Energy Resolution Studies in Simulation for the IDEA Dual-Readout Calorimeter Prototype

Round 1

Reviewer 1 Report

Thank you for submitting this article, it is an interesting paper with valuable observations and studies regarding this specific approach to calorimetry. I think the paper could use a few clarifications for readers who are not closely involved with this technology, but overall I believe it is in good shape. I have split my comments into two categories: one pertaining to substance and the other to style. Please see the attached file for details.

Comments for author File: Comments.pdf

Author Response

Thank you for your extremely detailed comments! A lot of them were very helpful. I hope to have addressed them adequately. Please see my detailed answers below.

Substance:

Point 1: IDEA is in the title but is not mentioned in the paper. I think it either needs to be dropped from the title or (preferably) should be mentioned in the abstract/introduction.
I have added it to the abstract and again in the first sentence of section 2 with a citation to the IDEA detector concept.

Point 2: Line 4-7: From the abstract alone, I fail to understand the benefits of dual-readout technology, which I think is essential to convey. More specifically, what do you mean by “compensating for this effect” on line 4? From the context provided in the abstract, the “effect” is the “event-by-event fluctuations in shower development” but what is the compensation strategy and how does a dual-readout calorimeter help? It is also unclear to me how the dual-readout is relevant to granularity specifically, as the readout strategy has in principle nothing to do with granularity.
Thanks for your comment. Indeed, most of your questions are answered in the introduction. Regarding the compensation strategy, there is more than one (see compensating calorimeters). Dual-readout allows to calculate the fluctuation on an event-by-event basis. Regarding the granularity: yes the readout strategy has nothing to do with the granularity, but the way this dual-readout calorimeter is built offers high transverse granularity which is needed to effectively use particle flow algorithms. To at least allude to this in the abstract I have modified an exisiting sentence to: The approach of dual-readout calorimetry has emerged as candidate to fulfil both these requirements, by allowing to calculate the fluctuations in the shower development event-by-event and offering a high transverse granularity.

Point 3: Line 7-11: There is a disconnect between the discussion earlier in the abstract about challenges related to hadronic shower reconstruction and the end where the EM shower energy resolution is discussed. Are these challenges also true for EM showers?
Thanks, this is a good point! The EM showers do not face the problems to the same extent the hadronic showers do. But nonetheless, the EM energy resolution is an important benchmark, which is why this prototype has been built. I have modified the sentence starting in line 7 to: As an important benchmark of such a calorimeter is the electromagnetic energy resolution, a prototype of the IDEA calorimeter has been built for use in testbeams.

Point 4: Introduction: I believe that the introduction does not clearly convey what the benefit of dual-readout calorimetry is to an outsider, which I think is the one thing it needs to do. I understand it’s probably explained at length in your references, but I hope it can be summarized in one or two sentences in your introduction.
I have now opened my Introduction with 'The main purpose of a dual-readout calorimeter is to significantly improve the resolution of the energy measurement for hadronic showers. This is obtained thanks to an event-by-event determination of the electromagnetic fraction of the shower.' to make clear waht the benefit of the dual-readout method is compared to other calorimeters. The rest of the introduction now serves as explanation of the problems that arise from the fluctuations

Point 5: Introduction: The dual-readout technology is presented as a 25 years old idea (pun intended), but also treated as novel, as it requires prototyping. What is new about IDEA compared to previous implementations of the technology?
That is a great question. So far no dual-readout calorimeter has been built for the use in a collider experiment. In fact, none of the prototypes built so far were large enough for hadronic containment. IDEA now proposes to use this technology for a high energy collider setting.
Regarding what is novel about this specific prototype: Apart from testing out new fibre layouts and assembly structure (the Bucatini structure using brass tubes), the use of SiPMs for readout is also relatively new and requires prototyping. Of couse a prototype large enough for hadronic containment would be preferable, but unfortunately cost is also a factor that needs to be taken into account.
I have made no changes to the text regarding this comment.

Point 6: Line 16: “Response” is a bit vague here, what do the “e” and “h” parameters characterize exactly here? A conversion factor from energy to observed signal? Please specify.
Yes, you're exactly right, it is the conversion efficiency from energy deposit to generated signal. I have added this explanation to the sentence introducing the response.

Point 7: Figure 1: This figure could use some annotations for clarity. E.g. point at fiber types (cherenkov vs scintillating), point at where the “back” of the detector is, point at what constitutes a “tower”, point at where the PMTs/SiPMs are located, etc.
I have added some annotations, but a clear indication of a tower is a bit difficult in this picture. In the description for Figure 1b I have put an emphasis that one tower is depicted. Other than being a collection of 20x16 fibres there is nothing special about a tower in general. In out prototype it's just that only the central tower is read out by SiPMs.

