http://cms.cern.ch/cds/HIN-15-009
Bilal
line 164–175 The technique to extract the signal shape v2 from V0 the a double Gaussian function is used and for the for the background 4th order polynomial function. The v2 values are extracted from V0 candidates from the peak region(which contain real V0’s and background V0’s from combinatorics) . I don’t understand how these combinatorics background is accounted in the analysis?
Linda
Abstract: Specify whose pt this is (charged particles, K0 and Lambda?)
35-36) for better understanding of –> for a better understanding of 98) are found to be selected –> are selected? 99) i.e., tracks –> without comma 107-124) It is not clear to me at this stage if (and where) tracks with 0.3< pT < 0.4 GeV are used 126) How do you correct for detector effects? Is it what is explained in 152? 136) pT range –> pTtrig range? (Otherwise pTtrig is not defined) 179) i.e., jet like –> without comma 181) shape of –> shapes/shape of … are/is invariant 236) canceled –> cancelled 338) v2 data –> v2 values? 342) different approaches –> what are they? ATLAS vs CMS measurements? 414-416) I find this sentence very difficult to understand 422) i.e., –> without comma
Nazar
General comments: across the paper the word “multiplicity” or “event multiplicity” is extensively used, while it refers to the “number of tracks in the event”. It sounds ambiguous in the text, especially the “event multiplicity”. I'd rather use “track-multiplicity” everywhere in the text. if possible, use a single notation consistently in equations and text: either v_{n}^{sub} or v_{n}^{signal}
Line-specific comments: l.98 in both of the eta ranges → in any of the eta ranges: one particle can't be in both ranges at the same time l.114 on the multiplicity is found l.118 which → that l.256 remove results l.263 to be described in → described in l.269 for pairs of a charged → for pairs of charged Figure 2. Are the error bars shown on the plots? If not, state in the text. l.336 for all the three systems l.337 at a slower rate than observed l.341 about a 10% difference → should better state the difference in standard deviations since uncertainties on the measurements are not negligible l.369 the dependence of the elliptic flow harmonics on particle species can shed l.375 over → across l.389 charge → charged l.26 high-density, but spatially localized, medium l.36 In particular, their dependence on the particle species and l.39 are not well understood, but l.47 from very-low-multiplicity pp events. l.84 layers of pixel tracking detector, requiring the track origin to be located within
Valentina
In general the paper is well written, with complete and detailed description of the physics. It represents an high quality measurement giving lot of valuable information on heavy-ion physics applied to pp collisions.
In the following, some minor comments:
ll.38-39: 'subnucleonic structure of a proton' → Does it mean quark and gluon densities? ll.127-129: Why this effect does not apply for 13 TeV data? Why the efficiency corrections applied to 13 TeV data sample are bigger than the ones for 5 TeV data? Figs.5-6: Are the systematic uncertanties shown just for 13 TeV data but equivalent for all samples? Please, specify in the caption. Fig.10: What variable is named as v2{LYZ}. Please, specify or include a reference.
Marco M
General comments on the draft
The aim of this article is a careful study of the features of the long-range di-hadron correlations (h-h, K0s-h or Lambda-h) observed in pp collisions at three energies: 5.02, 7 and 13 TeV. The observed features are qualitatively similar to those observed at the LHC in another “small” colliding system (pPb collisions), and also in “large” colliding systems (PbPb collisions), both already studied by CMS in several previous papers. However, the obtained results are interesting, the analysis methods are thorougly described, and overall the paper is very well written.
A few comments on specific parts of the writeup are given in the following:
ABSTRACT A few lines before the end we find the sentences: “A clear particle species dependence of v_2 is observed for high-multiplicity pp events at sqrt(s) = 13 TeV. For p_T < 2 GeV, the v_2 values of K_0S are larger than those of Λ/Λbar at a given p_T.” However (apart from the wrong units of p_T), by inspecting the figures (7.right and 8.top) showing such effect, we can see that the “mass ordering” involves also the inclusive charged hadrons, which on average, being mostly pions, have a lighter mass. Indeed in the text of section 5.2 this is taken into account, when you write: “In the lower p_T region of 2.5 GeV/c, the v_2 value of K_0S is greater than that of Λ/Λ at a given p_T value. Both are consistently below the inclusive charged particle v_2 values. Since most charged particles are pions in this p_T range, this indicates that lighter particle species exhibit a stronger azimuthal anisotropy signal. So, we suggest to correct slightly the sentence in the abstract, as you did in sect. 5.2, or otherwise as follows (or something similar): “For p_T < 2 GeV/c, a mass-ordering scheme is observed for the v_2 values of charged hadrons (mostly pions), K_0S and Λ/Λbar, with lighter particle species exhibiting a stronger azimuthal anisotropy signal.”
