REVIEW
The production of doubly charmed exotic hadrons in heavy ion collisions
High Energy Physics - Phenomenology · Submitted 16 Sep 2021 · 76 references
Pith's verdict: The qualitative claim — central Pb-Pb is a far better source of T_cc than pp — is almost certainly right, but the 'three orders of magnitude' headline rests on an uncalibrated coalescence window and a polynomial extrapolation, not on a pp simulation.
T0 · AI review
Why Pith says this
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Referee report
The paper
The paper claims
Heavy ion collisions should produce the doubly charmed tetraquark $T_{cc}^+$ at roughly a thousand times the proton-proton rate, with a centrality dependence that distinguishes it from the $X(3872)$.
The reading
What carries the argument
The machinery is charm-meson coalescence inside an AMPT transport simulation: after hadronization, $D$ and $D^*$ mesons are paired into molecules when their relative distance falls in the window $[5~mathrm{fm}, 7~mathrm{fm}]$ and their invariant mass falls in $[2M_D, 2M_{D^*}]$. Because AMPT tracks flavor but not spin, the $D^*/D$ ratio is fixed separately by a thermal-model relation $R(AB) = e^{-(m_A - m_B)/T_{mathrm{freezeout}}}$ at $T_{mathrm{freezeout}} simeq 160$ MeV, giving about $29%$ $D^{*+}$ versus $71%$ $D^+$ and $29%$ $D^{*0}$ versus $71%$ $D^0$, with a $pm 10%$ variation used as the uncertainty band. The second load-bearing piece is the combinatorial threshold argument: with $N$ charm quarks and $D^*$ fraction $R$, the $X(3872)$ formation probability is $P_X = 1 - R^{2N} - (1-R)^{2N}$ while the $T_{cc}$ probability is $P_T = 1 - R^{N} - (1-R)^{N}$, so the doubly charmed state is suppressed whenever the charm multiplicity per event is small, which is exactly the peripheral regime.
Extended reading notes
Core claim
Within the $D D^*$ molecular picture, the $T_{cc}^+$ yield in Pb-Pb collisions at $2.76$ TeV is enhanced by roughly three orders of magnitude over the proton-proton yield, and in the most central events it is about equal to the $X(3872)$ yield computed in the same simulation. The authors report inclusive yields per million minimum-bias events of about 49000 for the $X(3872)$, 44000 for $T_{cc}^0$, 44000 for $T_{cc}^{++}$ and 50000 for $T_{cc}^+$. They explain why these four numbers sit at the same order despite naive charm counting suggesting more $cbar c$ than $cc$ pairs: molecular formation consumes a $D$ together with a $D^*$, so both the $X(3872)$ and the $T_{cc}$ rates scale as $N^2 R(1-R)$ in the charm-pair number $N$ and the $D^*$ fraction $R$. The distinguishing prediction is in the centrality dependence: the $T_{cc}^+$-to-$X(3872)$ ratio falls steeply from central to peripheral collisions because forming a $T_{cc}$ requires at least one $D$ and one $D^*$ pair, a probability $P_T = 1 - R^N - (1-R)^N$ that vanishes as $N to 1$, whereas the $X(3872)$ needs only a $D$ and a $bar D^*$. Extrapolating the ratio to the ultra-peripheral limit gives at least three orders of magnitude suppression relative to the $X(3872)$, consistent with the LHCb proton-proton ratio, and at least two orders of magnitude suppression of the iso-triplet partners relative to $T_{cc}^+$, which the authors offer as a possible reason the iso-triplet states have not been seen.
Load-bearing premise
The claim rests on counting a $D$ and a $D^*$ as bound whenever their relative distance lands in the hand-picked window of 5 to 7 femtometres, a choice inherited from earlier work that fixes the absolute size of every yield and, because the fireball shrinks in peripheral collisions, also drives the reported centrality slope.
Editorial extensions
If this is right
- Central and semi-central Pb-Pb collisions should contain $T_{cc}^+$ at a rate comparable to the $X(3872)$, making a simultaneous measurement of both feasible with the same data set.
- The $T_{cc}^+$-to-$X(3872)$ yield ratio should drop by roughly three orders of magnitude from the most central to the most peripheral events, a much steeper centrality slope than either state shows on its own.
- The iso-triplet partners $T_{cc}^0$ and $T_{cc}^{++}$ should be at least two orders of magnitude rarer than $T_{cc}^+$ in the ultra-peripheral limit, which would explain their absence from current LHCb data.
- Rapidity distributions should be flat near mid-rapidity and fall off at forward and backward rapidity, while $p_T$ spectra should fall steeply, as expected for thermal emission with radial flow.
