Approaches to Measuring Direct Photon Yield in A–A Collisions

Round 1

Reviewer 1 Report

Manuscript is fine and acceptable in the current format

  I would like to mention that the article needs some miner corrections like: Introduction part should be enhanced It would be interesting to add some discussion on the previous measurements and their results.    English language should be checked throughout the Manuscript   Overall Manuscript is well written and suitable for Publication

Author Response

Dear Referee,

This discussion would be indeed interesting, but a) this talk was devoted to methods of measurement, not review of obtained results; b)as this is relatively large field, I picked up only some key measurements and discussed them in section 7.

       Best regards, Dmitri

Reviewer 2 Report

Dear Authors,

This article is written in a very clean and nice way. I think it is a good review of the methods employed in direct photon measurements and can be used as a starting point by begginners.

I only have a few minor comments below.

Best regards,

 

 

 

Introduction, page1

L6 "charged particles" -> "charged partons" ?

   "in course" -> "in"

L8-9 "information about the innermost part of the collision."

    Its not clear what "innermost" means here. Does it refer to the spatially innermost part of the collision?

    I think that photons are not restricted to just the innermost part of a collision, but provide information from all the 

    spatial extent of the collision. Please rephrase.

L11-12 "quark and gluon fragmentation functions"

    Its not clear how the direct photons provide information about the quark and gluon fragmentation functions.

    It was just said that they are created in parton interactions, not fragmentation of quark/gluons.

 

Statistical method

Formula 3: This needs to be better explained. What kind of parameterization is pi0_param ?

page7

"fill 3-dimensional correlation" -> "full" (typo)

Author Response

Dear referee,

Thank you for reading. All your suggestions implemented except

L6 "charged particles" -> "charged partons" ?

Left as is because direct photons can also be emitted e.g. by charged hadrons in hadronic gas.

    Best regards, Dmitri

 

Reviewer 3 Report

 

Direct photons are a powerful tool for exploring the QGP because they are produced at all stages of the collision without being affected by the final-state interactions, as they participate only in the electromagnetic interactions. A variety of QGP signatures are also present in smaller systems, therefore direct photon measurements are studied for all collision systems.

The paper reviews the experimental methods for measuring direct photons in different collision systems and at different energies. I recommend the paper for publication, however the text should be improved. Below I have listed some comments/questions/suggestions.

 

Abstract: Suggestion: “An overview of experimental methods for measuring direct photon spectra in pp, p-A and A-A collisions is presented. The advantages and limitations of each method are discussed and illustrated by the results of various experiments.”

Introduction: Some important references are missing.

Sectioning: Methods can be sub-sections

 

Line 19: “percents” à percent

Eq. 1: “E_cone” à “cone” should not be italic

Paragraph before Eq. 2 à Suggestion: “The advantages and limitations of the method follow directly from this idea: this method works well at high pT in pp and p-A collisions, but at medium and especially at small pT the contribution of large-angle π0 decays increases and more and more decay photons can pass the isolation criterion.”

Line 30: “The purity rapidly decreases with decrease of pT with rapid increase of systematic uncertainties” à I do not see a rapid increase of systematic uncertainties. Maybe rephrase.

Line 31: “most of the measurements … ” à most measurements of the isolated photon spectrum …

Line 33: “Nevertheless …” à Sentence is too long. Maybe split.

Line 41-44: Sentence is too long and does not read well. Split and reformulate.

Line 45: “into” à to

Eq. 3: gamma^{Meas} à gamma^{meas} to be consistent

Line 48: What is meant by the parameterization is not clear.

Line 49: “calculated yield of decay photons” à Calculation in which framework?

Line 53: “what” à which

Line 55: Suggestion: “Later, this method was used by all experiments that measured direct photon spectra.”

Line 56: “Advantage of this method is…” à I would not call it as a disadvantage. Suggestion:  “This method can be applied to any kind of collisions, high or low multiplicity, however it requires excellent quality of the electromagnetic calorimeter and precise measurement of neutral meson spectra.”

Line 60: Suggestion: “This makes the application of the statistical method for low pT extremely difficult.”

Line 63: Suggestion: “The energy and position resolutions of electromagnetic calorimeters improve with increasing energy of the reconstructed photon.”

Line 64: “In addition,…” à Sentence is too long.

Line 65: “increase of the photon energy” à “with increasing photon energy”

Line 65: “becomes better” à improves?

Line 69: What is “hard tracks”?

Line 75: “, see fig. 5” à (see Figure. 5)? Be consistent in the whole document.

Line 76-77: “Electromagnetic calorimeter PHOS is built of” à The Photon Spectrometer (PHOS) is a high resolution electromagnetic calorimeter built of … while Electromagnetic Calorimeter (EMCAL) …

Line 76-77: It would be good to have references for the PHOS and EMCAL.

Line 78: “As a result” is redundant here. You can omit it.

Line 79: “However, conversion…” à The conversion method ..

