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REVIEW 3 major objections 3 minor 33 references

Measurement of azimuthal anisotropy of muons from charm and bottom hadrons in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

T0 review · 3 major / 3 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read ATLAS observes a significant nonzero elliptic-flow coefficient $v_2$ for muons from charm decays in high-multiplicity $pp$ collisions, while bottom-decay muons are consistent with zero.

desk verdict First pp measurement separating charm and bottom heavy-flavor muon v2: careful analysis, load-bearing nonflow assumption handled honestly but with a couple of soft spots worth a referee's attention. read the letter →

arxiv 1909.01650 v2 pith:JZJAOKMK submitted 2019-09-04 nucl-ex hep-ex

ATLAS Collaboration This is my paper · ORCID
classification nucl-exhep-ex
keywords ellipticflowazimuthalanisotropycharmquarkbottomheavy-flavormuonsproton-protoncollisionshigh-multiplicityeventstemplatefitmethod
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports the first separation of elliptic-flow measurements for muons from charm versus bottom hadron decays in proton-proton collisions at $\sqrt{s}=13$ TeV. In high-multiplicity events, muons from charm decays show a significant nonzero elliptic anisotropy coefficient $v_2$, while muons from bottom decays are consistent with zero over the measured transverse momentum range. If correct, this means charm quarks are dragged into the collective, geometry-driven expansion pattern in the smallest collision system, whereas the heavier bottom quarks are not in this momentum window. The result gives a new, mass-resolved handle on how quark mass shapes interactions with the medium formed in high-multiplicity $pp$ events.

What carries the argument

The central object is the template-fit decomposition of per-muon two-particle azimuthal correlation functions into a nonflow component and a flow-modulated ridge: $C_{\mathrm{templ}}(\Delta\phi)=F\,C_{\mathrm{LM}}(\Delta\phi)+G\left[1+\sum_{n=2}^4 2v_{n,n}\cos(n\Delta\phi)\right]$. This carries the argument by attributing any multiplicity-dependent excess in the correlation function to collective elliptic flow, after subtracting the low-multiplicity baseline that is assumed to contain only nonflow. The charm-bottom separation is then carried by template fits to the muon transverse impact parameter $d_0$, and the muon $v_2$ is obtained from the pair anisotropy through flow factorization $v_n^{\mu}=v_{n,n}/v_n^h$.

What would settle it

Repeat the extraction with the low-multiplicity baseline shifted from $N_{\mathrm{rec}}^{\mathrm{ch}}<40$ to $N_{\mathrm{rec}}^{\mathrm{ch}}<20$, and compare the resulting charm and bottom $v_2$ values; if the shift moves $v_2$ by more than the quoted uncertainties, the nonflow-shape-independence assumption is falsified.

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Extended reading notes

Core claim

The central claim is that the elliptic anisotropy $v_2$ of muons from charm-hadron decays in $pp$ collisions at $\sqrt{s}=13$ TeV is significantly nonzero for high-multiplicity events, while $v_2$ of muons from bottom decays is consistent with zero. Using 150 pb$^{-1}$ of ATLAS data and muons with $4<p_T<7$ GeV and $|\eta|<2.4$, the analysis separates heavy-flavor muons from light-hadron decay backgrounds through the momentum imbalance $\Delta p/p_{\mathrm{ID}}$, and separates charm from bottom muons through template fits to the transverse impact parameter $d_0$. The inclusive heavy-flavor muon $v_2$ shows no strong multiplicity dependence in the 60$\le N_{\mathrm{rec}}^{\mathrm{ch}}<120$ range and decreases as $p_T$ rises from 4 to 7 GeV. The bottom-decay muon $v_2$ is zero within uncertainties, whereas the charm-decay muon $v_2$ is nonzero at lower $p_T$, indicating that bottom quarks do not appear to participate in the collective behavior in these smallest collision systems.

Load-bearing premise

The analysis assumes the shape of the nonflow background, made of back-to-back dijets and resonance decays, does not change with event multiplicity, so that the low-multiplicity correlation function can be subtracted from the high-multiplicity one; if that shape changes, the extracted $v_2$ would be contaminated.