- The elliptic flow coefficient $v_2$ of $T_{cc}$ should closely track that of the $X(3872)$ and of the underlying charm mesons, because all of them inherit similar spatial distributions in the fireball.
Reading between the lines
- If the threshold argument is right, the centrality slope of the $T_{cc}^+$-to-$X(3872)$ ratio is a direct handle on the number of $cbar c$ pairs per event, so the ratio could double as an independent charm-multiplicity counter in heavy ion runs.
- The same combinatorial logic applies to any exotic whose valence content requires a fixed number of heavy quarks, so triply charmed or $cc$-plus-strange candidates should show even steeper centrality slopes and could be separated from single-charm backgrounds on that basis alone.
- Because the absolute normalization is set by a hand-chosen coalescence window rather than by a derived binding probability, the predicted enhancement factor is the softest number in the paper; the ratio between states and its centrality shape are more robust than the absolute yield.
- A measurement of $v_2$ for $T_{cc}$ that differs from that of the $X(3872)$ would point to a compact tetraquark component rather than a loosely bound molecule, since a compact state need not inherit the spatial distribution of its would-be constituents.
Original abstract
Hadron spectroscopy provides direct physical measurements that shed light on the non-perturbative behavior of quantum chromodynamics (QCD). In particular, various exotic hadrons such as the newly observed $T_{cc}^+$ by the LHCb collaboration, offer unique insights on the QCD dynamics in hadron structures. In this letter, we demonstrate how heavy ion collisions can serve as a powerful venue for hadron spectroscopy study of doubly charmed exotic hadrons by virtue of the extremely charm-rich environment created in such collisions. The yields of $T_{cc}^+$ as well as its potential isospin partners are computed within the molecular picture for Pb-Pb collisions at center-of-mass energy $2.76~\mathrm{TeV}$. We find about three-order-of-magnitude enhancement in the production of $T_{cc}^+$ in Pb-Pb collisions as compared with the yield in proton-proton collisions, with a moderately smaller enhancement in the yields of the isospin partners $T_{cc}^0$ and $T_{cc}^{++}$. The $T_{cc}^+$ yield is comparable to that of the $X(3872)$ in the most central collisions while shows a considerably stronger decrease toward peripheral collisions, due to a "threshold" effect of the required double charm quarks for $T_{cc}^+$. Final results for their rapidity and transverse momentum $p_T$ dependence as well as the elliptic flow coefficient are reported and can be tested by future experimental measurements.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
Hadron spectroscopy provides direct physical measurements that shed light on the non-perturbative behavior of quantum chromodynamics (QCD). In particular, various exotic hadrons such as the newly observed $T_{cc}^+$ by the LHCb collaboration, offer unique insights on the QCD dynamics in hadron structures. In this letter, we demonstrate how heavy ion collisions can serve as a powerful venue for hadron spectroscopy study of doubly charmed exotic hadrons by virtue of the extremely charm-rich environment created in such collisions. The yields of $T_{cc}^+$ as well as its potential isospin partners are computed within the molecular picture for Pb-Pb collisions at center-of-mass energy $2.76~\mathrm{TeV}$. We find about three-order-of-magnitude enhancement in the production of $T_{cc}^+$ in Pb-Pb collisions as compared with the yield in proton-proton collisions, with a moderately smaller enhancement in the yields of the isospin partners $T_{cc}^0$ and $T_{cc}^{++}$. The $T_{cc}^+$ yield is comparable to that of the $X(3872)$ in the most central collisions while shows a considerably stronger decrease toward peripheral collisions, due to a "threshold" effect of the required double charm quarks for $T_{cc}^+$. Final results for their rapidity and transverse momentum $p_T$ dependence as well as the elliptic flow coefficient are reported and can be tested by future experimental measurements.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Hadron spectroscopy provides direct physical measurements that shed light on the non-perturbative behavior of quantum chromodynamics (QCD). | background 1.00 | 0.04 | 0.02 | asserted 0.99 |
| s1 | In particular, various exotic hadrons such as the newly observed $T_{cc}^+$ by the LHCb collaboration, offer unique insights on the QCD dynamics in hadron structures. | background 1.00 | 0.04 | 0.02 | asserted 0.93 |
| s2 | In this letter, we demonstrate how heavy ion collisions can serve as a powerful venue for hadron spectroscopy study of doubly charmed exotic hadrons by virtue of the extremely charm-rich environment created in such collisions. | contribution 1.00 | 0.51 | 0.19 | asserted 1.00 |
| s3 | The yields of $T_{cc}^+$ as well as its potential isospin partners are computed within the molecular picture for Pb-Pb collisions at center-of-mass energy $2.76~\mathrm{TeV}$. | method 0.78 | 0.93 | 0.07 | asserted 0.99 |
| s4 | We find about three-order-of-magnitude enhancement in the production of $T_{cc}^+$ in Pb-Pb collisions as compared with the yield in proton-proton collisions, with a moderately smaller enhancement in the yields of the isospin partners $T_{cc}^0$ and $T_{cc}^{++}$. | result 1.00 | 0.96 | 0.60 | asserted 1.00 |
| s5 | The $T_{cc}^+$ yield is comparable to that of the $X(3872)$ in the most central collisions while shows a considerably stronger decrease toward peripheral collisions, due to a "threshold" effect of the required double charm quarks for $T_{cc}^+$. | result 0.89 | 0.92 | 0.15 | asserted 1.00 |
| s6 | Final results for their rapidity and transverse momentum $p_T$ dependence as well as the elliptic flow coefficient are reported and can be tested by future experimental measurements. | result 0.45 | 0.92 | 0.08 | asserted 0.95 |
What was examined, and what was not
- arXiv listing abstract v1
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:2109.07733 by the authors.
