General questions:
- line 20 we mention the reduced central event hadron activity, but our selection does not seem to make explicit use of that. Is that due to the pile-up, or for some other reason?
- Both the ttb scale and normalisation systematic errors are among the most dominant ones. Are we fully sure there is no double counting here? I know you say the scale uncertainties only look at the change in shape, not the total even numbers..
Requiring a reduced hadron activity between the two tagging jets would mean applying some kinematic selection to variables related to the third jet. This would in turn mean explicitly relying on the parton shower modeling for the definition of our phase space, which is not very convenient. Instead, we decided to select our signal region according to centrality of the di-lepton system with respect to the two jets, since it has a good discrimination power and is scarcely correlated to the third jet kinematic. Concerning the systematic uncertainties, there is no double counting between the ttb normalization and QCD scale variations.
Detailed comments:
- "First" in the title: is this conform with our publication rules? This was under discussion some time ago, whether we should use it (or not) in titles.
Dropped "First". Title changed in: "Observation of electroweak W+W- pair production in association with two jets in proton-proton collisions at sqrt{s} = 13 TeV".
- ref [1] is a recent paper of course, but not the one that originally pointed out what is reported in this statement, Shouldn't we refer (also) to the original paper?
We changed the Ref[1] to:
@article{PhysRevD.16.1519,
title = {Weak interactions at very high energies: The role of the Higgs-boson mass},
author = {Lee, Benjamin W. and Quigg, C. and Thacker, H. B.},
journal = {Phys. Rev. D},
volume = {16},
issue = {5},
pages = {1519--1531},
numpages = {0},
year = {1977},
month = {Sep},
publisher = {American Physical Society},
doi = {10.1103/PhysRevD.16.1519},
url = {https://link.aps.org/doi/10.1103/PhysRevD.16.1519}
}
- line 39: We give no comment on the CMS trigger here at the end of the section as we usually do in our papers? space limitations of PRL?.
We will add the standard CMS description:
"The central feature of the CMS apparatus is a superconducting solenoid of 6\unit{m} internal diameter, providing a magnetic field of 3.8\unit{T}. A silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections, are installed within the solenoid. Forward calorimeters extend the pseudorapidity coverage provided by the barrel and endcap detectors. Muons are detected in gas-ionization chambers embedded in the steel flux-return yoke outside the solenoid. A more detailed description of the CMS detector, together with a definition of the coordinate system and the relevant kinematic variables, can be found in Ref.~\cite{CMS_detector}.
Events of interest are selected using a two-tiered trigger system. The first level (L1), composed of custom hardware processors, uses information from the calorimeters and muon detectors to select events at a rate of around 100\unit{kHz} within a fixed latency of about 4\mus~\cite{CMS:2020cmk}. The second level, known as the high-level trigger (HLT), consists of a farm of processors running a version of the full event reconstruction software optimized for fast processing, and reduces the event rate to around 1\unit{kHz} before data storage~\cite{CMS:2016ngn}.
During the 2016 and 2017 data-taking, a gradual shift in the timing of the inputs of the ECAL L1 trigger in the region at $\abs{\eta} > 2.0$ caused a specific trigger inefficiency. For events containing an electron (a jet) with \pt larger than $\approx$50\GeV ($\approx$100\GeV), in the region $2.5 < \abs{\eta} < 3.0$ the efficiency loss is $\approx$10--20\%, depending on \pt, $\eta$, and time. Correction factors were computed from data and applied to the acceptance evaluated by simulation.
The particle-flow (PF) algorithm~\cite{CMS:2017yfk} aims to reconstruct and identify each individual particle in an event, with an optimized combination of information from the various elements of the CMS detector. The energy of photons is obtained from the ECAL measurement. The energy of electrons is determined from a combination of the electron momentum at the primary interaction vertex as determined by the tracker, the energy of the corresponding ECAL cluster, and the energy sum of all bremsstrahlung photons spatially compatible with originating from the electron track. The energy of muons is obtained from the curvature of the corresponding track. The energy of charged hadrons is determined from a combination of their momentum measured in the tracker and the matching ECAL and HCAL energy deposits, corrected for the response function of the calorimeters to hadronic showers. Finally, the energy of neutral hadrons is obtained from the corresponding corrected ECAL and HCAL energies. The candidate vertex with the largest value of summed physics-object $\pt^2$ is taken to be the primary $\Pp\Pp$ interaction vertex. The physics objects used for this determination are the jets and the associated missing transverse momentum, taken as the negative vector sum of the \pt of those jets.