Point 8: Line 66-68: I don't quite understand this procedure: “ [...] peak around 20 GeV when averaged over all 19 simulated positions” is a somewhat vague statement. Why “around” and not exactly ? If I look at figure 4, it does not look like the central value is on average 20 GeV, as the majority of the points is below 20 GeV in both channels and each point should carry the same weight (same number of electrons). It also looks like the two channels might have different means. Could you please clarify?
I have changed "around" to "at" since this is what I meant. In Figure 4 it seems that the axis label is misleading. It should say "Reconstructed peak in GeV" instead of "Mean in GeV" (I have changed this now). There lies the reason for the 'asymmetry'. The calibration procedure uses the true mean, while the figure is plotting the reconstructed peak, which due to a slight asymmetry in the distribution is not exactly the average of all entries.

Point 9: Line 83: What is “shower maximum” in this context?
Changed it to "region of maximal energy deposit"

Points 10 & 11:
Line 84: “quite narrow”: please avoid vague qualifiers and quantify the radial extent of the shower, e.g. use Moliere radius or another representative quantity. Line 85: “Within a few mm”, same comment as line 84. Also the use of the word “shower center” implies that all the energy is deposited within a few mm of a point, but you probably mean the shower central axis or something equivalent to that?
Reading this sentence again I have decided to scrap it, since it was a bit meaningless. I have replaced it with 'From simulation studies, roughly 90 % of the shower energy is deposited within a distance of 14 mm of the shower axis and even 50 % within 5 mm.'
I am hesitant to call this a Moliere radius because it has not yet been determined exactly for the prototype and this is just an estimation.

Point 12: Figure 4&5: Do the markers in these figures have error bars? I believe some of the points (e.g. the 1st and the 13th, or 2nd and 14th) should essentially be identical, within errors, but they are not exactly. It might be worth showing what kind of statistical fluctuations are expected.
Thanks, well spotted! The plots do include error bars, but due to the large statistics the errors are below the 1% level. The discrepancy between these points has not yet been investigated properly. I strongly suspect that it is due to lateral leakage. As the entire prototype has a containment of 94%, the relative leakage between the points can change since they are not symmetrically around the centre of the prototype. But this point has not been brought up yet and would be interesting to invesitgate with a simulation of a full calorimeter with close to 100% containment. I don't think this discrepancy is too relevant for my conclusion though, as I don't quantitatively use the periodicity of the signal. But thank you for bringing this to our attention! 
I have made no changes to the text regarding this comment.

Point 13: Line 99-101: Is there an understanding as to why the scintillation channel performs better when the shower does not impinge on a scintillation fiber? Was this investigated? Is this observed in data? Maybe this is addressed in lines 102-110 but I fail to understand the argument.
Yes, this is addressed in lines 102-110. The argument is that the distribution when hitting a scintillation fibre is much wider, thus yielding a worse sigma/E. I can try to improve the wording a bit to make the point clearer.
In data so far we do not have the precision to accurately pinpoint the position of the beam such that we would be able to see this effect to the same extent.

Point 14: Line 119: “ [...] fluctuations in the shower development [...]”. I think the parameter “a” (stochastic term) should also encompass irreducible stochastic variations in the amount of energy visible in the active part of your sampling calorimeter. But maybe that’s what you meant and I misunderstand. I would state at this point in the text that “a” is the stochastic term as you refer to it as such later in the section
Yes, you're correct. I have added the name "stochastic term" to the explanation.

Point 15: Line 120-121: By “shower leakage”, do you mean parts of the EM shower escaping the detector?
Yes, I have changed "shower leakage" to "shower leakage escaping the detector"

Point 16: Line 145: What do you mean by “sub-mm sampling of the showers is likely possible”?
I mean that it is likely possible to determine the shower axis to a sub-mm precision by analysing the shower profile. I have changed the text accordingly.

Style:

Point 1: Line 3: While not strictly incorrect, it is considered informal to use “especially” at the beginning of a sentence, especially in academic writing. You could simply remove it or change the phrasing, that’s up to you.
Thanks, I have reordered the sentence.

Point 2: Line 5: I would not use the adverb “largely” in this context, as it is essentially synonymous to “mostly” or “primarily”, while you might have meant “greatly” or “significantly”.
Changed it to greatly

Point 3: Line 8-9: I would restructure the sentence “[...] simulation of [...] developed.” to “[...] simulation of this prototype has been developed in Geant4 for a testbeam environment”
Thanks,  that is a really good suggestion.

