estimate of 3%, but the reference, Ref. [42],
appears to be a talk available in the proceedings from two Quark
Matter conferences ago. Is there a more definitive, peer-reviewed
reference? Or is that presentation in the proceedings considered the
definitive reference for this STAR measurement?

In the discussion of the fitted v2 values for the two systems, there
is a claim that the difference originated from the initial collision
geometries, as modeled by energy density functional theory
calculations. Is there a quantitative comparison with these initial
geometries coupled to a modern hydro code, where one can there see
this 1.02 difference? If not, how do we know the difference is fully
understood?

By the way, in Fig 6 (right), is there a reason that there are lines
connecting the red markers (Group-3 FE), but not the others?

In the last paragraph of Sec 3.2, the topic of nuclear deformation is
raised. It would be good to add some additional citations in the
literature which more broadly represent the work in this area. For
example, there are some insightful studies on this paper by Giacalone
and collaborators (but those are not the only ones).

Due to the difficulty in studying the 3p background in data, the
authors study the HIJING generator. The justification is the agreement
between data and HIJING in Fig. 8. But this is only true for the set
of cuts for the left panel, and only in 70-80% events. Already in the
right panel, the C_sp in data undershoots that in HIJING, which may
also happen if one goes to more peripheral events (>80%) in the left
panel. So the conclusions seem somewhat model-dependent. (Maybe that
is the best one can do at the moment.)

As an alternative, there is a well-known HIJING "flow afterburner",
which would then result in a flow-induced background and thus may give
a much more realistic C_3p in HIJING. I believe this was even
developed by a STAR Collaboration member. Did the authors investigate
the impact of adding flow to HIJING?

In addition to HIJING, one may gain significant insight by performing
these studies in AMPT (and perhaps some other generator such as
HYDJET), which would have different relative contributions of flow and
non-flow. Did the authors consider using these, either now or in the
future?

Regarding the last sentence of Sec. 4: "We speculate that the
peripheral collision results are likely due to fluctuations." Is there
any supporting argument that can be added -- such as a quantitative
P-value or a good citation?

Regarding the confidence limit on f_CME, this is clearly dependent on
the modeling assumption of dB^2 / B^2 (as is nicely shown in Fig. 14).
I think it would be good to update the final sentence of the abstract
to note that the upper limit is dB^2/B^2-dependent but is 10% at 95%
CL for a particular dB^2/B^2 value. (Maybe the authors can even state
that is the expected or plausible value -- if true.)

Finally, in the conclusions, it may be useful for the reader to know
what the next steps in the CME search are. This paper represents a
concrete step, but is mainly restricted to checking the specific
observables, cuts, etc., of Ref. [25]. Where does the CME research
program go from here?