REVIEW
Effects of Multiple Scattering in Cold Nuclear Matter on J/psi Suppression and <p_T^2> in Heavy Ion Collisions
Nuclear Theory · Submitted 24 Feb 2006 · 30 references
Pith's verdict: A careful MC study that credibly rules out 2D Gaussian cold-matter rescattering as the sole source of J/psi suppression, but the stronger 'any value of <k_T^2>' claim is tied to that specific kick model.
T0 · AI review
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Referee report
The paper
The paper claims
A Monte-Carlo study of incoherent multiple scattering of charm quark pairs in cold nuclear matter concludes that no value of the transverse-momentum fluctuation strength can simultaneously reproduce the measured J/psi suppression and…
The reading
What carries the argument
The load-bearing mechanism is a momentum-space random walk: each charm quark and antiquark receives a set of independent, small transverse momentum kicks whose total is Gaussian-distributed (Eq. 6), with the strength per unit pathlength set by $\langle k_T^2\rangle$. The accumulated change in the c-cbar relative momentum is driven by the nonrelativistic approximation $\Delta q^2 \approx \Delta k_T^2 + 2 \Delta k_T q \cos\theta$ (Eq. 7), which couples the suppression mechanism (growth of $q^2$ reduces the J/psi formation probability $F(q^2)$) to the observable broadening $\Delta\langle p_T^2\rangle$. The effective formation probabilities in Eq. (8) show how including fluctuations in the magnitude and direction of the kicks converts exponential suppression into a milder power-law behavior. A Monte-Carlo model of the nuclear collision geometry provides the pathlengths, and the event generator for proton-proton collisions provides the initial $q^2$ distribution of the pairs.
Extended reading notes
Core claim
The central claim is that in the incoherent multiple-scattering framework, with independent two-dimensional Gaussian transverse momentum kicks on the charm quark and the charm antiquark, the experimentally measured J/psi suppression and transverse momentum broadening from nucleus-nucleus collisions at the SPS and at RHIC cannot be described simultaneously for any value of the fluctuation strength parameter $\langle k_T^2\rangle$. The argument runs through the nonrelativistic relation between the change in the pair's relative momentum and the transverse momentum broadening (Eq. 7), which forces increasing suppression to come with increasing $\langle p_T^2\rangle$. In the $(R_{AA}, \Delta\langle p_T^2\rangle)$ plane, the model traces a one-parameter curve; the measured points from central SPS and RHIC collisions lie away from that curve. The same conclusion holds for both color-singlet and color-octet J/psi formation, and the paper stresses that fluctuations make suppression much weaker than the earlier constant-shift treatment, for a given rescattering strength.
Load-bearing premise
The conclusion rests on the assumption that the charm quark and antiquark are kicked independently with Gaussian-distributed transverse momenta, and that only the transverse relative momentum determines J/psi formation; if the kicks are correlated, or longitudinal momentum transfer matters, or the kick distribution is not Gaussian, the curve linking suppression and <$p_T^{2}$> could be different.
Editorial extensions
If this is right
- Any incoherent rescattering model of this type predicts a one-parameter family of (suppression, <p_T^2>) pairs, so measuring both observables in the same system tightly constrains or excludes the model.
- The earlier coherent-scattering conclusion that cold matter alone explains Pb+Pb suppression is shown to depend on ignoring fluctuations, since fluctuations dramatically weaken the suppression at a fixed rescattering strength.
- With fluctuations included, color-singlet and color-octet formation probabilities become nearly indistinguishable, so J/psi suppression data alone cannot discriminate the production mechanism in this framework.
- Since cold-matter rescattering cannot account for the data, additional suppression from a hot deconfined medium, or some other mechanism, is required to explain the observed J/psi yields in nucleus-nucleus collisions.
- The same suppression-broadening correlation can be computed for other quarkonium states, such as the Upsilon, and compared with future data as a test of the incoherent multiple-scattering picture.
Reading between the lines
- A testable extension is to model correlated kicks, where the quark and antiquark receive equal-and-opposite transverse momenta so the pair's total p_T is unchanged; this would break the tight relation between suppression and <p_T^2> and could in principle rescue this class of models.
- The paper's no-fit statement is for the 2D Gaussian case; since the 3D random walk gives stronger suppression at moderate pathlengths, a full 3D analysis might shift the curve, although the mismatch for central events would likely survive.
- The (R_AA, Delta<p_T^2>) curve could serve as a reference line for future p+Pb or Pb+Pb measurements at the LHC, offering a quick way to decide whether incoherent rescattering contributes significantly to quarkonium suppression.
