# An introduction to axions and their detection

### Submission summary

 As Contributors: Igor Garcia Irastorza Arxiv Link: https://arxiv.org/abs/2109.07376v1 (pdf) Date submitted: 2021-09-16 12:54 Submitted by: Garcia Irastorza, Igor Submitted to: SciPost Physics Lecture Notes Academic field: Physics Specialties: Gravitation, Cosmology and Astroparticle Physics High-Energy Physics - Experiment High-Energy Physics - Phenomenology

### Abstract

In these notes I try to introduce the reader to the topic of axions: their theoretical motivation and expected phenomenology, their role in astrophysics and as dark matter candidate, and the experimental techniques to detect them. Special emphasis is made in this last point, for which a relatively updated review of worldwide efforts and future prospects is made. The material is intended as an introduction to the topic, and it was prepared as lecture notes for Les Houches summer school 2021. Abundant references are included to direct the reader to deeper insight on the different aspects of axion physics.

###### Current status:
Editor-in-charge assigned

### Submission & Refereeing History

Submission 2109.07376v1 on 16 September 2021

## Reports on this Submission

### Strengths

1) Extremely nice, comprehensive and thorough overview of the field.
2) Includes an exhaustive list of references, which is perfect for beginners in the field, but also very helpful to everybody else.

### Weaknesses

1) Due to the nature of the manuscript it is partially of list-like character (in particular in sections 7.1 (end) to 7.4). This part could use a bit more guidance for the reader.
2) The part on IAXO is a bit too detailed/enthusiastic compared to the other discussed experiments.

### Report

The manuscript definitely covers a subject of ongoing (rather increasing) interest to the research community and provides a correct, systematic and intelligible presentation of the material. Thus the journal's acceptance criteria are met, actually particularly well.

The manuscript is an extremely nice, systematic, comprehensive and thorough overview of the field of axions and experiments for their detection for beginners to the field. What should be mentioned in particular is the exhaustive list of up-to-date references, which is helpful to beginners and other readers alike.

I would definitely be happy to see these lecture notes published.

### Requested changes

1) Chapters 7.1-7.4 (mainly the part on the haloscopes) are a bit of a list-like character. This is not necessarily bad as it is helpful to have a close-to exhaustive list of the relevant experiments. Still, it would be more useful, if there could be something to guide the reader, e.g. an actual list or plot showing the regions which the different experiments probe.
2) In some parts there is references to "later on" or similar - please replace that by the reference to the chapter.
3) The manuscript should be proof-read by an English native speaker.
4)Figures should appear approximately where they are mentioned for the first time.
5) A plot showing the (in particular astrophysical) hints might be helpful. Some of them are included in Figure 7. It might also be sufficient to make clearer in this plot, what are hints, experiments, exclusions.
6) List of minor corrections, typos etc:
-page 2: SN==>SM
-page 3: pNB==>pNG
-page 5: being g==> g being
-page 6: first term and second term mixed up, "that the electroweak scale" ==> "than the electroweak scale", "that do not contain" ==> "that they do not contain"
-page 7: "issue that will be commented later"
-page 8: contraint==>constrain, arise==>arises
-page 9: repetition of convention a/A, I would consider once sufficient; constraint==>constrain
-page 10: green dot barely visible in plot
-page 11: (VR)mechanism==>(VR) mechanism, mechanism is==>mechanism in
-page 12: mention the $\Omega_{A,VR}$ and $\Omega_{DM}$ in the text when introducing them. Explain why assume $\theta^2_iF \sim1$ at all.
-page 13: refer to table 1 in text, Becasue==>Because
-page 14: take==>takes
-page 15: introducing anthropic window once is sufficient
-page 16: that the age==> than the age
-page 17: of the alter the , the drove ==> that drove, Earch==>Earth, chamaleons==>chameleons
-page 18: section==>Section
-page 19: ref [73]==>[73]
-page 20: brehmsstrahlung==>bremsstrahlung
-page 21: one example of "see later", please name section, but however==>however
-page 22: field==>fields, Earch==>Earth, is magnetic field ==> magnetic field is
-page 23: add reference to third bullet point
-page 24: another techniques==>another technique; LSW also possible with resonators only in production cavity
-page 25: explanation of $\beta_R$? , that is more complex admittedly, but not explained as implied
-page 27: ALPs==>ALPS, ALPS-II==>ALPS II, length of ALPS now about 2*125m (see e.g. 2021 PATRAS presentation)
-page 30: proportional the ==>proportional to the
-page 31: the the==> the
-page 32: reference to CULTASK?, proving proven
-page 33: haloscopes is ==> are
-page 34: explain why containing axion quasiparticles, exist==>exists, similar is done==>; nucleai==>nuclei, in the in the==> in the
-page 35: by a virtually==>by virtually; rephrase sentence with axions travelling to Earth, it is clear, what is meant, but as written is not true for all axions
-page 36: ALP-II==>ALPS II , explanation of ABC is repetition
-page 37: later on==>refer to relevant section, similar that==>similar to
-page 38: with q=... being
-page 39: use of buffer gas for other experiments? LSW?, x-ray==>X-ray; one bracket missing at the end of first paragraph
-page 40: Figure 7 (without bracket)
-page 41: bit too enthusiastic, previous section==>mention section

• validity: top
• significance: high
• originality: ok
• clarity: high
• formatting: good
• grammar: good

### Strengths

These lecture notes give a comprehensive yet very readable overview of the cutting edge in the field of axions and their detection. The level of detail is appropriate for an introduction and the majority of the relevant literature, including other reviews, is cited.