Editorial extensions

If this is right

  • In high-multiplicity $pp$ collisions, charm quarks participate in the same collective elliptic flow pattern that has been observed for light hadrons, reinforcing the hydrodynamic description of the smallest collision systems.
  • Bottom quarks show no measurable elliptic flow in the $4$\,--\,$7$ GeV muon $p_T$ range, implying a mass-dependent threshold for heavy-quark thermalization in $pp$ events.
  • The inclusive heavy-flavor muon $v_2$ is roughly flat with multiplicity and falls with increasing $p_T$, providing a new differential constraint on heavy-quark transport models.
  • The measured charm-bottom gap provides the first $pp$-system data point in a regime where transport calculations predict larger $D$-meson than $B$-meson $v_2$ at low $p_T$.
  • The demonstrated ability to separate charm and bottom contributions via $d_0$ templates can be applied to larger datasets and other collision systems to sharpen the mass-dependence picture.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Beyond the paper, if the charm-bottom gap persists with more data, the $p_T$ at which bottom flow turns on would map the mass threshold for thermalization in small systems.
  • Beyond the paper, replacing the low-multiplicity template with a rapidity-separated subevent estimator would directly test whether the nonflow-shape assumption biases the reported charm $v_2$.
  • Beyond the paper, a fully reconstructed $D$-meson measurement in the same $pp$ dataset could corroborate the charm flow signal without relying on muon decay template ambiguities.
  • Beyond the paper, an analogous measurement in $p$+Pb collisions at the same muon $p_T$ would show whether the charm-bottom gap widens or narrows as the system size grows.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 3 minor

Summary. This Letter reports measurements of the elliptic anisotropy coefficient v2 of muons from heavy-flavor hadron decays in pp collisions at sqrt(s)=13 TeV using 150 pb^-1 of ATLAS data, with separation of charm- and bottom-decay muons via the transverse impact parameter. The analysis uses two-particle muon-hadron correlations with a pseudorapidity gap, subtracts nonflow with a template method that scales the low-multiplicity correlation shape, and divides by the previously measured charged-hadron v2 to obtain the muon v2. The main results are a decreasing inclusive heavy-flavor muon v2 with pT, a charm-muon v2 that is claimed to be significantly nonzero at low pT, and a bottom-muon v2 consistent with zero within uncertainties.

Significance. If the result holds, it provides a first indication that charm quarks participate in the collective elliptic flow of high-multiplicity pp collisions while bottom quarks do not over the measured pT range, giving new constraints on heavy-quark transport in the smallest collision system. The paper is careful and quantitative in several respects: the heavy-flavor separation is cross-checked against FONLL and Pythia8, the muon-hadron correlation analysis includes efficiency corrections and pileup checks, the dominant systematic from the low-multiplicity window choice is quantified, and a Pythia8 closure test shows the extracted v2 is consistent with zero in the absence of flow. The significance of the measurement is, however, tempered by the reliance on the multiplicity-independence of the nonflow shape, which is tested mainly in simulation, and by the absence of a quoted statistical significance for the nonzero charm v2 claim.

major comments (3)
  1. [Template fit method (C_templ equation)] The template subtraction assumes that the shape of nonflow correlations is independent of multiplicity, so that the low-multiplicity correlation C_LM can be scaled and subtracted from the high-multiplicity correlation. The only validation reported is the Pythia8 closure check and the variation of the LM window, which gives the largest systematic uncertainty (15-35%). Neither test directly constrains a multiplicity-dependent change of dijet or resonance shapes in data. Since the reported charm-muon v2 is of order 0.05-0.1, a few-percent multiplicity evolution of the nonflow shape could create or cancel the signal. I request a data-driven cross-check (for example, an alternative nonflow-subtraction approach or a comparison using correlations in different event-shape or jet-activity classes) or a quantitative estimate of the maximal nonflow contamination under a controlled multiplicity-dependent model.
  2. [Summary paragraph and Figure 4] The abstract and summary claim a 'significant non-zero' v2 for muons from charm decays, but no significance is quoted anywhere in the text. Figure 4 shows only points with statistical and systematic bands; the number of standard deviations by which the charm v2 deviates from zero, and by which the charm and bottom v2 values differ, should be stated explicitly, including the correlated systematic component. Without this quantitative statement, the central claim is not fully supported.
  3. [Summary paragraph and Figure 4] The conclusion that bottom quarks 'do not participate in the collective behavior' is stronger than what a null measurement at this precision supports. The bottom-muon v2 is consistent with zero within sizeable uncertainties in each Nrec_ch and pT bin, but consistency with zero is not evidence of absence. The wording should be softened to 'no significant v2 is observed for bottom muons,' or an upper limit on the bottom-muon v2 should be provided.
minor comments (3)
  1. [Title page] The CERN header contains typographical artifacts ('ORGANISA TION' and 'A TLAS') that should be corrected in the final version.
  2. [Figure 3 and Figure 4 captions] The figures show pT axes extending beyond the measured range, and the caption text would be clearer if the pT ranges for the multiplicity-dependence panels and the multiplicity range for the pT-dependence panels were repeated in the captions rather than only in the main text.
  3. [Text near d0 fit] The statement that the background fraction is 'fixed in accord with the fit results in Delta(p)/p_ID' is vague; specify how the fixed value and its uncertainty are propagated into the final v2 uncertainties.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the charm/bottom muon v2 is an extracted measurement whose inputs (template shapes, reference hadron v2) do not determine the reported result by construction.