Original abstract
Coherent multiple scatterings of c and cbar quark pairs in the environment of heavy ion collisions have been used in a previous work by Qiu et al. [Qiu:1998rz] to study J/psi suppression. That model suggests that heavy quark re-scatterings in a cold nuclear medium can completely explain the centrality dependence of the observed J/psi suppression in Pb+Pb collisions at the SPS [Abreu:2001kd]. Their calculations also revealed significant differences under the assumptions of a color singlet or color octet production mechanism. A more recent analytic calculation [Fujii:2003fq], which includes incoherent final-state re-scatterings with explicit momentum transfer fluctuations in three dimensions, indicates much less suppression and little sensitivity to the production mechanism. In this article, we study simultaneously both the J/psi suppression and p_T modifications, at SPS and RHIC energies. We mainly focus on incoherent momentum transfer fluctuations in two dimensions, which is more appropriate for the heavy-ion collision kinematics. Our analytic and Monte-Carlo calculations reinforce the analytic results in [Fujii:2003fq]. Additionally, we find that the experimental J/psi suppression and <p_T^2> from nucleus-nucleus collisions at the SPS or RHIC cannot simultaneously be described in this incoherent multiple scattering framework for any value of the fluctuation strength parameter <k_T^2>.
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
Coherent multiple scatterings of c and cbar quark pairs in the environment of heavy ion collisions have been used in a previous work by Qiu et al. [Qiu:1998rz] to study J/psi suppression. That model suggests that heavy quark re-scatterings in a cold nuclear medium can completely explain the centrality dependence of the observed J/psi suppression in Pb+Pb collisions at the SPS [Abreu:2001kd]. Their calculations also revealed significant differences under the assumptions of a color singlet or color octet production mechanism. A more recent analytic calculation [Fujii:2003fq], which includes incoherent final-state re-scatterings with explicit momentum transfer fluctuations in three dimensions, indicates much less suppression and little sensitivity to the production mechanism. In this article, we study simultaneously both the J/psi suppression and p_T modifications, at SPS and RHIC energies. We mainly focus on incoherent momentum transfer fluctuations in two dimensions, which is more appropriate for the heavy-ion collision kinematics. Our analytic and Monte-Carlo calculations reinforce the analytic results in [Fujii:2003fq]. Additionally, we find that the experimental J/psi suppression and <p_T^2> from nucleus-nucleus collisions at the SPS or RHIC cannot simultaneously be described in this incoherent multiple scattering framework for any value of the fluctuation strength parameter <k_T^2>.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Coherent multiple scatterings of c and cbar quark pairs in the environment of heavy ion collisions have been used in a previous work by Qiu et al. [Qiu:1998rz] to study J/psi suppression. | background 1.00 | 0.03 | 0.01 | asserted 1.00 |
| s1 | That model suggests that heavy quark re-scatterings in a cold nuclear medium can completely explain the centrality dependence of the observed J/psi suppression in Pb+Pb collisions at the SPS [Abreu:2001kd]. | background 0.99 | 0.04 | 0.04 | hedged possibility 0.89 |
| s2 | Their calculations also revealed significant differences under the assumptions of a color singlet or color octet production mechanism. | background 0.96 | 0.04 | 0.02 | asserted 1.00 |
| s3 | A more recent analytic calculation [Fujii:2003fq], which includes incoherent final-state re-scatterings with explicit momentum transfer fluctuations in three dimensions, indicates much less suppression and little sensitivity to the production mechanism. | background 1.00 | 0.04 | 0.04 | hedged possibility 0.88 |
| s4 | In this article, we study simultaneously both the J/psi suppression and p_T modifications, at SPS and RHIC energies. | contribution 1.00 | 0.77 | 0.03 | asserted 0.97 |
| s5 | We mainly focus on incoherent momentum transfer fluctuations in two dimensions, which is more appropriate for the heavy-ion collision kinematics. | method 0.89 | 0.54 | 0.03 | asserted 0.87 |
| s6 | Our analytic and Monte-Carlo calculations reinforce the analytic results in [Fujii:2003fq]. | result 0.89 | 0.90 | 0.07 | asserted 1.00 |
| s7 | Additionally, we find that the experimental J/psi suppression and <p_T^2> from nucleus-nucleus collisions at the SPS or RHIC cannot simultaneously be described in this incoherent multiple scattering framework for any value of the fluctuation strength parameter <k_T^2>. | result 0.98 | 0.96 | 0.89 | asserted 1.00 |
What was examined, and what was not
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Figures
Showing 5 of the paper's 7 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:nucl-th/0602068 by the authors.
References
Discussion
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