Point 4: Line 15: Point of detail but I am not used to seeing electromagnetic abbreviated with a lower-case “em” instead of “EM”, is this the standard in collider physics?
I'm not sure what the stadard in collider physics is. So far I have mainly encountered it as lower-case em. But another reviewer has pointed out the same, so I have changed it to upper case EM for now.

Point 5: Line 18: Unclear to me what “this case” is. The previous sentence makes a general statement, which implies it’s in all cases, so the “In this case” should be dropped.
"This case" refers to e/h =/= 1. This is the default case unless it is a specially designed calorimeter which counteract this, for example the compensating calorimeters introduced in the following sentence. I have added a "Generally" to the beginning of the sentence talking about e/h =/= 1 to make clear this is not always the case.

Point 6: “This effect” now refers to something discussed two sentences ago (before the ZEUS discussion) and should probably be explicitly stated, e.g. “fluctuations in f_EM”.
The sentence now states: to mitigate the effect of fluctuations in f_EM

Point 7: Line 26: “latter” should refer to something listed in the previous sentence but it does not, please state what “the latter” is explicitly.
It refers to the total shower energy which is technically the 'latter' object in the previous sentence. But I see that this is probalby an incorrect use of 'latter'. I have now merged the two sentenced using 'which' making: This allows to measure f_EM event-by-event and correct for it when reconstructing the total shower energy, which can be calculated from the Cherenkov and scintillation signals [...]

Point 8:  Line 33: “this calorimeter”, which calorimeter?
This should be more clear now that I changed 'a dual readout calorimeter ' to  'the IDEA dual-readout calorimeter' following your first Point in the Substance section.

Point 9: Line 53: “Study the EM energy”, do you mean energy resolution? Maybe “The first step in the EM energy resolution study was [...]”
Yes good catch, I missed the word 'resolution' here.

Reviewer 2 Report

The paper describes simulation studies related to a dual-readout calorimeter with an attempt to measure the energy resolution that could be obtained.  Assuming there is a page limit restricting the length, the paper does a reasonable job describing the work.

Some general comments on ways the paper could be improved:

In Section 2, testbeam studies are mentioned and the text states that the simulation was validated from this data.  There should at least be a citation related to this and preferably some details on how well the simulation and data matched.  Right now, the reader is left with only accepting or rejecting (without evidence) that the simulation describes the data.

In Section 3, the simulation setup is described and very specific points for the incoming beam were chosen.  However, based on Figure 2, these points don't seem representative of the distribution of random impacts as would be seen in an actual experiment.  A random impact seems like it would distribute the showers differently and the oscillating behavior of Figures 4 and 5 appear to be an artifact of the choice of impact.  This is likely amplified by the slight angle of impact such that for a particle hitting a tube, which side of the tube is hit will influence the shower development (e.g. is the shower developing towards or away from a fibre or a gap?).  Further studies could help explore such influences (varying angle, randomized impact).  As it stands, the conclusions are likely limited by these factors.

Some specific comments:

Section 3: I don't think the comma in the first sentence "em energy, was to" should be there, but I could be wrong there.

"em": I am used to the abbreviation "EM" for electromagnetic.

Section 3.1: the wording would be improved by adding "fibre" after scintillation in the 3rd line.

Section 3.1: in section 2 the description of the test setup has two configurations: the central tower and the eight surrounding towers.  But the simulation section doesn't describe portion is used for these studies.

Figure 2: it would have helped me to indicate which direction the impact angle would send the shower development.

Section 3.2, p. 5: "where the fibre exceeds to a radial distance". I'm not sure I understand the word "exceeds", might it be "extends"?

Author Response

Thank you for your nice comments! I hope I have addressed the adequately. Please see my detailed answers below.

Point 1: In Section 2, testbeam studies are mentioned and the text states that the simulation was validated from this data.  There should at least be a citation related to this and preferably some details on how well the simulation and data matched.
Thank you, this is a good point. I can add a citation to a presentation at CALOR2022 from a colleague who has done these studies. Of course I would prefer to cite his proceedings for the conference, as they surely are more detailed than the presentation, but unfortunately the proceedings has not been published yet. Regarding how well the simulation and data matched: the validation was done comparing shower profiles as function of distance from the shower axis. There most data points were within the errors. For now maybe I can add one sentence explaining the validation and adding a reference to the presentation. Maybe his proceedings with the methodology are published soon.