INTRODUCTION The text of this section is written very well and in a very clear way. A small remark only about the following sentence at L46-47:“The contribution of back-to-back jet correlations is estimated and removed by subtracting correlations obtained from very low-multiplicity pp events.” We would suggest to slightly rephrase, as: “The residual contribution to long-range correlations of back-to-back jet correlations is estimated and removed… ”
L52 “Within the superconducting solenoid volume are a silicon pixel and strip tracker detector” are –> there are L59 nonisolated –> non-isolated L125 fractional cross section –> fractional inelastic cross section L132 for analysis –> for the analysis of the new pp data samples. Table 1: Values of Fraction are indicated using a scientific exponential notation in which the base 10 is replaced by “e”. Isn't it ugly?? L136 “trigger” particles are defined as charged particles or V_0 candidates originating from the primary vertex within a given p_T range.” Wouldn't it be better to add the charged tracks eta range? I mean: “trigger” particles are defined as charged particles or V_0 candidates with |eta|<2.4 and originating from the primary vertex within a given p_T range.“ L138 The same for “with the remaining charged primary tracks…”; possibly add: with |eta|<2.4 and… L140 add a semi-colon after analyses L150 At the end of this sentence, for sake of clarity about method and definitions (for which a reader must refer to previous papers), I would add something on B(0,0). For example, somehow as you did in sect. 6.1 of your analysis note (CMS AN-16-037), if it is not too long: “The normalization factor B(0,0) is the value of B(∆η,∆φ) at ∆η = 0 and ∆φ = 0, representing the mixed-event associated yield for both particles of the pair going in approximately the same direction, thus having full pair acceptance. Therefore, the ratio B (0,0)/B(∆η,∆φ) is the pair-acceptance correction factor used to derive the corrected per-trigger-particle associated yield distribution. L152 “is weighted by a correction factor…” I would add (after factor): “derived from MC simulations” L155 “The azimuthal anisotropy harmonics v_n of charged particles, K_0S and Λ/Λ particles can be extracted via a Fourier decomposition of two-particle ∆φ correlation functions averaged over |∆η| > 2,” Here I would cancel v_n, that may give confusion to the reader, since formula (4) includes the Fourier coefficient V_nDelta (for pair distribution). Also the average on |∆η|>2 of a 1D ∆φ distribution is confusing… Indeed, the average of the 2D correlation function on |∆η|>2 is a projection to a 1D ∆φ distribution. Moreover, I would add something about the reason of the long-range selection. Therefore, taking all of that into account, one could change the above sentence as follows: The azimuthal anisotropy harmonics of charged particles, K_0S and Λ/Λ particles can be extracted via a Fourier decomposition of long-range two-particle ∆φ correlation functions, obtained by averaging the 2D two-particle correlation functions over |∆η|>2, to remove most of the short-range correlations induced by jet fragmentation, or otherwise, splitting into two sentences: To remove most of the short-range correlations, especially those induced by jet fragmentation, the two-particle ∆η∆φ correlation functions are averaged on |∆η|>2. The azimuthal anisotropy harmonics of charged particles, K_0S and Λ/Λ particles can be extracted via a Fourier decomposition of the resulting “long-range” two-particle ∆φ correlation functions, or something similar, as more convenient for you. Formula (5) The use of this formula to extract azimuthal anisotropy harmonics for single-particles from the Fourier coefficients (fitted from pair distributions), is based on the assumption of “factorization”. May you add something about how we have assessed/verified the validity of this assumption? L166, L168, L175 v_2 –> v_n (consistently with all formulas in this section) L172 ”…, where σ represents the standard deviation of the double Gaussian fit.” So, what is this sigma? An average between the sigmas of the two gaussians? Or simply the r.m.s. of the distribution? L173 defined as mass window —> defined as the mass window L175 (1.155) GeV —> (1.095) GeV IF IT MUST BE A LOWER LIMIT!! THE UPPER LIMIT IS ALREADY: 1.135 GeV 183 results of low multiplicity events –> results for low multiplicity events 184 from those of high multiplicity events –> from those for high multiplicity events 187 Even if the following formula clarifies what it was done, maybe it would better to rewrite “subtracted from the data in the higher multiplicity region…” change to: “subtracted from the V_nDelta coefficients extracted in the higher multiplicity region…” Formula (7) and lines 188-191 Nassoc (10 ≤ Ntrk <20) Y_jet Formula (7) V_n∆ = V_n∆ − V_n∆ (10 ≤ Ntrk < 20) * ———————- *
Nassoc Y_jet
(10 ≤ Ntrk <20) L188-191 “Here, Y_jet represents the near-side jet yield obtained by integrating the difference of the short- and long-range event-normalized associated yields as shown in Fig. 2 (to be described in Section 5.1) over |∆φ| < 1.2. The ratio, Y_jet /Y_jet (10 ≤ N_trkoffline < 20), is introduced to account for the enhanced jet correlations resulting from the selection of higher multiplicity events.” Our comment: Y_jet (in the above formula and in the text) should be Y_jet(105 ≤ Ntrk <150), i.e. representing the near-side jet yield for the high multiplicity event class. Consistently, in the same formula, V_nDelta and Nassoc correspond to the same high multiplicity class. Even if it can seem obvious, for sake of clarity, at least for Nassoc and Y_jet it would be useful to indicate the multiplicity range to which they refer.
Piotr
L166-175 notation in this paragraph is a bit confusing, looks like you meant v_n in all cases where you write v_2 L215-217 sentence needs to be reworked, seems to say that v2sub{2} and v2{4} (as opposed to their systematics) “are quoted to be constant percentages” L219 track selections → track selection thresholds L271 add “left” and “right ” in the parentheses, like in the caption of Figure 1. L315 its → their L338 the discussion of the ATLAS result is not entirely clear.. the constant v2 value of 6% should be compared to which CMS result (maybe it's worth to state that explicitly in the paper)? v2{2} from Figure 6? This doesn't look like something that can be made a constant 6% by changing it by 10% here and there.. v2sub{2} looks closer and is probably the one to compare.