Digging into Axion Physics with (Baby)IAXO
Round 1
Reviewer 1 Report
I have read this nice paper and have only very few comments.
Cosmological constraints, around lines 138-143:
The phrase "would overclose the universe" is often used but makes no
sense: The curvature of the universe is set early (by inflation?), so if the
universe was set to be flat it remains flat forever, no matter which
physical processes (such as axion creation) happen along the way.
The question of the required axion mass depends on the physics of the
early universe. For example, if inflation happened after the PQ
transition, the axion abundance depends on the initial misalignment
angle and could be anything. If inflation happened before, cosmic
axion strings form and the abundance is in principle well defined,
but hard to calculate.
So perhaps this paragraph could be more nuanced.
Line 163-164:
The age of the Sun is fixed by the ages of meteorites
from the era of solar-system formation, not by helioseismology. The
age of the Sun is an input parameter for solar modeling.
Line 178-179:
Some recent papers have stressed that the dominant axion emission
process from SN cores could be from pion-nucleon scattering, see
arXiv:2108.13726 and references in this paper.
Line 189 onward:
The most restrictive constraint on the axion-electron coupling is not
from the WDLF, but from the tip of the red-giant branch (TRGB), for
the latest papers see: arXiv:2010.03833 and arXiv:2007.03694. They
strongly constrain the WD cooling hint. It is anyway not very clear
how that gets translated into a region of axion-photon coupling in
Fig.2.
The rest are just cosmetic details:
In the abstract, it is not clear what is an "axion specific magnet"
and "axion sensitive cross section".
Perhaps the acronym CAPA should be explained in the abstract.
The authors use the acronym DM, the word Dark Matter and dark matter
at random. Perhaps always DM once the acronym has been introduced?
The Sun should be always capitalized or never.
Line 56: It is not clear what is meant with a "baseline" axion search.
Line 188: "as axion-electron coupling" -> "such as ..."
Line 189: constrain -> constraint
Caption of Fig.1, last line: "being the flux ..." -> "the flux being
..."
Line 196: The coupling g_ae is dimensionless, should not have GeV^-1
Line 211: HESS -> H.E.S.S.
Figure 2: Should one not show the haloscope limits all the way down?
Later in the text the reach of these bounds is described, but probably
should be shown?
Caption of Fig.2: astrophysical constrains -> ... constraints
Line 228: probably again the word order with "being" at the beginning
should be changed.
Line 241-242: "This separation is based on the origin of the axions
..." somehow this sounds as if the following experiments were the
origin of the axions.
Line 491: of which is a host -> of which it is a host
Line 493: funding member -> founding member
Line 543: Why not simply name the spokesperson?
When describing the CAPA activities about axions, should one also
mention the theory part (Redondo et al.)?
Author Response
We would like to thank the referree for their constructive comments, we believe that parts of this paper have been improved thanks to them.
Cosmological constraints, around lines 138-143:
The phrase "would overclose the universe" is often used but makes no
sense: The curvature of the universe is set early (by inflation?), so if the
universe was set to be flat it remains flat forever, no matter which
physical processes (such as axion creation) happen along the way.
We have rephrased the sentence to avoid using overclosure arguments.
=============
Line 163-164:
The age of the Sun is fixed by the ages of meteorites
from the era of solar-system formation, not by helioseismology. The
age of the Sun is an input parameter for solar modeling.
>>> The dating of meteorites defines a maximum age on the solar system and therefore the Sun's age under the assumption that their formation was simultaneous. The age of the Sun is introduced as a parameter into the solar modeling, and as such it can be constrained using experimental measurements, i.e. helioseismology, with reasonable precision.
We added the following reference to support our sentence.
[39] Bonanno, A.; Fröhlich, H.E. A Bayesian estimation of the helioseismic solar age. 2015, 580, A130, [arXiv:astro-ph.SR/1507.05847].
doi:10.1051/0004-6361/201526419. (LINE 164)
=============
Line 178-179:
Some recent papers have stressed that the dominant axion emission
process from SN cores could be from pion-nucleon scattering, see
arXiv:2108.13726 and references in this paper.
>>> We have added the following sentence to eliminate this inaccuracy:
"However, those limits could be superseded by a new production channel, involving pions, and identified latterly in reference\,\cite{PhysRevD.104.103012}." (LINE 179-180)
Line 189 onward:
The most restrictive constraint on the axion-electron coupling is not
from the WDLF, but from the tip of the red-giant branch (TRGB), for
the latest papers see: arXiv:2010.03833 and arXiv:2007.03694. They
strongly constrain the WD cooling hint. It is anyway not very clear
how that gets translated into a region of axion-photon coupling in
Fig.2.
>>> We have added a sentence to the paragraph to include the information remarked by the referee. The sentence reads: "These WD cooling arguments should be contrasted with recent studies of the RGB tip of galactic globular clusters resulting in a revised bound to the axion-electron coupling, $g_{ae}<1.48-1.6\times10^{-13}^{-1}$\,\cite{Straniero:2020iyi,PhysRevD.102.083007}, shrinking the parameter space favored by WDs cooling hints, and interestingly providing an additional hint at
$0.60_{-0.58}^{+0.32}\times10^{-13}^{-1}$\,\cite{Straniero:2020iyi}." (LINE 199-203)
============
In the abstract, it is not clear what is an "axion specific magnet"
and "axion sensitive cross section".