Point 2: In Section 3, the simulation setup is described and very specific points for the incoming beam were chosen.  However, based on Figure 2, these points don't seem representative of the distribution of random impacts as would be seen in an actual experiment.  A random impact seems like it would distribute the showers differently and the oscillating behavior of Figures 4 and 5 appear to be an artifact of the choice of impact.
Yes, that is correct. The point of choosing specific impact points instead of random ones, was to see how well the prototype performs if you know the exact position. In an actual experiment we hope to implement particle flow algorithms to help us find the impact point on the calorimeter, which is why the transverse granularity of the calorimeter is important. Now that we know how the calorimeter response varies with the position, we can account for the effects seen in this study once we retrieve the impact point through particle flow.
I will add a sentence in the introduction of Section 3 stating this:
In an actual experiment, knowing the behaviour in the calorimeter response as function of the particle impact point allows to correct for any in this study observed effects once the impact point has been determined, e.g. by the use of particle flow algorithms \cite{citation_to_particle_flow}.

Point 3: Section 3: I don't think the comma in the first sentence "em energy, was to" should be there, but I could be wrong there.
Yes, rereading this again, I agree.

Point 4: "em": I am used to the abbreviation "EM" for electromagnetic.
I have changed it now. Another reviewer has pointed out the same.

Point 5:  Section 3.1: the wording would be improved by adding "fibre" after scintillation in the 3rd line.
Thanks, I have added it.

Point 6: Section 3.1: in section 2 the description of the test setup has two configurations: the central tower and the eight surrounding towers.  But the simulation section doesn't describe portion is used for these studies.
The simulation internally converts the output from all towers into photoelectrons. As I don't investigate individual channels in this study, but only the total energy amount, I don't explicitly need the tower structure, but I wanted to mention it for the prototype anyway. For now I have changed the sentence in section 3.2 where I talk about the output of the simulation. It was previously: The output of the simulation is given in the number of photoelectrons detected by each SiPM. The expected light yield per deposited GeV has to be calibrated back to an energy in order to calculate the combined channel response.
Now it is: As the output of the simulation is internally converted into the number of photoelectrons detected by the SiPMs and PMTs of all towers, the expected light yield per deposited GeV has to be calibrated back to an energy in order to calculate the combined channel response.

Point 7: Figure 2: it would have helped me to indicate which direction the impact angle would send the shower development.
It is not clear to me if you mean the direction of the beam with respect to the prototype, or the rotation angle of 1 degree of the prototype with respect to beam axis.
I have extended the description saying 'The beam travels in (negative) $z$ direction.'
I have also added a small sketch of the direction of rotation.
Additionally, I have now slightly changed the wording where I introduce the rotation angle in section 2 to: This configuration includes a rotation angle of \SI{1}{\degree} around the vertical axis which was also present at the testbeam activities.

Point 8: Section 3.2, p. 5: "where the fibre exceeds to a radial distance". I'm not sure I understand the word "exceeds", might it be "extends"?
Yes indeed, I mixed up the words here.

Reviewer 3 Report

A very nice paper.  Clearly transverse non-uniformity issues are relevant, and the paper discusses these very adequately and well.   I was interested in noting the resolution issues vis a vis scintillation and Cherenkov signals, and that both benefit if the specific fibers in question are not struck directly.  My naive expectation would have been: best resolution if the beam strikes the brass absorber directly, initiating a more rapid showering, but you have clearly thought deeply about it.   For me the key and new information was you discussion in the paragraphs on lines 102-116.  This is interesting and informative to the field using fiber calorimetry techniques.

The final expectation for EM resolution of 14%/root(E) for the stochastic term is a nice result and hopefully future measurements will confirm this.

There are minor text items suggested for correction: In the caption for figure 3, reorder some words near the end to: ...distribution which later on is used to calculate...

Lines 79-80.  Suggest removing "(" before word plotted and removing the ")" after the word direction, and make this a new (non-parenthetical) sentence.

Figure 4.  End caption with plural:  channels.

Line 95.  Again end sentence with plural: channels.

Line 108.  change the word "exceeds" to "extends"

A very good paper, describing an interesting detector concept. 

Randy

 

Author Response

Thank you for your nice comments! I hope I have addressed the adequately. Please see my detailed answers below.

Point 1: In the caption for figure 3, reorder some words near the end to: ...distribution which later on is used to calculate...
Thanks, I have changed the wording!

Point 2: Lines 79-80.  Suggest removing "(" before word plotted and removing the ")" after the word direction, and make this a new (non-parenthetical) sentence.
I have moved the part in the parentheses to a new sentence like this: Figure 4 shows the fit result for the peak as a function of the beam positions for all three channels (scintillation, Cherenkov and combined). The position is indicated with the y-coordinate due to the alternating row structure in y direction.

Point 3: Figure 4.  End caption with plural:  channels.
Changed it, and also same mistake in Figure 5.

Point 4: Line 95.  Again end sentence with plural: channels.
Changed it.

Point 5: Line 108.  change the word "exceeds" to "extends"
Thanks, I mixed up the words here.