Hadronic jets are clustered from all the PF candidates in an event using the infrared and collinear safe anti-\kt algorithm~\cite{Cacciari:2008gp, Cacciari:2011ma} with a distance parameter of 0.4. Jet momentum is determined as the vectorial sum of all particle momenta in the jet, and is found from simulation to be, on average, within 5 to 10% of the true momentum over the whole \pt spectrum and detector acceptance. Additional proton-proton interactions within the same or nearby bunch crossings can contribute additional tracks and calorimetric energy depositions, increasing the apparent jet momentum. To mitigate this effect, tracks identified to be originating from pileup vertices are discarded and an offset correction is applied to correct for remaining contributions. Jet energy corrections are derived from simulation studies so that the average measured energy of jets becomes identical to that of particle level jets. In situ measurements of the momentum balance in dijet, $\text{photon} + \text{jet}$, $\PZ + \text{jet}$, and multijet events are used to determine any residual differences between the jet energy scale in data and in simulation, and appropriate corrections are made~\cite{CMS:2016lmd}. Additional selection criteria are applied to each jet to remove jets potentially dominated by instrumental effects or reconstruction failures.
The missing transverse momentum vector \ptvecmiss is computed as the negative vector sum of the transverse momenta of all the PF candidates in an event, and its magnitude is denoted as \ptmiss~\cite{CMS:2019ctu}. The \ptvecmiss is modified to account for corrections to the energy scale of the reconstructed jets in the event. The pileup per particle identification (PUPPI) algorithm~\cite{Bertolini:2014bba} is applied to reduce the pileup dependence of the \ptmissvec observable. The \ptmissvec is computed from the PF candidates weighted by their probability to originate from the primary interaction vertex~\cite{CMS:2019ctu}."
- line 56: complicated way of saying that you re-weight the events according to the pile-up distribution as observed in data
"For each data set, simulated events are reweighted according to the pileup profile distribution as observed in data."
- what is the "dipole recoil setting"? Is that a MC parameter? is there a reference for it? please specify. It is not clear here if this is just detail or relevant for this study. This should be better explained.
The sentence was rephrased as "The dipole approach~\cite{sjostrand_2018} is used to model the inital-state radiation, rather than the standard \pt-ordered one used in the \PYTHIA parton shower.". We added this reference:
@article{sjostrand_2018,
title={Some dipole shower studies},
volume={78},
ISSN={1434-6052},
url={http://dx.doi.org/10.1140/epjc/s10052-018-5645-z},
DOI={10.1140/epjc/s10052-018-5645-z},
number={3},
journal={Eur. Phys. J. C},
publisher={Springer Science and Business Media LLC},
author={Cabouat, Baptiste and Sjöstrand, Torbjörn},
year={2018},
month={Mar}
}
- What tool was used for this study to estimate the effect of the interference? Or from which reference was this extracted reference? Please specify..
The interference contribution was estimated by subtraction: from the inclusive sample, which is simulated at LO by generating both EWK and QCD diagrams, as well as their interference, the individual EWK and QCD samples are subtracted. The remaining term, that can go negative, is the interference itself.
- line 76 and line 77: these correction procedures need a reference where these have been presented in detail.
The reweighting has been applied in order to get a better data/MC agreement. We added the following references to these methods.