We have rephrased the sentence, which now reads "The distinguishing characteristic of IAXO is that it will feature a magnet that is designed to maximise the relevant parameters in sensitivity and which will be equipped with x-ray focusing devices and detectors that have been developed for axion physics."
===========
Perhaps the acronym CAPA should be explained in the abstract.
>>> We added the definition in the abstract
===========
The authors use the acronym DM, the word Dark Matter and dark matter
at random. Perhaps always DM once the acronym has been introduced?
>>> We fixed as suggested
===========
The Sun should be always capitalized or never.
>>> In our manuscript in sentences where the word "sun" could be replaced by "star" we used the lowercase version. When we refer to our sun/star, we write it with its proper name: the Sun. Still, we have changed the text to use the uppercase version, i.e. replacing "our sun" by "the Sun".
============
Line 56: It is not clear what is meant with a "baseline" axion search.
>>> We rephrased the sentence to: "The aim of this article is to motivate axion searches, describe the planned physics research program of the International AXion Observatory (IAXO), and give an outlook of its further potential." (LINE 56-58)
============
Line 188: "as axion-electron coupling" -> "such as ..."
>>> Fixed
============
Line 189: constrain -> constraint
>>> Fixed
============
Line 196: The coupling g_ae is dimensionless, should not have GeV^-1
>>> Fixed.
============
Line 211: HESS -> H.E.S.S.
>>> Fixed
============
Figure 2: Should one not show the haloscope limits all the way down?
Later in the text the reach of these bounds is described, but probably
should be shown?
>>> We prefer to focus on the region of the parameter space covered by axion helioscope searches. However, we have added a new reference in figure 2 where the curious reader will find more detailed drawings of an extended version of the axion parameter space. Reference [61].
============
Caption of Fig.2: astrophysical constrains -> ... constraints
>>> Fixed
============
Line 228: probably again the word order with "being" at the beginning
should be changed.
>>> We updated the text using: "to lower values, with the stringent globular"
============
Line 241-242: "This separation is based on the origin of the axions
..." somehow this sounds as if the following experiments were the
origin of the axions.
>>> We have replaced it by the "source of the axions" to avoid confusion
============
Line 491: of which is a host -> of which it is a host
>>> Fixed.
============
Line 493: funding member -> founding member
>>> Fixed.
============
Line 543: Why not simply name the spokesperson?
>>> We have avoided mentioning the name simply because what we find relevant is the fact that this person comes from CAPA; if included, it would seem strange not mentioning the names of other people within CAPA with responsibilities in other parts of IAXO (radiopurity, detectors, software, analysis...). Similarly, for our colleagues from theory and phenomenology
============
When describing the CAPA activities about axions, should one also
mention the theory part (Redondo et al.)?
>>> We agree that the paragraph mentioning our theory and phenomenology experts was not giving full credit to the extent of their contribution; we have introduced a paragraph in which a more detailed reference to their work is given. (LINES 548-559, 581-584).
Reviewer 2 Report
This could be an interesting paper on recent axion experiments.
Maybe, at the beginning the review
Models of lepton and quark masses
Steven Weinberg
Phys. Rev. D 101, 035020 – Published 19 February 2020
should be cited. Moreover, the preprint "The 11-years solar cycle as the manifestation of the dark Universe" of K. Zioutas et al.
What is also not mentioned are ultra-low axion models, which may be out of the range of proposed new experiments: See e.g. Mielke, Physics Letters B 807 (2020) 135538 and
the refeences therein.
Author Response
We thank the referee for reading the article and for their suggestions. Here we comment briefly on them.
This could be an interesting paper on recent axion experiments.
Maybe, at the beginning the review
Models of lepton and quark masses Steven Weinber Phys. Rev. D 101, 035020 – Published 19 February 2020 should be cited.
>>>> With regard to the reference on the paper by S. Weinberg, we do not see how it is directly related to the content of our article which motivates axions and ALPs that fall within the sensitivities of the next generation helioscopes, haloscopes and laboratory experiments.
Moreover, the preprint "The 11-years solar cycle as the manifestation of the dark Universe" of K. Zioutas et al.
>>> This reference is a very interesting piece of work and we know it very well. However, we believe that it is outside of the scope of our manuscript. Gravitationally-trapped axions and KK-axions are probably better suited for gaseous TPCs targeting DM WIMP searches and our publication focuses on axion helioscopes. We will of course have in mind that reference for future publications connected to underground DM detectors, such as TREX-DM.
What is also not mentioned are ultra-low axion models, which may be out of the range of proposed new experiments: See e.g. Mielke, Physics Letters B 807 (2020) 135538 and
the refeences therein.
>>> In our manuscript we have covered the axion models and hints that are within the experimental reach. We believe adding the reference suggested would require a deeper contextualization of those models. If new experiments would have been proposed to probe those ultra-low axion models it would make sense to us to explore all the possible ultra-low mass axion models that could be tested.
Reviewer 3 Report
I apologize but I don't feel qualified to judge about this Axion Physics phenomenology domains.
Author Response
We thank the referree.
