Axion Searches at the CERN SPS: From Their Dawn to Current Prospects

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is a mini-review of beam dump and similar high-energy methods for the detection of WIMPs & ALPs. Some of them were suggested by our common friend Guido Barbiellini who passed away recently and this tribute is a very fit article to be included in a special issue of this journal in memory of Guido. The article is well written by two experts in the field and given the very good job they have done in presenting their material, I have no improvements to suggest.  

Author Response

Comment: This is a mini-review of beam dump and similar high-energy methods for the detection of WIMPs & ALPs. Some of them were suggested by our common friend Guido Barbiellini who passed away recently and this tribute is a very fit article to be included in a special issue of this journal in memory of Guido. The article is well written by two experts in the field and given the very good job they have done in presenting their material, I have no improvements to suggest.  

Reply: We are very thankful for the positive feedback of the reviewer. 

Reviewer 2 Report

Comments and Suggestions for Authors

This is a nice mini-review, on a very natural topic to include in this special issue.  I recommend publication, provided a couple minor improvements are made.

The sentences, "However, in the high-coupling region, ALPs decay inside the ECAL and cannot be distinguished from a SM electromagnetic shower.  Since this loss is exponential, increasing statistic only leads to a marginal improvement in sensitivity," at lines 129–131 is a bit unclear to me.  It would seem to me that if few ALPs make it through to the HCAL, that limits only the overall sensitivity, while improved numbers of electrons on target will lead still lead to the same factor of improvement.  The authors should clarify what they mean here.

Near the end of section 4, the manuscript mentions the FASER and SHiP experiments.  It would help if the authors gave some indications of the mass ranges these experiments will cover.  (The earliest FASER results are shown in figure 3, but it is not clear how much the constraints shown there will expand further along the mass axis as it takes further data.)

Author Response

This is a nice mini-review, on a very natural topic to include in this special issue.  I recommend publication, provided a couple minor improvements are made.

We thank the referee for the very positive feedback and very useful comments (see our replies below).

Comment 1:

The sentences, "However, in the high-coupling region, ALPs decay inside the ECAL and cannot be distinguished from a SM electromagnetic shower.  Since this loss is exponential, increasing statistic only leads to a marginal improvement in sensitivity," at lines 129–131 is a bit unclear to me.  It would seem to me that if few ALPs make it through to the HCAL, that limits only the overall sensitivity, while improved numbers of electrons on target will lead still lead to the same factor of improvement.  The authors should clarify what they mean here.

Reply 1: In NA64, we search for both invisible and visible signatures. 

- Visible signature: The upper boundary of our sensitivity (i.e., at large couplings) is set by the minimum decay distance we can probe when looking for displaced vertices. Very short-lived ALPs decay promptly (either in the production target, the ECAL, or early in the HCAL), therefore, the sensitivity is driven by the ALP lifetime and not by statistics, meaning that increasing the number of electrons on target brings only marginal improvement. Improving reach in this region would require a different experimental configuration, such as a shorter dump and a shorter veto calorimeter (the first HCAL module).

- Invisible signature: In contrast, the lower boundary of sensitivity (at small couplings) is determined by the production cross-section, since ALPs in this region typically decay after passing through the detector. For these long-lived ALPs, increasing the number of electrons on target does lead to a significant improvement in the sensitivity.

We thank the referee to point out that this paragraph was not clear and we therefore changed it to 

“
However, in the high-coupling region, ALPs decay promptly inside the ECAL target, producing fully contained electromagnetic showers that are indistinguishable from standard electron signatures, or in the first HCAL module, which is used as a veto for electro-nuclear background events. In this regime, sensitivity is limited by the ALP decay length rather than statistics: most decays occur too early to produce a detectable displaced vertex. Since the decay probability follows an exponential distribution, increasing statistics leads to only marginal gains in sensitivity. Improving reach in this region would instead require modifications to the experimental setup. Nevertheless, this region remains theoretically compelling, particularly within the QCD axion band (yellow region in Fig.~\ref{fig:exclusionPlot}).”

Comment 2:

Reply 2: FASER should be able to probe the ALPs masses put to 0.3 GeV and couplings strength around 10^-5 while SHiP will be able to probe up to 1 GeV and epsilon 10^-6 (see (https://arxiv.org/pdf/1901.09966 Fig.38, page 109).
We changed the text to:

“The FASER experiment at LHC has recently released their first results in the search for ALPs~\cite{FASER:2024bbl}. Located 480 meters downstream of the ATLAS interaction point~\cite{Feng:2017uoz}, FASER explores a similar region of parameter space, targeting light, long-lived particles. Its sensitivity complements that of NA64, particularly in the regime of smaller couplings ($\epsilon \sim 10^{-5}$) and higher masses (up to 0.3 GeV) \cite{Beacham:2019nyx}.

Looking further ahead, the SHiP experiment~\cite{SHiP:2021nfo}, currently under construction at the CERN SPS, aims to probe heavier masses (up to 1 GeV) and even smaller couplings ($<\epsilon \sim 10^{-6}$) ~\cite{Beacham:2019nyx}. 
“

Reviewer 3 Report

Comments and Suggestions for Authors

This tribute paper to Guido Barbiellini’s scientific legacy provides a brief but complete and clear overview of the history of beam-dump experiments at CERN. I particularly liked the explanation and beautiful graphics illustrating different possible signatures of an axion in experiments of this kind.

I recommend acceptance of the manuscript in its current form. 

Author Response

Comment: 

This tribute paper to Guido Barbiellini’s scientific legacy provides a brief but complete and clear overview of the history of beam-dump experiments at CERN. I particularly liked the explanation and beautiful graphics illustrating different possible signatures of an axion in experiments of this kind.

I recommend acceptance of the manuscript in its current form. 

Reply: We thank the reviewer for the very positive assessment.