References
How the literature uses it
38 arXiv papers cite this in 43 sentences
Papers citing it · 14 sentences without a confident label
Cites it as background 28 papers
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This observation triggered many related theoretical and experimental studies and many groups performed analyses to figure out the spectroscopic parameters, decay modes, magnetic dipole moments, and production mechanisms of doubly-heavy tetraquark states including $T_cc^+$ via various approaches and models [ref] .
Exploring the magnetic dipole moments of $T_{QQ \bar q \bar s}$ and $T_{QQ \bar s \bar s}$ states in the framework of QCD light-cone sum rules · Introduction · 0.90
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As a chain reaction, this announcement has stimulated numerous works [ref] , with different outcomes depending on the analysis.
$T_{cc}^{+}$ coupled channel analysis and predictions · Introduction · 0.96
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The mass of the $T^+_cc$ state with respect to the $D^*+D^0$ mass threshold and the width are [equation] For the doubly charmed tetraquarks, their properties have been studied extensively in the past decades, and there exists abundant experimental and theoretical works [ref] .
Electromagnetic properties of the $T^+_{cc}$ molecular states · Introduction · 0.90
What it cites · 31 sentences without a confident label
Uses the method or tool of 1 paper
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Framework For this study, we adopt the framework developed in Ref. [ref] to generate a total of one million minimum bias events from the default version of AMPT transport model for Pb-Pb collisions at $√(s)=2.76 TeV$ for simulating the production of both $X(3872)$ and the $T_cc$ in these collisions.
Deciphering the nature of X(3872) in heavy ion collisions · Introduction · 0.99
Compares with 8 papers
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We note that the extrapolated result in that limit shows consistency with the corresponding ratio between $T_cc^+$ and $X(3872)$ in $pp$ collisions from LHCb measurements [ref] .
Observation of X(3872) production in pp collisions at sqrt(s)= 7 TeV · Introduction · 0.87
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We note that the extrapolated result in that limit shows consistency with the corresponding ratio between $T_cc^+$ and $X(3872)$ in $pp$ collisions from LHCb measurements [ref] .
Measurement of the X(3872) production cross section via decays to J/psi pi pi in pp collisions at sqrt(s) = 7 TeV · Introduction · 0.87
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We note that the extrapolated result in that limit shows consistency with the corresponding ratio between $T_cc^+$ and $X(3872)$ in $pp$ collisions from LHCb measurements [ref] .
2009.06619 · Introduction · 0.87
Cites as background 38 papers
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The observation of the exotics with two or more heavy quarks also provides a way to shed light on the potential symmetry (such as diquark-antiquark symmetry [ref] ).
Doubly Heavy Baryons, Heavy Quark-DiQuark Symmetry and NRQCD · Introduction · 0.82
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The observation of the exotics with two or more heavy quarks also provides a way to shed light on the potential symmetry (such as diquark-antiquark symmetry [ref] ).
Effective Field Theory Lagrangians for Baryons with Two and Three Heavy Quarks · Introduction · 0.82
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There are many theoretical studies of the open double heavy tetraquark system in the literature, focusing on key issues such as the formation mechanism (i.e. whether the double heavy tetraquark system is bound or not in either molecular picture or compact tetraquark picture) [ref] , the double heavy exotic spectrum [ref] and the decay modes/production mechanism [ref] and their magnetic dipole moments [ref] .
Stable multiquark states with heavy quarks in a diquark model · Introduction · 1.00
Discussion
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