@article{Khachatryan:2016mnb,
author = "Khachatryan, Vardan and others",
title = "Measurement of differential cross sections for top quark
pair production using the lepton+jets final state in
proton-proton collisions at {13\TeV}",
collaboration = "CMS",
journal = "Phys. Rev. D",
volume = "95",
year = "2017",
pages = "092001",
doi = "10.1103/PhysRevD.95.092001",
eprint = "1610.04191",
archivePrefix = "arXiv",
primaryClass = "hep-ex",
reportNumber = "CMS-TOP-16-008, CERN-EP-2016-227",
SLACcitation = "%%CITATION = ARXIV:1610.04191;%%"
}
@article{Sirunyan:2019bzr,
author = "Sirunyan, Albert M and others",
title = "Measurements of differential {\PZ} boson production cross
sections in proton-proton collisions at {$\sqrt{s}=13\TeV$}",
collaboration = "CMS",
journal = "JHEP",
volume = "12",
year = "2019",
pages = "061",
doi = "10.1007/JHEP12(2019)061",
eprint = "1909.04133",
archivePrefix = "arXiv",
primaryClass = "hep-ex",
reportNumber = "CMS-SMP-17-010, CERN-EP-2019-175",
SLACcitation = "%%CITATION = ARXIV:1909.04133;%%"
}
- Some missing information (I guess due to PRL space pressure??) * We do not define how we select a primary vertex, which I assume we do in the analysis. (we do talk about pile-up vertices later) * pTmiss is not defined; we usually define that in papers. * I assume we use particle flow in the analysis? Not mentioned here.
See the comment about the CMS trigger.
- line 131: I assume this conclusion is drawn from MC studies, or does it come from the real data experience? Perhaps good to spell that out.
"A large fraction of the DY background" -> "A large fraction of the DY MC background"
- line 138: "Regardless of the final state, nonprompt leptons are mainly produced via W + jets events" somewhat unlucky phrasing, as the W boson delivered a prompt lepton, but I imagine you talk about the additional lepton here...
Indeed the nonprompt lepton is not the one coming from the W boson, but rather it can be either a jet mis-reconstructed as a lepton or a real lepton coming from a B meson decay produced in the jet itself. We will rephrase as: "Nonprompt leptons, i.e.\,either leptons produced in decays of hadrons or jets misidentified as leptons, are mainly due to \wj events."
- line 182-188: so the signal is shown twice on the plots, once as a contribution to the distribution, and once as a stacked contribution to compare the sum with data..? One can guess that, but it is not clear from the text.
The signal is shown both as stacked and superimposed histogram, those lines have been removed and the caption of Figure 2 now reads: "The contributions from background and signal processes are shown as stacked histograms; the signal template is also displayed as a superimposed line to highlight the difference in shape with respect to the background distribution. Systematic uncertainties are plotted as dashed gray bands. This description holds for Figures 3 and 4 as well."
- line 191: how does this 2.1% lumi normalization uncertainty relates the 1.6% lumi uncertainty reported in line 43? Perhaps I am missing something...
Indeed we apply a 1.6% uncertainty in the luminosity over the full Run 2 data set, keeping correlations into account as recommended by the LUMI group. The 2.0% value (for some reason the text mistakenly reports 2.1%, but the actual number is 2.0%) is the contribution of such uncertainty to the cross section measurement. This number differs from the 1.6% a priori value, but it should be noticed that this nuisance parameter is only defined for the signal sample and for those backgrounds whose normalization is not measured in data. The luminosity uncertainty in the cross section measurement can ultimately depend on two effects: the correlation among different processes, that can slightly pull the nuisance parameter during the fit procedure, and the error propagation to the final result. Eventually, the combined action of these effects makes the 1.6% a priori uncertainty in the luminosity a greater contribution when computing the cross section measurement.
- line 227: we derive a fiducial and a more inclusive cross section. It is clear to me how we defined the fiducial cross section, measured within a certain phase space region. The more inclusive cross section is defined with parton level cuts, which is fine on a MC. But how do we actually derive this from the measurement? We are not unfolding the data, are we? And is this a cross section for W^+W^- or for lepton^+lepton^- channel for the fiducial cross section? The discussion here needs to be expanded on how we derive this experimental number to make this information more significant and usable.
The measured cross section in the more inclusive fiducial volume comes from the MC cross section multiplied by the measured signal strength. The cross section refers to the W+W-jj -> l+l-vvjj electroweak production as this is how the sample was generated.
- line 252: I suggest to repeat the fiducial cuts that have been imposed here, as this cross section is defined only within that regions, as is done in the abstract or just with reference to table 3
We added the reference to Table 3.