full rationale

This is an experimental measurement, not a model-derived prediction, and the derivation chain is self-contained. The heavy-flavor muon v2 is obtained from the template fit to muon-hadron correlations as v_mu_2 = v_2,2(mu,h)/v_h_2(h), where v_2,2 is the fitted cosine amplitude of the ridge term in the high-multiplicity correlation after subtracting the scaled low-multiplicity correlation, and v_h_2 is taken from a previous ATLAS charged-hadron measurement. This factorization is not circular: the numerator is measured in this analysis and the denominator is an independent published observable for a different particle species; neither quantity is defined in terms of the other. The low-multiplicity template subtraction relies on the explicitly stated assumption that the shape of nonflow correlations is multiplicity independent, and the paper reports a simulation-based test as well as systematic variations of the LM window, the Delta-eta gap, and the f_b extraction. That is a scientific assumption subject to systematic uncertainty, not a logical reduction of the result to its inputs: the fit has free parameters and could have returned a v2 consistent with zero. The charm-versus-bottom separation is likewise obtained from measured d0 distributions and two independent v_sig measurements with different bottom fractions, not from a pre-imposed input. The self-citations to previous ATLAS analyses are methodological or provide the reference v_h_2; they do not carry the central physics conclusion, and no manufactured circularity should be inferred from their presence.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The central claim depends on two fitted fractions (f_sig and f_b_to_mu) and on assumptions about nonflow shape and simulation fidelity, rather than on model parameters tuned to produce the result.

free parameters (3)
  • Signal muon fraction f_sig = Extracted per Delta p/pID bin; not listed numerically
    Obtained by fitting the Delta p/pID distribution with simulation templates; used to extrapolate v_{2,2} to pure heavy-flavor muons.
  • Bottom-muon fraction f_b_to_mu = About 0.4 at pT=4 GeV rising to 0.6 at pT=7 GeV
    Extracted from the d0 template fit; propagates into the charm and bottom v2 separation.
  • Template fit coefficients F, G, v_{n,n} = Fitted per multiplicity and pT bin
    Free parameters of the template fit to the correlation function; the reported v2 is built from v_{2,2} and the external charged-hadron v2.
assumptions (4)
  • domain assumption The shape of nonflow correlations is independent of event multiplicity.
    Required for the template fit subtraction of nonflow in high-multiplicity events; stated in the text and tested only with simulation (Ref [27]).
  • domain assumption Flow factorization v_mu_n = v_{n,n} / v_h_n holds.
    Used to convert muon-hadron correlation v_{2,2} into muon v2 using charged-hadron v2 from ATLAS.
  • domain assumption Pythia8 with A14 tune and Geant4 simulation accurately model the detector response and the Delta p/pID and d0 distributions.
    Signal and background templates for the f_sig and f_b_to_mu fits are taken from simulation; mismodeling would bias the charm/bottom separation.
  • domain assumption The non-heavy-flavor signal muon fraction is about 2.5% and can be fixed in the d0 fit.
    This fraction is taken from Pythia8 simulation and varied only within 0-5% for systematics; it affects the charm/bottom decomposition.

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Cite this review

Pith. "Pith review of Measurement of azimuthal anisotropy of muons from charm and bottom hadrons in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector." pith.science (2026). https://pith.science/paper/JZJAOKMK

@misc{pith2026190901650,
  author       = {Pith},
  title        = {Pith review of: Measurement of azimuthal anisotropy of muons from charm and bottom hadrons in $pp$ collisions at $\sqrts=13$ TeV with the ATLAS detector},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JZJAOKMK}},
  note         = {Machine review of arXiv:1909.01650}
}
abstract

The elliptic flow of muons from the decay of charm and bottom hadrons is measured in $pp$ collisions at $\sqrt{s}=13$ TeV using a data sample with an integrated luminosity of 150 pb$^{-1}$ recorded by the ATLAS detector at the LHC. The muons from heavy-flavor decay are separated from light-hadron decay muons using momentum imbalance between the tracking and muon spectrometers. The heavy-flavor decay muons are further separated into those from charm decay and those from bottom decay using the distance-of-closest-approach to the collision vertex. The measurement is performed for muons in the transverse momentum range 4-7 GeV and pseudorapidity range $|\eta|<2.4$. A significant non-zero elliptic anisotropy coefficient $v_{2}$ is observed for muons from charm decays, while the $v_{2}$ value for muons from bottom decays is consistent with zero within uncertainties.

Discussion (0). Continue with ORCID to comment.

Reference graph

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