### Weaknesses

The notes are in good shape but are in good need of a proof read, and to fill in some gaps where references are missing (I have provided a list).

I think there is a slight over-weighting towards the discussion of helioscopes. Much more time is spent on the technical details of them than on other aspects (this could be pointed out in the introduction for instance).

### Report

I can recommend this manuscript for publication, I only have relatively minor comments and corrections that primarily centre around typos and missing references/discussion.

### Requested changes

Global change: the figures do not line up well to the order they appear in the text.

Page 1:
* Abstract: "and as dark matter" → "and as a dark matter"

Page 2:
* "SN" should be "SM"

Page 3:
* "pNB" should be "pNG"

Page 4:
* "Viewed the QCD Lagrangian in isolation" → "Viewing the QCD Langrangian in isolation"

Page 5:
* 10^7 GeV^-1 → 10^7 GeV
* Eq. 3, there is an updated EDM measurement here https://arxiv.org/pdf/2001.11966.pdf

Page 6:
* "Are that do not" → "are that they do not"

Page 7:
* "issue that will..." → "an issue that will..."

Page 9:

Page 10:
* Figure 1: Thee minimum of the potential would usually be at an angle of 0.

Page 11:
* Space between "(VR)mechanism"
* "production mechanism is each scenario" → in each scenario

Page 12:
* Just above Eq. 21, it says Omega^pre rather than Omega^post

Page 13:
* Table 1: "up to several times it"?
* "dilte them away" → "dilutes them away"

Page 14:
* Discussion on TD simulations seems tobe missing references to e.g. https://arxiv.org/abs/1809.09241 https://arxiv.org/abs/1906.00967 https://arxiv.org/abs/2108.05368

Page 15:
* N_DM instead of N_DM is used a few times

Page 16:
* There was another more recent telescope search done in https://arxiv.org/abs/2009.01310

Page 17:
* Miniclusters bullet point: no references anywhere here
* "as byproduct" → "as a byproduct"

Page 18:
* "Classical" → "Classic"
* No CL or significance given fro the 7e-10 GeV^-1 stellar bound

Page 20:
* "Whose Feynman diagram" → diagrams
* Eq. 25: The more stringent bound is 1.3e-13 taken from omega centauri in the recent Capozzi and Raffelt paper

Page 21:
* Eq. 27: This doesn't seem to match Eq.(3.3) of https://arxiv.org/pdf/1906.11844.pdf
* No citation given for the NS cooling bounds.

Page 22:
* "If the field extend" → extends

Section 4.3:
One could also mention here:
* DM axions falling into neutron star magnetospheres and producing a radio line https://arxiv.org/abs/2004.00011 https://arxiv.org/abs/2008.11188 https://arxiv.org/abs/2107.01225
* stellar axions converting in MW B-field from star clusters https://arxiv.org/abs/2008.03305
* SN distance ladder measurements https://arxiv.org/abs/2011.05993
* Diffuse background of ALPs https://arxiv.org/abs/2008.11741

Page 23:
* Superradiance: There are also lighter masses that are constrained from Supermassive black holes rather than just the stellar mass ones where this bound comes from. See e.g. https://arxiv.org/abs/2009.07206
* Text beginning "The search for low energy axions..." is a repetition of earlier text

Page 24:
* "Haloscopes rely on the assumption of the" → "that"

Page 28:
* Eq. 32: personally i don’t believe one can use N-body simulations to estimate this. Certainly there are other works using different simulaitons that find different functional forms e.g. https://arxiv.org/abs/1811.11763

Page 30:
* "poses a experimental" → an experimental
* Soleoidal → Solenoidal

Page 32:
* "this type of searches" → "these types of searches"

Page 33:
* 10.000 → 10,000
* It should be noted that the principle behind BEAST---namely that you can sense axion-induced electric fields down in the long-wavelength quasi-static regime---has been seriously doubted by the community, see e.g. https://arxiv.org/abs/1809.10709 https://arxiv.org/abs/1812.05487
* "and few T" → "and a few T"

Page 34
* "similar dynamics than" → "similar dynamics to"
* nuclear spin expts: could mention the recent NASDUCK experiment here https://arxiv.org/abs/2105.04603
* nucleai → nuclei
* "use of Zeeman" → "use of the Zeeman"
* "split he" → "split the"
* "Sensitiviy" → "Sensitivity"
* "in the in the"
* XENON1T excess: this is only true when interpreted as a solar axion flux, i think the DM ALP interpretation is not constrained

Page 35
* Last paragraph is repeated text

Page 36:
* Doesn't Figure 7 also have IAXO+ shown too?
* "Despite of the"

Page 38:
* "Allows to use"

Page 41:
* "Which give a useful" → gives

• validity: high
• significance: good
• originality: ok
• clarity: good
• formatting: acceptable
• grammar: acceptable