Section 6

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Section 6.1 Last line: What does ”alternatively” mean ? If one choice is for electrons and the other for muons, the authors should state it clearly.

Footnote: this statement deserves a complete section, with the list of parameters used in the default CMS and ATLAS configurations, as well as possible explanations as to why this parameter choice was made, and a comparison with the actual CMS and ATLAS resolutions and granularities.

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Figure 3

• It is not clear what the grey bands are in this plot. Shouldn’t they be

removed? In CMS, they are supposed to cover differences between the simulation and the data in CMS, not the difference between Delphes and CMS. This comment is valid for all plots Strangely enough, the ATLAS fred band width is way smaller than that for CMS. Does it represent the same thing ?

• For all plots, it is important to have the statistical uncertainty bars

indicated, or to state that they are covered by the size of the markers. In the latter case, an explanation is needed for the apparent scatter of the DELPHES points, and to compare this scatter with the input resolution function.

• For all plots, label, legends, etc... are way too small to be readable.

Figure 4

Looking at Ref. [20], and in particular its slide 33, my impression is that the DELPHES resolution is a factor 2 optimistic with respect to the CMS resolution. It seems that DELPHES parametrized the Gaussian width of the core of the CMS resolution, rather than the effective 68% width.

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Figure 5

On the CMS side, it would be nice to have an explanation for the following effects (already alluded to above):

• why does the calo jet resolution curve saturates at low jet pT ?
• why does the PF jet resolution curve of Delphes departs from CMS at high pT? (it’s probably because the tracker pT resolution is used to

determine the PF track pT) And at low pT ? Actually, the DELPHES jet pT resolution is almost independent with pT: it goes from 10 to 7% when varying the pt from 30 to 500 GeV/c, while the actual CMS PF varies from 14% to 5%.

• why does the PF curve show a discontinuity between the 1st and 2nd points ?

Once the effects are understood, it would be important to fix the imple- mentation in DELPHES. These differences are important for data analysis, and may to DELPHES user draw wrong conclusions from his DELPHES studies (of particular importance if DELPHES is used to define the upgrade strategy of the expensive LHC detectors).

Section 6.3

Par 2:

It is not clear if the CMS study is made with of without pile-up. As pile-up and its particle-flow mitigation are an important adds-on to DELPHES 3.0, it would be nice to have an illustration of their performance here.

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Par 1:

L3: The anti-kT algorithm has no ”cone” - and ”a cone R = 0.5” has no meaning even for a cone algorithm. (Note: the same mistake occurs in Section 7.2)

L9/11: The slight difference of efficiency is a large difference (20%), which might the DELPHES user draw incorrect conclusions from the abilities of his/her analysis. The explanation given here should be checked, and the cul- prit (jet energy correction or b tagging efficiency should be fixed in DELPHES.

Par 2:

Bullet 1: ”any parton from the top quark decay” ! ”any parton from the decay of either of top quarks”. I find the definition of unmatched rather un- natural. If the three jets from the ”hadronically-decaying top” were matched, I fail to understand why the event is classified as unmatched, even if the other b for the other top is unmatched.

Par 3:

L5: Why are the distributions not normalized to the number of events ? Is it because the number of permutations/event is vastly different in DELPHES and in CMS ? If it is the case, shouldn’t DELPHES be fixed ?

L9/10: ”Pile-up, not considered in the present study, can degrade the jet energy resolution.” : A strong emphasis is put on the ability of Delphes 3.0 to simulate pile up, and to simulate its mitigation procedures based on the PF reconstruction. Since Delphes is fast, I would assume that it would take no time to redo this study with pile-up. It is disappointing for the reader not to see this validation in the paper. It actually casts doubts on the DELPHES ability to accurately simulate pile-up and its mitigation, which is surely not what the authors aim at.

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Figure 7

The bottom inserts of all plots are difficult to understand. The label ”rel. diff.” makes the reader guess that they show the relative difference (i.e., the ratio - 1) of the two distributions, but a quick look at the distributions leads the reader to doubt about it. For exampe, the right plot shows a 20% difference between the two distributions around the maximum, which is not visible in the ”rel. diff.” plot. Maybe the wrong scale was chosen for the bottom inserts ?

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Par 4: The reader is again disappointed to see that, in the search for VBF- produced Higgs boson with pile-up, for which it is said that pile-up has a pretty large and negative impact, the authors decided to use calorimeter jets instead of particle-flow reconstruction, aimed exactly at mitigating pile-up effects. Again, it casts doubts on the ability of DELPHES to simulate pile-up in particle reconstruction, and to simulate its mitigation with particle-flow reconstruction. The paper has therefore the effect opposite to what the authors are aiming at.

Criterion 2.

The pT cut used to count light jets between j1 and j2 ought to be given. Are these four cuts used in the CMS analysis which the authors are using for comparison ?

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Par 2:

L4: ”accurate productions in high pile-up scenarios should solely rely on full simulation tools” is very strong, and probably incorrect statement, and should probably be removed (or seriously rephrased).

First, it casts again large doubts on the pertaining DELPHES ability to simulate pile up interactions.

Second, the fact that, maybe, DELPHES cannot deal with high PU en- vironment does not mean that other fast simulation tools cannot do. For example, it seems the CMS fast simulation deals pretty well with the high pile-up produced by LHC in 2012.