Line 80: “Note, that the contamination of the conversion method is completely different from the one in calorimeter, what is important for cross-check of the results.” à do you mean “Note that the contamination in the conversion method is completely different from that in the statistical method, which is important for cross-checking the results.”

Line 82: “However,…” à "The reduction of contamination uncertainties in the conversion method is achieved at the price of lower photon reconstruction efficiency and large uncertainties associated with the very precise description of the material budget of the detector." reads better, however it is not clear to me why a precise description of the material budget causes large uncertainties.

Line 85: “Conversion method” à The conversion method

After Line 87 before Line 88: Do you mean “The idea of the internal conversion method to measure the direct photon yield is based on the decomposition of the invariant mass distribution of e+e- pairs into a hadron and a direct (virtual) photon contribution. Thanks to the Kroll-Wada formula [10], one can relate the yield of e+e- pairs coming from the same source at different pair masses and even extrapolate to the yield of real photons emitted by that source:”

Figure 6 caption: “The dashed (black)” à The dashed black …

Figure 6 caption: Explain what f_dir(m_ee) and f_c(m_ee) either in the caption of in the text.

Line 90: Suggestion: “This method solves two problems: first, one gets rid of the contamination of the photon spectrum that is common in photon detection with calorimeters, and second, by choosing a proper region of pair masses, one can considerably reduce the contribution of hadron decays to the photon spectrum.”

Line 94: Suggestion: “Technically, the invariant e+e- mass distribution, f(mee), calculated for each pT bin is fitted by the sum …”

Line 98: “Minimum Bias” à minimum bias

Line 99: Suggestion: “One can see an excellent description of the shape of the measured invariant mass distribution and that the cocktail contribution is dramatically reduced just above the π0 mass.”. Also, here, can you clarify/rephrase what is meant by “the cocktail contribution is dramatically reduced just above the π0 mass.”

Line 102: Suggestion: “The signal-to-background ratio decreases rapidly with increasing collision multiplicity and is of the order of ∼10-3-10-4 for A-A collisions.”

Line 104: “of combinatorial background” à of the combinatorial background

Line 105: “Another problem … ”. You start with “first” and then you count two more items starting with “Another problem”. I would rephrase this part. Also “external conversion contribution” is not explained before.

Line 107: “mass or width” à mass and width?

First paragraph after Line 109: Suggestion: “Direct photon intensity interferometry can be used to explore the space-time dimensions of the hottest part of the collision and also to extract the direct photon yield.”

First paragraph after Line 109: “As lifetime of hadrons … ” à The sentence is too long and does not read well. I would split the sentence. Are you referring to a "bump" in Figure 7? Also, it would be better to explain the physical meaning of "bump" beforehand in a separate sentence.

Figure 7 caption: The caption can be modified/clarified. It is identical with the cited paper.

Equation 5: Subscript “inv” should not be in math mode.

Sentence before Equation 5: “if one parametrizes …” à Sentence is incomplete.

Sentence after Equation 5: “fill 3-dimensional correlation” à It is not clear what is meant by “fill”. Actually, this sentence as a whole is not clear. Please rephrase it.

Line 113: Suggestion: “This experiment was perfect for this type of measurement. It was a fixed-target experiment, and thanks to the Lorentz boost, the photon energy reconstructed in the calorimeter was high enough to reliably identify photons. In addition, since the electromagnetic calorimeter was 21 m from the interaction point, photon showers with small opening angles could be separated. Finally, the material budged in front of the calorimeter was very low, which reduced the contribution of photon conversion at low relative momentum.”

Line 141: “appears from” à originate from?

Line 142: Suggestion: “At higher pT, the difference between isolated and direct photons diminishes. Decay photons have a considerable probability of meeting the isolation criterion even at high pT, which limits the purity of isolated photons to modest values, see fig. 1.”

Line 146: Suggestion: “Direct photons provide unique and valuable information about the nucleon structure functions in pp and p-A collisions, as well as about the temperature, space-time dimensions, and development of the collective flow in hot matter created in A-A collisions.”

Line 148: “proportion” à the fraction

Line 149: “small while” à small, while

Line 151: “were proposed” à have been proposed

 

Comments for author File: Comments.pdf

Author Response

Dear referee,

thank you for careful reading and useful suggestions. I implemented most of them except of the following:

>Introduction: Some important references are missing.

I tried to avoid writing another one review of direct photon results and concentrated on methods of measurements. So I preferred to have short introduction.

> Line 30: “The purity rapidly decreases with decrease of pT with rapid increase of systematic uncertainties” à I do not see a rapid increase of systematic uncertainties. Maybe rephrase.

 Added "relative systematic uncertainties"

> Line 41-44: Sentence is too long and does not read well. Split and reformulate.

-> Sentence clearly structured into similar steps, should not cause problems for reader.

> Line 49: “calculated yield of decay photons” à Calculation in which framework?

Any framework: from Pythia to self-made phase space decayer can be used. I think, it is obvious from context.

   Thank you once more for suggestions, best regards, Dmitri